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Guice (pronounced 'juice') is a lightweight dependency injection framework for Java 5, brought to you by Google.
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/**
 * 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 com.google.inject.util.StackTraceElements;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

/**
 * Injects constructors.
 *
 * @author [email protected] (Bob Lee)
 */
class ConstructorInjector {

  final Class implementation;
  final InjectorImpl.SingleMemberInjector[] memberInjectors;
  final InjectorImpl.SingleParameterInjector[] parameterInjectors;
  final ConstructionProxy constructionProxy;

  ConstructorInjector(InjectorImpl injector, Class implementation) {
    this.implementation = implementation;
    Constructor constructor = findConstructorIn(injector, implementation);
    parameterInjectors = createParameterInjector(injector, constructor);
    memberInjectors = injector.injectors.get(implementation)
        .toArray(new InjectorImpl.SingleMemberInjector[0]);
    constructionProxy = injector.constructionProxyFactory.get(constructor);
  }

  /**
   * Used to create an invalid injector.
   */
  private ConstructorInjector() {
    implementation = null;
    memberInjectors = null;
    parameterInjectors = null;
    constructionProxy = null;
  }

  InjectorImpl.SingleParameterInjector[] createParameterInjector(
      InjectorImpl injector, Constructor constructor) {
    try {
      return constructor.getParameterTypes().length == 0
          ? null // default constructor.
          : injector.getParametersInjectors(
              constructor,
              constructor.getParameterAnnotations(),
              constructor.getGenericParameterTypes()
          );
    }
    catch (InjectorImpl.MissingDependencyException e) {
      e.handle(injector.errorHandler);
      return null;
    }
  }

  private Constructor findConstructorIn(InjectorImpl injector,
      Class implementation) {
    Constructor found = null;
    @SuppressWarnings("unchecked")
    Constructor[] constructors
        = (Constructor[]) implementation.getDeclaredConstructors();
    for (Constructor constructor : constructors) {
      Inject inject = constructor.getAnnotation(Inject.class);
      if (inject != null) {
        if (inject.optional()) {
          injector.errorHandler.handle(
              StackTraceElements.forMember(constructor),
              ErrorMessages.OPTIONAL_CONSTRUCTOR);
        }

        if (found != null) {
          injector.errorHandler.handle(
              StackTraceElements.forMember(found),
              ErrorMessages.TOO_MANY_CONSTRUCTORS);
          return InjectorImpl.invalidConstructor();
        }
        found = constructor;
      }
    }
    if (found != null) {
      return found;
    }

    // If no annotated constructor is found, look for a no-arg constructor
    // instead.
    try {
      return implementation.getDeclaredConstructor();
    }
    catch (NoSuchMethodException e) {
      injector.errorHandler.handle(
          StackTraceElements.forMember(
              implementation.getDeclaredConstructors()[0]),
          ErrorMessages.MISSING_CONSTRUCTOR,
          implementation);
      return InjectorImpl.invalidConstructor();
    }
  }

  /**
   * Construct an instance. Returns {@code Object} instead of {@code T} because
   * it may return a proxy.
   */
  Object construct(InternalContext context, Class expectedType) {
    ConstructionContext constructionContext
        = context.getConstructionContext(this);

    // We have a circular reference between constructors. Return a proxy.
    if (constructionContext.isConstructing()) {
      // TODO (crazybob): if we can't proxy this object, can we proxy the
      // other object?
      return constructionContext.createProxy(expectedType);
    }

    // If we're re-entering this factory while injecting fields or methods,
    // return the same instance. This prevents infinite loops.
    T t = constructionContext.getCurrentReference();
    if (t != null) {
      return t;
    }

    try {
      // First time through...
      constructionContext.startConstruction();
      try {
        Object[] parameters
            = InjectorImpl.getParameters(context, parameterInjectors);
        t = constructionProxy.newInstance(parameters);
        constructionContext.setProxyDelegates(t);
      }
      finally {
        constructionContext.finishConstruction();
      }

      // Store reference. If an injector re-enters this factory, they'll
      // get the same reference.
      constructionContext.setCurrentReference(t);

      // Inject fields and methods.
      for (InjectorImpl.SingleMemberInjector injector : memberInjectors) {
        injector.inject(context, t);
      }

      return t;
    }
    catch (InvocationTargetException e) {
      throw new RuntimeException(e);
    }
    finally {
      constructionContext.removeCurrentReference();
    }
  }

  /**
   * Returns an invalid constructor. This enables us to keep running and
   * reporting legitimate errors.
   */
  static  ConstructorInjector invalidConstructor() {
    return new ConstructorInjector() {
      Object construct(InternalContext context, Class expectedType) {
        throw new UnsupportedOperationException();
      }
      public T get() {
        throw new UnsupportedOperationException();
      }
    };
  }
}