What is com sun proxy Proxy

Question

I have seen that when errors occur deep in different frameworks  e g frameworks implementing the EJB specification or some JPA providers  the stacktrace contain classes like com sun proxy  Proxy  I know what a Proxy is  but I am looking for a more technical and more java specific answer    What are they  How are they created  What is there relationship to the JVM  Are they JVM implementation specific

User · Accepted Answer

Proxies are classes that are created and loaded at runtime. There is no source code for these classes. I know that you are wondering how you can make them do something if there is no code for them. The answer is that when you create them, you specify an object that implements InvocationHandler, which defines a method that is invoked when a proxy method is invoked.
You create them by using the call
```
Proxy.newProxyInstance(classLoader, interfaces, invocationHandler)
```
The arguments are:
1. classLoader. Once the class is generated, it is loaded with this class loader.
2. interfaces. An array of class objects that must all be interfaces. The resulting proxy implements all of these interfaces.
3. invocationHandler. This is how your proxy knows what to do when a method is invoked. It is an object that implements InvocationHandler. When a method from any of the supported interfaces, or hashCode, equals, or toString, is invoked, the method invoke is invoked on the handler, passing the Method object for the method to be invoked and the arguments passed.
For more on this, see the documentation for the Proxy class.
Every implementation of a JVM after version 1.3 must support these. They are loaded into the internal data structures of the JVM in an implementation-specific way, but it is guaranteed to work.

User · Answer

What are they   Nothing special  Just as same as common Java Class Instance   But those class are Synthetic proxy classes created by java lang reflect Proxy newProxyInstance  What is there relationship to the JVM  Are they JVM implementation specific   Introduced in 1 3  http   docs oracle com javase 1 3 docs relnotes features html reflection  It is a part of Java  so each JVM should support it    How are they created  Openjdk7 source    In short   they are created using JVM ASM tech   defining javabyte code at runtime     something using same tech    asm  http   asm ow2 org    cglib  http   cglib sourceforge net      What happens after calling java lang reflect Proxy newProxyInstance   reading the source you can see newProxyInstance call getProxyClass0 to obtain a  Class      after lots of cache or sth it calls the magic ProxyGenerator generateProxyClass which return a byte   call ClassLoader define class to load the generated  Proxy Class  the classname you have seen  just instance it and ready for use   What happens in magic sun misc ProxyGenerator   draw a class bytecode  combining all methods in the interfaces into one each method is build with same bytecode like   get calling Method meth info  stored while generating  pass info into invocation handler s invoke   get return value from invocation handler s invoke   just return it  the class bytecode  represent in form of byte     How to draw a class  Thinking your java codes are compiled into bytecodes  just do this at runtime  Talk is cheap show you the code  core method in sun misc ProxyGenerator java   generateClassFile          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  lt  interfaces length  i              Method   methods   interfaces i  getMethods            for  int j   0  j  lt  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 lt ProxyMethod gt  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 lt ProxyMethod gt  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    gt  65535            throw new IllegalArgumentException  method limit exceeded              if  fields size    gt  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  lt  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  lt  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        addProxyMethod         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 lt ProxyMethod gt  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 lt Class lt   gt  gt  legalExceptions   new ArrayList lt Class lt   gt  gt                     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 lt ProxyMethod gt  3           proxyMethods put sig  sigmethods             sigmethods add new ProxyMethod name  parameterTypes  returnType                                     exceptionTypes  fromClass        Full code about gen the proxy method      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  lt  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  gt  0                 code ipush parameterTypes length  out                out writeByte opc anewarray               out writeShort cp getClass  java lang Object                  for  int i   0  i  lt  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 lt Class lt   gt  gt  catchList   computeUniqueCatchList exceptionTypes           if  catchList size    gt  0                 for  Class lt   gt  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                     lt init gt      Ljava lang Throwable  V                  out writeByte opc athrow

[java] What is com.sun.proxy.$Proxy

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