什么是jdk动态代理

JDK的动态代理,就是在程序运行的过程中,根据被代理的接口来动态生成代理类的class文件,并加载运行的过程。JDK从1.3开始支持动态代理。那么JDK是如何生成动态代理的呢?JDK动态代理为什么不支持类的代理,只支持接口的代理?

首先来看一下如何使用JDK动态代理。JDK提供了java.lang.reflect.Proxy类来实现动态代理的,可通过它的newProxyInstance来获得代理实现类。同时对于代理的接口的实际处理,是一个java.lang.reflect.InvocationHandler,它提供了一个invoke方法供实现者提供相应的代理逻辑的实现。可以对实际的实现进行一些特殊的处理,像Spring AOP中的各种advice。

简单使用

1.定义被代理接口

public interface HelloWorld {
    void sayHello(String name);
}

2.接口实现类

public class HelloWorldImpl implements HelloWorld{
    @Override
    public void sayHello(String name) {
        System.out.println("hello"+name);
    }
}

3.实现一个java.lang.reflect.InvocationHandler:对方法的增强

public class CustomInvocationHandler implements InvocationHandler {

    private Object target;
    public CustomInvocationHandler(Object target){
        this.target=target;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("Before invocation");
        Object retVal = method.invoke(target, args);
        System.out.println("After invocation");
        return retVal;
    }
}

4.使用代理

public class ProxyTest {
    public static void main(String[] args) {
        //可以拿到动态生成的代理类class文件用于返编译
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        CustomInvocationHandler handler = new CustomInvocationHandler(new HelloWorldImpl());
        HelloWorld proxy = (HelloWorld) Proxy.newProxyInstance(
                ProxyTest.class.getClassLoader(),
                new Class[]{HelloWorld.class},
                handler);
        proxy.sayHello("Mikan");
    }
}

5.输出

Before invocation
helloMikan
After invocation

原理

<1>

@CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
            throws IllegalArgumentException
    {
        Objects.requireNonNull(h);

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

        /*
         * <2>生成代理类的class
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         *  使用我们实现的InvocationHandler作为参数调用构造方法来获得代理类的实例
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }

           // 有了代理类的实例 使用我们实现的InvocationHandler作为参数调用构造方法来获得代理类的实例
            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;
                    }
                });
            }
            //构造函数创建
            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);
        }
    }

<2>

//缓存是弱引用 每次gc后都会回收 具体创建看ProxyClassFactory<3>
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
            proxyClassCache = new WeakCache<>(new Proxy.KeyFactory(), new Proxy.ProxyClassFactory());
    private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        // 代理的接口数量不能超过65535
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }
        //JDK对代理进行了缓存,如果已经存在相应的代理类,则直接返回,否则才会通过ProxyClassFactory来创建代理
        return proxyClassCache.get(loader, interfaces);
    }

<3>

//Proxy内部静态类
    private static final class ProxyClassFactory
            implements BiFunction<ClassLoader, Class<?>[], Class<?>>
    {
        // 所有代理类名字的前缀 如class com.sun.proxy.$Proxy0
        private static final String proxyClassNamePrefix = "$Proxy";

        //  // 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);
            for (Class<?> intf : interfaces) {

                Class<?> interfaceClass = null;
                try {
                    //根据classLoader获取被代理接口的class
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) {
                }
                //不相等表示传入的classLoader和被代理接口不一致校验失败
                if (interfaceClass != intf) {
                    throw new IllegalArgumentException(
                            intf + " is not visible from class loader");
                }
                //非接口类型报错
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                            interfaceClass.getName() + " is not an interface");
                }
                /*
                 * 避免重复定义被代理接口 如果从夫定义报错
                 */
                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                    throw new IllegalArgumentException(
                            "repeated interface: " + interfaceClass.getName());
                }
            }

            String proxyPkg = null;     //存储代理类的包名
            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;


            // 对于非公共接口,代理类的包名与接口的相同
            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");
                    }
                }
            }

            //公共接口统一用com.sun.proxy作为包名
            if (proxyPkg == null) {
                // if no non-public proxy interfaces, use com.sun.proxy package
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            /*
             * 计数器+1
             */
            long num = nextUniqueNumber.getAndIncrement();
            //获得代理类的全名称
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * 这里才是真正的生成代理类的字节码的地方<4>
           */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                    proxyName, interfaces, accessFlags);
            try {
                //根据二进制字节码返回相应的Class实例
                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());
            }
        }
    }

如何动态生成字节码

ProxyGenerator是sun.misc包中的类,它没有开源,但是可以反编译来一探究竟:

public static byte[] generateProxyClass(final String var0, Class[] var1) {
        ProxyGenerator var2 = new ProxyGenerator(var0, var1);
        final byte[] var3 = var2.generateClassFile();
        // 这里根据参数配置,决定是否把生成的字节码(.class文件)保存到本地磁盘,我们可以通过把相应的class文件保存到本地,再反编译来看看具体的实现,这样更直观
        if(saveGeneratedFiles) {
            AccessController.doPrivileged(new PrivilegedAction() {
                public Void run() {
                    try {
                        FileOutputStream var1 = new FileOutputStream(ProxyGenerator.dotToSlash(var0) + ".class");
                        var1.write(var3);
                        var1.close();
                        return null;
                    } catch (IOException var2) {
                        throw new InternalError("I/O exception saving generated file: " + var2);
                    }
                }
            });
        }
        return var3;
    }

saveGeneratedFiles这个属性的值从哪里来呢:

private static final boolean saveGeneratedFiles = ((Boolean)AccessController.doPrivileged(new GetBooleanAction("sun.misc.ProxyGenerator.saveGeneratedFiles"))).booleanValue();

GetBooleanAction实际上是调用Boolean.getBoolean(propName)来获得的,而Boolean.getBoolean(propName)调用了System.getProperty(name),所以我们可以设置sun.misc.ProxyGenerator.saveGeneratedFiles这个系统属性为true来把生成的class保存到本地文件来查看
这里要注意,当把这个属性设置为true时,生成的class文件及其所在的路径都需要提前创建,否则会抛出FileNotFoundException异常。如:

Exception in thread "main" java.lang.InternalError: I/O exception saving generated file: java.io.FileNotFoundException: com/sun/proxy/$Proxy0.class (No such file or directory)
at sun.misc.ProxyGenerator$1.run(ProxyGenerator.java:336)
at sun.misc.ProxyGenerator$1.run(ProxyGenerator.java:327)
at java.security.AccessController.doPrivileged(Native Method)
at sun.misc.ProxyGenerator.generateProxyClass(ProxyGenerator.java:326)
at java.lang.reflect.Proxy$ProxyClassFactory.apply(Proxy.java:672)
at java.lang.reflect.Proxy$ProxyClassFactory.apply(Proxy.java:592)
at java.lang.reflect.WeakCache$Factory.get(WeakCache.java:244)
at java.lang.reflect.WeakCache.get(WeakCache.java:141)
at java.lang.reflect.Proxy.getProxyClass0(Proxy.java:455)
at java.lang.reflect.Proxy.newProxyInstance(Proxy.java:738)
at com.mikan.proxy.ProxyTest.main(ProxyTest.java:15)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

即我们要在运行当前main方法的路径下创建com/sun/proxy目录,并创建一个$Proxy0.class文件,才能够正常运行并保存class文件内容。

 

反编译$Proxy0.class文件,如下所示:

package com.sun.proxy;
 
import com.mikan.proxy.HelloWorld;
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 HelloWorld {
  private static Method m1;
  private static Method m3;
  private static Method m0;
  private static Method m2;
 
  public $Proxy0(InvocationHandler paramInvocationHandler) {
    super(paramInvocationHandler);
  }
 
  public final boolean equals(Object paramObject) {
    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 void sayHello(String paramString) {
    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() {
    try {
      return ((Integer)this.h.invoke(this, m0, null)).intValue();
    }
    catch (Error|RuntimeException localError) {
      throw localError;
    }
    catch (Throwable localThrowable) {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }
 
  public final String toString() {
    try {
      return (String)this.h.invoke(this, m2, null);
    }
    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") });
      m3 = Class.forName("com.mikan.proxy.HelloWorld").getMethod("sayHello", new Class[] { Class.forName("java.lang.String") });
      m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
      m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
      return;
    }
    catch (NoSuchMethodException localNoSuchMethodException) {
      throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
    }
    catch (ClassNotFoundException localClassNotFoundException) {
      throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
    }
  }
}

可以看到,动态生成的代理类有如下特性:

继承了Proxy类,实现了代理的接口,由于java不能多继承,这里已经继承了Proxy类了,不能再继承其他的类,所以JDK的动态代理不支持对实现类的代理,只支持接口的代理。
提供了一个使用InvocationHandler作为参数的构造方法。
生成静态代码块来初始化接口中方法的Method对象,以及Object类的equals、hashCode、toString方法。
重写了Object类的equals、hashCode、toString,它们都只是简单的调用了InvocationHandler的invoke方法,即可以对其进行特殊的操作,也就是说JDK的动态代理还可以代理上述三个方法。
代理类实现代理接口的sayHello方法中,只是简单的调用了InvocationHandler的invoke方法,我们可以在invoke方法中进行一些特殊操作,甚至不调用实现的方法,直接返回。

CGLIB导致的空指针问题