One of the most important principles of the Maven build system is that there are standard locations for all of the files in the Maven project. There are several advantages to this principle. One advantage is that Maven projects normally have an identical directory layout, making it easy to find files in a project. Another advantage is that the various tools integrated with Maven need almost no initial configuration. For example, the Java compiler knows that it should compile all of the source files under src/main/java and put the results into target/classes.

Example 3 shows the elements of the standard Maven directory layout that are relevant to building OSGi bundle projects. In addition, the standard locations for Spring-DM and Blueprint configuration files (which are not defined by Maven) are also shown.

ProjectDir/
    pom.xml
    src/
        main/
            java/
                ...
            resources/
                META-INF/
                    spring/
                        *.xml
                OSGI-INF/
                    blueprint/
                        *.xml
        test/
            java/
            resources/
    target/
        ...

	Note
	It is possible to override the standard directory layout, but this is not a recommended practice in Maven.

The pom.xml file is the Project Object Model (POM) for the current project, which contains a complete description of how to build the current project. A pom.xml file can be completely self-contained, but frequently (particular for more complex Maven projects) it can import settings from a parent POM file.

After building the project, a copy of the pom.xml file is automatically embedded at the following location in the generated JAR file:

META-INF/maven/groupId/artifactId/pom.xml

The src/ directory contains all of the code and resource files that you will work on while developing the project.

The target/ directory contains the result of the build (typically a JAR file), as well as all all of the intermediate files generated during the build. For example, after performing a build, the target/classes/ directory will contain a copy of the resource files and the compiled Java classes.

The src/main/ directory contains all of the code and resources needed for building the artifact.

The src/test/ directory contains all of the code and resources for running unit tests against the compiled artifact.

Each java/ sub-directory contains Java source code (*.java files) with the standard Java directory layout (that is, where the directory pathnames mirror the Java package names, with / in place of the . character). The src/main/java/ directory contains the bundle source code and the src/test/java/ directory contains the unit test source code.

If you have any configuration files, data files, or Java properties to include in the bundle, these should be placed under the src/main/resources/ directory. The files and directories under src/main/resources/ will be copied into the root of the JAR file that is generated by the Maven build process.

The files under src/test/resources/ are used only during the testing phase and will not be copied into the generated JAR file.

By default, Fuse ESB Enterprise installs and activates support for Spring Dynamic Modules (Spring DM), which integrates Spring with the OSGi container. This means that it is possible for you to include Spring configuration files, META-INF/spring/*.xml, in your bundle. One of the key consequences of having Spring DM enabled in the OSGi container is that the lifecycle of the Spring application context is automatically synchronized with the OSGi bundle lifecycle:

In practice, this means that you can treat your Spring-enabled bundle as if it is being deployed in a Spring container. Using Spring DM, the features of the OSGi container and a Spring container are effectively merged. In addition, Spring DM provides additional features to support the OSGi container environment—some of these features are discussed in OSGi Services.

OSGi R4.2 defines a blueprint container, which is effectively a standardized version of Spring DM. Fuse ESB Enterprise has built-in support for the blueprint container, which you can enable simply by including blueprint configuration files, OSGI-INF/blueprint/*.xml, in your project. For more details about the blueprint container, see OSGi Services.