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java - 什么是 com.sun.proxy.$Proxy

转载 作者:IT老高 更新时间:2023-10-28 20:31:21 28 4
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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:

调用java.lang.reflect.Proxy#newProxyInstance

之后会发生什么
  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[]
  3. 的魔法 ProxyGenerator.generateProxyClass
  4. 调用ClassLoaderdefine class来加载生成的$Proxy类(你见过的类名)
  5. 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 handlerinvoke()
    3. invocation handlerinvoke()
    4. 获取返回值
    5. 直接返回
  3. 类(字节码)以byte[]

  4. 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/

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