I have a proxy (generated either as a JDKProxy or a CGLIB one) that is generated at runtime in the JVM. I wanted to know if there is a way to write the contents of this class (which looks like com.sun.proxy$Proxy123.class) to a file so that I may use a jd-eclipse like decompiler to see the kind of code generated. Since the class is present in the JVM, I wanted to know if there is a way we can ask the ClassLoader to provide an InputStream/URL to the actual class that can then be used to write contents to disk - and this file on the disk can be read using either jd-eclipse or javap. I know that this is not a production use case, but I was curious to see the contents of this dynamically generated class.
Thanks!
You can use Instrumentation to register a retransform-capable ClassFileTransformer and request a re-transformation of the Proxy class. Then, within the transformer’s transform method you have hands on the byte array which makes up the class. After saving the array to a class file you can simply return the array unmodified to let the JVM proceed unaffected.
But I’m not sure what you expect from looking into these classes. They are implemented straight-forward and offer no surprises. Here is an example of javap’s output after doing the steps described above on the java.lang.Runnable Proxy as generated with Oracle’s jdk1.7.0_40:
public final class com.sun.proxy.$Proxy0 extends java.lang.reflect.Proxy implements java.lang.Runnable {
private static java.lang.reflect.Method m1;
private static java.lang.reflect.Method m3;
private static java.lang.reflect.Method m0;
private static java.lang.reflect.Method m2;
public com.sun.proxy.$Proxy0(java.lang.reflect.InvocationHandler);
Code:
0: aload_0
1: aload_1
2: invokespecial #8 // Method java/lang/reflect/Proxy."<init>":(Ljava/lang/reflect/InvocationHandler;)V
5: return
public final int hashCode();
Code:
0: aload_0
1: getfield #16 // Field java/lang/reflect/Proxy.h:Ljava/lang/reflect/InvocationHandler;
4: aload_0
5: getstatic #55 // Field m0:Ljava/lang/reflect/Method;
8: aconst_null
9: invokeinterface #28, 4 // InterfaceMethod java/lang/reflect/InvocationHandler.invoke:(Ljava/lang/Object;Ljava/lang/reflect/Method;[Ljava/lang/Object;)Ljava/lang/Object;
14: checkcast #57 // class java/lang/Integer
17: invokevirtual #60 // Method java/lang/Integer.intValue:()I
20: ireturn
21: athrow
22: astore_1
23: new #42 // class java/lang/reflect/UndeclaredThrowableException
26: dup
27: aload_1
28: invokespecial #45 // Method java/lang/reflect/UndeclaredThrowableException."<init>":(Ljava/lang/Throwable;)V
31: athrow
Exception table:
from to target type
0 21 21 Class java/lang/Error
0 21 21 Class java/lang/RuntimeException
0 21 22 Class java/lang/Throwable
public final boolean equals(java.lang.Object);
Code:
0: aload_0
1: getfield #16 // Field java/lang/reflect/Proxy.h:Ljava/lang/reflect/InvocationHandler;
4: aload_0
5: getstatic #20 // Field m1:Ljava/lang/reflect/Method;
8: iconst_1
9: anewarray #22 // class java/lang/Object
12: dup
13: iconst_0
14: aload_1
15: aastore
16: invokeinterface #28, 4 // InterfaceMethod java/lang/reflect/InvocationHandler.invoke:(Ljava/lang/Object;Ljava/lang/reflect/Method;[Ljava/lang/Object;)Ljava/lang/Object;
21: checkcast #30 // class java/lang/Boolean
24: invokevirtual #34 // Method java/lang/Boolean.booleanValue:()Z
27: ireturn
28: athrow
29: astore_2
30: new #42 // class java/lang/reflect/UndeclaredThrowableException
33: dup
34: aload_2
35: invokespecial #45 // Method java/lang/reflect/UndeclaredThrowableException."<init>":(Ljava/lang/Throwable;)V
38: athrow
Exception table:
from to target type
0 28 28 Class java/lang/Error
0 28 28 Class java/lang/RuntimeException
0 28 29 Class java/lang/Throwable
public final java.lang.String toString();
Code:
0: aload_0
1: getfield #16 // Field java/lang/reflect/Proxy.h:Ljava/lang/reflect/InvocationHandler;
4: aload_0
5: getstatic #65 // Field m2:Ljava/lang/reflect/Method;
8: aconst_null
9: invokeinterface #28, 4 // InterfaceMethod java/lang/reflect/InvocationHandler.invoke:(Ljava/lang/Object;Ljava/lang/reflect/Method;[Ljava/lang/Object;)Ljava/lang/Object;
14: checkcast #67 // class java/lang/String
17: areturn
18: athrow
19: astore_1
20: new #42 // class java/lang/reflect/UndeclaredThrowableException
23: dup
24: aload_1
25: invokespecial #45 // Method java/lang/reflect/UndeclaredThrowableException."<init>":(Ljava/lang/Throwable;)V
28: athrow
Exception table:
from to target type
0 18 18 Class java/lang/Error
0 18 18 Class java/lang/RuntimeException
0 18 19 Class java/lang/Throwable
static {};
Code:
0: ldc #70 // String java.lang.Object
2: invokestatic #76 // Method java/lang/Class.forName:(Ljava/lang/String;)Ljava/lang/Class;
5: ldc #77 // String equals
7: iconst_1
8: anewarray #72 // class java/lang/Class
11: dup
12: iconst_0
13: ldc #70 // String java.lang.Object
15: invokestatic #76 // Method java/lang/Class.forName:(Ljava/lang/String;)Ljava/lang/Class;
18: aastore
19: invokevirtual #81 // Method java/lang/Class.getMethod:(Ljava/lang/String;[Ljava/lang/Class;)Ljava/lang/reflect/Method;
22: putstatic #20 // Field m1:Ljava/lang/reflect/Method;
25: ldc #83 // String java.lang.Runnable
27: invokestatic #76 // Method java/lang/Class.forName:(Ljava/lang/String;)Ljava/lang/Class;
30: ldc #84 // String run
32: iconst_0
33: anewarray #72 // class java/lang/Class
36: invokevirtual #81 // Method java/lang/Class.getMethod:(Ljava/lang/String;[Ljava/lang/Class;)Ljava/lang/reflect/Method;
39: putstatic #50 // Field m3:Ljava/lang/reflect/Method;
42: ldc #70 // String java.lang.Object
44: invokestatic #76 // Method java/lang/Class.forName:(Ljava/lang/String;)Ljava/lang/Class;
47: ldc #85 // String hashCode
49: iconst_0
50: anewarray #72 // class java/lang/Class
53: invokevirtual #81 // Method java/lang/Class.getMethod:(Ljava/lang/String;[Ljava/lang/Class;)Ljava/lang/reflect/Method;
56: putstatic #55 // Field m0:Ljava/lang/reflect/Method;
59: ldc #70 // String java.lang.Object
61: invokestatic #76 // Method java/lang/Class.forName:(Ljava/lang/String;)Ljava/lang/Class;
64: ldc #86 // String toString
66: iconst_0
67: anewarray #72 // class java/lang/Class
70: invokevirtual #81 // Method java/lang/Class.getMethod:(Ljava/lang/String;[Ljava/lang/Class;)Ljava/lang/reflect/Method;
73: putstatic #65 // Field m2:Ljava/lang/reflect/Method;
76: return
77: astore_1
78: new #90 // class java/lang/NoSuchMethodError
81: dup
82: aload_1
83: invokevirtual #93 // Method java/lang/Throwable.getMessage:()Ljava/lang/String;
86: invokespecial #96 // Method java/lang/NoSuchMethodError."<init>":(Ljava/lang/String;)V
89: athrow
90: astore_1
91: new #100 // class java/lang/NoClassDefFoundError
94: dup
95: aload_1
96: invokevirtual #93 // Method java/lang/Throwable.getMessage:()Ljava/lang/String;
99: invokespecial #101 // Method java/lang/NoClassDefFoundError."<init>":(Ljava/lang/String;)V
102: athrow
Exception table:
from to target type
0 77 77 Class java/lang/NoSuchMethodException
0 77 90 Class java/lang/ClassNotFoundException
public final void run();
Code:
0: aload_0
1: getfield #16 // Field java/lang/reflect/Proxy.h:Ljava/lang/reflect/InvocationHandler;
4: aload_0
5: getstatic #50 // Field m3:Ljava/lang/reflect/Method;
8: aconst_null
9: invokeinterface #28, 4 // InterfaceMethod java/lang/reflect/InvocationHandler.invoke:(Ljava/lang/Object;Ljava/lang/reflect/Method;[Ljava/lang/Object;)Ljava/lang/Object;
14: pop
15: return
16: athrow
17: astore_1
18: new #42 // class java/lang/reflect/UndeclaredThrowableException
21: dup
22: aload_1
23: invokespecial #45 // Method java/lang/reflect/UndeclaredThrowableException."<init>":(Ljava/lang/Throwable;)V
26: athrow
Exception table:
from to target type
0 16 16 Class java/lang/Error
0 16 16 Class java/lang/RuntimeException
0 16 17 Class java/lang/Throwable
}