This document provides some details on how to convert legacy Plexus components into modern JSR-330 components.

For brevity of examples imports are omitted. The following table defines the meanings and fully-qualified-class-names of the annotations references in the following examples.

References

http://www.jcp.org/en/jsr/detail?id=330

http://www.jcp.org/en/jsr/detail?id=305

Plexus @Component annotations are replaced by standard @Named, @Singleton, etc annotations.

For example this Plexus component:

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component
{
}

can be converted to:

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
}

package components;
@Named
@Singleton
public class MyComponent
    implements Component
{
}

@Component(role=Component.class)
public class DefaultComponent
    implements Component
{
}

package components;
@Named
@Singleton
public class DefaultComponent
    implements Component
{
}

By default in Plexus, components are singletons, but this is not the case for every component. This Plexus component is not a singleton:

@Component(role=Component.class, hint="my", instantiationStrategy="per-lookup")
public class MyComponent
    implements Component
{
}

and is converted to:

@Named("my")
public class MyComponent
    implements Component
{
}

Notice this is the same as the example above, except with-out the @Singleton annotation.

By default the type of the component is determined automatically, though in some rare cases an explicit type is required. To specify the explicit binding type use the @Typed annotation:

@Named("my")
@Typed(Component.class)
public class MyComponent
    extends SomeSupportClassHardToGuessTypeFrom
{
}

In some cases component descriptions are required. There is no standard annotation to provide this, however Sisu provides a custom annotation for this.

@Component(role=Component.class, hint="my", description="My custom component")
public class MyComponent
    implements Component
{
}

becomes:

@Named("my")
@Singleton
@Description("My custom component")
public class MyComponent
    implements Component
{
}

@Requirement defines injection points for legacy Plexus components. These more-or-less line-up directly with replacement with @Inject, though there are more options available as @Inject is support for fields, constructors and methods, where @Requirement only worked with fields. The recommended option is to replace legacy Plexus injection with constructor injection where possible.

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component
{
    @Requirement
    private AnotherComponent another;
}

becomes:

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    private final AnotherComponent another;

    @Inject
    public MyComponent(final AnotherComponent another) {
    this.another = another;
}
}

Use of constructor injection in this fashion has some impact on replacing legacy Plexus lifecycle Initializable and Contextualizable interfaces, which often only exist to perform setup once injection is performed.

Other options for conversions using field injection:

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    @Inject
    private AnotherComponent another;
}

or method injection:

@Named("my")
@Singleton
public class MyComponent
implements Component
{
    private AnotherComponent another;

    @Inject
    public setAnotherComponent(final AnotherComponent another) {
        this.another = another;
    }
}

Optional components are configured to be @Nullable

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component
{
    @Requirement(optional=true)
    private AnotherComponent another;
}

becomes:

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    private final AnotherComponent another;

    @Inject
    public MyComponent(final @Nullable AnotherComponent another) {
        this.another = another;
    }
}

Legacy Plexus component hints become @Named:

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component
{
    @Requirement(hint="foo")
    private AnotherComponent another;
}
                

becomes:

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    private final AnotherComponent another;

    @Inject
    public MyComponent(final @Named("foo") AnotherComponent another) {
        this.another = another;
    }
}

Legacy Plexus component roles, which are normally only used for collection types are generally not needed:

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component
{
    @Requirement(role=AnotherComponent.class)
    private List components;
}

becomes:

@Named("my")
@Singleton
public class MyComponent
implements Component
{
    private final List components;

    @Inject
    public MyComponent(final List components) {
        this.components = components;
    }
}

Plexus configuration injection is handled by `@Inject @Named("${expression}")` injection.

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component
{
    @Configuration(name="configDir")
    private File configDir;
}

becomes:

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    private final File configDir;

    @Inject
    public MyComponent(final @Named("${configDir}") configDir) {
        this.configDir = configDir;
    }
}

Default values are provided by expression syntax.

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component
{
    @Configuration(name="configDir", value="defaultDir")
    private File configDir;
}

becomes:

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    private final File configDir;

    @Inject
    public MyComponent(final @Named("${configDir:-defaultDir}") configDir) {
        this.configDir = configDir;
    }
}

This section is specific to how to adapt legacy Plexus component lifecycle interfaces. There is no hard-fast way to adapt these, but there are some guidelines to follow.

By and far this can be replaced by using constructor-inject, and performing the initialize() at the end of the constructor.

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component, Initializable
{
    @Requirement
    private AnotherComponent another;

    public initialize() throws InitializationException {
        another.init();
    }
}

becomes:

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    private final AnotherComponent another;

    @Inject
    public MyComponent(final AnotherComponent another) {
    this.another = another;
        another.init();
    }
}

Similar to Initializable, though if the context is needed you can inject the container context parameters with:

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    @Inject
    public MyComponent(final @Parameters Map params) {
        // do something with _context_ params
    }
}

Support for JSR250 lifecycle annotations is available when enabled for the Plexus wrapper ([Maven 3.5+](https://issues.apache.org/jira/browse/MNG-6084) enables JSR250 support). If JSR250 support is not enabled then applications can use other techniques to handle start/stop behavior. The examples below show the solution used in Sonatype Nexus, which relies on a modified implementation of the Google-Guava EventBus to manage lifecycle events.

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component, Startable
{
    public start() throws StartingException {
        // do something to "start"
    }

    public stop() throws StoppingException {
        // do something to "stop"
    }
}

becomes:

import org.sonatype.sisu.goodies.eventbus.EventBus;
import com.google.common.eventbus.Subscribe;

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    private final EventBus eventBus;

    @Inject
    public MyComponent(final EventBus eventBus) {
        this.eventBus = eventBus;
        eventBus.register(this);
    }

    @Subscribe
    public on(final NexusStartedEvent event) throws Exception {
        // do something to "start"
    }

    @Subscribe
    public on(final NexusStoppedEvent event) throws Exception {
        // do something to "stop"

        eventBus.unregister(this);
    }
}

Similar to Startable use of events are used to handle replacement for Disposable components.

@Component(role=Component.class, hint="my")
public class MyComponent
    implements Component, Disposable
{
    public dispose() {
        // do something to "dispose"
    }
}

becomes:

import org.sonatype.sisu.goodies.eventbus.EventBus;
import com.google.common.eventbus.Subscribe;

@Named("my")
@Singleton
public class MyComponent
    implements Component
{
    private final EventBus eventBus;

    @Inject
    public MyComponent(final EventBus eventBus) {
        this.eventBus = eventBus;
        eventBus.register(this);
    }

    @Subscribe
    public on(final NexusStoppedEvent event) throws Exception {
        // do something to "dispose"

        eventBus.unregister(this);
    }
}

Plugins which require additional custom bindings can provide a @Named Guice module to configure components bindings further.

Sisu will automatically load modules which are @Named and apply them to the injectors bindings. These modules are really no different than normal Guice modules, except that they need to have the @Named annotation on them so that Sisu can locate them when initializing.

@Named
public class MyPluginModule
    extends com.google.inject.AbstractModule
{
    public void configure() {
        bind(Component.class).to(MyComponent.class);
    }
}