Producers

Work in progress. This page is the canonical reference for Producer<T> and @Produces. Full content lands alongside the producer-vs-resolver tutorial (see the roadmap).

When you need a producer

Direct class instantiation works for the common case: DDI calls the no-arg constructor, injects fields, runs @PostConstruct, done. You need a producer when:

• The instance requires constructor arguments DDI can’t supply.
• The factory call has side effects you want explicit (opening a connection, reading config).
• The decision which subtype to instantiate is data-driven and you don’t want a ClassResolver for it.

The shape

package com.svenruppert.ddi.producer;

public interface Producer<T> {
  T create();
}

Annotate the producer with @Produces to advertise it to the container:

@Produces
public class JdbcConnectionProducer implements Producer<Connection> {
  public Connection create() {
    return DriverManager.getConnection(System.getenv("DB_URL"));
  }
}

Multiple producers per type

You can have many producers for the same impl. If you do, you must also declare a ProducerResolver that picks between them — same rule as multiple impls needing a ClassResolver.

See also

• Resolution rules — the full decision table for impls × producers × resolvers.
• ClassResolver — picks a class; producers wrap a factory.