JDK中的proxy动态代理

最新推荐文章于 2024-12-28 15:12:51 发布

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本文详细介绍了如何使用Java动态代理创建代理对象的过程，并深入探讨了其内部实现机制，包括Proxy类和InvocationHandler接口的作用。

今天无意看proxy的使用。了解一下动态代理。

实例分析如下：

声明一个接口：

[java] view plain copy print ?

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

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

实现该接口

[java] view plain copy print ?

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;
}
}

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;       
    }
}

实现InvocationHandler 接口，关键之处在这

[java] view plain copy print ?

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;
}

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：

[java] view plain copy print ?

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());
}
}

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：

method:public Java.lang.String java.lang.Object.toString()

pm.toString:PackageManagerImpl

method:public abstract java.lang.String com.czq.proxy.IPackageManager.getPackageInfo()

com.czq.proxy

得出结论：

1 pm.getPackageInfo()方法会走到PackageManagerWoker的invoke方法。但是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？还需要能强转成IPackageManager

打印pm的className

[java] view plain copy print ?

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

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

[java] view plain copy print ?

/**
* 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<ClassLoader, Class<?>[], 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<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
/*
* 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());
}
}
}

  /**
     * 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<ClassLoader, Class<?>[], 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<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
 
            /*
             * 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());
            }
        }
    }

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

[java] view plain copy print ?

/**
* 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<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;
}

 
 /**
     * 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<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;
    }

发现 saveGeneratedFiles 为true报错生成的class的源码。

这个saveGeneratedFiles 怎么赋值呢？

[java] view plain copy print ?

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

 /** 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。

[java] view plain copy print ?

// 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");

// 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，需要再工程更目录下，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

[java] view plain copy print ?

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());
}
}

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。

源码如下：http://download.youkuaiyun.com/detail/chenzhiqin20/9462544

原文地址：http://blog.youkuaiyun.com/chenzhiqin20/article/details/50900439