Struts example source code file (ConstructionContext.java)

This example Struts source code file (ConstructionContext.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Struts tags/keywords

arraylist, constructioncontext, delegatinginvocationhandler, delegatinginvocationhandler, dependencyexception, illegalaccessexception, illegalstateexception, object, object, please, reflection, runtimeexception, runtimeexception, t, t, util

The Struts ConstructionContext.java source code

/**
 * 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.opensymphony.xwork2.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 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<? super T> 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()) {
      throw new DependencyException(
          expectedType.getName() + " is not an interface.");
    }

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

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

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

  void setProxyDelegates(T delegate) {
    if (invocationHandlers != null) {
      for (DelegatingInvocationHandler<T> invocationHandler
          : invocationHandlers) {
        invocationHandler.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(
            "Not finished constructing. Please don't call methods on this"
                + " object until the caller's construction is complete.");
      }

      try {
        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;
    }
  }
}