src/com/google/inject/ConstructionContext.java

/**
 * Copyright (C) 2006 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.inject;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.ArrayList;
import java.util.List;

/**
 * Context of a dependency construction. Used to manage circular references.
 *
 * @author crazybob@google.com (Bob Lee)
 */
class ConstructionContext<T> {

  T currentReference;
  boolean constructing;

  List<DelegatingInvocationHandler<T>> invocationHandlers;

  T getCurrentReference() {
    return currentReference;
  }

  void removeCurrentReference() {
    this.currentReference = null;
  }

  void setCurrentReference(T currentReference) {
    this.currentReference = currentReference;
  }

  boolean isConstructing() {
    return constructing;
  }

  void startConstruction() {
    this.constructing = true;
  }

  void finishConstruction() {
    this.constructing = false;
    invocationHandlers = null;
  }

  Object createProxy(Class<?> expectedType) {
    // TODO: if I create a proxy which implements all the interfaces of
    // the implementation type, I'll be able to get away with one proxy
    // instance (as opposed to one per caller).

    if (!expectedType.isInterface()) {
      // TODO: Report better error.
      throw new ConfigurationException("Tried proxying "
          + expectedType.getName() + " to support a circular dependency, but"
          + " it is not an interface.");
    }

    if (invocationHandlers == null) {
      invocationHandlers = new ArrayList<DelegatingInvocationHandler<T>>();
    }

    DelegatingInvocationHandler<T> invocationHandler
        = new DelegatingInvocationHandler<T>();
    invocationHandlers.add(invocationHandler);

    Object object = Proxy.newProxyInstance(expectedType.getClassLoader(),
        new Class[] { expectedType }, invocationHandler);
    return expectedType.cast(object);
  }

  void setProxyDelegates(T delegate) {
    if (invocationHandlers != null) {
      for (DelegatingInvocationHandler<T> handler : invocationHandlers) {
        handler.setDelegate(delegate);
      }
    }
  }

  static class DelegatingInvocationHandler<T> implements InvocationHandler {

    T delegate;

    public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable {
      if (delegate == null) {
        throw new IllegalStateException("This is a proxy used to support"
            + " circular references involving constructors. The object we're"
            + " proxying is not constructed yet. Please wait until after"
            + " injection has completed to use this object.");
      }

      try {
        // This appears to be not test-covered
        return method.invoke(delegate, args);
      }
      catch (IllegalAccessException e) {
        throw new RuntimeException(e);
      }
      catch (IllegalArgumentException e) {
        throw new RuntimeException(e);
      }
      catch (InvocationTargetException e) {
        throw e.getTargetException();
      }
    }

    void setDelegate(T delegate) {
      this.delegate = delegate;
    }
  }
}