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☕️【Java 技术之旅】深入分析 JDK 动态代理的分析(源码深入)

发布于: 2021 年 06 月 07 日
☕️【Java 技术之旅】深入分析JDK动态代理的分析(源码深入)

动态代理步骤

  1. 创建一个实现接口 InvocationHandler 的类,它必须实现 invoke 方法

  2. 创建被代理的类以及接口

  3. 通过 Proxy 的静态方法


通过 Proxy 的静态方法


    ProxyObject proxyObject = new ProxyObject();    InvocationHandler invocationHandler = new DynamicProxy(proxyObject);    ClassLoader classLoader = proxyObject.getClass().getClassLoader();    ProxyObjectInterface proxy = (IRoom) Proxy.newProxyInstance(classLoader,new Class[]  {ProxyObjectInterface.class},invocationHandler);    proxy.execute();
public class DynamicProxy implements InvocationHandler { private Object object;
public DynamicProxy(Object object){ this.object = object; }
@Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { Object result = method.invoke(object,args); return result; }}
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创建一个代理 newProxyInstance

    public static Object newProxyInstance(ClassLoader loader,                                          Class<?>[] interfaces,                                          InvocationHandler h)        throws IllegalArgumentException    {        //检验h不为空,h为空抛异常        Objects.requireNonNull(h);        //接口的类对象拷贝一份        final Class<?>[] intfs = interfaces.clone();        //进行一些安全性检查        final SecurityManager sm = System.getSecurityManager();        if (sm != null) {            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);        }        /*         * Look up or generate the designated proxy class.         *  查询(在缓存中已经有)或生成指定的代理类的class对象。         */        Class<?> cl = getProxyClass0(loader, intfs);        /*         * Invoke its constructor with the designated invocation handler.         */        try {            if (sm != null) {                checkNewProxyPermission(Reflection.getCallerClass(), cl);            }            //得到代理类对象的构造函数,这个构造函数的参数由constructorParams指定            //参数constructorParames为常量值:      private static final Class<?>[] constructorParams = { InvocationHandler.class };            final Constructor<?> cons = cl.getConstructor(constructorParams);            final InvocationHandler ih = h;            if (!Modifier.isPublic(cl.getModifiers())) {                AccessController.doPrivileged(new PrivilegedAction<Void>() {                    public Void run() {                        cons.setAccessible(true);                        return null;                    }                });            }            //这里生成代理对象,传入的参数new Object[]{h}后面讲            return cons.newInstance(new Object[]{h});        } catch (IllegalAccessException|InstantiationException e) {            throw new InternalError(e.toString(), e);        } catch (InvocationTargetException e) {            Throwable t = e.getCause();            if (t instanceof RuntimeException) {                throw (RuntimeException) t;            } else {                throw new InternalError(t.toString(), t);            }        } catch (NoSuchMethodException e) {            throw new InternalError(e.toString(), e);        }    }
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  1. 先对 h 进行判空处理。


这段代码核心就是通过 getProxyClass0(loader, intfs)得到代理类的 Class 对象,然后通过 Class 对象得到构造方法,进而创建代理对象。下一步看 getProxyClass0 这个方法。从 1 可知,先接口得到接口类,当接口的数量超过 65535,则报异常。


    //此方法也是Proxy类下的方法    private static Class<?> getProxyClass0(ClassLoader loader,                                           Class<?>... interfaces) {        if (interfaces.length > 65535) {            throw new IllegalArgumentException("interface limit exceeded");        }
// If the proxy class defined by the given loader implementing // the given interfaces exists, this will simply return the cached copy; // otherwise, it will create the proxy class via the ProxyClassFactory //意思是:如果代理类被指定的类加载器loader定义了,并实现了给定的接口interfaces, //那么就返回缓存的代理类对象,否则使用ProxyClassFactory创建代理类。 return proxyClassCache.get(loader, interfaces); }
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  1. proxyClassCache 是一个弱引用的缓存


这里看到 proxyClassCache,有 Cache 便知道是缓存的意思,正好呼应了前面 Look up or generate the designated proxy class。查询(在缓存中已经有)或生成指定的代理类的 class 对象这段注释。


在进入 get 方法之前,我们看下 proxyClassCache 是什么?高能预警,前方代码看起来可能有乱,但我们只需要关注重点即可。


private static final WeakCache<ClassLoader, Class<?>[], Class<?>>        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
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//K代表key的类型,P代表参数的类型,V代表value的类型。// WeakCache<ClassLoader, Class<?>[], Class<?>>  proxyClassCache  说明proxyClassCache存的值是Class<?>对象,正是我们需要的代理类对象。final class WeakCache<K, P, V> {    private final ReferenceQueue<K> refQueue        = new ReferenceQueue<>();    // the key type is Object for supporting null key    private final ConcurrentMap<Object, ConcurrentMap<Object, Supplier<V>>> map        = new ConcurrentHashMap<>();    private final ConcurrentMap<Supplier<V>, Boolean> reverseMap        = new ConcurrentHashMap<>();    private final BiFunction<K, P, ?> subKeyFactory;    private final BiFunction<K, P, V> valueFactory;    public WeakCache(BiFunction<K, P, ?> subKeyFactory,                     BiFunction<K, P, V> valueFactory) {        this.subKeyFactory = Objects.requireNonNull(subKeyFactory);        this.valueFactory = Objects.requireNonNull(valueFactory);    }
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其中 map 变量是实现缓存的核心变量,他是一个双重的 Map 结构: (key, sub-key) -> value。其中 key 是传进来的 Classloader 进行包装后的对象,sub-key 是由 WeakCache 构造函数传人的 KeyFactory()生成的。value 就是产生代理类的对象,是由 WeakCache 构造函数传人的 ProxyClassFactory()生成的。如下,回顾一下:


proxyClassCache 是个 WeakCache 类的对象,调用 proxyClassCache.get(loader, interfaces); 可以得到缓存的代理类或创建代理类(没有缓存的情况)。


说明 WeakCache 中有 get 这个方法。先看下 WeakCache 类的定义(这里先只给出变量的定义和构造函数),继续看它的 get();


//K和P就是WeakCache定义中的泛型,key是类加载器,parameter是接口类数组public V get(K key, P parameter) {        //检查parameter不为空        Objects.requireNonNull(parameter);         //清除无效的缓存        expungeStaleEntries();        // cacheKey就是(key, sub-key) -> value里的一级key,        Object cacheKey = CacheKey.valueOf(key, refQueue);        // lazily install the 2nd level valuesMap for the particular cacheKey        //根据一级key得到 ConcurrentMap<Object, Supplier<V>>对象。如果之前不存在,则新建一个ConcurrentMap<Object, Supplier<V>>和cacheKey(一级key)一起放到map中。        ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);        if (valuesMap == null) {            ConcurrentMap<Object, Supplier<V>> oldValuesMap                = map.putIfAbsent(cacheKey,                                  valuesMap = new ConcurrentHashMap<>());            if (oldValuesMap != null) {                valuesMap = oldValuesMap;            }        }
// create subKey and retrieve the possible Supplier<V> stored by that // subKey from valuesMap //这部分就是调用生成sub-key的代码,上面我们已经看过怎么生成的了 Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter)); //通过sub-key得到supplier Supplier<V> supplier = valuesMap.get(subKey); //supplier实际上就是这个factory Factory factory = null;
while (true) { //如果缓存里有supplier ,那就直接通过get方法,得到代理类对象,返回,就结束了,一会儿分析get方法。 if (supplier != null) { // supplier might be a Factory or a CacheValue<V> instance V value = supplier.get(); if (value != null) { return value; } } // else no supplier in cache // or a supplier that returned null (could be a cleared CacheValue // or a Factory that wasn't successful in installing the CacheValue) // lazily construct a Factory //下面的所有代码目的就是:如果缓存中没有supplier,则创建一个Factory对象,把factory对象在多线程的环境下安全的赋给supplier。 //因为是在while(true)中,赋值成功后又回到上面去调get方法,返回才结束。 if (factory == null) { factory = new Factory(key, parameter, subKey, valuesMap); }
if (supplier == null) { supplier = valuesMap.putIfAbsent(subKey, factory); if (supplier == null) { // successfully installed Factory supplier = factory; } // else retry with winning supplier } else { if (valuesMap.replace(subKey, supplier, factory)) { // successfully replaced // cleared CacheEntry / unsuccessful Factory // with our Factory supplier = factory; } else { // retry with current supplier supplier = valuesMap.get(subKey); } } } }
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所以接下来我们看 Factory 类中的 get 方法。接下来看 supplier 的 get()


        public synchronized V get() { // serialize access            // re-check            Supplier<V> supplier = valuesMap.get(subKey);            //重新检查得到的supplier是不是当前对象            if (supplier != this) {                // something changed while we were waiting:                // might be that we were replaced by a CacheValue                // or were removed because of failure ->                // return null to signal WeakCache.get() to retry                // the loop                return null;            }            // else still us (supplier == this)            // create new value            V value = null;            try {                 //代理类就是在这个位置调用valueFactory生成的                 //valueFactory就是我们传入的 new ProxyClassFactory()                //一会我们分析ProxyClassFactory()的apply方法                value = Objects.requireNonNull(valueFactory.apply(key, parameter));            } finally {                if (value == null) { // remove us on failure                    valuesMap.remove(subKey, this);                }            }            // the only path to reach here is with non-null value            assert value != null;
// wrap value with CacheValue (WeakReference) //把value包装成弱引用 CacheValue<V> cacheValue = new CacheValue<>(value);
// put into reverseMap // reverseMap是用来实现缓存的有效性 reverseMap.put(cacheValue, Boolean.TRUE);
// try replacing us with CacheValue (this should always succeed) if (!valuesMap.replace(subKey, this, cacheValue)) { throw new AssertionError("Should not reach here"); }
// successfully replaced us with new CacheValue -> return the value // wrapped by it return value; } }
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拨云见日,来到 ProxyClassFactory 的 apply 方法,代理类就是在这里生成的。首先看 proxyClassCache 的定义 WeakCache<ClassLoader, Class<?>[], Class<?>>,泛型里面第一个表示加载器 K,第二个表示接口类 P,第三个则是生成的代理类 V。而 V 的生成则是通过 ProxyClassFactory 生成的。调用其 apply();


 //这里的BiFunction<T, U, R>是个函数式接口,可以理解为用T,U两种类型做参数,得到R类型的返回值private static final class ProxyClassFactory        implements BiFunction<ClassLoader, Class<?>[], Class<?>>    {        // prefix for all proxy class names        //所有代理类名字的前缀        private static final String proxyClassNamePrefix = "$Proxy";        // next number to use for generation of unique proxy class names        //用于生成代理类名字的计数器        private static final AtomicLong nextUniqueNumber = new AtomicLong();        @Override        public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);            //验证代理接口,可不看            for (Class<?> intf : interfaces) {                /*                 * Verify that the class loader resolves the name of this                 * interface to the same Class object.                 */                Class<?> interfaceClass = null;                try {                    interfaceClass = Class.forName(intf.getName(), false, loader);                } catch (ClassNotFoundException e) {                }                if (interfaceClass != intf) {                    throw new IllegalArgumentException(                        intf + " is not visible from class loader");                }                /*                 * Verify that the Class object actually represents an                 * interface.                 */                if (!interfaceClass.isInterface()) {                    throw new IllegalArgumentException(                        interfaceClass.getName() + " is not an interface");                }                /*                 * Verify that this interface is not a duplicate.                 */                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {                    throw new IllegalArgumentException(                        "repeated interface: " + interfaceClass.getName());                }            }            //生成的代理类的包名             String proxyPkg = null;     // package to define proxy class in            //代理类访问控制符: public ,final            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;            /*             * Record the package of a non-public proxy interface so that the             * proxy class will be defined in the same package.  Verify that             * all non-public proxy interfaces are in the same package.             */            //验证所有非公共的接口在同一个包内;公共的就无需处理            //生成包名和类名的逻辑,包名默认是com.sun.proxy,      // 类名默认是$Proxy 加上一个自增的整数值            //如果被代理类是 non-public proxy interface ,则用和被代理类接口一样的包名            for (Class<?> intf : interfaces) {                int flags = intf.getModifiers();                if (!Modifier.isPublic(flags)) {                    accessFlags = Modifier.FINAL;                    String name = intf.getName();                    int n = name.lastIndexOf('.');                    String pkg = ((n == -1) ? "" : name.substring(0, n + 1));                    if (proxyPkg == null) {                        proxyPkg = pkg;                    } else if (!pkg.equals(proxyPkg)) {                        throw new IllegalArgumentException(                            "non-public interfaces from different packages");                    }                }            }            if (proxyPkg == null) {                // if no non-public proxy interfaces, use com.sun.proxy package                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";            }
/* * Choose a name for the proxy class to generate. */ long num = nextUniqueNumber.getAndIncrement(); //代理类的完全限定名,如com.sun.proxy.$Proxy0.calss String proxyName = proxyPkg + proxyClassNamePrefix + num;
/* * Generate the specified proxy class. */ //核心部分,生成代理类的字节码 byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags); try { //把代理类加载到JVM中,至此动态代理过程基本结束了 return defineClass0(loader, proxyName, proxyClassFile, 0, proxyClassFile.length); } catch (ClassFormatError e) { /* * A ClassFormatError here means that (barring bugs in the * proxy class generation code) there was some other * invalid aspect of the arguments supplied to the proxy * class creation (such as virtual machine limitations * exceeded). */ throw new IllegalArgumentException(e.toString()); } } }
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然后调用 getMethod(),将 equals(),hashcode(),toString()等方法添加进去。然后遍历所有接口的方法,添加到代理类中。最后将这些方法进行排序。


private static List<Method> getMethods(Class<?>[] interfaces) {        List<Method> result = new ArrayList<Method>();        try {            result.add(Object.class.getMethod("equals", Object.class));            result.add(Object.class.getMethod("hashCode", EmptyArray.CLASS));            result.add(Object.class.getMethod("toString", EmptyArray.CLASS));        } catch (NoSuchMethodException e) {            throw new AssertionError();        }
getMethodsRecursive(interfaces, result); return result; }private static void getMethodsRecursive(Class<?>[] interfaces, List<Method> methods) { for (Class<?> i : interfaces) { getMethodsRecursive(i.getInterfaces(), methods); Collections.addAll(methods, i.getDeclaredMethods()); } }
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最后输出相关 proxy class

package com.zhb.jdk.proxy;import java.io.FileOutputStream;import java.io.IOException;import java.lang.reflect.Proxy;
import com.zhb.jdk.dynamicProxy.HelloworldImpl;
import sun.misc.ProxyGenerator;
/** * @author ZHB * @date 2018年8月31日下午11:35:07 * @todo TODO */public class DynamicProxyTest {
public static void main(String[] args) {
IUserService target = new UserServiceImpl(); MyInvocationHandler handler = new MyInvocationHandler(target); //第一个参数是指定代理类的类加载器(我们传入当前测试类的类加载器) //第二个参数是代理类需要实现的接口(我们传入被代理类实现的接口,这样生成的代理类和被代理类就实现了相同的接口) //第三个参数是invocation handler,用来处理方法的调用。这里传入我们自己实现的handler IUserService proxyObject = (IUserService) Proxy.newProxyInstance(DynamicProxyTest.class.getClassLoader(), target.getClass().getInterfaces(), handler); proxyObject.add("陈粒");
String path = "D:/$Proxy0.class"; byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", HelloworldImpl.class.getInterfaces()); FileOutputStream out = null;
try { out = new FileOutputStream(path); out.write(classFile); out.flush(); } catch (Exception e) { e.printStackTrace(); } finally { try { out.close(); } catch (IOException e) { e.printStackTrace(); } }
}}
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// Decompiled by Jad v1.5.8e2. Copyright 2001 Pavel Kouznetsov.// Jad home page: http://kpdus.tripod.com/jad.html// Decompiler options: packimports(3) fieldsfirst ansi space 
import com.zhb.jdk.proxy.IUserService;import java.lang.reflect.*;
public final class $Proxy0 extends Proxy implements IUserService{
private static Method m1; private static Method m2; private static Method m3; private static Method m0; //代理类的构造函数,其参数正是是InvocationHandler实例, //Proxy.newInstance方法就是通过通过这个构造函数来创建代理实例的 public $Proxy0(InvocationHandler invocationhandler) { super(invocationhandler); } // Object类中的三个方法,equals,toString, hashCode public final boolean equals(Object obj) { try { return ((Boolean)super.h.invoke(this, m1, new Object[] { obj })).booleanValue(); } catch (Error ) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } }
public final String toString() { try { return (String)super.h.invoke(this, m2, null); } catch (Error ) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } //接口代理方法 public final void add(String s) { try { // invocation handler的 invoke方法在这里被调用 super.h.invoke(this, m3, new Object[] { s }); return; } catch (Error ) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } }
public final int hashCode() { try { // 在这里调用了invoke方法。 return ((Integer)super.h.invoke(this, m0, null)).intValue(); } catch (Error ) { } catch (Throwable throwable) { throw new UndeclaredThrowableException(throwable); } }
// 静态代码块对变量进行一些初始化工作 static { try { m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") }); m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]); m3 = Class.forName("com.zhb.jdk.proxy.IUserService").getMethod("add", new Class[] { Class.forName("java.lang.String") }); m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]); } catch (NoSuchMethodException nosuchmethodexception) { throw new NoSuchMethodError(nosuchmethodexception.getMessage()); } catch (ClassNotFoundException classnotfoundexception) { throw new NoClassDefFoundError(classnotfoundexception.getMessage()); } }}
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发布于: 2021 年 06 月 07 日阅读数: 33
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☕️【Java 技术之旅】深入分析JDK动态代理的分析(源码深入)