JDK动态代理之源码分析

绝地灬酷狼 2022-05-12 03:34 223阅读 0赞

一、代理模式是什么？

代理模式就是给一个对象提供一个代理对象，并由代理对象管理着被代理对象的引用。就像生活中的代理律师，你只需要找好代理律师，剩下的都交给代理律师来打理。

Spring MVC 有两大特性，IoC 和 AOP。IoC为控制反转，这里不做介绍；AOP（Aspect Oriented Programming 面向切面编程）的实现就是基于代理技术。

二、静态代理

先来看下什么是静态代理。静态代理在程序运行前，就已经编写好代理类，实现静态代理需要以下：

1、定义业务接口

2、被代理对象实现业务接口

3、代理对象实现业务接口并持有被代理对象的引用

码上看

2.1 业务接口

package cn.zjm.show.proxy;
    
    /**
     * @author Zhao JinMing
     * @date 2018年9月14日 09:49
     */
    
    public interface IUserService {
    
        void add(String name);
    }

2.2 被代理对象实现接口

package cn.zjm.show.proxy;
    
    /**
     * @author Zhao JinMing
     * @date 2018年9月14日 09:49
     */
    public class UserServiceImpl implements IUserService {
    
        @Override
        public void add(String name) {
            System.out.println("添加用户“" + name + "”成功。");
        }
    }

2.3 代理对象实现接口并持有代理对象引用

package cn.zjm.show.proxy;
    
    /**
     * @author Zhao JinMing
     * @date 2018年9月14日 09:50
     */
    public class UserServiceProxy implements IUserService {
    
        private IUserService proxy;
    
        public UserServiceProxy(IUserService proxy) {
            this.proxy = proxy;
        }
    
        @Override
        public void add(String name) {
            System.out.println("开始插入");
            proxy.add(name);
            System.out.println("结束插入");
        }
    }

2.4 代码测试

package cn.zjm.show.proxy;
    
    public class SourceProxy {
    
        public static void main(String[] args) {
            IUserService userService = new UserServiceImpl();
            IUserService proxy = new UserServiceProxy(userService);
    
            proxy.add("王五");
        }
    }

2.5 输出结果

开始插入
    添加用户“王五”成功。
    结束插入

2.6 总结

既然已经有动态代理了，那为什么还需要动态代理。毋庸置疑肯定是静态代理与动态代理相比有着很多的缺陷。从扩展性的角度考虑，如果业务接口添加或者删除方法，那么不仅仅是被代理类需要修改，代理类也需要修改。而且对于不同的业务接口，需要写不同的代理类，这么麻烦的操作，怕麻烦的大佬肯定受不了，于是乎动态代理来了。

三、动态代理

所谓的动态代理，就是在程序运行的时候，根据需要动态的创建代理类及其实例来完成具体的操作。动态代理又分为JDK动态代理和cglib动态代理两大类，下面介绍的是JDK动态代理。

3.1 使用JDK动态代理的步骤

①定义业务接口

②被代理对象实现业务接口

③通过Proxy的静态方法newProxyInstance( ClassLoaderloader, Class\[\] interfaces, InvocationHandler h)创建一个代理对象

④使用代理对象

3.1 定义业务接口

package cn.zjm.show.proxy;
    
    /**
     * @author Zhao JinMing
     * @date 2018年9月14日 09:49
     */
    public interface IUserService {
    
        void add(String name);
    }

3.2 被代理对象实现业务接口

3.3 创建代理对象并使用

package cn.zjm.show.proxy;
    
    import java.lang.reflect.InvocationHandler;
    import java.lang.reflect.Method;
    import java.lang.reflect.Proxy;
    
    public class SourceProxy {
    
        public static void main(String[] args) {
            IUserService userService = new UserServiceImpl();
            IUserService proxy = (IUserService) Proxy.newProxyInstance(SourceProxy.class.getClassLoader(), userService.getClass().getInterfaces(), new InvocationHandler() {
                @Override
                public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                    System.out.println("准备使用动态代理插入数据");
                    Object invoke = method.invoke(userService, args);
                    System.out.println("使用动态代理插入对象结束");
                    return invoke;
                }
            });
            proxy.add("王五");
        }
    }

3.4 运行结果

准备使用动态代理插入数据
    添加用户“王五”成功。
    使用动态代理插入对象结束

运行结果和静态代理一样，说明成功了。但是，我们注意到，我们并没有像静态代理那样去自己定义一个代理类，并实例化代理对象。实际上，动态代理的代理对象是在内存中的，是JDK根据我们传入的参数生成好的。那动态代理的代理类和代理对象是怎么产生的呢？

四、源码分析

首先来看 ***newProxyInstance(ClassLoader loader, Class<?>\[\] interfaces, InvocationHandler h)***

public static Object newProxyInstance(ClassLoader loader,
                                              Class<?>[] interfaces,
                                              InvocationHandler h)
            throws IllegalArgumentException
        {
            //判断h是否为空
            Objects.requireNonNull(h);
            //对接口数组的拷贝
            final Class<?>[] intfs = interfaces.clone();
            //安全检查
            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。
             *     --核心代码
             */
            Class<?> cl = getProxyClass0(loader, intfs);
    
            /*
             * Invoke its constructor with the designated invocation handler.
             * 用invocationHandler生成构造函数
             */
            try {
                if (sm != null) {
                    checkNewProxyPermission(Reflection.getCallerClass(), cl);
                }
                //得到代理对象的构造函数
                //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;
                        }
                    });
                }
                //生成代理对象
                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);
            }
        }

代码核心是通过getProxyClass0(loader, intfs)得到代理类的Class对象，然后通过Class对象获得构造方法，进而创建代理对象。再看getProxyClass0(loader, intfs)方法。

***getProxyClass0(loader, intfs)***

private static Class<?> getProxyClass0(ClassLoader loader,
                                               Class<?>... interfaces) {
            //接口数量不能大于65535
            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
            /*
                如果由实现给定接口的给定加载器定义的代理类存在，则这将简单地返回缓存的副本；
                否则，它将通过ProxyClassFactory创建代理类
            */
            return proxyClassCache.get(loader, interfaces);
        }
    
    
        // private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
    
        // proxyClassCache是WeakCache类的实例，WeakCache又是什么？

再看看 ***WeakCache*** 是什么

//K为key类型，P为参数类型，V为value类型
    //根据传进来的参数，K为ClassLoader类型，P为Class<?>[]类型，V为Class<?>类型
    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);
        }

map变量为实现缓存的核心变量，他是一个双重map结构（key，sub-key。

在进入***proxyClassCache.get(loader, interfaces)***

public V get(K key, P parameter) {
            //检查参数是否为空
            Objects.requireNonNull(parameter);
            //清除无效的缓存
            expungeStaleEntries();
            //cacheKey 为上述map中的一级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，如果不存在，如果不存在就新建一个ConcurrentMap<Object, Supplier<V>>对象
            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
            // 创建sub-key、获取存储的Supplier<V>
            Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
            // 通过sub-key得到supplier
            Supplier<V> supplier = valuesMap.get(subKey);
            Factory factory = null;
    
            while (true) {
                //如果缓存中有supplier，就直接通过get方法得到代理对象返回
                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
                //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。
                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);
                    }
                }
            }
        }

再来看Factory中的 ***get()*** 方法

public synchronized V get() { // serialize access
        // re-check
        Supplier<V> supplier = valuesMap.get(subKey);
        //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 {
            //代理类在这里得到代理对象
            //重点方法valueFactory.apply(key, parameter)
            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)
        CacheValue<V> cacheValue = new CacheValue<>(value);
    
        // put into reverseMap
        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;
    }

再看 ***ProxyClassFactory*** 的 ***apply()*** 方法

//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());
               }
           }
       }

获取字节码文件

package cn.zjm.show.proxy;
    
    import sun.misc.ProxyGenerator;
    
    import java.io.FileOutputStream;
    import java.io.IOException;
    import java.lang.reflect.InvocationHandler;
    import java.lang.reflect.Method;
    import java.lang.reflect.Proxy;
    
    public class SourceProxy {
    
        public static void main(String[] args) {
            IUserService userService = new UserServiceImpl();
            IUserService proxy = (IUserService) Proxy.newProxyInstance(SourceProxy.class.getClassLoader(), userService.getClass().getInterfaces(), new InvocationHandler() {
                @Override
                public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                    System.out.println("动态代理开始");
                    Object invoke = method.invoke(userService, args);
                    System.out.println("动态代理结束");
                    return invoke;
                }
            });
    
            proxy.add("王五");
    
            String path = "D:/$Proxy0.class";
            byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", userService.getClass().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();
                }
            }
        }
    }

反编译后

import cn.zjm.show.proxy.IUserService;
    import java.lang.reflect.InvocationHandler;
    import java.lang.reflect.Method;
    import java.lang.reflect.Proxy;
    import java.lang.reflect.UndeclaredThrowableException;
    
    public final class $Proxy0
      extends Proxy
      implements IUserService
    {
      private static Method m1;
      private static Method m2;
      private static Method m4;
      private static Method m3;
      private static Method m0;
      
      public $Proxy0(InvocationHandler paramInvocationHandler)
        throws 
      {
        super(paramInvocationHandler);
      }
      
      public final boolean equals(Object paramObject)
        throws 
      {
        try
        {
          return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
      
      public final String toString()
        throws 
      {
        try
        {
          return (String)this.h.invoke(this, m2, null);
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
      
      public final void del(String paramString)
        throws 
      {
        try
        {
          this.h.invoke(this, m4, new Object[] { paramString });
          return;
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
      
      public final void add(String paramString)
        throws 
      {
        try
        {
          this.h.invoke(this, m3, new Object[] { paramString });
          return;
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
      
      public final int hashCode()
        throws 
      {
        try
        {
          return ((Integer)this.h.invoke(this, m0, null)).intValue();
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
      
      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]);
          m4 = Class.forName("cn.zjm.show.proxy.IUserService").getMethod("del", new Class[] { Class.forName("java.lang.String") });
          m3 = Class.forName("cn.zjm.show.proxy.IUserService").getMethod("add", new Class[] { Class.forName("java.lang.String") });
          m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
          return;
        }
        catch (NoSuchMethodException localNoSuchMethodException)
        {
          throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
        }
        catch (ClassNotFoundException localClassNotFoundException)
        {
          throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
        }
      }
    }