动态代理（一）

系统的整理一下动态代理的知识，打好基础知识，越学越快，要不然就淹死在知识的海洋里了。

文章参考地址：https://www.cnblogs.com/zhangxinly/p/6974283.html

一、总叙

代理，顾名思义，就是讲真正的逻辑进行包装，之后交由一个事物进行处理，这是我个人的理解。代理分为静态代理和动态代理。静态代理，比如说：遥控器（按下按键，里面进行实际的操作），汽车（踩下油门，发动机加速运转）.etc.动态代理，是程序运行时动态动态加载的。AOP的原理就是动态代理。

二、动态代理

动态代理实现方式分为两种，一种是实现InvocationHandler接口，另一种就是Cglib，这里接下来首先讲解第一种方法InvocationHandler方式。

1.InvocationHandler

首先看看这个接口

/**
 * {@code InvocationHandler} is the interface implemented by
 * the <i>invocation handler</i> of a proxy instance.
      // 它是个接口，被代理实例的Invocation处理器来实现
 *
 * <p>Each proxy instance has an associated invocation handler.
 * When a method is invoked on a proxy instance, the method
 * invocation is encoded and dispatched to the {@code invoke}
 * method of its invocation handler.
 */
     // 每个代理实例有个相关的invocation处理器。当原本的方法在一个代理实例中被调用的时候，
     // 方法调用会被编码，分发到它invocation处理器的方法。
public interface InvocationHandler {

    /**
     * Processes a method invocation on a proxy instance and returns
     * the result.  This method will be invoked on an invocation handler
     * when a method is invoked on a proxy instance that it is
     * associated with.
     *
          // 调用方法的代理实例
     * @param   proxy the proxy instance that the method was invoked on
     *
　　　　　　　　 // 调用方法的代理实例的接口方法（原本的方法）
     * @param   method the {@code Method} instance corresponding to
     * the interface method invoked on the proxy instance.  The declaring
     * class of the {@code Method} object will be the interface that
     * the method was declared in, which may be a superinterface of the
     * proxy interface that the proxy class inherits the method through.
     *
　　　　　　　　　// 调用方法的代理实例的接口方法的方法参数（原本的方法参数）
     * @param   args an array of objects containing the values of the
     * arguments passed in the method invocation on the proxy instance,
     * or {@code null} if interface method takes no arguments.
     * Arguments of primitive types are wrapped in instances of the
     * appropriate primitive wrapper class, such as
     * {@code java.lang.Integer} or {@code java.lang.Boolean}.
     *
     * @return  the value to return from the method invocation on the
     * proxy instance.  If the declared return type of the interface
     * method is a primitive type, then the value returned by
     * this method must be an instance of the corresponding primitive
     * wrapper class; otherwise, it must be a type assignable to the
     * declared return type.  If the value returned by this method is
     * {@code null} and the interface method's return type is
     * primitive, then a {@code NullPointerException} will be
     * thrown by the method invocation on the proxy instance.  If the
     * value returned by this method is otherwise not compatible with
     * the interface method's declared return type as described above,
     * a {@code ClassCastException} will be thrown by the method
     * invocation on the proxy instance.
     *
     *
     * @see     UndeclaredThrowableException
     */
    public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable;
}

2.示例代码

package com.lee.demo.springSourceLearn.demo;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class JDKProcyDemo {
     
    interface DoSomething {
        void originalMethod(String s);
    }
 
    static class Original implements DoSomething {
        public void originalMethod(String s) {
            System.out.println(s);
        }
    }
 
    static class Handler implements InvocationHandler {
        private final DoSomething original;
 
        public Handler(DoSomething original) {
            this.original = original;
        }
 
　　　　　　　　　　//这就是代理类要做的事情，在真实方法调用的前后加上before、after的输出
        @Override
        public Object invoke(Object proxy, Method method, Object[] args)
                throws IllegalAccessException, IllegalArgumentException,
                InvocationTargetException {
            System.out.println("BEFORE");
            method.invoke(original, args);
            System.out.println("AFTER");
            return null;
        }
    }
 
    public static void main(String[] args){
           // 设置属性这段代码的作用就是让生成的动态代理类,保存到本地，在user目录的工程根目录下，实在找不到你就搜一个Proxy0，我相信会出来的
        System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true"); 
        Original original = new Original();
        Handler handler = new Handler(original);
           // 下面红色的代码中，生成代理类
        DoSomething f = (DoSomething) Proxy.newProxyInstance(DoSomething.class.getClassLoader(),
                new Class[] { DoSomething.class },
                handler);
        System.out.println(f.getClass().getName());
47         f.originalMethod("Hallo");
    }
 
}


看到这里，希望大家讲上面的例子运行一下，之后，再Debug运行一下，运行过程中产生的疑问，带着疑问，再进行下面的阅读。

Proxy.newProyInstance

/**
　　　　　　　// 返回一个指定接口的代理类实例，这个接口将方法的调用分发给了指定的invocation处理器

     * Returns an instance of a proxy class for the specified interfaces
     * that dispatches method invocations to the specified invocation
     * handler.
     *
     * <p>{@code Proxy.newProxyInstance} throws
     * {@code IllegalArgumentException} for the same reasons that
     * {@code Proxy.getProxyClass} does.
     *
        // 定义代理类的class loader
     * @param   loader the class loader to define the proxy class
　　　　　　　// 代理类实现的接口
     * @param   interfaces the list of interfaces for the proxy class
     *          to implement
        // 分发给被调用方法的invocation处理器
     * @param   h the invocation handler to dispatch method invocations to
     * @return  a proxy instance with the specified invocation handler of a
     *          proxy class that is defined by the specified class loader
     *          and that implements the specified interfaces
     * @throws  IllegalArgumentException if any of the restrictions on the
     *          parameters that may be passed to {@code getProxyClass}
     *          are violated
     * @throws  SecurityException if a security manager, <em>s</em>, is present
     *          and any of the following conditions is met:
     *          <ul>
     *          <li> the given {@code loader} is {@code null} and
     *               the caller's class loader is not {@code null} and the
     *               invocation of {@link SecurityManager#checkPermission
     *               s.checkPermission} with
     *               {@code RuntimePermission("getClassLoader")} permission
     *               denies access;</li>
     *          <li> for each proxy interface, {@code intf},
     *               the caller's class loader is not the same as or an
     *               ancestor of the class loader for {@code intf} and
     *               invocation of {@link SecurityManager#checkPackageAccess
     *               s.checkPackageAccess()} denies access to {@code intf};</li>
     *          <li> any of the given proxy interfaces is non-public and the
     *               caller class is not in the same {@linkplain Package runtime package}
     *               as the non-public interface and the invocation of
     *               {@link SecurityManager#checkPermission s.checkPermission} with
     *               {@code ReflectPermission("newProxyInPackage.{package name}")}
     *               permission denies access.</li>
     *          </ul>
     * @throws  NullPointerException if the {@code interfaces} array
     *          argument or any of its elements are {@code null}, or
     *          if the invocation handler, {@code h}, is
     *          {@code null}
     */
    @CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);

        final Class<?>[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        /*
         * Look up or generate the designated proxy class.
         */
　　　　　　　　　　// 下面一二三就是这个类的主线
           // 这里是关键，①生成代理类
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
　　　　　　　　　　 // ②获取构造方法
            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;
                    }
                });
            }
　　　　　　　　　　　　　// ③生成代理类的实例
            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);
        }
    }

getProxyClass0

/**
     * Generate a proxy class.  Must call the checkProxyAccess method
     * to perform permission checks before calling this.
     */
    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
        return proxyClassCache.get(loader, interfaces);
    }

/**
     * Look-up the value through the cache. This always evaluates the
     * {@code subKeyFactory} function and optionally evaluates
     * {@code valueFactory} function if there is no entry in the cache for given
     * pair of (key, subKey) or the entry has already been cleared.
     *
     * @param key       possibly null key
     * @param parameter parameter used together with key to create sub-key and
     *                  value (should not be null)
     * @return the cached value (never null)
     * @throws NullPointerException if {@code parameter} passed in or
     *                              {@code sub-key} calculated by
     *                              {@code subKeyFactory} or {@code value}
     *                              calculated by {@code valueFactory} is null.
     */
    public V get(K key, P parameter) {
        Objects.requireNonNull(parameter);

        expungeStaleEntries();

        Object cacheKey = CacheKey.valueOf(key, refQueue);

        // lazily install the 2nd level valuesMap for the particular cacheKey
        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
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
               // 从下面的代码分析后得知，supplier是factory new生成的或是从缓存中取出来的
            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
            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);
                }
            }
        }
    }

supplier.get

　　这个方法中会调用ProxyClassFactory的apply方法，这个地方我没有找到代码上的依据是看到别人的文章后，写上去的，如果您能帮我解答这里的疑问，不胜感激！！！

ProxyClassFactory.apply

public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {

        Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
        for (Class<?> intf : interfaces) {
                /*
                 * Verify that the class loader resolves the name of this interface to the same Class object.
                 * 类加载器和接口名解析出的是同一个
                 */
            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.
                 * 确保是一个接口
                 */
            if (!interfaceClass.isInterface()) {
                throw new IllegalArgumentException( interfaceClass.getName() + " is not an interface");
            }
                /*
                 * Verify that this interface is not a duplicate.
                 * 确保接口没重复
                 */
            if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                throw new IllegalArgumentException( "repeated interface: " + interfaceClass.getName());
            }
        }

        String proxyPkg = null;     // package to define proxy class in
        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
             */
        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.
             * proxyClassNamePrefix = $Proxy
             * nextUniqueNumber 是一个原子类，确保多线程安全，防止类名重复，类似于：$Proxy0，$Proxy1......
             */
        long num = nextUniqueNumber.getAndIncrement();
        String proxyName = proxyPkg + proxyClassNamePrefix + num;
            /*
             * Generate the specified proxy class.
             * 生成类字节码的方法:重点
             */
        byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags);
        try {
            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());
        }
    }

ProxyGenerator.generateProxyClass

public static byte[] generateProxyClass(final String name, Class<?>[] interfaces, int accessFlags) {
        ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
        //真正生成字节码的方法
        final byte[] classFile = gen.generateClassFile();
        // 如果saveGeneratedFiles为true 则生成字节码文件，所以在开始我们要设置这个参数
        // 代码中哪里设置这个变量不知道，所以通过硬编码的方法，用system.setProperty来设置
        if (saveGeneratedFiles) {
            java.security.AccessController.doPrivileged( new java.security.PrivilegedAction<Void>() {
                        public Void run() {
                            try {
                                int i = name.lastIndexOf('.');
                                Path path;
                                if (i > 0) {
                                    Path dir = Paths.get(name.substring(0, i).replace('.', File.separatorChar));
                                    Files.createDirectories(dir);
                                    path = dir.resolve(name.substring(i+1, name.length()) + ".class");
                                } else {
                                    path = Paths.get(name + ".class");
                                }
                                Files.write(path, classFile);
                                return null;
                            } catch (IOException e) {
                                throw new InternalError( "I/O exception saving generated file: " + e);
                            }
                        }
                    });
        }
        return classFile;
    }

到这里呢，我们会生成动态代理类的字节码文件$Proxy0

这时我们如何反编译这个文件，来看到生成动态类到底是什么样子的呢？

我也是首先尝试了用javap命令，到class文件所以目录下，javap XXXX（字节码文件名），但是这时生成的反编译文件的内容，只有方法，方法里面没有具体的逻辑。

我又尝试的使用反编译插件。在Eclipse marketPlace中，搜索Decompiler并进行安装，安装并重启Eclipse后，将.class文件拖到Eclipse中，就可以看到了。

参考地址：https://www.cnblogs.com/JealousGirl/p/setup_Decompiler.html

接下来，看看反编译后的文件。

$Proxy0.class

package com.lee.demo.springSourceLearn.demo;

import com.lee.demo.springSourceLearn.demo.JDKProcyDemo.DoSomething;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

final class $Proxy0 extends Proxy implements DoSomething {
    private static Method m1;
    private static Method m2;
    private static Method m0;
    private static Method m3;

    public $Proxy0(InvocationHandler arg0) throws  {
      super(arg0);
   }

    public final boolean equals(Object arg0) throws  {
      try {
         return ((Boolean)super.h.invoke(this, m1, new Object[]{arg0})).booleanValue();
      } catch (RuntimeException | Error arg2) {
         throw arg2;
      } catch (Throwable arg3) {
         throw new UndeclaredThrowableException(arg3);
      }
   }

    public final String toString() throws  {
      try {
         return (String)super.h.invoke(this, m2, (Object[])null);
      } catch (RuntimeException | Error arg1) {
         throw arg1;
      } catch (Throwable arg2) {
         throw new UndeclaredThrowableException(arg2);
      }
   }

    public final int hashCode() throws  {
      try {
         return ((Integer)super.h.invoke(this, m0, (Object[])null)).intValue();
      } catch (RuntimeException | Error arg1) {
         throw arg1;
      } catch (Throwable arg2) {
         throw new UndeclaredThrowableException(arg2);
      }
   }

    public final void originalMethod(String arg0) throws  {
      try {
　　　　　　　　　　// 这行就是关键，它调用重写后的invoke方法。
         super.h.invoke(this, m3, new Object[]{arg0});
      } catch (RuntimeException | Error arg2) {
         throw arg2;
      } catch (Throwable arg3) {
         throw new UndeclaredThrowableException(arg3);
      }
   }

    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]);
            m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
            m3 = Class.forName("com.lee.demo.springSourceLearn.demo.JDKProcyDemo$DoSomething")
                    .getMethod("originalMethod", new Class[]{Class.forName("java.lang.String")});
        } catch (NoSuchMethodException arg1) {
            throw new NoSuchMethodError(arg1.getMessage());
        } catch (ClassNotFoundException arg2) {
            throw new NoClassDefFoundError(arg2.getMessage());
        }
    }
}

最后的执行结果，如下：

com.lee.demo.springSourceLearn.demo.$Proxy0
BEFORE
Hallo
AFTER

以上就这些，希望对您有帮助，不恰当的地方希望各位指正。