Commons Inject – Using Commons Inject

To use Commons Inject, you have to create an IInjector. The purpose of the IInjector is the creation of POJO's, according to the rules you specify. The IInjector is the direct equivalent of the Guice Injector.

To create an Injector, you must have some modules that provide the injectors configurtion. There are a few predefined modules:

The PostConstruct module provides support for an application lifecycle: Your POJO's may be initialized when the aplication starts. And there is also the possibility for a shutdown. See this document for details. [[Log4jLoggerModule]] The Log4j Logger module allows to have Log4j Loggers injected into your POJO's. Details are described here.

Of course, the predefined modules will never be sufficient. In practice, you will also have custom modules like this:

package com.foo.myapp;

import org.apache.commons.inject.api.IModule;
import org.apache.commons.inject.api.bind.IBinder;

public class MyModule implements IModule {
  @Override
  public void configure(IBinder pBinder) {
    pBinder.setDefaultScope(Scopes.PER_CALL);
    pBinder.bind(List.class).to(ArrayList.class);
    pBinder.bind(List.class, "linked").to(LinkedList.class);
    ... // More bindings.
  }
}

Assuming that you have created your modules, and you wish to use one or more of the predefined module, then you can create an IInjector like this:

import org.apache.commons.inject.api.CommonsInject;
import org.apache.commons.inject.api.IInjector;

public IInjector newInjector() {
  PostConstructModule module0 = new PostConstructModule();
  Log4jLoggerModule module1 = new Log4jLoggerModule();
  MyModule module2 = new MyModule();
  return CommonsInject.build(module0, module1, module2);

That Injector can create POJO's for you, like this:

  List<Foo> fooList = injector.requireInstance(List.class);
  List<Bar> barList = injector.requireInstance(List.class, "linked");

According to the above configuration, fooList will be an instance of java.util.ArrayList, but barList will be a java.util.LinkedList.

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