Module.java

package com.dkellycollins.ioc;

import java.util.HashMap;
import java.util.Map;

/**
 * Manages class instances.
 */
public abstract class Module {

    private Map<String, Provider> _registry;

    public Module() {
        _registry = new HashMap<String, Provider>();
    }

    public <T> T get(String key) {
        if(_registry.containsKey(key)) {
            return (T) _registry.get(key).getInstance();
        }
        return null;
    }

    protected void init() {}

    protected void register(String key, Provider provider) {
        _registry.put(key, provider);
    }

    protected <T> Provider<T> singleton(Provider<T> provider) {
        return new SingletonProvider<T>(provider);
    }
}

Provider.java

package com.dkellycollins.ioc;

/**
 * Represents a class that can provide an instance of another class.
 */
public interface Provider<T> {

    /**
     * Returns an instance of the class.
     * @return
     */
    T getInstance();
}

SingletonProvider.java

package com.dkellycollins.ioc;

/**
 * Manages a single instance of a class.
 * @param <T> The type of the class.
 */
class SingletonProvider<T> implements Provider<T> {

    private final Provider<T> _inner;

    private T _instance;

    public SingletonProvider(Provider<T> inner) {
        _inner = inner;
    }

    @Override
    public T getInstance() {
        if(_instance == null) {
            _instance = _inner.getInstance();
        }
        return _instance;
    }
}