Java中常用的代理实现

Java中常用的代理实现

代理模式是开发框架中常用的设计模式，比如ORM声明式事务的实现方式、中心化的异常处理等。Java中有几种常见中的代理实现的方法：

1. 静态代理；
2. Java中java.lang.reflect.Proxy;
3. 使用cglib这样的第三方库；

静态代理

静态代理是直接实现被代理的接口，并接受一个真正的接口功能实现对象作为被代理者。就实现的方法来说，静态代理与设计模式中的Decorator模式几乎一样，不过Decorator会多一些扩展方法。例如：

package com.iluwatar.proxy;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * 
 * The proxy controlling access to the {@link IvoryTower}.
 * 
 */
public class WizardTowerProxy implements WizardTower {

  private static final Logger LOGGER = LoggerFactory.getLogger(WizardTowerProxy.class);

  private static final int NUM_WIZARDS_ALLOWED = 3;

  private int numWizards;

  private final WizardTower tower;

  public WizardTowerProxy(WizardTower tower) {
    this.tower = tower;
  }

  @Override
  public void enter(Wizard wizard) {
    if (numWizards < NUM_WIZARDS_ALLOWED) {
      tower.enter(wizard);
      numWizards++;
    } else {
      LOGGER.info("{} is not allowed to enter!", wizard);
    }
  }
}

代码来自：https://github.com/iluwatar/java-design-patterns

在这个例子中，WizardTowerProxy实现了接口WizardTower, 代理了对象WizardTower tower。

在实际的开发过程中，使用静态代理的场景并不多见，静态代理也远远没有发挥出代理模式的灵活性。

动态代理

动态代理有种实现的方式，常见的两种是：

1. 使用java.lang.reflect.Proxy来处理interface生成新的对象，以实现对接口的代理，例如：Spring-data-jpa中对Dao接口的代理；
2. 通过继承已有类，生成子类的方式实现代理，例如：Spring中声明式事务的实现。

方式1

首先，我们以myBatis中的MapperProxyFactory为例来说明第一种动态代理：

/**
 *    Copyright 2009-2015 the original author or authors.
 *
 *    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 org.apache.ibatis.binding;

import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.ibatis.session.SqlSession;

/**
 * @author Lasse Voss
 */
public class MapperProxyFactory<T> {

  private final Class mapperInterface;
  private final Map methodCache = new ConcurrentHashMap();

  public MapperProxyFactory(Class mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  public Class getMapperInterface() {
    return mapperInterface;
  }

  public Map getMethodCache() {
    return methodCache;
  }

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

}

重点是方法T newInstance(MapperProxy mapperProxy)。然后我们再看看MapperProxy是什么：

/**
 *    Copyright 2009-2015 the original author or authors.
 *
 *    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 org.apache.ibatis.binding;

import java.io.Serializable;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.util.Map;

import org.apache.ibatis.reflection.ExceptionUtil;
import org.apache.ibatis.session.SqlSession;

/**
 * @author Clinton Begin
 * @author Eduardo Macarron
 */
public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  private final Class mapperInterface;
  private final Map methodCache;

  public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

}

可知，MapperProxy类是一个InvocationHandler接口的实现。

方式2

方式2我们可以以声明式事务的实现为例子来分析：

/*
 * Copyright 2002-2014 the original author or authors.
 *
 * 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 org.springframework.transaction.interceptor;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Properties;

import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;

import org.springframework.aop.support.AopUtils;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.transaction.PlatformTransactionManager;

/**
 * AOP Alliance MethodInterceptor for declarative transaction
 * management using the common Spring transaction infrastructure
 * ({@link org.springframework.transaction.PlatformTransactionManager}).
 *
 * 
Derives from the {@link TransactionAspectSupport} class which
 * contains the integration with Spring's underlying transaction API.
 * TransactionInterceptor simply calls the relevant superclass methods
 * such as {@link #invokeWithinTransaction} in the correct order.
 *
 * 
TransactionInterceptors are thread-safe.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @see TransactionProxyFactoryBean
 * @see org.springframework.aop.framework.ProxyFactoryBean
 * @see org.springframework.aop.framework.ProxyFactory
 */
@SuppressWarnings("serial")
public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {

    /**
     * Create a new TransactionInterceptor.
     * 
Transaction manager and transaction attributes still need to be set.
     * @see #setTransactionManager
     * @see #setTransactionAttributes(java.util.Properties)
     * @see #setTransactionAttributeSource(TransactionAttributeSource)
     */
    public TransactionInterceptor() {
    }

    /**
     * Create a new TransactionInterceptor.
     * @param ptm the default transaction manager to perform the actual transaction management
     * @param attributes the transaction attributes in properties format
     * @see #setTransactionManager
     * @see #setTransactionAttributes(java.util.Properties)
     */
    public TransactionInterceptor(PlatformTransactionManager ptm, Properties attributes) {
        setTransactionManager(ptm);
        setTransactionAttributes(attributes);
    }

    /**
     * Create a new TransactionInterceptor.
     * @param ptm the default transaction manager to perform the actual transaction management
     * @param tas the attribute source to be used to find transaction attributes
     * @see #setTransactionManager
     * @see #setTransactionAttributeSource(TransactionAttributeSource)
     */
    public TransactionInterceptor(PlatformTransactionManager ptm, TransactionAttributeSource tas) {
        setTransactionManager(ptm);
        setTransactionAttributeSource(tas);
    }


    @Override
    public Object invoke(final MethodInvocation invocation) throws Throwable {
        // Work out the target class: may be {@code null}.
        // The TransactionAttributeSource should be passed the target class
        // as well as the method, which may be from an interface.
        Class targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

        // Adapt to TransactionAspectSupport's invokeWithinTransaction...
        return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
            @Override
            public Object proceedWithInvocation() throws Throwable {
                return invocation.proceed();
            }
        });
    }


    //---------------------------------------------------------------------
    // Serialization support
    //---------------------------------------------------------------------

    private void writeObject(ObjectOutputStream oos) throws IOException {
        // Rely on default serialization, although this class itself doesn't carry state anyway...
        oos.defaultWriteObject();

        // Deserialize superclass fields.
        oos.writeObject(getTransactionManagerBeanName());
        oos.writeObject(getTransactionManager());
        oos.writeObject(getTransactionAttributeSource());
        oos.writeObject(getBeanFactory());
    }

    private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
        // Rely on default serialization, although this class itself doesn't carry state anyway...
        ois.defaultReadObject();

        // Serialize all relevant superclass fields.
        // Superclass can't implement Serializable because it also serves as base class
        // for AspectJ aspects (which are not allowed to implement Serializable)!
        setTransactionManagerBeanName((String) ois.readObject());
        setTransactionManager((PlatformTransactionManager) ois.readObject());
        setTransactionAttributeSource((TransactionAttributeSource) ois.readObject());
        setBeanFactory((BeanFactory) ois.readObject());
    }

}

Object invoke(final MethodInvocation invocation)是处理事务的逻辑，这个MethodInterceptor是通过org.springframework.cglib.proxy.MethodProxy#invoke，实际就是cglib实现的。

具体代理生成的代码可以参考org.springframework.aop.framework.CglibAopProxy#getProxy(java.lang.ClassLoader):

    @Override
    public Object getProxy(ClassLoader classLoader) {
        if (logger.isDebugEnabled()) {
            logger.debug("Creating CGLIB proxy: target source is " + this.advised.getTargetSource());
        }

        try {
            Class rootClass = this.advised.getTargetClass();
            Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");

            Class proxySuperClass = rootClass;
            if (ClassUtils.isCglibProxyClass(rootClass)) {
                proxySuperClass = rootClass.getSuperclass();
                Class[] additionalInterfaces = rootClass.getInterfaces();
                for (Class additionalInterface : additionalInterfaces) {
                    this.advised.addInterface(additionalInterface);
                }
            }

            // Validate the class, writing log messages as necessary.
            validateClassIfNecessary(proxySuperClass, classLoader);

            // Configure CGLIB Enhancer...
            Enhancer enhancer = createEnhancer();
            if (classLoader != null) {
                enhancer.setClassLoader(classLoader);
                if (classLoader instanceof SmartClassLoader &&
                        ((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
                    enhancer.setUseCache(false);
                }
            }
            enhancer.setSuperclass(proxySuperClass);
            enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
            enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
            enhancer.setStrategy(new ClassLoaderAwareUndeclaredThrowableStrategy(classLoader));

            Callback[] callbacks = getCallbacks(rootClass);
            Class[] types = new Class[callbacks.length];
            for (int x = 0; x < types.length; x++) {
                types[x] = callbacks[x].getClass();
            }
            // fixedInterceptorMap only populated at this point, after getCallbacks call above
            enhancer.setCallbackFilter(new ProxyCallbackFilter(
                    this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
            enhancer.setCallbackTypes(types);

            // Generate the proxy class and create a proxy instance.
            return createProxyClassAndInstance(enhancer, callbacks);
        }
        catch (CodeGenerationException ex) {
            throw new AopConfigException("Could not generate CGLIB subclass of class [" +
                    this.advised.getTargetClass() + "]: " +
                    "Common causes of this problem include using a final class or a non-visible class",
                    ex);
        }
        catch (IllegalArgumentException ex) {
            throw new AopConfigException("Could not generate CGLIB subclass of class [" +
                    this.advised.getTargetClass() + "]: " +
                    "Common causes of this problem include using a final class or a non-visible class",
                    ex);
        }
        catch (Throwable ex) {
            // TargetSource.getTarget() failed
            throw new AopConfigException("Unexpected AOP exception", ex);
        }
    }

就是使用Enhancer这个类来生成子类实现代理的。