JDK动态代理源码实现分析SLH

---恢复内容开始---

谈一谈JDK动态代理学习的一些坑，基于JDK8。

先来看一下JDK的动态是怎么用的。

package dynamic.proxy;   
  
import java.lang.reflect.InvocationHandler;  
import java.lang.reflect.Method;  
import java.lang.reflect.Proxy;  
  
/** 
 * 实现自己的InvocationHandler 
 * @author ShiLinghuai 
 * @since 2012-8-9 
 * 
 */  
public class MyInvocationHandler implements InvocationHandler {  
      
    // 目标对象   
    private Object target;  
      
    /** 
     * 构造方法 
     * @param target 目标对象  
     */  
    public MyInvocationHandler(Object target) {  
        super();  
        this.target = target;  
    }  
  
  
    /** 
     * 执行目标对象的方法 
     */  
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {  
          
        // 在目标对象的方法执行之前简单的打印一下  
        System.out.println("------------------before------------------");  
          
        // 执行目标对象的方法  
        Object result = method.invoke(target, args);  
          
        // 在目标对象的方法执行之后简单的打印一下  
        System.out.println("-------------------after------------------");  
          
        return result;  
    }  
  
    /** 
     * 获取目标对象的代理对象 
     * @return 代理对象 
     */  
    public Object getProxy() {  
　　　　　//调用JDK Proxy类的方法，方法参数第一个为一个类加载器，第二个方法为实现类的接口，第三个
　　　　　//参数是一个实现InvocationHandler的类
        return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),   
                target.getClass().getInterfaces(), this);  
    }  
}  
  
package dynamic.proxy;  
  
/** 
 * 目标对象实现的接口，用JDK来生成代理对象一定要实现一个接口 
 * @author zyb 
 * @since 2012-8-9 
 * 
 */  
public interface UserService {  
  
    /** 
     * 目标方法  
     */  
    public abstract void add();  
  
}  
  
package dynamic.proxy;   
  
/** 
 * 目标对象 
 * @author zyb 
 * @since 2012-8-9 
 * 
 */  
public class UserServiceImpl implements UserService {  
  
    /* (non-Javadoc) 
     * @see dynamic.proxy.UserService#add() 
     */  
    public void add() {  
        System.out.println("--------------------add---------------");  
    }  
}  
  
package dynamic.proxy;   
  
import org.junit.Test;  
  
/** 
 * 动态代理测试类 
 * @author zyb 
 * @since 2012-8-9 
 * 
 */  
public class ProxyTest {  
  
    @Test  
    public void testProxy() throws Throwable {  
        // 实例化目标对象  
        UserService userService = new UserServiceImpl();  
          
        // 实例化InvocationHandler  
        MyInvocationHandler invocationHandler = new MyInvocationHandler(userService);  
          
        // 根据目标对象生成代理对象  
        UserService proxy = (UserService) invocationHandler.getProxy();  
          
        // 调用代理对象的方法  
        proxy.add();  
          
    }  
}  

执行结果如下： 
------------------before------------------ 
--------------------add--------------- 
-------------------after------------------ 

首先来看一下JDK是怎样生成代理对象的。既然生成代理对象是用的Proxy类的静态方newProxyInstance，那么我们就去它的源码里看一下它到底都做了些什么？ 但是我们不先分析源码，我们先看下生成的代理类是什么样子的：

import dynamic.proxy.UserService;  
import java.lang.reflect.*;  
  
public final class $Proxy11 extends Proxy  
    implements UserService  
{  
  
    // 构造方法，参数就是刚才传过来的MyInvocationHandler类的实例  
    public $Proxy11(InvocationHandler invocationhandler)  
    {  
        super(invocationhandler);  
    }  
  
    public final boolean equals(Object obj)  
    {  
        try  
        {  
            return ((Boolean)super.h.invoke(this, m1, new Object[] {  
                obj  
            })).booleanValue();  
        }  
        catch(Error _ex) { }  
        catch(Throwable throwable)  
        {  
            throw new UndeclaredThrowableException(throwable);  
        }  
    }  
  
    /** 
     * 这个方法是关键部分 
     */  
    public final void add()  
    {  
        try  
        {  
            // 实际上就是调用MyInvocationHandler的public Object invoke(Object proxy, Method method, Object[] args)方法，第二个问题就解决了  
            super.h.invoke(this, m3, null);  
            return;  
        }  
        catch(Error _ex) { }  
        catch(Throwable throwable)  
        {  
            throw new UndeclaredThrowableException(throwable);  
        }  
    }  
  
    public final int hashCode()  
    {  
        try  
        {  
            return ((Integer)super.h.invoke(this, m0, null)).intValue();  
        }  
        catch(Error _ex) { }  
        catch(Throwable throwable)  
        {  
            throw new UndeclaredThrowableException(throwable);  
        }  
    }  
  
    public final String toString()  
    {  
        try  
        {  
            return (String)super.h.invoke(this, m2, null);  
        }  
        catch(Error _ex) { }  
        catch(Throwable throwable)  
        {  
            throw new UndeclaredThrowableException(throwable);  
        }  
    }  
  
    private static Method m1;  
    private static Method m3;  
    private static Method m0;  
    private static Method m2;  
  
    // 在静态代码块中获取了4个方法：Object中的equals方法、UserService中的add方法、Object中的hashCode方法、Object中toString方法  
    static   
    {  
        try  
        {  
            m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {  
                Class.forName("java.lang.Object")  
            });  
            m3 = Class.forName("dynamic.proxy.UserService").getMethod("add", new Class[0]);  
            m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);  
            m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);  
        }  
        catch(NoSuchMethodException nosuchmethodexception)  
        {  
            throw new NoSuchMethodError(nosuchmethodexception.getMessage());  
        }  
        catch(ClassNotFoundException classnotfoundexception)  
        {  
            throw new NoClassDefFoundError(classnotfoundexception.getMessage());  
        }  
    }  
}  

这个类就是最后JDK帮我们实现的，当然他有特定的规则的。我们重点关注一下add()方法，它实际上就是调用MyInvocationHandler的public Object invoke(Object proxy, Method method, Object[] args)方法，在这个方法里你可以在调用实现类的特定方法之前之后都可以进行一些统一的操作。

最后我们看下源码，到底JDK是怎么实现这么个类的。

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);
        }

        /*
         * 这里首先通过类加载器和接口生成代理类模板。
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
　　　　　　　//获取cl模板的构造函数，准备实例化对象，
            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;
                    }
                });
            }
　　　　　　　//这里最后返回模板的实例化对象，参数就是那个实现InvacationHandler的类
            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);
        }
    }

我们接着看怎么实现那个代理类模板。

    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);
    }

再进入get(loader, interfaces)方法

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;
            }
        }

        //这个apply方法就是创建模板的主要方法
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {
                // supplier might be a Factory or a CacheValue<V> instance
                V value = supplier.get();
                if (value != null) {
　　　　　　　　　　　　//3，取出模板类
                    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) {
　　　　　　　　　　//1，封装一下模板类
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) {
　　　　　　　　　　/,2，放到缓存里
                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);
                }
            }
        }
    }

看下subKeyFactory.apply(key, parameter)方法

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.
                 */
                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());
                }
            }

            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.
             */
            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();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * 这里生成模板的字节码
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try {
　　　　　　　　　　//返回模板类 
                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());
            }
        }
    }

接下来肯定要看下如何生成字节码，和如何把字节码生成模板类

public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
        ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
        final byte[] var4 = var3.generateClassFile();
        if (saveGeneratedFiles) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    try {
                        int var1 = var0.lastIndexOf(46);
                        Path var2;
                        if (var1 > 0) {
                            Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar));
                            Files.createDirectories(var3);
                            var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                        } else {
                            var2 = Paths.get(var0 + ".class");
                        }
　　　　　　　　　　　　　　　//这里看着像是只是把接口什么的一些基本信息写进流里
                        Files.write(var2, var4, new OpenOption[0]);
                        return null;
                    } catch (IOException var4x) {
                        throw new InternalError("I/O exception saving generated file: " + var4x);
                    }
                }
            });
        }

        return var4;
    }

那defineClass0这个方法应该是生成字节码，再生成Class