JDK中的proxy动态代理原理剖析

主要API类是:

Proxy.newProxyInstance

public static Object newProxyInstance(ClassLoader loader,
                                      Class[] interfaces,
                                      InvocationHandler h)
                               throws IllegalArgumentException
返回一个指定接口的代理类实例,该接口可以将方法调用指派到指定的调用处理程序。此方法相当于:
     Proxy.getProxyClass(loader, interfaces).
         getConstructor(new Class[] { InvocationHandler.class }).
         newInstance(new Object[] { handler });

Proxy.newProxyInstance 抛出 IllegalArgumentException,原因与 Proxy.getProxyClass 相同。
参数:
loader - 定义代理类的类加载器
interfaces - 代理类要实现的接口列表
h - 指派方法调用的调用处理程序
返回:
一个带有代理类的指定调用处理程序的代理实例,它由指定的类加载器定义,并实现指定的接口
抛出:
IllegalArgumentException - 如果违反传递到 getProxyClass 的参数上的任何限制
NullPointerException - 如果 interfaces 数组参数或其任何元素为 null,或如果调用处理程序 h 为 null

先声明一个接口

package com.czq.proxy;
public interface IPackageManager {
     String getPackageInfo() ;
}

实现该接口

package com.czq.proxy;
public class PackageManagerImpl implements IPackageManager {
    @Override
    public String getPackageInfo() {
        String s = "com.czq.proxy";
//        System.out.println(s);
        return s;
    }
    @Override
    public String toString() {
        return "PackageManagerImpl";
    }
}

实现InvocationHandler 接口,关键之处在这

package com.czq.proxy;

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

public class PackageManagerWoker implements InvocationHandler {
    private Object mTarget = null;
    public PackageManagerWoker(Object target) {
        super();
        this.mTarget = target;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System. out.println("1" );
        System. out.println("method:" +method);
        if (args != null) {
            for (int i = 0; i < args.length; i++) {
                System. out.println("args[" + i + "]:" + args[i]);
            }
        }
        Object result = method.invoke( mTarget, args);
        System. out.println("2" );
        return result;       
    }

测试:

package com.czq.proxy;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.Proxy;

public class Test {

    public static void main(String[] args) {
        // 从源码中得知,设置这个值,可以把生成的代理类,输出出来。
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

       
        IPackageManager pkgManger = new PackageManagerImpl();
//        System.out.println("pkgManger.toString:"+pkgManger.toString());
        PackageManagerWoker woker = new PackageManagerWoker(pkgManger);
        IPackageManager pm = (IPackageManager) Proxy.newProxyInstance(pkgManger.getClass().getClassLoader(), pkgManger
                .getClass().getInterfaces(), woker);
       //  System.out.println("pm.getName:" +pm.getClass().getName())
    
        System. out.println("pm.toString:" +pm.toString());
        System.out.println(pm.getPackageInfo());
    } 
}

输出结果如下:

1
method:public java.lang.String java.lang.Object.toString()
2
pm.toString:PackageManagerImpl
1
method:public abstract java.lang.String com.czq.proxy.IPackageManager.getPackageInfo()
2
com.czq.proxy

得出结论:
pm.getPackageInfo()方法会走到PackageManagerWoker的invoke方法。
思考问题:
PackageManagerWoker不继承IPackageManager。不能强转成IPackageManager。
也就是pm对象不是PackageManagerWoker对象。
那pm 是哪个对象,是什么类呢?为什么还能强转成IPackageManager

打印pm的className

System.out.println("pm.getName:" +pm.getClass().getName())

得出的结果:

pm.getName:com.sun.proxy.$Proxy0

也就是pm对象是com.sun.proxy.$Proxy0这个类new出的对象。这个类是刚刚Proxy.newProxyInstance自动生成的class
那这个class里面写的是什么呢?
查看源码:Proxy.java

/**
     * A factory function that generates, defines and returns the proxy class given
     * the ClassLoader and array of interfaces.
     */
    private static final class ProxyClassFactory
        implements BiFunction[], Class>
    {
        // prefix for all proxy class names
        private static final String proxyClassNamePrefix = "$Proxy";

        // next number to use for generation of unique proxy class names
        private static final AtomicLong nextUniqueNumber = new AtomicLong();

        @Override
        public Class apply(ClassLoader loader, Class[] interfaces) {

            Map, 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

            /*
             * 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)) {
                    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;

            /*
             * Generate the specified proxy class.
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces);
            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());
            }
        }
    }

通过ProxyGenerator 生成了这个class。
查看ProxyGenerator源码:

/**
     * Generate a proxy class given a name and a list of proxy interfaces.
     *
     * @param name        the class name of the proxy class
     * @param interfaces  proxy interfaces
     * @param accessFlags access flags of the proxy class
    */
    public static byte[] generateProxyClass(final String name,
                                            Class[] interfaces,
                                            int accessFlags)
    {
        ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
        final byte[] classFile = gen.generateClassFile();

        if (saveGeneratedFiles) {
            java.security.AccessController.doPrivileged(
            new java.security.PrivilegedAction() {
                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;
    }

发现 saveGeneratedFiles 为true报错生成的class的源码。
这个saveGeneratedFiles 怎么赋值呢?

 /** debugging flag for saving generated class files */
    private final static boolean saveGeneratedFiles =
        java.security.AccessController.doPrivileged(
            new GetBooleanAction(
                "sun.misc.ProxyGenerator.saveGeneratedFiles")).booleanValue();

也就是把sun.misc.ProxyGenerator.saveGeneratedFiles 改成true就可以输出结果了。

        // 从源码中得知,设置这个值,可以把生成的代理类,输出出来。      

System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

注意,需要再工程根目录下,增加 com/sun/proxy目录,否则会报错如下:

Exception in thread "main" java.lang.InternalError: I/O exception saving generated file: java.io.FileNotFoundException : com\sun\proxy\$Proxy0.class (系统找不到指定的路径。)
     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.czq.proxy.Test.main( Test.java:18)

把proxy0输出的结果如下:
反编译看看proxy0是内容是啥,有什么秘密

package com.sun.proxy;

import com.czq.proxy.IPackageManager;
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 IPackageManager
{
  private static Method m3; // 生成对应的方法对象
  private static Method m1;
  private static Method m0;
  private static Method m2;
// proxy0 继承Proxy,实现IPackageManager 接口,需要传入 InvocationHandler,初始化对应的h对象。
// 我们的h对象就是PackageManagerWoker,所以我们会调用到PackageManagerWoker的 invoke方法。
// 所以是proxy0,调用InvocationHandler的 invoke 方法,传入对应的方法。InvocationHandler 放射调用对应的tagret中的方法。
  public $Proxy0(InvocationHandler paramInvocationHandler)
    throws
  {
    super(paramInvocationHandler);
  }

  public final String getPackageInfo()
    throws
  {
    try
    {
      return (String)this.h.invoke(this, m3, null);
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
    }
    throw new UndeclaredThrowableException(localThrowable);
  }

  public final boolean equals(Object paramObject)
    throws
  {
    try
    {
      return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
    }
    throw new UndeclaredThrowableException(localThrowable);
  }

  public final int hashCode()
    throws
  {
    try
    {
      return ((Integer)this.h.invoke(this, m0, null)).intValue();
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
    }
    throw new UndeclaredThrowableException(localThrowable);
  }

  public final String toString()
    throws
  {
    try
    {
      return (String)this.h.invoke(this, m2, null);
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
    }
    throw new UndeclaredThrowableException(localThrowable);
  }

  static
  {
    try
    {
     // 把各个方法,对应到成员变量上
      m3 = Class.forName("com.czq.proxy.IPackageManager").getMethod("getPackageInfo", new Class[0]);
      m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
      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());
  }
}

结论如下:

  1. proxy0 继承Proxy,实现IPackageManager 接口,需要传入 InvocationHandler,初始化对应的h对象。
  2. 我们的h对象就是PackageManagerWoker,所以我们会调用到PackageManagerWoker的 invoke方法。
  3. 所以是proxy0,调用InvocationHandler的 invoke 方法,传入对应的方法。InvocationHandler 放射调用对应的tagret中的方法。套了2层
    整体类图如下
JDK中的proxy动态代理原理剖析_第1张图片
类图.png

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