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Class Loading Algorithm
Class Loading Algorithm
Overview
The OSGi bundle class loading algorithm plays a key role in the OSGi framework.
Ultimately, it is this algorithm that defines the relationship between a bundle and
its dependencies.
Requirements on a bundle class loader
Here are some of the requirements that a bundle class loader must satisfy in order
to support a flexible and consistent class loading architecture:
In order to avoid loading multiple copies of a class, the bundle class
loader must first of all try to find the class in one of its dependent
bundles.
The bundle class loader must never load a duplicate
copy of a class into its class space.
Bundle class loading algorithm
The following is a simplified description of the bundle class loading algorithm
(for example, it does not enumerate all of the ways in which class loading can
fail). For a full description of the algorithm, see the Runtime Class
Loading section of the OSGi Core
Specification.
If the class belongs to one of the java.* packages or any
packages listed in the org.osgi.framework.bootdelegation
property, the bundle class loader delegates to the parent class
loader.
If the class belongs to one of the packages listed in
Import-Package, the bundle class loader delegates loading
to the corresponding exporter bundle.
If the class belongs to one of the packages imported by
Require-Bundle, the bundle class loader delegates loading
to the corresponding exporter bundle.
Note
It is strongly recommended that you avoid using the
Require-Bundle header. OSGi dependencies are meant to
be defined at package granularity, not bundle
granularity.
Next, the bundle class loader looks for the class amongst its internal
classes (inside its own JAR file).
Next, the bundle class loader searches the internal classes of any
fragments attached to the bundle.
Finally, if the class belongs to one of the packages imported using
DynamicImport-Package, the bundle class loader delegates
loading to the corresponding exporter bundle (if there is one).
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tHAF OEBPS/BCL-ClassLoader.html
Class Loader Basics
Class Loader Basics
Overview
If you are concerned about which version of a class your application uses, you
inevitably have to start dealing with class loaders. In Java, the class loader is
the mechanism that controls where class definitions come from and decides whether or
not to allow a particular class definition to be used. So, when it comes to
versioning and security, class loaders play a central role.
Because version control is also important in OSGi, it follows that class loaders
play a key role in the OSGi architecture. One of the main aims of OSGi is to make
class loading more flexible and more manageable.
Bundle class loader
A fundamental rule in OSGi is that every bundle has its own class
loader and this class loader is known as the bundle class
loader.
You might wonder whether it is really necessary to have a class loader for every
bundle. The class loader per bundle architecture is chosen for the following
reasons:
A class loader encapsulates knowledge about where to find classes and a
particular bundle might have knowledge about finding classes that other
bundles do not possess. For example, a bundle can have private classes, not
directly visible to other bundles.
In order to support the flexible OSGi class loading architecture, each
bundle needs to be in control of how and where it finds its classes.
Identity of a loaded class
In the context of OSGi, where a typical application depends on multiple class
loaders (one for each bundle), it is important to understand that the
fully-qualified class name—for example, org.foo.Hello—is
not sufficient to identify a loaded class uniquely. In
general, a loaded class is uniquely identified by combining the classloader identity
with the fully-qualified class name.
For example, if the class, org.foo.Hello, gets loaded by the bundle
classloader for version 1.3 of bundle A, the loaded class is uniquely
identified by the combination of the class loader, A;1.3, and the class
name, org.foo.Hello. In this chapter, the following notation is used
for the unique identity of the loaded class:
A;1.3/org.foo.Hello
Bundle class space
The OSGi specification defines a bundle's class space to be
the set of all classes accessible to the bundle's class loader, where each class in
the set is a unique instance, qualified by the class loader that loaded it.
A bundle class space includes a bundle's private classes, as well as any public
classes imported from the bundle's dependencies.
Multiple copies of a loaded class
When you have multiple class loaders at parallel locations in the class loader
hierarchy (which is the case with the bundle class loaders), it is actually possible
to load multiple, distinct, copies of the same class.
For example, given the appropriate conditions, it is possible that version 1.3 of
bundle A and version 2.0 of bundle B could both independently load the class,
org.foo.Hello. In this case, you would have two distinct loaded
classes, as follows:
A;1.3/org.foo.Hello
B;2.0/org.foo.Hello
The OSGi framework is designed to prevent multiple copies of
a class ever appearing in the same class space, because this almost always causes
errors at run time. On the other hand, it is permissible for different copies of a
class to be loaded into different class spaces. For example, it
is perfectly alright for separate applications running in an OSGi container to use
different versions of the same class.
Class loader delegation
One of the key class loader concepts is class loader
delegation, which is where a class loader, instead of loading a
class itself, asks another class loader to load the class. When class loader
delegation occurs, you need to distinguish between:[1]
Initial class loader
The class loader that is initially asked to load the class.
Effective class loader
The class loader that actually loads the class,
after delegation is taken into account.
[1] Terminology coined by Andreas Schaefer, an O'Reilly Java author.
One of the main aims of the OSGi class loading architecture is to avoid loading
conflicting class definitions into the same bundle class space. This section
explains how such conflicts can arise in practice.
Symbolic references
To explain the difficulties that can be caused by having multiple copies of a
loaded class, we need the concept of a symbolic reference to
a class or interface. A symbolic reference is a point in your code where a class
name is used, except the actual definition of the class. For example, when a class
name is used to specify a return type, a parameter type, or a variable type, the
class name is a symbolic reference.
At run time, the JVM must resolve each symbolic reference by linking it to a
specific instance of a loaded class. Symbolic references are resolved using the same
class loader as the class in which they appear. For example, Example 1.1 shows the definition of the
TestHello class, which is loaded by the class loader,
A.
Example 1.1. Symbolic Reference Used in a Class Cast
// Java
package org.bar;
// TestHello loaded by ClassLoader 'A'
public class TestHello {
public void useHelloObj(Object hello) {
org.foo.Hello h = (org.foo.Hello) hello;
System.out.println(h.getGreeting());
}
}
Important
Although the symbolic reference, org.foo.Hello, is initially
loaded by class loader A, this does not imply
that the symbolic references must be resolved to A/org.foo.Hello.
If the initial class loader A decides to delegate to another class
loader, B, the symbolic reference could be resolved to
B/org.foo.Hello instead (so that B is the
effective class loader). The delegation mechanism is crucial, because it enables
an OSGi bundle to re-use existing loaded classes and avoid class space
inconsistencies.
Class cast exceptions
When multiple class loaders are used in parallel (as happens in OSGi), there is a
danger that a class could be loaded more than once. This is undesirable, because it
almost inevitably causes class cast exceptions at run time.
For example, consider the TestHello class shown in Example 1.1, where the org.foo.Hello
symbolic reference has been resolved to A/org.foo.Hello. If you also
have a Hello object that is an instance of B/org.foo.Hello
type, you will get a class cast exception when you pass this object to the
TestHello.useHelloObj(Object) method. Specifically, in the line of
code that performs the following cast:
org.foo.Hello h = (org.foo.Hello) hello;
The org.foo.Hello symbolic reference has been resolved to the
A/org.foo.Hello type, but the hello object is of
B/org.foo.Hello type. Because the types do not match, you get the
class cast exception.
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tHAX OEBPS/BCL.html
Chapter 1. Bundle Class Loader
Chapter 1. Bundle Class Loader
This chapter introduces some basic OSGi class loading concepts. Ultimately, in
order to understand the relationship between a bundle and its dependencies, and to
understand the meaning of importing and exporting packages, it is essential to learn
about the bundle class loader.
This section explains how to set up a Maven project for a typical API
bundle.
The hello-paris bundle exemplifies an API bundle, which contains only
public Java interfaces. Hence, the API bundle should export all of its own packages
and associate a version number with the exported packages.
Directory structure
The hello-paris bundle has the following directory
structure:
The Java source code is located under the src/main/java
sub-directory. The org.fusesource.example.hello.paris package is public
and all of its classes and interfaces can be exported from the bundle.
There are no blueprint resources associated with this bundle.
Sample API
The hello-paris bundle is a pure API, which means it contains
only Java interfaces. In this example, there is a single interface,
HelloParis, which returns a localized greeting and a
Clock object that tells the local time. The
HelloParis interface is defined as follows:
// Java
package org.fusesource.example.hello.paris;
import org.fusesource.example.time.Clock;
public interface HelloParis {
public String getGreeting();
public Clock getLocalTime();
}
Maven dependencies
In the Maven POM file, the hello-paris bundle defines
dependencies on the following Maven artifact:
time-util
Import and export rules
The following import and export rules apply to the hello-paris
bundle:
Exporting own packages—the
org.fusesource.example.hello.paris package is public,
and must be exported.
Importing own packages—none of the bundle's
own packages should be imported.
Importing dependent packages—any external
package dependencies must be imported.
Maven bundle plug-in settings
The Maven bundle plug-in is configured to export the API package,
org.fusesource.example.hello.paris (coded as
${project.groupId}.hello.paris*). The
Export-Package instruction also contains entries to block the
export of any packages containing .impl or .internal.
In this case, the bundle plug-in instructions are as follows:
The Import-Package header lists one external package dependency,
org.fusesource.example.time. None of the bundle's own packages
are imported.
The Export-Package header is used to export the API package,
org.fusesource.example.hello.paris, while the uses
clause declares a transitive dependency on the
org.fusesource.example.time package.
This section explains how to set up a Maven project for a consumer bundle.
The hello-consumer bundle exemplifies a consumer bundle, which
imports OSGi services and accesses the services through the relevant API packages.
This is the bundle that drives the sample application and, therefore, it relies on a
blueprint lifecycle callback (through the blueprint bean element's
init-method attribute) to initiate processing.
Note
Being a consumer is just a role, not an absolute category, so you will
commonly come across bundles that behave both as a consumer and a provider. In
the current example, however, hello-consumer behaves as a pure
consumer.
Directory structure
The hello-consumer bundle has the following directory
structure:
The org.fusesource.example.hello.consumer package is public and
all of its classes and interfaces are exported from the bundle. It would not
matter, however, if you made this package private instead, because it is not
needed by any other bundles.
The src/main/resources/OSGI-INF/blueprint directory contains a
single blueprint file, client.xml. Any file matching the pattern,
*.xml, in this directory is assumed to be a blueprint
configuration file.
Sample consumer code
The hello-consumer bundle effectively drives the sample
application, obtaining references to the HelloBoston and
HelloParis OSGi services, and then invoking methods on these
services to obtain localised greetings and times.
The hello-consumer bundle contains the class,
ConsumeHello, which is a client of the OSGi services,
HelloBoston and HelloParis. To gain access to the
OSGi services, ConsumeHello defines the setter methods,
getHelloBoston() and getHelloParis(), and relies
on the blueprint framework to inject the references. The entry point is the
init() method, which gets invoked after the
ConsumeHello bean is created and injected with the service
references. The ConsumeHello class is defined as follows:
// Java
package org.fusesource.example.hello.consumer;
import org.fusesource.example.hello.boston.HelloBoston;
import org.fusesource.example.hello.paris.HelloParis;
public class ConsumeHello {
protected HelloBoston helloBoston = null;
protected HelloParis helloParis = null;
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
}
public void init() {
if (helloBoston==null || helloParis==null) {
System.out.println("Initialization failed. Injected objects are null.");
return;
}
String enGreeting = helloBoston.getGreeting();
String bostonTime = helloBoston.getLocalTime().getLocalTime();
System.out.println("Boston says:" + enGreeting + " at " + bostonTime);
String frGreeting = helloParis.getGreeting();
String parisTime = helloParis.getLocalTime().getLocalTime();
System.out.println("Paris says:" + frGreeting + " at " + parisTime);
}
public HelloBoston getHelloBoston() {
return helloBoston;
}
public void setHelloBoston(HelloBoston helloBoston) {
this.helloBoston = helloBoston;
}
public HelloParis getHelloParis() {
return helloParis;
}
public void setHelloParis(HelloParis helloParis) {
this.helloParis = helloParis;
}
}
Access OSGi service
The ConsumeHello class needs to obtain a reference to the
HelloBoston service and a reference to the
HelloParis service. Use the reference element to
create proxies for the HelloBoston service and for the
HelloParis service. Use the bean element to create
a ConsumeHello instance and inject the helloBoston and
helloParis proxies.
The ConsumeHello bean also requires an entry point to initiate
processing. By setting the bean element's init-method
attribute to init, you ensure that the blueprint framework calls
the ConsumeHello.init() method after all of the bean's properties
have been injected.
For example, the blueprint file, OSGI-INF/blueprint/client.xml,
has the following contents:
In the Maven POM file, the hello-consumer bundle defines
dependencies on the following Maven artifacts:
time-util
hello-paris
hello-boston
Import and export rules
The following import and export rules apply to the hello-consumer
bundle:
Exporting own packages—a client typically
does not need to export its own packages, because a client does not
usually expose an API.
Importing own packages—a client does not
import its own packages.
Importing dependent packages—any external
package dependencies must be imported.
Maven bundle plug-in settings
The Maven bundle plug-in is configured to export the package,
org.fusesource.example.hello.consumer, although the export is
unnecessary in this particular example. The Export-Package
instruction also contains entries to block the export of any packages containing
.impl or .internal. In this case, the bundle
plug-in instructions are as follows:
The Import-Package header imports the external package
dependencies—for example,
org.fusesource.example.hello.boston.
The Export-Package header exports the package,
org.fusesource.example.hello.consumer. In this case, however,
the export is not really needed and the package could have been declared private
instead (for example, using the Private-Package
instruction).
The Import-Service header declares the OSGi services accessed by
this bundle. The services are accessed respectively through the
HelloBoston interface and through the HelloParis
interface.
This section explains how to set up a Maven project for a typical provider
bundle.
The hello-paris-impl bundle exemplifies a provider bundle, which
provides the implementation of an API. The provider does not export any of its own
packages, because the implementation classes are private. But the provider does
instantiate and export an OSGi service, which is accessed through the
HelloParis interface.
Directory structure
The hello-paris-impl bundle has the following directory
structure:
The org.fusesource.example.hello.paris.impl package is private.
By default, the Maven bundle plug-in treats any packages containing the segments
.impl or .internal, as private packages.
The src/main/resources/OSGI-INF/blueprint directory contains a
single blueprint file, paris-svc.xml. Any file matching the
pattern, *.xml, in this directory is assumed to be a blueprint
configuration file.
Sample implementation
The hello-paris-impl bundle is intended to implement all of the
interfaces appearing in the hello-paris API bundle. In this
example, a single HelloParisImpl class implements the
HelloParis interface, as follows:
// Java
package org.fusesource.example.hello.paris.impl;
import org.fusesource.example.hello.paris.HelloParis;
import org.fusesource.example.time.Clock;
import org.fusesource.example.time.TimeUtil;
public class HelloParisImpl implements HelloParis {
protected Clock localTime = null;
public String getGreeting() {
return new String("Bonjour!");
}
public Clock getLocalTime() {
if (localTime==null) {
localTime = TimeUtil.createClock(TimeUtil.TimeZone.PARIS);
}
return localTime;
}
}
Publish OSGi service
The natural way to bootstrap the HelloParis implementation in
OSGi is to publish the class, HelloParisImpl, as an OSGi service.
Use the bean element to create a HelloParisImpl
instance and use the service element to publish the bean,
advertising it as a service of HelloParis type.
For example, the blueprint file,
OSGI-INF/blueprint/paris-svc.xml, has the following
contents:
In the Maven POM file, the hello-paris-impl bundle defines
dependencies on the following Maven artifacts:
time-util
hello-paris
Import and export rules
The following import and export rules apply to the
hello-paris-impl bundle:
Exporting own packages—none of the bundle's
own packages should be exported; they are all private.
Importing own packages—none of the bundle's
own packages should be imported.
Importing dependent packages—any external
package dependencies must be imported.
Maven bundle plug-in settings
You can use the default export rules (that is, omitting the
Export-Package instruction), as long as you take care to put all of
your code into packages containing .impl or .internal
(which are not exported by default). You should explicitly list the implemented API,
org.fusesource.example.hello.paris, in the
Import-Package instruction and add the provide:=true
clause to it. This signals that this bundle is acting as the
provider of the hello.paris package (and
ensures that the API is imported with the correct version range—see Automatic Import Versioning). In this case, the bundle plug-in instructions
are as follows:
The Import-Package header imports external package dependencies
only—for example, org.fusesource.example.hello.paris and
org.fusesource.example.time.
The Export-Service header advertises the OSGi service as a
HelloParis instance. This enables clients to find the
HelloParisImpl instance by searching for a service of
HelloParis type.
This section explains how to set up a Maven project for an API/provider bundle,
where the API packages and implementation packages are kept in
separate Maven projects, but then combined into a single
bundle at build-time.
You can demonstrate an API/provider build-time combination bundle by modifying the
Maven configuration of the hello-paris-impl bundle. By exploiting the
Bnd utility's provide:=true clause, you can modify the instructions for
the Maven bundle plug-in, so that the
org.fusesource.example.hello.paris API package gets included in the
hello-paris-impl bundle at build time.
provide:=true clause
Whenever a bundle plays the role of provider with respect to a particular API
package, you must indicate this explicitly by attaching the
provide:=true clause to the API package, either in an import
instruction or in an export instruction (otherwise, by default, the bundle plug-in
would assume that the bundle is a consumer of the API package).
In particular, for an API/provider build-time combination bundle, you must
export the org.fusesource.example.hello.paris
API package and attach the provide:=true clause, as follows:
Exporting a package with the provide:=true clause has a very
important side effect: if the code for the exported package is not in the
current Maven project, the Maven bundle plug-in adds the package code to the
bundle at build time.
Included Maven projects
Code from the following Maven projects is included in the
hello-paris-impl bundle:
hello-paris-impl
Contains the private package,
org.fusesource.example.hello.paris.impl. Build this
bundle to create the API/provider build-time combination bundle,
hello-paris-impl.
hello-paris
Contains the public API package,
org.fusesource.example.hello.paris. This project is a
prerequisite for hello-paris-impl and must be built (using
mvn install) before the hello-paris-impl
project. You do not deploy the
hello-paris bundle directly into the OSGi container,
however.
Import and export rules
The following import and export rules apply to the hello-paris-impl
bundle, when it is configured as an API/provider build-time combination:
Exporting own packages—no own packages to
export, because the hello-paris-impl Maven project contains
only private implementation packages.
Exporting dependent packages—export the public
API package, org.fusesource.example.hello.paris, with the
provide:=true clause, which causes the API code to be
included in the hello-paris-impl bundle at build time.
Importing own packages—import the public API
package, org.fusesource.example.hello.paris.
Importing dependent packages—any external
package dependencies must be imported.
Maven bundle plug-in settings
To include the API from the hello-paris bundle in the
hello-paris-impl bundle, add the
org.fusesource.example.hello.paris package to the
Export-Package instruction with the provide:=true
clause attached, as shown in Example 2.3. Compare this with the
regular API/provider case, Example 2.2, which
takes the same approach of exporting the API package with the
provide:=true clause. The semantics, however, are a bit different,
because the current example pulls in the API package from a separate Maven
project.
Example 2.3. Plug-In Settings for Build-Time Combination
In the preceding example, the included
org.fusesource.example.hello.paris API package is given the
same version as the hello-paris-impl Maven
project, by adding the version=${project.version} clause to the export
instruction. In practice, however, you might not always want to assign versions in
this way. You might prefer to assign distinct versions to the
API package (from the hello-paris Maven project) and the implementation
package (from the hello-paris-impl Maven project).
The alternative approach you can take is to store version information at
package granularity in a packageinfo file (see
Export versions at package granularity for details). The
Maven bundle plug-in automatically scans your source code, looking for
packageinfo files and extracting the version information from them.
In this case, you must omit the version clause from the export instruction, as
follows:
<Export-Package>
!${project.groupId}*.impl*,
!${project.groupId}*.internal*,
<!-- hello.paris version stored in packageinfo -->
${project.groupId}.hello.paris*;provide:=true,
</Export-Package>
Generated MANIFEST.MF file
When you build the bundle using Maven, the Maven bundle plug-in automatically
generates the following MANIFEST.MF file:
The Import-Package header imports both the public API package,
org.fusesource.example.hello.paris, and the external package
dependencies—for example, org.fusesource.example.time.
The Export-Package header exports the public API package,
org.fusesource.example.hello.paris, while the uses
clause declares a transitive dependency on the
org.fusesource.example.time package.
The Export-Service header advertises the OSGi service as a
HelloParis instance.
Bundle deployment
Because the hello-paris-impl bundle includes all of the code from the
hello-paris Maven project, it is unnecessary
to deploy the hello-paris bundle in this case.
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uHAߕG G # OEBPS/ImportExport-BundleTypes.html
Overview of Bundle Types
Overview of Bundle Types
Overview
The Maven bundle plug-in is a powerful tool for generating OSGi bundle header
information. When set up correctly, it can generate most of the bundle header
information for you automatically. The bundle plug-in has, however, a bewildering
array of options available. In order to make sense of the bundle plug-in
configuration, the first question question you need to answer is: what type of
bundle do I have?
Figure 2.1 shows the most important bundle
types and bundle relationships that you will come across in a typical OSGi
application.
Figure 2.1. Relationship between API, Provider, and Consumer Bundles
A well-designed application normally exposes a block of functionality through a
collection of abstract Java interfaces, which constitute a pure API package. To
provide a clean separation of implementation and interface description, the API
package is normally deployed in one bundle, the API bundle,
and the API implementation is deployed in a separate bundle, the provider
bundle. The code that actually uses the API is referred to as a
consumer and is packaged in a consumer bundle. Each of these
bundle types have differing requirements when it comes to configuring their Maven
bundle plug-in instructions.
Bundle Types
We can identify the following typical bundle types:
Library bundle
A library bundle contains Java classes and interfaces, which are
public and intended to be used by other bundles. Often, in a library,
there is no formal separation between API interfaces and implementation
classes. Instead, developers simply instantiate and use the classes
provided by the library.
A library bundle does not publish or access any OSGi services.
API bundle
A pure API bundle contains only Java interfaces (or abstract
classes), which are public. The implementation of the API is meant to be
provided in a separate bundle or JAR.
An API bundle does not publish or access any OSGi services.
Provider bundle
A provider bundle contains the classes that implement an API.
Usually, the classes in a provider bundle are all private and are
not exported from the bundle.
The natural mechanism for a provider bundle to expose its
functionality is to create and publish one or more OSGi services (where
the OSGi services are then accessed through the public API).
Consumer bundle
A consumer bundle contains code that uses an API.
A consumer bundle typically accesses one or more OSGi services; it
does not usually publish an OSGi service itself (unless it is acting as
a hybrid of a consumer and a provider).
API/provider combination bundle
In some cases, it can make sense to package API packages and
implementation packages together, resulting in an API/provider
combination bundle. For example, if you intend to provide only
one implementation of an API, it can be simpler
to package the API and its implementation in a single bundle.
For the API/provider combination bundle, it is assumed that the API
code and the implementation code belong to the same Maven
project.
API/provider build-time combination bundle
Even if you opt for a formal separation of API code and
implementation code—that is, where the API code and the
implementation code are developed in separate Maven projects—you
might nevertheless want to add the API interfaces to the provider
bundle. Peter Kriens (creator of the Bnd tool, on which the Maven bundle
plug-in is based) recommends that a provider bundle should always include
its API interfaces. One reason for this is that, when you
deploy a provider bundle without embedding the API, there always a risk
that the required version of the API will not be available in the OSGi
container at deploy time. By embedding the API interfaces in the
provider, you eliminate that risk and improve the reliability of your
deployment.
When you develop your API interfaces and your implementation classes
in separate Maven projects, the bundle plug-in supports a special
configuration that enables you to embed the API in the provider bundle
at build-time. Hence, this bundle is called an
API/provider build-time combination.
Summary of import/export rules
Table 2.1 summarizes the
import/export rules for the various bundle types.
Table 2.1. Import/Export Rules for Bundles
Bundle Type
Export Own Packages
Import Own Packages
Publish OSGi Service
Access OSGi Service
Library
Yes
API
Yes
Provider
Yes
Consumer
Maybe
Maybe
Yes
API/provider combination
Yes(API only)
Yes(API only)
Yes
Importing and exporting own packages
The basic rule for exporting a bundle's own packages is
as follows: a bundle should export any of its own packages that are public and
intended to be used by other bundles; private packages should not be
exported.
The basic rule for importing a bundle's own packages is as
follows: a bundle does not import any of its own packages,
because those packages are already available from the bundle's own classpath
(external dependencies, on the other hand, must of course be imported).
But there is a special case where it makes sense for a bundle both to export
and to import its own packages. The OSGi container
interprets this in a special way: the bundle either imports the
package (thus using the classes from another bundle, and ignoring its own copy of
those classes) or exports the package (thus using its own copy
of the classes and making these classes available to other bundles).
For example, an API/provider combination bundle exports and imports its own API
packages. This ensures the bundle is able to use the API interfaces from another
bundle, if a suitable version is available. Re-using existing packages and classes
is an important strategy to avoid duplicate class instances in the same class space
(see Conflicting Classes).
When to publish an OSGi service
An OSGi service is essentially a plain Java object, made accessible to other
bundles by being published in the OSGi service registry (see Exporting a Service in Deploying into the Container).
Provider bundles demonstrate a typical use case of an OSGi service. The classes in
a provider bundle are only meant to be accessed
indirectly—that is, through the relevant API interfaces.
But there must be some mechanism that enables you to instantiate one or more of the
implementation classes, in order to bootstrap the implementation. Typically, the
mechanism for bootstrapping an implementation depends on the framework or container
you are currently using. For example, in Spring you would typically use a
bean element to create an instance of an implementation
class.
In the context of OSGi, however, the natural mechanism for bootstrapping an
implementation is to create and publish one or more OSGi services.
Default bundle plug-in settings
Example 2.1 shows a minimal configuration
of the Maven bundle plug-in, which you can use for basic Maven projects.
Although the preceding minimal configuration is not ideal for all bundle projects,
it does have a useful default behavior that is often good enough to get your project
started:
Import rules—when you build your Maven project,
the bundle plug-in tries to discover all of the bundle's package
dependencies automatically. By default, the bundle plug-in then adds
all of the discovered package dependencies to the
Import-Package header in the Manifest file (equivalent to
the wildcard instruction, *).
However, the effectiveness of the initial scan depends on where the
dependencies actually arise, as follows:
Java source code—Bnd scans the Java
source code in your project to discover package dependencies. There
is one blind spot in this process, however: the scan cannot detect
dependencies that arise from using Java reflection and explicit
calls on a class loader.
Spring configuration file—Bnd is
not capable of scanning Spring
configuration files to discover package dependencies. Any
dependencies arising from Spring configuration must be added
manually to the Import-Package instruction.
Blueprint configuration file—the
Apache Karaf implementation of blueprint is capable of mapping XML
namespaces to Java packages and dynamically resolving the resulting
dependencies at run time. Hence, there is no need for blueprint's
dependencies to be added to the Import-Package
instruction.
This is one of the major benefits of using blueprint
configuration.
Provided that the dependent bundles define a version for their exported
packages, the bundle plug-in also automatically assigns a version range to
the imported packages (see Automatic Import Versioning).
Export rules—by default, the Maven bundle
plug-in exports all of the packages in your project, except for packages
that match the patterns *.impl or *.internal. It
is therefore recommended that you follow the convention of naming all of
your private packages with the suffix *.impl or
*.internal.
By default, the bundle plug-in does not assign a
version to the exported packages. This is a major disadvantage of the
default export instruction (see Export Versioning).
This section explains how to set up a Maven project for an API/provider
bundle.
The hello-boston bundle exemplifies an API/provider combination
bundle. The API/provider bundle exports only its API packages, while the
implementation packages are kept private. The API/provider bundle also instantiates
and exports an OSGi service, which is accessed through the HelloBoston
interface.
Directory structure
The hello-boston bundle has the following directory
structure:
The org.fusesource.example.hello.boston package is public and all
of its classes and interfaces are exported from the bundle.
The org.fusesource.example.hello.boston.impl package is private.
By default, the Maven bundle plug-in treats any packages containing the segments
.impl or .internal, as private packages.
The src/main/resources/OSGI-INF/blueprint directory contains a
single blueprint file, boston-svc.xml. Any file matching the
pattern, *.xml, in this directory is assumed to be a blueprint
configuration file.
Sample API
The hello-boston bundle contains an API, which consists of the
interface, HelloBoston. The HelloBoston interface
returns a localized greeting and a Clock object that tells the
local time. It is defined as follows:
// Java
package org.fusesource.example.hello.boston;
import org.fusesource.example.time.Clock;
public interface HelloBoston {
public String getGreeting();
public Clock getLocalTime();
}
Sample implementation
The hello-boston bundle also implements the API. In this example,
a single HelloBostonImpl class implements the
HelloBoston interface, as follows:
// Java
package org.fusesource.example.hello.boston.impl;
import org.fusesource.example.hello.boston.HelloBoston;
import org.fusesource.example.time.Clock;
import org.fusesource.example.time.TimeUtil;
public class HelloBostonImpl implements HelloBoston {
protected Clock localTime = null;
public String getGreeting() {
return new String("Hello!");
}
public Clock getLocalTime() {
if (localTime==null) {
localTime = TimeUtil.createClock(TimeUtil.TimeZone.BOSTON);
}
return localTime;
}
}
Publish OSGi service
The natural way to bootstrap the HelloBoston implementation in
OSGi is to publish the class, HelloBostonImpl, as an OSGi service.
Use the bean element to create a HelloBostonImpl
instance and use the service element to publish the bean,
advertising it as a service of HelloBoston type.
For example, the blueprint file,
OSGI-INF/blueprint/boston-svc.xml, has the following
contents:
In the Maven POM file, the hello-boston bundle defines
dependencies on the following Maven artifact:
time-util
Import and export rules
The following import and export rules apply to the hello-boston
bundle:
Exporting own packages—export the public
API package, org.fusesource.example.hello.boston. Do
not export the private package,
org.fusesource.example.hello.boston.impl.
Importing own packages—import the public
API package, org.fusesource.example.hello.boston.
Importing dependent packages—any external
package dependencies must be imported.
Maven bundle plug-in settings
You must export the API package,
org.fusesource.example.hello.boston, by including it in the
Export-Package instruction, and add the provide:=true
clause to it. This signals that this bundle is acting as the
provider of the hello.boston package (and
ensures that the API is imported with the correct version range—see Automatic Import Versioning). In this case, the bundle plug-in instructions
are as follows:
Example 2.2. Plug-In Settings for API/Provider Combination
The Import-Package header imports both the public API package,
org.fusesource.example.hello.boston, and the external package
dependencies—for example, org.fusesource.example.time.
The Export-Package header exports the public API package,
org.fusesource.example.hello.boston, while the uses
clause declares a transitive dependency on the
org.fusesource.example.time package.
The Export-Service header advertises the OSGi service as a
HelloBoston instance. This enables clients to find the
HelloBostonImpl instance by searching for a service of
HelloBoston type.
This section explains how to set up a Maven project for a typical library
bundle.
The time-util bundle exemplifies a library bundle, where the main
purpose of a library is to make interfaces and classes available to other bundles.
Hence, the library should export all of its own packages and associate a version
number with the exported packages.
Directory structure
Assuming it is built as a Maven project, the time-util bundle has
the following directory structure:
The Java source code is located under the src/main/java
sub-directory. The org.fusesource.example.time package is public and
all of its classes and interfaces can be exported from the bundle.
There are no blueprint resources associated with this bundle.
Sample implementation
The time-util bundle is essentially a wrapper around some of the
standard time utilities in Java. It provides a Clock class, which
returns the local time in a particular time zone when you invoke the
Clock.getLocalTime() method. The Clock class is
defined as follows:
// Java
package org.fusesource.example.time;
import java.util.Calendar;
import java.util.Locale;
import java.util.TimeZone;
public class Clock {
...
public Clock(TimeUtil.TimeZone tz) {
...
}
public String getLocalTime() {
return jcal.getTime().toString();
}
}
The TimeUtil class is a factory that is used to create
Clock instances for particular time zones. Two time zones are
supported: TimeZone.BOSTON and TimeZone.PARIS. The
TimeUtil class is defined as follows:
// Java
package org.fusesource.example.time;
public class TimeUtil {
public enum TimeZone {
BOSTON,
PARIS
}
public static Clock createClock(TimeZone tz) {
return new Clock(tz);
}
}
Import and export rules
The following import and export rules apply to the time-util
bundle:
Exporting own packages—the
org.fusesource.example.time package is public, and must
be exported.
Importing own packages—none of the bundle's
own packages should be imported, which is the usual case for a library
bundle.
Importing dependent packages—any external
package dependencies must be imported. In this particular example,
however, there are none (the time-util bundle depends only
on classes from the JVM).
Maven bundle plug-in settings
The Maven bundle plug-in is configured to export the library package,
org.fusesource.example.time (coded as
${project.groupId}.time). The Export-Package
instruction also contains entries to block the export of any packages containing
.impl or .internal. In this case, the bundle
plug-in instructions are as follows:
Figure 2.2 shows an overview of a sample
OSGi application consisting of five bundles, which illustrates the basic bundle
types discussed in the previous section. The application is driven by the
hello-consumer bundle, which imports a HelloParis
service from the hello-paris bundle and imports a
HelloBoston service from the hello-boston
bundle.
Figure 2.2. Sample OSGi Application
The sample bundles
The sample OSGi application consists of the following bundles:
time-util
Fits the pattern of a library bundle. The
time-util bundle is a utility library that can create
Clock instances that tell the time in a particular time
zone.
The time-util bundle is implemented using classes from
the JDK and thus has no external package dependencies.
hello-paris
Fits the pattern of an API bundle. The
hello-paris bundle consists of a single Java interface,
which returns a greeting, getGreeting(), and gives the
local time in Paris, getLocalTime().
The hello-paris bundle has the following external package
dependency:
org.fusesource.example.time
hello-paris-impl
Fits the pattern of a provider bundle. The
hello-paris-impl bundle implements the
hello-paris API bundle.
The hello-paris-impl bundle has the following external
package dependencies:
Fits the pattern of an API/provider combination
bundle. The hello-boston bundle combines a
Java interface and its implementation, where the Java interface returns
a greeting, getGreeting(), and gives the local time in
Boston, getLocalTime().
The hello-boston bundle has the following external
package dependency:
org.fusesource.example.time
hello-consumer
Fits the pattern of a consumer bundle. The
hello-consumer bundle imports the
HelloParis OSGi service and the
HelloBoston OSGi service and then invokes on these
services to report the local times in Paris and in Boston.
The hello-consumer bundle has the following external
package dependencies:
PK
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OEBPS/ImportExport.html
Chapter 2. Importing and Exporting Packages
Chapter 2. Importing and Exporting Packages
The key to understanding the rules for importing and exporting packages is to know
what type of bundle you are dealing with. This chapter aims to show that, once you
have figured out the role played by a particular bundle, you will quickly be able to
figure out the requisite Maven bundle configuration.
By default, the Maven bundle plug-in automatically assigns a version range to
imported packages, following the semantic versioning rules outlined in Semantic Versioning. You need to provide an additional hint to the
bundle plug-in, to indicate whether the bundle imports a package in the role of a
consumer or a provider (the bundle plug-in presumes the consumer role).
For automatic import versioning to work, the package dependency
must be versioned at build time, otherwise the importing
bundle cannot calculate the import range.
Automatic consumer import range
The Maven bundle plug-in automatically generates consumer
import ranges that conform to the rules of semantic versioning, as defined in
Consumer import range. The only
prerequisite is that the corresponding exporter actually defines a package
version.
For example, given that the hello-paris bundle consumes version
1.2.1 of the org.fusesource.example.time package,
the Maven bundle plug-in automatically generates a manifest with the following
import:
Because the Maven bundle plug-in assumes by default that an importer is acting
in the role of consumer, it is necessary to specify explicitly
when an importer is acting in the role of provider, using the
provide:=true clause. There are two different approaches you can
take to specifying the provider import range, depending on how you package the API
and provider bundles, as follows:
When the API and the provider are to be packaged in separate bundles, append
the provide:=true clause to the relevant API packages listed in the
Import-Package instruction.
For example, given that the hello-paris-impl bundle provides the
implementation of the org.fusesource.example.hello.paris package
from the hello-paris API bundle, you would define the
Import-Package instructions for the
hello-paris-impl bundle as follows:
When the API and the provider are to be combined in the same bundle, append
the provide:=true clause to the relevant API packages listed in the
Export-Package instruction (this is the correct setting to use both
for an API/provider combination bundle and for an API/provider build-time
combination bundle).
For example, given that the hello-boston bundle includes both the
org.fusesource.example.hello.boston API and its implementation
classes, you would define the Export-Package instructions for the
hello-boston bundle as follows:
Given that the org.fusesource.example.hello.boston package has
version 1.0, the Maven bundle plug-in generates a manifest with the
following highlighted import and export:
In the case where the API and the provider code are located in separate
Maven projects, setting provide:=true on an exported API package in
the provider's POM has the important side-effect that the API
interfaces are included in the provider bundle. For a detailed
explanation, see API/Provider Build-Time Combination.
Customize import range
Occasionally, it will be necessary to customize the import version
range—for example, if the corresponding exporter does not follow the OSGi
semantic versioning conventions. The simplest case is where you specify the
import range explicitly, as in the following example:
Sometimes, a third-party exporter might follow a consistent versioning convention,
but this convention is different from the OSGi convention. In this case, it make
sense to define a custom import rule that codifies the alternative convention. For
example, if a third-party exporter increments the minor version number whenever
binary compatibility with consumers is broken, you could use Bnd's
range macro to codify this rule on consumer imports, as
follows:
Where the Bnd macro is written in the format,
$<Macro> and must be enclosed in
a CDATA section to avoid clashing with the XML interpretations of <
and > (actually, Bnd supports a variety of different macro delimiters,
most of which cannot be used here: braces, ${...}, clash with Maven
properties; while square brackets, $[...], and parentheses,
$(...), clash with the syntax of version ranges).
Entries like == and =+ are masks that return a version
based on the version of the corresponding exporter. The equals sign, =,
returns the corresponding version part unchanged; the plus sign, +,
returns the corresponding version part plus one; and the minus sign, -,
returns the corresponding version part minus one. For example, if the corresponding
exporter has the version, 1.3.0.4, the range mask,
[==,=+), resolves to [1.3,1.4). For more details, consult
the Bnd documentation for the range macro.
The range macro is a relatively new addition to Bnd. In POMs that
were written before the range macro became available, you might come
across ranges written using the Bnd version macro. For example, the
range, $<range;[==,=+)>, could also be written using the
version macro as follows:
At the time of writing, the Bnd tool has a bug that causes two
NullPointerException exceptions, accompanied by lengthy stack
traces, to be generated whenever you build a Maven project featuring the
range or version macros as shown above. Although
alarming, these exceptions are harmless non-fatal errors.
If you check the generated bundle after building, you will see that the
Manifest.mf file is generated exactly as specified by the
bundle instructions.
Customize version policies
Bnd and the Maven bundle plug-in support directives that enable you to
override the default versioning policies. Occasionally, you might come across an
entire subsystem that consistently follows a different versioning convention
from OSGi and, in this case, it makes sense to override the default versioning
policies.
Since Bnd version 1.15 (version 2.2.0 of the Maven bundle plug-in), the
following directives can be used to specify versioning policies:
-consumer-policy
Specifies a macro to calculate the consumer import range.
-provider-policy
Specifies a macro to calculate the provider import range.
In the Maven bundle plug-in, these directives are set using the elements,
_consumer-policy and _provider-policy—for
example:
Where the macro, $<Macro>, is escaped
as, $<Macro>, in XML (or you could
use a CDATA section).
Tip
Any Bnd directives that start with the hyphen character, -,
can also be set in the Maven bundle plug-in using an element whose name is
obtained by changing the initial hyphen, -, to an underscore,
_.
OSGi allows you to associate a single version with an
exported package. The version that you choose for your package (or packages)
should conform to the conventions of semantic versioning, as defined in Semantic versioning rules.
Export version at bundle granularity
The simplest approach to export versioning is where you use the bundle version
as the export version and you assign the same export version to all exported
packages in the bundle.
Using the Maven bundle plug-in, you can implement this versioning policy using
instructions like the following:
Where the ${project.version} macro returns the the contents of
the project/version element in the POM file (the version of the
current Maven artifact).
Export versions at package granularity
Strictly speaking, importing and exporting works at the granularity level of
packages, not of bundles. In principle, therefore, it is possible to assign versions
at the level of individual packages, so that one bundle contains multiple packages
with different versions. There are some scenarios where it can be useful to assign
versions at package granularity.
For example, consider a bundle that contains both an API package and a package
that implements the API (see API/Provider Build-Time Combination). In this case, it makes more sense to use separate versions for the API package
and the implementation package.
Using the Maven bundle plug-in, you can specify the version of an individual Java
package by creating or modifying the standard Java packageinfo file in
the corresponding package directory. For example, if you want to assign version
1.2.1 to the org.fusesource.example.time package,
create a file called packageinfo (no suffix) in the
src/main/java/org/fusesource/example/time directory and add the
following line:
version 1.2.1
Alternatively, since Java 5 it is also possible to specify version information
using annotations in a package-info.java file, for example:
The basic principle of semantic versioning is that an increment to a major, minor,
or micro part of a version signals a particular degree of compatibility or
incompatibility with previous versions of a package. The Semantic
Versioning technical white paper from the OSGi Alliance makes specific
recommendations about how to interpret each of the major, minor, and micro parts of
a version. Based on these definitions, the paper explains how to assign a version
number to an exported package and how to assign a version range to an imported
package.
This section summarizes the recommendations of the OSGi technical white
paper.
Version fidelity
Consider a bundle (the importer) that has package
dependencies on another bundle (the exporter). At build
time, the importer is compiled and tested against a
specific version of the exporter. It follows that the
build-time version of the exporter bundle is the version least likely to cause
any problems or bugs, because it has already been tested against the
importer.
If you deploy the importer together with the original build-time version of
the exporter, you have perfect version fidelity between
the two bundles. Unfortunately, fidelity cannot usually be achieved in real
deployments, because the exporter bundle is often used by many other bundles,
compiled against slightly different versions of the exporter. Particularly in a
modern open-source programming environment, where an application might pull in
hundreds of bundles from many third-party projects and companies, it becomes
essential to allow for version flexibility when matching bundles to their
dependencies.
Backward compatibility in Java
To figure out how much version flexibility we can allow, we first need to
consider the rules for backward compatibility in Java. Java is one of the few
languages that explicitly considers issues of binary compatibility in its
specification and the detailed rules can be found in the Binary
Compatibility chapter of The Java Language
Specification. The following is an incomplete list of changes that
are binary compatibility with consumers of particular classes or
interfaces:
Adding new fields, methods, or constructors to an existing class or
interface.
Deleting private fields, methods, or constructors of a class.
Deleting package-only access fields, methods, or constructors of a
class or interface.
Re-ordering the fields, methods, or constructors in an existing type
declaration.
Moving a method upward in the class hierarchy.
Reordering the list of direct super-interfaces of a class or
interface.
Inserting new class or interface types in the type hierarchy.
Consumers and providers
In a well-structured Java application, bundles often fall into one of the
following categories: API,
consumer, and provider. The
consumer and provider roles can be defined as follows:
Consumer—a bundle that uses the
functionality exposed by an API, invoking methods on the API's Java
interfaces.
Provider—a bundle that provides the
functionality of an API, containing classes that implement the API's
Java interfaces.
Both the consumer and the provider import packages from an API bundle, as
illustrated in Figure 3.1. Hence, the consumer
and the provider are both sensitive to changes in the API bundle.
Figure 3.1. Consumers and Provider of API Interfaces
It turns out that the rules of binary compatibility are quite different for
consumers and providers. The coupling between consumer, provider and API can be
described as follows:
Compatibility between consumer and API—this
coupling obeys the rules from the Binary
Compatibility chapter of the Java Language
Specification. There is quite a lot of scope for altering
the API without breaking backward compatibility; in particular, it is
possible to add methods to Java interfaces without breaking
compatibility.
Compatibility between provider and API—this
coupling is not explicitly considered in the Java Language
Specification. It turns out that this coupling is
much more restrictive than the consumer case.
For example, adding methods to Java interfaces breaks backward
compatibility with the provider.
Callback interfaces
The role of a consumer is not always as clear cut as initially suggested here.
Although consumers mostly interact with an API by invoking methods on the API's
interfaces, there are some cases where a consumer actually
implements one of the API interfaces (thus muddying the
distinction between a consumer and a provider). A classic example of where this
might happen is when an API defines a callback interface,
which a consumer can implement in order to receive event notifications.
For example, consider the javax.jms.MessageListener interface
from the JMS API, which is defined as follows:
A consumer is expected to implement the onMessage() method in
order to receive messages from the underlying JMS service. So, in this case, the
more restrictive binary compatibility rules (the same ones that apply to a
provider) must be applied to the MessageListener interface.
Tip
It is recommended that you try to be as conservative as possible with
callback interfaces, changing them as infrequently as possible, in order to
avoid breaking binary compatibility with consumers.
OSGi version syntax
The OSGi version syntax defines a version to have up to four parts, as
follows:
Where <major>, <minor>, and
<micro> are positive integers and
<qualifier> is an arbitrary string. For example, the
following are valid OSGi versions:
4
2.1
3.99.12.0
2.0.0.07-Feb-2011
OSGi version ranges
When declaring dependencies on imported packages in OSGi, you can declare a
range of acceptable versions. An OSGi version range is
defined using a notation borrowed from mathematics: square brackets—that
is, [ and ]— denote an inclusive range; and
parentheses—that is, ( and )—denote an
exclusive range. You can also mix a parenthesis with a bracket to define a
half-inclusive range. Here are some examples:
OSGi Version Range
Restriction on Version, v
[1.0, 2.0)
1.0 <= v < 2.0
[1.0, 2.0]
1.0 <= v <= 2.0
(1.4.1, 1.5.5)
1.4.1 < v < 1.5.5
(1.5, 1.9]
1.5 < v <= 1.9
1.0
1.0 <= v <
Of the preceding examples, the most useful style is the half-inclusive range,
[1.0, 2.0). A simple version number on its own, 1.0,
is interpreted as an inclusive range up to positive infinity.
Semantic versioning rules
The fundamental idea of semantic versioning is that a bundle's version should
indicate when the bundle breaks binary compatibility with its preceding version,
bearing in mind that there are different kinds of binary compatibility:
compatibility of consumers and
compatibility of providers. The OSGi
Semantic Versioning technical white paper proposes the following
versioning conventions:
<major>
When a change breaks binary compabitility with both consumers
and providers, increment the bundle's major
version number. For example, a version change from 1.3
to 2.0 signals that the new bundle version is
incompatible with older consumers and providers.
<minor>
When a change breaks binary compatibility with providers, but
not consumers, increment the bundle's minor
version number. For example, a version change from 1.3
to 1.4 signals that the new bundle version is
incompatible with older providers, but compatible with older
consumers.
<micro>
A change in micro version does not signal any backward
compatibility issues. The micro version can be incremented for bug
fixes that affect neither consumers nor providers of the API.
<qualifier>
The qualifier is typically used as a build identifier—for
example, a time stamp.
Consumer import range
Assuming that an exporter bundle obeys the preceding OSGi semantic versioning
rules, it is possible to work out the range of versions that are compatible with
a consumer. For example, if a consumer is built against an exporter with version
1.3, the next version that would break binary compatibility
with the consumer is 2.0. It follows that all exporter versions up
to, but excluding, 2.0 ought to be compatible with the
consumer.
In general, the consumer import range is calculated from the build-time
version of the exporter according to the following rule: given the build-time
version, <major>.<minor>.<micro>.<qual>, define the
corresponding consumer import range to be [<major>.<minor>,
<major>+1).
Table 3.1 shows some examples of how to
choose the correct consumer import range for a given exporter version.
Table 3.1. Example Consumer Import Ranges
Build-Time Version of Exporter
Consumer Import Range
3.0
[3.0, 4)
2.0.1
[2.0, 3)
2.1.4
[2.1, 3)
2.1.5.2011-02-07-LATEST
[2.1, 3)
Provider import range
According to the OSGi semantic versioning rules, providers are compatible with
a narrower range of versions than consumers. For example, if a provider is built
against an exporter with version 1.3, the next version that would
break binary compatibility with the provider is 1.4. It follows
that all exporter versions up to, but excluding, 1.4 ought to be
compatible with the provider.
In general, the provider import range is calculated from the build-time
version of the exporter according to the following rule: given the build-time
version, <major>.<minor>.<micro>.<qual>, define the
corresponding provider import range to be [<major>.<minor>,
<major>.<minor>+1).
Table 3.2 shows some examples of how to
choose the correct provider import range for a given exporter version.
Table 3.2. Example Provider Import Ranges
Build-Time Version of Exporter
Provider Import Range
3.0
[3.0, 3.1)
2.0.1
[2.0, 2.1)
2.1.4
[2.1, 2.2)
2.1.5.2011-02-07-LATEST
[2.1, 2.2)
PK
vHAz z OEBPS/Versioning.html
Chapter 3. Versioning
Chapter 3. Versioning
For the stability of an OSGi application, it is essential that a bundle restricts
the range of acceptable versions of its dependencies; otherwise, the bundle could be
wired to an incompatible version of an imported package, with disastrous results. On
the other hand, an application cannot afford to be too restrictive: a complex
application might re-use a package in many different places. If a strict version
restriction is placed on such a package every time it occurs, it might prove
impossible to deploy the application, due to conflicting version requirements. Also,
a certain amount of version flexibility is required in order to deploy patched
bundles as hot fixes. This chapter describes the OSGi version policies that help you
to achieve these goals and explains how to implement version polices using the Maven
bundle plug-in.
![[Note]](imagesdb/note.gif)
![[Important]](imagesdb/important.gif)
![[Tip]](imagesdb/tip.gif)
