Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Using Producer Methods, Producer Fields, and Disposer Methods in CDI Applications

A producer method generates an object that can then be injected. Typically, you use producer methods in the following situations:

- When you want to inject an object that is not itself a bean

- When the concrete type of the object to be injected may vary at runtime

- When the object requires some custom initialization that the bean constructor does not perform

For more information on producer methods, see Injecting Objects by Using Producer Methods.

A producer field is a simpler alternative to a producer method; it is a field of a bean that generates an object. It can be used instead of a simple getter method. Producer fields are particularly useful for declaring Java EE resources such as data sources, JMS resources, and web service references.

A producer method or field is annotated with the `javax.enterprise.inject.Produces` annotation.

### Using Producer Methods

A producer method can allow you to select a bean implementation at runtime instead of at development time or deployment time. For example, in the example described in The producermethods Example: Using a Producer Method to Choose a Bean Implementation, the managed bean defines the following producer method:

    @Produces
    @Chosen
    @RequestScoped
    public Coder getCoder() {

        switch (coderType) {
            case TEST:
                return new TestCoderImpl();
            case SHIFT:
                return new CoderImpl();
            default:
                return null;
        }
    }

Here, `getCoder` becomes in effect a getter method, and when the `coder` property is injected with the same qualifier and other annotations as the method, the selected version of the interface is used.

    @Inject
    @Chosen
    @RequestScoped
    Coder coder;

Specifying the qualifier is essential: It tells CDI which `Coder` to inject. Without it, the CDI implementation would not be able to choose between `CoderImpl`, `TestCoderImpl`, and the one returned by `getCoder` and would cancel deployment, informing the user of the ambiguous dependency.

### Using Producer Fields to Generate Resources

A common use of a producer field is to generate an object such as a JDBC `DataSource` or a Java Persistence API `EntityManager` (see Chapter 40, "Introduction to the Java Persistence API," for more information). The object can then be managed by the container. For example, you could create a `@UserDatabase` qualifier and then declare a producer field for an entity manager as follows:

    @Produces
    @UserDatabase
    @PersistenceContext
    private EntityManager em;

The `@UserDatabase` qualifier can be used when you inject the object into another bean, `RequestBean`, elsewhere in the application:

        @Inject
        @UserDatabase
        EntityManager em;
        ...

The producerfields Example: Using Producer Fields to Generate Resources shows how to use producer fields to generate an entity manager. You can use a similar mechanism to inject `@Resource`, `@EJB`, or `@WebServiceRef` objects.

To minimize the reliance on resource injection, specify the producer field for the resource in one place in the application, and then inject the object wherever in the application you need it.

### Using a Disposer Method

You can use a producer method or a producer field to generate an object that needs to be removed when its work is completed. If you do, you need a corresponding disposer method, annotated with a `@Disposes` annotation. For example, you can close the entity manager as follows:

    public void close(@Disposes @UserDatabase EntityManager em) {
        em.close();
    }

The disposer method is called automatically when the context ends (in this case, at the end of the conversation, because `RequestBean` has conversation scope), and the parameter in the `close` method receives the object produced by the producer field.

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