【发布时间】：2013-11-07 03:37:11
【问题描述】：
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.$Proxy 这样的类。我知道代理是什么，但我正在寻找更技术性和更具体的 Java 答案。
- 它们是什么？
- 它们是如何创建的？
- 与JVM有什么关系？它们是特定于 JVM 实现的吗？
【问题讨论】：
【发布时间】：2013-11-07 03:37:11
【问题描述】：
它们是什么？
没什么特别的。和普通的 Java Class Instance 一样。
但是这些类是由java.lang.reflect.Proxy#newProxyInstance创建的Synthetic proxy classes
与JVM有什么关系？它们是特定于 JVM 实现的吗？
在 1.3 中引入
http://docs.oracle.com/javase/1.3/docs/relnotes/features.html#reflection
它是 Java 的一部分。所以每个JVM都应该支持它。
它们是如何创建的（Openjdk7 源代码）？
简而言之：它们是使用 JVM ASM 技术创建的（在运行时定义 javabyte 代码）
使用相同技术的东西：
- asm(http://asm.ow2.org/)
- cglib(http://cglib.sourceforge.net/)
调用java.lang.reflect.Proxy#newProxyInstance后会发生什么
- 阅读源码可以看到newProxyInstance调用
getProxyClass0获取了一个`Class`
- 经过大量缓存或某事后，它会调用魔术
ProxyGenerator.generateProxyClass，返回一个字节[] - 调用ClassLoader
define class加载生成的$ProxyClass（你见过的类名） - 只需实例化即可使用
魔法 sun.misc.ProxyGenerator 会发生什么
- 绘制一个类（字节码），将接口中的所有方法合二为一
-
每个方法都使用相同的字节码构建
- 获取调用方法方法信息（生成时存储）
- 将信息传递到
invocation handler的invoke() - 从
invocation handler的invoke()获取返回值 - 直接退货
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);
}
【讨论】：
代理是在运行时创建和加载的类。这些类没有源代码。我知道您想知道如果没有代码，如何让他们做某事。答案是，当你创建它们时，你指定一个实现
InvocationHandler的对象，它定义了一个在调用代理方法时调用的方法。-
你通过调用来创建它们
Proxy.newProxyInstance(classLoader, interfaces, invocationHandler)参数是：
-
classLoader。一旦生成了类，就会使用这个类加载器来加载它。 -
interfaces。必须都是接口的类对象数组。生成的代理实现了所有这些接口。 -
invocationHandler。这就是您的代理在调用方法时知道该做什么的方式。它是一个实现InvocationHandler的对象。当调用来自任何受支持接口或hashCode、equals或toString的方法时，将在处理程序上调用invoke方法，并为要调用的方法传递Method对象以及传递的参数。
有关这方面的更多信息，请参阅
Proxy类的文档。 -
JVM 1.3 之后的每个实现都必须支持这些。它们以特定于实现的方式加载到 JVM 的内部数据结构中，但保证可以工作。
【讨论】：