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OSGi demystified, Part 4: Don’t bungle your bundles

The OSGi demystified article series addresses common OSGi issues in CICS — offering insight into OSGi, discussing best practices, and providing setup and configuration advice. This article is a roundup of some best practices to use with OSGi.

Use common OSGi bundles

Inevitably, at some point in larger applications, you’ll realize that some of your OSGi bundles provide common services to a number of applications, or that you’re using the same third-party libraries in multiple applications. Either way, you may have come across the OSGI_BUNDLES option of the JVM profile and concluded that it allows you to install a list of common OSGi bundles. The OSGi bundles in that list are automatically installed when the OSGi JVM server is enabled. However, you may have also realized that the same OSGi bundles can be installed as CICS bundle parts in a CICS bundle. So what’s the difference?

Ultimately, both routes achieve the same result so it is a matter of preference and architecture. By adding the OSGi bundles to the OSGI_BUNDLES option, you have a simpler mechanism for installing libraries that does not require CICS bundle packaging. The library bundles are guaranteed to be available by the time the first application runs, and they are guaranteed to be unchanged for the duration of the JVM server’s enable status. Conversely, an OSGi bundle that’s installed using a CICS bundle provides more flexibility, an independent lifecycle (managed by the CICS bundle), and the potential to be updated at runtime. However, the install ordering of CICS bundles relies on you to ensure that libraries are installed before applications. It also requires a more complex deployment process to ensure that the libraries are installed regardless of the CICS start-up procedure — whether that be CSD group, warm restarts, SPI, or initial start.

OSGI_BUNDLES installed libraries are often referred to as middleware bundles. If you are confident that these OSGi bundles will not need to be dynamically updated or to have independent lifecycling, then using this option can reduce the complexity of deployment and the burden of ordering the install sequence.

For more information see JVM server profile options in the product documentation.

Properties files and OSGi

A common approach to configuring applications is to use a properties file. In the most basic case, a properties file resides in a hard-coded location outside of your application. A refinement to that basic approach is to pick up a system property that instructs your application where to find the properties file. Although this offers a little more flexibility, the configuration is still entirely separate from your application — and can lead to errors.

In this scenario, you might have a chunk of Java code like this:

    File file = new File("/u/ibm/test.properties");
    FileInputStream fileInput = new FileInputStream(file);
    Properties properties = new Properties();
    properties.load(fileInput);
    fileInput.close();

A natural evolution for OSGi is to place the properties file inside the OSGi bundle so that the application and configuration remain together. However, by placing the properties file inside the bundle, you are no longer in control of the actual file-system location of the file when the bundle is installed. To combat this problem, you can use use the bundle’s classloader in conjunction with the getResourceAsStream() method to find and load the properties, as shown below:

    InputStream inputStream = getClass().getResourceAsStream("/test.properties");
    Properties props = new Properties();
    props.load(inputStream);

Ultimately, you might consider using the OSGi Configuration Admin service, which defines a mechanism for managing, creating, and passing configuration settings to an OSGi bundle. Put simply, configuration is a list of name-value pairs that can be created and managed by a management application. The management application — perhaps a GUI front-end for configuring properties — creates or updates the configuration and passes it to the Configuration Admin service. The service acts like a central hub and distributes the configuration to interested parties, such as your OSGi bundle. These parties then register as ManagedService services and declare their interest in the configuration (via a unique ID).

Further reading:

Convert to OSGi bundle or use Bundle-ClassPath?

Migrating to OSGi isn’t always straightforward, and there are design and practicality trade-offs. For example, should you place existing JARs on an OSGi bundle-classpath and wrap them inside the OSGi bundle, or should you convert those JARs to OSGi bundles in their own right? As explained earlier in this series, each OSGi bundle has its own classloader. By using the Bundle-ClassPath header in the MANIFEST.MF of an OSGi bundle, you can add other JARs and classes to that bundle’s classpath. The JARs and classes become part of the bundle. One potential drawback of this approach is that those JARs lose version independence and granularity and they are no longer eligible to be shared libraries. However, there are situations where this approach is the prudent choice. Ultimately, it is a design choice. Our rule of thumb is this: For lightweight libraries with few dependencies where the library is unlikely to be used by other OSGi bundles in the system, Bundle-ClassPath is fine. You can even choose to export its packages through your own OSGi bundle. If a library is large, has other dependencies, many consumers, or might be versioned within a system — installing it as an OSGi bundle in its own right is the way to go. This can reduce the overall footprint of the system, promote reuse, and reduce both complexity and duplication.

Avoid Require-Bundle

You should avoid using the OSGi bundle MANIFEST.MF header Require-Bundle where possible because it ties you to a specific bundle implementation. You also lose package granularity because you are forced to consume other packages from the same required bundle. If those packages are not your preferred versions of an implementation, then your system can very quickly become constrained and inflexible. You’re better off taking a more fine-grained approach with Import-Package, which allows you to pick and choose each package/version individually. For example:

        Require-Bundle: com.ibm.multiple.packages        
        Import-Package: com.ibm.specific.package;version="[1.0.0, 2.0.0)"

Don’t split your packages

A split package is the situation where classes from the same package are provided by more than one JAR. In the normal Java world where you are dealing with a single hierarchical class loader, there is no issue. Even when accessing package-private classes, everything works quite well. However, if you bring this concept into the OSGi world, you are begging for trouble. In OSGi, each OSGi bundle has its own class loader. If you put two parts of a package in different OSGi bundles, those classes end up on different class loaders and are considered incompatible because Java uses the class loader/package/class combination as the unique identifier. Split packages in OSGi often result in a java.lang.IllegalAccessError exception at runtime.

In OSGi, a package is considered an atomic unit so it should be loaded by the same class loader to ensure consistency. If you really need to split packages and re-factoring isn’t an option, a workaround is to make one of the OSGi bundles an OSGi bundle fragment instead. A fragment always lives within a host OSGi bundle, so applying this fragment always lives within a host OSGi bundle and applying this technique ensures that both are loaded by the same class loader.

Know your execution environment

The OSGi bundle MANIFEST.MF header Bundle-RequiredExecutionEnvironment is often misunderstood. The value of this header is the minimum level of Java runtime in which the OSGi bundle can execute. It is often incorrectly set to an arbitrarily high level, for example JavaSE-1.8 when a value of JavaSE-1.5 might be the more appropriate choice. Setting it correctly gives your OSGi bundle greater flexibility to install in multiple environments. Here’s an example of this definition:

        Bundle-RequiredExecutionEnvironment: JavaSE-1.5

Avoid circular dependencies

Circular dependencies usually indicate sub-optimal packaging or application architecture problems. It is important to ensure that these dependencies are designed out of the structure of your applications. A number of approaches claim to remove these cycles, like inversion of control (dependency injection), interfaces, and creation of an inversion of control (dependency injection), interfaces, and creation of an intermediate object. Although these techniques can fix the problem, it is often better to look at the design between the modules and question why the cyclic dependency exists. Can the application be better designed to avoid them? Sometimes simply moving classes to a more appropriate package is enough. If you are developing in an IDE such as Eclipse, this type of error is highlighted with an error message similar to the following:

A cycle was detected in the build path of project 'com.company.package'.`

The following articles provide useful information on how to tackle these issues, along with some best practices for OSGi development:

Separate the API from the implementation

By putting your Java API in a separate OSGi bundle from the implementation of that API, you gain stability and flexibility. An implementation will generally require regular refresh due to bug fixes and improvements, while the API tends to remain stable. Keeping the two apart means that consumers of the API will not be required to refresh their dependencies each time a minor change comes along. A clear separation between API and implementation can also help avoid circular dependency problems.

Summary

This roundup of best practices has covered many different aspects of OSGi development, including guidance with common bundles, avoiding split packages, preventing cyclic dependencies, configuration properties files, and whether Bundle-Classpath or full OSGi conversion is best for you. These best practices can help you avoid difficulties and embrace the benefits. Be sure to explore the other articles in this series for an expanded understanding of OSGi.