小白努力学编程

JDK动态代理原理剖析

28 February 2014

昨天被问了个问题，问题的大意是这样的：为什么 Proxy.newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)方法的3个参数是这样的定义的？笔者一阵语塞，好生郁闷。在这里补充一下，记录下对这个问题的解答。

基本样例

接口类

package com.vavi.proxy;

public interface Sleepable {
void sleep();

void eat();
}

实现类

package com.vavi.proxy;

public class Person implements Sleepable {

public void sleep() {
System.out.println("He is sleeping");
}

public void eat() {
System.out.println("He is eating");

}

}

InvocationHandler实现类

package com.vavi.proxy.dynaimc.jdk;

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

public class PersonDynamicJDKProxyHandler implements InvocationHandler {
private final Object targetObject;

public PersonDynamicJDKProxyHandler(Object targetObject) {
this.targetObject = targetObject;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args)
    throws Throwable {
if (method.getName().equals("eat")) {

    System.out.println("wash hands before eating");
    method.invoke(targetObject, args);
    System.out.println("ready to sleep now..");
} else {

    System.out.println("take off clothes");
    method.invoke(targetObject, args);
    System.out.println("sweet dream now..");

}

return null;
}
}

客户端类

package com.vavi.proxy.dynaimc.jdk;

import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.Proxy;

import org.junit.Test;

import com.vavi.proxy.Person;
import com.vavi.proxy.Sleepable;

public class TestPersonDynamicJDKProxy {
@Test
public void testProxy() throws Exception {

// System.getProperties().put(
// "sun.misc.ProxyGenerator.saveGeneratedFiles", true);
Person person = new Person();
PersonDynamicJDKProxyHandler handler = new PersonDynamicJDKProxyHandler(
    person);

Sleepable proxy = (Sleepable) Proxy.newProxyInstance(person.getClass()
    .getClassLoader(), person.getClass().getInterfaces(), handler);

// 获取代理类的字节码
generateProxyClassFile();

proxy.eat();
proxy.sleep();

}

private void generateProxyClassFile() {
byte[] classFile = sun.misc.ProxyGenerator.generateProxyClass(
    "$MyProxy", Person.class.getInterfaces());

FileOutputStream out = null;
String path = "/Users/ghj/startup/$MyProxy.class";
try {
    out = new FileOutputStream(path);
    out.write(classFile);
    out.flush();
} catch (Exception e) {
    e.printStackTrace();
} finally {
    try {
    out.close();
    } catch (IOException e) {
    e.printStackTrace();
    }
}
}
}

源码分析

执行上述程序后，系统打印如下结果：

wash hands before eating
He is eating
ready to sleep now..
take off clothes
He is sleeping
sweet dream now..

现在程序运行正常，成功实现了动态代理的效果。但是这个为什么能够得到这样的效果呢？我们跟着一步步跟踪代码执行，首先发现最重要的是(Sleepable) Proxy.newProxyInstance(person.getClass().getClassLoader(), person.getClass().getInterfaces(), handler);这段代码。

public static Object Proxy.newProxyInstance(ClassLoader loader,
                                      Class<?>[] interfaces,
                                      InvocationHandler h)
    throws IllegalArgumentException
{
    if (h == null) {
        throw new NullPointerException();
    }

    /*
     * Look up or generate the designated proxy class.
     */
    Class<?> cl = getProxyClass0(loader, interfaces); // 标记1
    /*
     * Invoke its constructor with the designated invocation handler.
     */
    try {
        final Constructor<?> cons = cl.getConstructor(constructorParams);// 标记2
        final InvocationHandler ih = h;
        SecurityManager sm = System.getSecurityManager();
        if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(cl)) {
            // create proxy instance with doPrivilege as the proxy class may
            // implement non-public interfaces that requires a special permission
            return AccessController.doPrivileged(new PrivilegedAction<Object>() {
                public Object run() {
                    return newInstance(cons, ih);//标记3
                }
            });
        } else {
            return newInstance(cons, ih);//标记3
        }
    } catch (NoSuchMethodException e) {
        throw new InternalError(e.toString());
    }
}

在上面代码中，标记1生成代理类的class对象，标记2获得带有这个参数constructorParams的构造器，标记3完成对象实例化。 需要提前说明的是，由于在Proxy类中，硬编码了private final static Class[] constructorParams = { InvocationHandler.class };这个属性值，所以这个一定程度了约束了我们必须要和InvocationHandler打交道了。并且隐含在代理类中，有一个带有constructorParams参数的构造器。这个在后文也会提及到。

下面接着看标记1内部实现（笔者删除了大量注释），中间进行了一些安全校验，接口个数校验，重复的接口名称等校验。

private static Class<?> getProxyClass0(ClassLoader loader,
                                       Class<?>... interfaces) {
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
        final int CALLER_FRAME = 3; // 0: Reflection, 1: getProxyClass0 2: Proxy 3: caller
        final Class<?> caller = Reflection.getCallerClass(CALLER_FRAME);
        final ClassLoader ccl = caller.getClassLoader();
        checkProxyLoader(ccl, loader);
        ReflectUtil.checkProxyPackageAccess(ccl, interfaces);
    }

    if (interfaces.length > 65535) {
        throw new IllegalArgumentException("interface limit exceeded");
    }

    Class<?> proxyClass = null;

    /* collect interface names to use as key for proxy class cache */
    String[] interfaceNames = new String[interfaces.length];

    // for detecting duplicates
    Set<Class<?>> interfaceSet = new HashSet<>();

    for (int i = 0; i < interfaces.length; i++) {

        String interfaceName = interfaces[i].getName();
        Class<?> interfaceClass = null;
        try {
            interfaceClass = Class.forName(interfaceName, false, loader);
        } catch (ClassNotFoundException e) {
        }
        if (interfaceClass != interfaces[i]) {
            throw new IllegalArgumentException(
                interfaces[i] + " 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.contains(interfaceClass)) {
            throw new IllegalArgumentException(
                "repeated interface: " + interfaceClass.getName());
        }
        interfaceSet.add(interfaceClass);

        interfaceNames[i] = interfaceName;
    }


    List<String> key = Arrays.asList(interfaceNames);

    /*
     * Find or create the proxy class cache for the class loader.
     */
    Map<List<String>, Object> cache;
    synchronized (loaderToCache) {
        cache = loaderToCache.get(loader);
        if (cache == null) {
            cache = new HashMap<>();
            loaderToCache.put(loader, cache);
        }
        /*
         * This mapping will remain valid for the duration of this
         * method, without further synchronization, because the mapping
         * will only be removed if the class loader becomes unreachable.
         */
    }


    synchronized (cache) {

        do {
            Object value = cache.get(key);
            if (value instanceof Reference) {
                proxyClass = (Class<?>) ((Reference) value).get();
            }
            if (proxyClass != null) {
                // proxy class already generated: return it
                return proxyClass;
            } else if (value == pendingGenerationMarker) {
                // proxy class being generated: wait for it
                try {
                    cache.wait();
                } catch (InterruptedException e) {

                }
                continue;
            } else {
                 cache.put(key, pendingGenerationMarker);
                break;
            }
        } while (true);
    }

    try {
        String proxyPkg = null;     // package to define proxy class in


        for (int i = 0; i < interfaces.length; i++) {
            int flags = interfaces[i].getModifiers();
            if (!Modifier.isPublic(flags)) {
                String name = interfaces[i].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 + ".";
        }

        { 
            long num;
            synchronized (nextUniqueNumberLock) {
                num = nextUniqueNumber++;
            }
            String proxyName = proxyPkg + proxyClassNamePrefix + num; // 标记1.1

            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces); // 标记1.2
            try {
                proxyClass = defineClass0(loader, proxyName, 
                    proxyClassFile, 0, proxyClassFile.length);// 标记1.3
            } catch (ClassFormatError e) {

                throw new IllegalArgumentException(e.toString());
            }
        }
        // add to set of all generated proxy classes, for isProxyClass
        proxyClasses.put(proxyClass, null);

    } finally {

        synchronized (cache) {
            if (proxyClass != null) {
                cache.put(key, new WeakReference<Class<?>>(proxyClass));
            } else {
                cache.remove(key);
            }
            cache.notifyAll();
        }
    }
    return proxyClass;
}

标记1.1完成包名计算。 这里解释了为什么包名是类似com.sun.proxy.$Proxy.NUM 或者PKG.$Proxy.NUM的形式了。

标记1.2完成字节码数组拼接，这个稍后分析。

标记1.3完成字节码数组拼接，最终返回Class对象。

标记1.2的代码如下:

public static byte[] ProxyGenerator.generateProxyClass(final String name,
                                        Class[] interfaces)
{
    ProxyGenerator gen = new ProxyGenerator(name, interfaces);
    final byte[] classFile = gen.generateClassFile(); //标记1.2.1

    if (saveGeneratedFiles) { //标记1.2.2
        java.security.AccessController.doPrivileged(
        new java.security.PrivilegedAction() {
            public Object run() {
                try {
                    FileOutputStream file =
                        new FileOutputStream(dotToSlash(name) + ".class");
                    file.write(classFile);
                    file.close();
                    return null;
                } catch (IOException e) {
                    throw new InternalError(
                        "I/O exception saving generated file: " + e);
                }
            }
        });
    }

    return classFile;
}

在上面代码的标记1.2.1中完成了实际的字节码拼接操作。

在上面代码的标记1.2.2中，使用了这个saveGeneratedFiles变量。而这个变量是这么定义的private final static boolean saveGeneratedFiles = java.security.AccessController.doPrivileged( new GetBooleanAction("sun.misc.ProxyGenerator.saveGeneratedFiles")).booleanValue();。这个参数可以帮助我们把字节码写到文件中了。

现在，接着看下标记1.2.1处的代码。从下面的代码我们可以看到。该方法先后完成了hashCodeMethod，equalsMethod，toStringMethod的数据准备，所有接口的所有方法。在标记1.2.1.1处完成了带InvocationHandler参数的构造器和静态代码块的数据准备。

为避免正文过长，我把generateConstructor和generateStaticInitializer挪到附录章节。

private byte[] generateClassFile() {

    /* ============================================================
     * Step 1: Assemble ProxyMethod objects for all methods to
     * generate proxy dispatching code for.
     */

    /*
     * Record that proxy methods are needed for the hashCode, equals,
     * and toString methods of java.lang.Object.  This is done before
     * the methods from the proxy interfaces so that the methods from
     * java.lang.Object take precedence over duplicate methods in the
     * proxy interfaces.
     */
    addProxyMethod(hashCodeMethod, Object.class);
    addProxyMethod(equalsMethod, Object.class);
    addProxyMethod(toStringMethod, Object.class);

    /*
     * Now record all of the methods from the proxy interfaces, giving
     * earlier interfaces precedence over later ones with duplicate
     * methods.
     */
    for (int i = 0; i < interfaces.length; i++) {
        Method[] methods = interfaces[i].getMethods();
        for (int j = 0; j < methods.length; j++) {
            addProxyMethod(methods[j], interfaces[i]);
        }
    }

    /*
     * For each set of proxy methods with the same signature,
     * verify that the methods' return types are compatible.
     */
    for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
        checkReturnTypes(sigmethods);
    }

    /* ============================================================
     * Step 2: Assemble FieldInfo and MethodInfo structs for all of
     * fields and methods in the class we are generating.
     */
    try {
        methods.add(generateConstructor()); //标记1.2.1.1

        for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
            for (ProxyMethod pm : sigmethods) {

                // add static field for method's Method object
                fields.add(new FieldInfo(pm.methodFieldName,
                    "Ljava/lang/reflect/Method;",
                     ACC_PRIVATE | ACC_STATIC));

                // generate code for proxy method and add it
                methods.add(pm.generateMethod());
            }
        }

        methods.add(generateStaticInitializer()); //标记1.2.1.2

    } catch (IOException e) {
        throw new InternalError("unexpected I/O Exception");
    }

    if (methods.size() > 65535) {
        throw new IllegalArgumentException("method limit exceeded");
    }
    if (fields.size() > 65535) {
        throw new IllegalArgumentException("field limit exceeded");
    }

    /* ============================================================
     * Step 3: Write the final class file.
     */

    /*
     * Make sure that constant pool indexes are reserved for the
     * following items before starting to write the final class file.
     */
    cp.getClass(dotToSlash(className));
    cp.getClass(superclassName);
    for (int i = 0; i < interfaces.length; i++) {
        cp.getClass(dotToSlash(interfaces[i].getName()));
    }

    /*
     * Disallow new constant pool additions beyond this point, since
     * we are about to write the final constant pool table.
     */
    cp.setReadOnly();

    ByteArrayOutputStream bout = new ByteArrayOutputStream();
    DataOutputStream dout = new DataOutputStream(bout);

    try {
        /*
         * Write all the items of the "ClassFile" structure.
         * See JVMS section 4.1.
         */
                                    // u4 magic;
        dout.writeInt(0xCAFEBABE);
                                    // u2 minor_version;
        dout.writeShort(CLASSFILE_MINOR_VERSION);
                                    // u2 major_version;
        dout.writeShort(CLASSFILE_MAJOR_VERSION);

        cp.write(dout);             // (write constant pool)

                                    // u2 access_flags;
        dout.writeShort(ACC_PUBLIC | ACC_FINAL | ACC_SUPER);
                                    // u2 this_class;
        dout.writeShort(cp.getClass(dotToSlash(className)));
                                    // u2 super_class;
        dout.writeShort(cp.getClass(superclassName));

                                    // u2 interfaces_count;
        dout.writeShort(interfaces.length);
                                    // u2 interfaces[interfaces_count];
        for (int i = 0; i < interfaces.length; i++) {
            dout.writeShort(cp.getClass(
                dotToSlash(interfaces[i].getName())));
        }

                                    // u2 fields_count;
        dout.writeShort(fields.size());
                                    // field_info fields[fields_count];
        for (FieldInfo f : fields) {
            f.write(dout);
        }

                                    // u2 methods_count;
        dout.writeShort(methods.size());
                                    // method_info methods[methods_count];
        for (MethodInfo m : methods) {
            m.write(dout);
        }

                                     // u2 attributes_count;
        dout.writeShort(0); // (no ClassFile attributes for proxy classes)

    } catch (IOException e) {
        throw new InternalError("unexpected I/O Exception");
    }

    return bout.toByteArray();
}

我们最后使用反编译工具JAD-UI看下生成的代理对象字节码。完整的代码见附录，这里关注下里面的eat()方法。该方法内部的this.h属性就是InvocationHandler的实现类。然后调到invoke方法，完成了最终的执行。

public final void eat()

{
try {
    this.h.invoke(this, m3, null); 
    return;
} catch (Error localError) {
    throw localError;
} catch (Throwable localThrowable) {
    throw new UndeclaredThrowableException(localThrowable);
}
}

总结

  1. 最后，我们再回头看看本篇提的问题。这3个参数分别解决了如下几个问题：
    1. ClassLoader loader 解决了使用什么classloader来加载这个代理类
    2. Class<?>[] interfaces 首先约束了JDK动态代理机制是基于接口实现的，它要求我们被代理的类必须实现相应的接口；其次JDK动态代理机制会帮我们完成接口方法的代理方法的实现，并通过硬编码把代理职责委托给了InvocationHandler h 这个参数。
    3. InvocationHandler h 完成了实际的代理职责。InvocationHandler.invoke(Object proxy, Method method, Object[] args)的3个参数：
      1. proxy是生成的代理实例，里面不包含target对象实例。所以，我们一般在实现InvocationHandler接口时，会通过构造方法传入target对象。
      2. method和args 分别对应了 target对象的方法和参数
      3. 在InvocationHandler.invoke内部再利用反射完成target对象的方法执行。
  2. 在源码面前，一切毫无遁形。
  3. 知其然，尽量要知其所以然。

附录：

  1. ProxyGenerator源码

2.构造器数据准备

 private MethodInfo generateConstructor() throws IOException {
    MethodInfo minfo = new MethodInfo(
        "<init>", "(Ljava/lang/reflect/InvocationHandler;)V",
        ACC_PUBLIC);

    DataOutputStream out = new DataOutputStream(minfo.code);

    code_aload(0, out);

    code_aload(1, out);

    out.writeByte(opc_invokespecial);
    out.writeShort(cp.getMethodRef(
        superclassName,
        "<init>", "(Ljava/lang/reflect/InvocationHandler;)V"));

    out.writeByte(opc_return);

    minfo.maxStack = 10;
    minfo.maxLocals = 2;
    minfo.declaredExceptions = new short[0];

    return minfo;
}

3.静态块数据准备

private MethodInfo generateStaticInitializer() throws IOException {
    MethodInfo minfo = new MethodInfo(
        "<clinit>", "()V", ACC_STATIC);

    int localSlot0 = 1;
    short pc, tryBegin = 0, tryEnd;

    DataOutputStream out = new DataOutputStream(minfo.code);

    for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
        for (ProxyMethod pm : sigmethods) {
            pm.codeFieldInitialization(out);
        }
    }

    out.writeByte(opc_return);

    tryEnd = pc = (short) minfo.code.size();

    minfo.exceptionTable.add(new ExceptionTableEntry(
        tryBegin, tryEnd, pc,
        cp.getClass("java/lang/NoSuchMethodException")));

    code_astore(localSlot0, out);

    out.writeByte(opc_new);
    out.writeShort(cp.getClass("java/lang/NoSuchMethodError"));

    out.writeByte(opc_dup);

    code_aload(localSlot0, out);

    out.writeByte(opc_invokevirtual);
    out.writeShort(cp.getMethodRef(
        "java/lang/Throwable", "getMessage", "()Ljava/lang/String;"));

    out.writeByte(opc_invokespecial);
    out.writeShort(cp.getMethodRef(
        "java/lang/NoSuchMethodError", "<init>", "(Ljava/lang/String;)V"));

    out.writeByte(opc_athrow);

    pc = (short) minfo.code.size();

    minfo.exceptionTable.add(new ExceptionTableEntry(
        tryBegin, tryEnd, pc,
        cp.getClass("java/lang/ClassNotFoundException")));

    code_astore(localSlot0, out);

    out.writeByte(opc_new);
    out.writeShort(cp.getClass("java/lang/NoClassDefFoundError"));

    out.writeByte(opc_dup);

    code_aload(localSlot0, out);

    out.writeByte(opc_invokevirtual);
    out.writeShort(cp.getMethodRef(
        "java/lang/Throwable", "getMessage", "()Ljava/lang/String;"));

    out.writeByte(opc_invokespecial);
    out.writeShort(cp.getMethodRef(
        "java/lang/NoClassDefFoundError",
        "<init>", "(Ljava/lang/String;)V"));

    out.writeByte(opc_athrow);

    if (minfo.code.size() > 65535) {
        throw new IllegalArgumentException("code size limit exceeded");
    }

    minfo.maxStack = 10;
    minfo.maxLocals = (short) (localSlot0 + 1);
    minfo.declaredExceptions = new short[0];

    return minfo;
}

4.代理类反编译后的源码

package com.vavi.proxy.dynaimc.jdk;

import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

import com.vavi.proxy.Sleepable;

public final class MyProxy extends Proxy implements Sleepable {
private static Method m1;
private static Method m3;
private static Method m0;
private static Method m4;
private static Method m2;

public MyProxy()

{
super(paramInvocationHandler);
}

public final boolean equals()

{
try {
    return ((Boolean) this.h.invoke(this, m1,
        new Object[] { paramObject })).booleanValue();
} catch (Error localError) {
    throw localError;
} catch (Throwable localThrowable) {
    throw new UndeclaredThrowableException(localThrowable);
}
}

@Override
public final void eat()

{
try {
    this.h.invoke(this, m3, null);
    return;
} catch (Error localError) {
    throw localError;
} catch (Throwable localThrowable) {
    throw new UndeclaredThrowableException(localThrowable);
}
}

@Override
public final int hashCode()

{
try {
    return ((Integer) this.h.invoke(this, m0, null)).intValue();
} catch (Error localError) {
    throw localError;
} catch (Throwable localThrowable) {
    throw new UndeclaredThrowableException(localThrowable);
}
}

@Override
public final void sleep()

{
try {
    this.h.invoke(this, m4, null);
    return;
} catch (Error localError) {
    throw localError;
} catch (Throwable localThrowable) {
    throw new UndeclaredThrowableException(localThrowable);
}
}

@Override
public final String toString()

{
try {
    return ((String) this.h.invoke(this, m2, null));
} catch (Error 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.vavi.proxy.Sleepable").getMethod("eat",
        new Class[0]);
    m0 = Class.forName("java.lang.Object").getMethod("hashCode",
        new Class[0]);
    m4 = Class.forName("com.vavi.proxy.Sleepable").getMethod("sleep",
        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());
}
}
}

% include JB/setup %}

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