Javassist实现动态代理

 

动态代理模式简述：之所以会出现代理这种模式就是因为我们常有这么一种需求：在被代理类的方法调用前后执行一些其它的逻辑，这些逻辑不适合由被代理类来实现，那这些逻辑谁来实现？当然是代理类。那代理类是谁？从哪里来？代理类是我们利用字节码生成工具动态创建的，然后利用反射实例化而得到代理对象。

 

tips:这篇文章讲的不是动态代理模式的思想而是怎么实现(简单实现)一个和java.lang.reflect.Proxy类相似的类。

 

按照JDK动态代理的套路来，先定义一个InvocationHandler来给客户端添加代理的逻辑(这个类和JDK中的一模一样)：

 

 

package cc.lixiaohui.demo.javassist.proxy;

import java.lang.reflect.Method;

/**
 * @author lixiaohui
 * @date 2016年9月27日 下午9:53:59
 *
 */
public interface InvocationHandler {

    /**
     * 业务逻辑填充
     *
     * @param proxy 生成的代理对象
     * @param method 调用的方法
     * @param args 调用该方法的参数
     * @return 调用该方法的返回值
     * @throws Throwable throws if any exception
     */
    Object invoke(Object proxy, Method method, Object[] args) throws Throwable;

}

 

 

接下来是Proxy类，代理类由该类动态生成并且都继承自该类,和JDK的一样，这也是为什么JDK的动态代理只能代理接口而不能代理父类，这里我就按照JDK的套路来，当然理解了这个后要实现代理父类也没什么难度了。Proxy类是怎么生成类的呢？首先代理的是接口，所以先遍历所有接口，再遍历接口的所有方法，为代理类生成与这些方法同签名同返回值的方法，也就是相当于实现(implement)这些接口的方法，至于生成过程具体是怎么样的我们不用管这也是用javassist的好处（字节码是怎么生成的是javassist的事，我们只需要用它的API即可）。

 

 

package cc.lixiaohui.demo.javassist.proxy;

import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.CtField;
import javassist.CtMethod;
import javassist.Modifier;
import javassist.NotFoundException;

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

import cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyWeakHashMap;

/**
 * 负责代理类类的生成
 *
 * <ul>
 * 生成的代理类:
 * <li>public final 修饰(和JDK动态代理不同的是:JDK生成的代理类也是final的, 但不一定是public的, 当所代理的接口中有至少一个以上的接口不是public时生成的代理就不是public的)</li>
 *
 * <li>类名以$Proxy为前缀, 后缀为数字, 如cc.lixiaohui.$Proxy0, cc.lixiaohui.$Proxy1...</li>
 *
 * <li>生成的代理类继承自{@link Proxy}</li>
 *
 * <li>生成的代理类所在package只有一种情况下才是确定的: 当所有接口中有且只有一个接口是non-public时.其他情况所在package不确定</li>
 * </ul>
 *
 * @author lixiaohui
 * @date 2016年9月27日 下午9:51:24
 *
 */
public class Proxy {

    private static final Logger logger = LoggerFactory.getLogger(Proxy.class);

    /**
     * 生成的代理类名前缀
     */
    private static final String PROXY_CLASSNAME_PREFIX = "$Proxy";

    /**
     * 类后缀数字生成器
     */
    private static final AtomicInteger SUFFIX_GENERATOR = new AtomicInteger();

    private static final boolean SHOULD_BE_FINAL = true;
    private static final boolean SHOULD_BE_ABSTRACT = false;
    private static final boolean SHOULD_BE_PUBLIC = true;

    protected InvocationHandler invocationHandler;

    /**
     * 弱引用已生成的Class的缓存, ClassLoader和被代理Class都相同时生成的代理Class才是相同的(这个类自己实现的,简单扩展一下java.util.Map就可以实现)
     * <类加载器, 被代理Class, 生成的代理Class>
     */
    private static CompoundKeyWeakHashMap<ClassLoader, Class<?>, Class<?>> proxyClassCache = new CompoundKeyWeakHashMap<ClassLoader, Class<?>, Class<?>>();

    protected Proxy(InvocationHandler invocationHandler) {
        this.invocationHandler = invocationHandler;
    }

    public static Object newProxyInstance(ClassLoader classLoader, Class<?> targetClass, InvocationHandler invocationHandler)
            throws Exception {
        // check not null
        classLoader = Objects.requireNonNull(classLoader, "classLoader cannot be null");
        targetClass = Objects.requireNonNull(targetClass, "targetClass cannot be null");
        invocationHandler = Objects.requireNonNull(invocationHandler, "invocationHandler cannot be null");

        Class<?> proxyClass = proxyClassCache.get(classLoader, targetClass);
        // 有缓存
        if (proxyClass != null) {
            logger.debug("get proxy from cache");
            return proxyClass.getConstructor(InvocationHandler.class).newInstance(invocationHandler);
        }

        // singleton instance of classpool
        ClassPool pool = ClassPool.getDefault();
        //生成代理类的全限定名
        String qualifiedName = generateQualifiedName(targetClass);
        // 创建代理类
        CtClass proxy = pool.makeClass(qualifiedName);
        // 设被代理类继承自Proxy
        setSuperClass(pool, proxy);
        // 获取被代理类的所有接口
        CtClass[] interfaces = pool.get(targetClass.getName()).getInterfaces();

        int methodIndex = 0;
        // 遍历这些接口
        for (CtClass parent : interfaces) {
            proxy.addInterface(parent);

            // 获取该接口的所有方法
            CtMethod[] methods = parent.getDeclaredMethods();
            for (int j = 0; j < methods.length; ++j) {
                CtMethod method = methods[j];
                String fieldSrc = String.format("private static java.lang.reflect.Method method%d = Class.forName(\"%s\").getDeclaredMethods()[%d];"
                        , methodIndex, parent.getName(), j);
                logger.debug("field src for method {}: {}", method.getName(), fieldSrc);
                // 生成字段
                CtField ctField = CtField.make(fieldSrc, proxy);
                // 添加字段
                proxy.addField(ctField);
                // 生成对应的Method
                generateMethod(pool, proxy, method, methodIndex);

                ++methodIndex;
            }
        }
        // 设置代理类的类修饰符
        setModifiers(proxy, SHOULD_BE_PUBLIC, SHOULD_BE_FINAL, SHOULD_BE_ABSTRACT);
        // 生成构造方法
        generateConstructor(pool, proxy);
        // 持久化class到硬盘, for use of debug
        proxy.writeFile(".");
        // to java.lang.Class
        proxyClass = proxy.toClass(classLoader, null);
        // 缓存
        proxyClassCache.put(classLoader, targetClass, proxyClass);
        return proxyClass.getConstructor(InvocationHandler.class).newInstance(invocationHandler);
    }

    /**
     * 生成代理类的全限定名
     */
    private static String generateQualifiedName(Class<?> targetClass) throws Exception {
        CtClass theInterface = null;
        for (CtClass parent : ClassPool.getDefault().get(targetClass.getName()).getInterfaces()) {
            if (theInterface == null) {
                theInterface = parent;
            }
            if (!Modifier.isPublic(parent.getModifiers())) {
                theInterface = parent;
                break;
            }
        }
        String name = theInterface.getPackageName() + "." + PROXY_CLASSNAME_PREFIX + SUFFIX_GENERATOR.getAndIncrement();
        return name;
    }


    /**
     * 设置类的修饰符
     */
    private static void setModifiers(CtClass proxy, boolean shouldBePublic, boolean shouldBeFinal, boolean shouldBeAbstract) {
        int modifier = 0;
        modifier = shouldBePublic ? modifier | Modifier.PUBLIC : modifier;
        modifier = shouldBeFinal ? modifier | Modifier.FINAL : modifier;
        modifier = shouldBeAbstract ? modifier | Modifier.ABSTRACT : modifier;
        logger.error(Modifier.toString(modifier));
        proxy.setModifiers(modifier);
    }

    /**
     * 生成构造函数
     */
    private static void generateConstructor(ClassPool pool, CtClass proxy) throws NotFoundException, CannotCompileException {
        CtConstructor ctConstructor = new CtConstructor(new CtClass[]{pool.get(InvocationHandler.class.getName())}, proxy);
        String methodBodySrc = String.format("super(%s);", "$1");
        logger.debug("constructor body for constructor {}: {}", ctConstructor.getName(), methodBodySrc);
        ctConstructor.setBody(methodBodySrc);
        proxy.addConstructor(ctConstructor);
    }


    /**
     * 生成代理方法
     */
    private static void generateMethod(ClassPool pool, CtClass proxy, CtMethod method, int methodIndex) throws NotFoundException, CannotCompileException {
        CtMethod ctMethod = new CtMethod(method.getReturnType(), method.getName(), method.getParameterTypes(), proxy);
        String methodBodySrc = String.format("return super.invocationHandler.invoke(this, method%d, $args);", methodIndex);
        logger.debug("method body for method {}: {}", method.getName(), methodBodySrc);
        ctMethod.setBody(methodBodySrc);
        proxy.addMethod(ctMethod);
    }

    /**
     * 把proxy类的父类设置为Proxy
     *
     */
    private static void setSuperClass(ClassPool pool, CtClass proxy) throws CannotCompileException, NotFoundException {
        proxy.setSuperclass(pool.get(Proxy.class.getName()));
    }

}

 

 

使用测试

 使用方式和JDK的没啥区别

先定义两个被代理的接口：

 

package cc.lixiaohui.demo.javassist.proxy.example;

public interface Talkable {
    Object talk(String words) throws Exception;
}

 

package cc.lixiaohui.demo.javassist.proxy.example;

interface Smileable {
    Object smile() throws Exception;
}

 

Person实现上面两接口：

 

package cc.lixiaohui.demo.javassist.proxy.example;

public class Person implements Smileable, Talkable {

    private String name;

    public Person(String name) {
        this.name = name;
    }

    public Object talk(String words) throws Exception {
        System.out.println(name + " says: " + words);
        return words;
    }

    public Object smile() throws Exception {
        System.out.println(name + " start smiling");
        System.out.println(name + " stop smiling");
        return null;
    }

}

 

 

代理逻辑的实现：

 

package cc.lixiaohui.demo.javassist.proxy.example;

import java.lang.reflect.Method;

import cc.lixiaohui.demo.javassist.proxy.InvocationHandler;
import cc.lixiaohui.demo.javassist.proxy.Proxy;

public class JavassistProxyFactory implements InvocationHandler{

    //被代理类的对象
    private Object target;

    public JavassistProxyFactory(Object target) {
        this.target = target;
    }

    /*
     * @see cc.lixiaohui.demo.javassist.proxy.InvocationHandler#invoke(java.lang.Object, java.lang.reflect.Method, java.lang.Object[])
     */
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("------- intercept before --------");
                // 调用原来的方法
        Object result = method.invoke(target, args);
        System.out.println("--------intercept after ---------");
        return result;
    }
    // 获取代理类的对象
    public Object getProxy() throws Exception {
        return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(), target.getClass(), this);
    }

}

 

测试主程序：

package cc.lixiaohui.demo.javassist.proxy.example;

import java.lang.reflect.Modifier;

import org.junit.Test;

public class Tester {

    @Test
    public void testJavassist() throws Exception {
        Person person = new Person("小明");
        Object proxy = new JavassistProxyFactory(person).getProxy();
                // System.gc(); // 主动触发gc
        Object proxy1 = new JavassistProxyFactory(person).getProxy();
        ((Talkable) proxy).talk("hello world");
        ((Smileable) proxy).smile();

        System.out.println("package: " + proxy.getClass().getPackage().getName());
        System.out.println("classname: " + proxy.getClass().getName());
        System.out.println("modifiers: " + Modifier.toString(proxy.getClass().getModifiers()));
                System.out.println(proxy.getClass() == proxy1.getClass()); // 测试缓存是否起作用
    }

}

 

结果输出：

 
 

 

在 Proxy类中生成代理Class时把这个Class持久化了到硬盘中，通过反编译工具查看生成的代理类的源码：

 

/*** Eclipse Class Decompiler plugin, copyright (c) 2016 Chen Chao ([e-mail]) ***/
package cc.lixiaohui.demo.javassist.proxy.example;

import cc.lixiaohui.demo.javassist.proxy.InvocationHandler;
import cc.lixiaohui.demo.javassist.proxy.Proxy;
import java.lang.reflect.Method;

public final class $Proxy0 extends Proxy implements Smileable, Talkable {
    private static Method method0 = java.lang.Class.forName("cc.lixiaohui.demo.javassist.proxy.example.Smileable").getDeclaredMethods()[0];
    private static Method method1 = java.lang.Class.forName("cc.lixiaohui.demo.javassist.proxy.example.Talkable").getDeclaredMethods()[0];

    public Object smile() {
        return this.invocationHandler.invoke(this, method0, new Object[0]);
    }

    public Object talk(String paramString) {
        return this.invocationHandler.invoke(this, method1, new Object[] { paramString });
    }

    public $Proxy0(InvocationHandler paramInvocationHandler) {
        super(paramInvocationHandler);
    }
}

评论

2 楼 莫名的拉风 2017-05-27  

minisnoopy2u 写道

您好，ClassLoader, targetClass, proxyClass为什么说要保存“弱”引用呢？直接put进去的不是引用吗？CompoundKeyWeakHashMap的定义能贴一下吗？

package cc.lixiaohui.demo.javassist.proxy.util;

import java.lang.ref.WeakReference;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Objects;
import java.util.Set;

/**
* 基于{@link java.util.HashMap}的CompoundKeyMap的实现, 所有key为弱引用
*
* @author lixiaohui
* @date 2016年10月1日 下午12:37:08
*
*/
public class CompoundKeyWeakHashMap<K, P, V> implements CompoundKeyMap<K, P, V> {

private Map<KeyHolder<CompoundKey<K, P>>, V> map = new HashMap<KeyHolder<CompoundKey<K, P>>, V>();


/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap#get(java.lang.Object, java.lang.Object)
*/
public V get(K key, P param) {
key = Objects.requireNonNull(key, "key cannot be null");
param = Objects.requireNonNull(param, "param cannot be null");

return map.get(newKey(key, param));
}

private KeyHolder<CompoundKey<K, P>> newKey(K key, P param) {
return new KeyHolder<CompoundKey<K, P>>(new CompoundKeyImpl<K, P>(key, param));
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap#get(java.lang.Object, java.lang.Object, java.lang.Object)
*/
public V get(K key, P param, V defValue) {
key = Objects.requireNonNull(key, "key cannot be null");
param = Objects.requireNonNull(param, "param cannot be null");

V value = get(key, param);
return value == null ? defValue : value;
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap#put(java.lang.Object, java.lang.Object, java.lang.Object)
*/
public V put(K key, P param, V value) {
return map.put(newKey(key, param), value);
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap#putIfAbsent(java.lang.Object, java.lang.Object, java.lang.Object)
*/
public V putIfAbsent(K key, P param, V value) {
return map.putIfAbsent(newKey(key, param), value);
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap#entrySet()
*/
public Set<Entry<CompoundKeyMap.CompoundKey<K, P>, V>> entrySet() {
throw new UnsupportedOperationException();
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap#keys()
*/
public Set<CompoundKeyMap.CompoundKey<K, P>> keys() {
throw new UnsupportedOperationException();
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap#values()
*/
public Collection<V> values() {
return map.values();
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap#size()
*/
public int size() {
return map.size();
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap#isEmpty()
*/
public boolean isEmpty() {
return map.isEmpty();
}

static class KeyHolder<T> extends WeakReference<T> {

public KeyHolder(T referent) {
super(referent);
}

@SuppressWarnings("unchecked")
@Override
public boolean equals(Object obj) {
return get().equals(((KeyHolder<T>) obj).get());
}

@Override
public int hashCode() {
return get().hashCode();
}

}

static class CompoundKeyImpl<K, P> implements CompoundKey<K, P> {

private K key;

private P param;

CompoundKeyImpl(K key, P param) {
super();
this.key = key;
this.param = param;
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap.CompoundKey#getKey()
*/
public K getKey() {
return key;
}

/*
* @see cc.lixiaohui.demo.javassist.proxy.util.CompoundKeyMap.CompoundKey#getParam()
*/
public P getParam() {
return param;
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((key == null) ? 0 : key.hashCode());
result = prime * result + ((param == null) ? 0 : param.hashCode());
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
CompoundKeyImpl<?, ?> other = (CompoundKeyImpl<?, ?>) obj;
if (key == null) {
if (other.key != null)
return false;
} else if (!key.equals(other.key))
return false;
if (param == null) {
if (other.param != null)
return false;
} else if (!param.equals(other.param))
return false;
return true;
}


}

}

1 楼 minisnoopy2u 2017-05-26  

您好，ClassLoader, targetClass, proxyClass为什么说要保存“弱”引用呢？直接put进去的不是引用吗？CompoundKeyWeakHashMap的定义能贴一下吗？