我已经看到，当不同框架(例如实现 EJB 规范的框架或某些 JPA 提供程序)中发生错误时，堆栈跟踪包含像 com.sun.proxy.$Proxy 这样的类。我知道代理是什么，但我正在寻找更技术性和更具体的 Java 答案。

1. 它们是什么？
2. 它们是如何创建的？
3. 与JVM有什么关系？它们是特定于 JVM 实现的吗？

最佳答案

它们是什么？

没什么特别的。和普通的 Java Class Instance 一样。

但是这些类是由java.lang.reflect.Proxy#newProxyInstance 创建的合成代理类

与JVM有什么关系？它们是特定于 JVM 实现的吗？

在 1.3 中引入

http://docs.oracle.com/javase/1.3/docs/relnotes/features.html#reflection

它是 Java 的一部分。所以每个JVM都应该支持它。

它们是如何创建的(Openjdk7 源代码)？

简而言之:它们是使用 JVM ASM 技术创建的(在运行时定义 javabyte 代码)

使用相同技术的东西:

• asm(http://asm.ow2.org/)
• cglib(http://cglib.sourceforge.net/)

调用java.lang.reflect.Proxy#newProxyInstance

1. 阅读源码可以看到newProxyInstance调用getProxyClass0获取了一个`Class

   `

2. 经过大量缓存或某事后，它会调用返回 byte[]
的魔法 ProxyGenerator.generateProxyClass
3. 调用ClassLoaderdefine class来加载生成的$Proxy类(你见过的类名)
4. 只需实例化即可使用

魔法 sun.misc.ProxyGenerator 会发生什么

1. 绘制一个类(字节码)，将接口(interface)中的所有方法合二为一

2. 每个方法都使用相同的字节码构建

   1. 获取调用方法信息(生成时存储)
   2. 将信息传递到 invocation handler 的 invoke()
   3. 从invocation handler的invoke()
   获取返回值
   4. 直接返回

3. 类(字节码)以byte[]

的形式表示

如何绘制类

认为您的 java 代码被编译成字节码，只需在运行时执行此操作

说话很便宜，给你看代码

sun/misc/ProxyGenerator.java 中的核心方法

生成类文件

/**
 * Generate a class file for the proxy class.  This method drives the
 * class file generation process.
 */
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());

        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());

    } 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();
}

添加代理方法

/**
 * Add another method to be proxied, either by creating a new
 * ProxyMethod object or augmenting an old one for a duplicate
 * method.
 *
 * "fromClass" indicates the proxy interface that the method was
 * found through, which may be different from (a subinterface of)
 * the method's "declaring class".  Note that the first Method
 * object passed for a given name and descriptor identifies the
 * Method object (and thus the declaring class) that will be
 * passed to the invocation handler's "invoke" method for a given
 * set of duplicate methods.
 */
private void addProxyMethod(Method m, Class fromClass) {
    String name = m.getName();
    Class[] parameterTypes = m.getParameterTypes();
    Class returnType = m.getReturnType();
    Class[] exceptionTypes = m.getExceptionTypes();

    String sig = name + getParameterDescriptors(parameterTypes);
    List<ProxyMethod> sigmethods = proxyMethods.get(sig);
    if (sigmethods != null) {
        for (ProxyMethod pm : sigmethods) {
            if (returnType == pm.returnType) {
                /*
                 * Found a match: reduce exception types to the
                 * greatest set of exceptions that can thrown
                 * compatibly with the throws clauses of both
                 * overridden methods.
                 */
                List<Class<?>> legalExceptions = new ArrayList<Class<?>>();
                collectCompatibleTypes(
                    exceptionTypes, pm.exceptionTypes, legalExceptions);
                collectCompatibleTypes(
                    pm.exceptionTypes, exceptionTypes, legalExceptions);
                pm.exceptionTypes = new Class[legalExceptions.size()];
                pm.exceptionTypes =
                    legalExceptions.toArray(pm.exceptionTypes);
                return;
            }
        }
    } else {
        sigmethods = new ArrayList<ProxyMethod>(3);
        proxyMethods.put(sig, sigmethods);
    }
    sigmethods.add(new ProxyMethod(name, parameterTypes, returnType,
                                   exceptionTypes, fromClass));
}

关于生成代理方法的完整代码

    private MethodInfo generateMethod() throws IOException {
        String desc = getMethodDescriptor(parameterTypes, returnType);
        MethodInfo minfo = new MethodInfo(methodName, desc,
            ACC_PUBLIC | ACC_FINAL);

        int[] parameterSlot = new int[parameterTypes.length];
        int nextSlot = 1;
        for (int i = 0; i < parameterSlot.length; i++) {
            parameterSlot[i] = nextSlot;
            nextSlot += getWordsPerType(parameterTypes[i]);
        }
        int localSlot0 = nextSlot;
        short pc, tryBegin = 0, tryEnd;

        DataOutputStream out = new DataOutputStream(minfo.code);

        code_aload(0, out);

        out.writeByte(opc_getfield);
        out.writeShort(cp.getFieldRef(
            superclassName,
            handlerFieldName, "Ljava/lang/reflect/InvocationHandler;"));

        code_aload(0, out);

        out.writeByte(opc_getstatic);
        out.writeShort(cp.getFieldRef(
            dotToSlash(className),
            methodFieldName, "Ljava/lang/reflect/Method;"));

        if (parameterTypes.length > 0) {

            code_ipush(parameterTypes.length, out);

            out.writeByte(opc_anewarray);
            out.writeShort(cp.getClass("java/lang/Object"));

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

                out.writeByte(opc_dup);

                code_ipush(i, out);

                codeWrapArgument(parameterTypes[i], parameterSlot[i], out);

                out.writeByte(opc_aastore);
            }
        } else {

            out.writeByte(opc_aconst_null);
        }

        out.writeByte(opc_invokeinterface);
        out.writeShort(cp.getInterfaceMethodRef(
            "java/lang/reflect/InvocationHandler",
            "invoke",
            "(Ljava/lang/Object;Ljava/lang/reflect/Method;" +
                "[Ljava/lang/Object;)Ljava/lang/Object;"));
        out.writeByte(4);
        out.writeByte(0);

        if (returnType == void.class) {

            out.writeByte(opc_pop);

            out.writeByte(opc_return);

        } else {

            codeUnwrapReturnValue(returnType, out);
        }

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

        List<Class<?>> catchList = computeUniqueCatchList(exceptionTypes);
        if (catchList.size() > 0) {

            for (Class<?> ex : catchList) {
                minfo.exceptionTable.add(new ExceptionTableEntry(
                    tryBegin, tryEnd, pc,
                    cp.getClass(dotToSlash(ex.getName()))));
            }

            out.writeByte(opc_athrow);

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

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

            code_astore(localSlot0, out);

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

            out.writeByte(opc_dup);

            code_aload(localSlot0, out);

            out.writeByte(opc_invokespecial);

            out.writeShort(cp.getMethodRef(
                "java/lang/reflect/UndeclaredThrowableException",
                "<init>", "(Ljava/lang/Throwable;)V"));

            out.writeByte(opc_athrow);
        }

关于java - 什么是 com.sun.proxy.$Proxy，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19633534/