Dynamic proxy in Java loses generic type information. Demo with CLHSDB, run on JDK8b99/Mac OS X 10.7.5. This gist is to answer question from https://gist.github.com/kdlan/6128766

clhsdb_session

$ jps
78131 Jps
78118 Test
$ clhsdb
Password:
hsdb> verbose true
hsdb> attach 78118
Attaching to process 78118, please wait...
hsdb> jseval "t = jvm.threads[jvm.threads.length-1]"
Thread (address=0x00000007eaa852b8, name=main) 
hsdb> jseval "f = t.frames[t.frames.length-1]"
Frame (method=Test.main(java.lang.String[]), bci=42, line=39) 
hsdb> jseval "f.locals"
{proxyBean=Object 0x00000007eabc9d68, args=Object 0x00000007eabb7d08, rawBean=Object 0x00000007eabb9dd8} 
hsdb> jseval "sa.objHeap.newOop(any2addr('0x00000007eabc9d68').addOffsetToAsOopHandle(0))"
sun.jvm.hotspot.oops.Instance@eabc9d68 
hsdb> jseval "p = sa.objHeap.newOop(any2addr('0x00000007eabc9d68').addOffsetToAsOopHandle(0))"
sun.jvm.hotspot.oops.Instance@eabc9d68 
hsdb> jseval "p.klass.name.asString()"
com/sun/proxy/$Proxy0 
hsdb> jseval "pk = p.klass"
sun.jvm.hotspot.oops.InstanceKlass@0x00000007b9c60a50 
hsdb> print 0x00000007b9c60a50

public final class com.sun.proxy.$Proxy0 @0x00000007b9c60a50



Super Class
public class java.lang.reflect.Proxy @0x00000007b9c4ad50
Interfaces
public abstract interface Test$TestService @0x00000007b9c60240

Fields

private static  java.lang.reflect.Method m1; (offset = 104)
private static  java.lang.reflect.Method m0; (offset = 108)
private static  java.lang.reflect.Method m3; (offset = 112)
private static  java.lang.reflect.Method m2; (offset = 116)


Methods
public void <init>(java.lang.reflect.InvocationHandler) @0x0000000104fc3b40;
static void <clinit>() @0x0000000104fc4100;
public final boolean equals(java.lang.Object) @0x0000000104fc3c30;
public final java.lang.String toString() @0x0000000104fc3ef0;
public final int hashCode() @0x0000000104fc3d20;
public final java.util.List list(java.util.List) @0x0000000104fc3e10;

Constant Pool
Constant Pool of [public final class com.sun.proxy.$Proxy0 @0x00000007b9c60a50] @0x0000000104fc36a0
hsdb> jseval "b = sa.objHeap.newOop(any2addr('0x00000007eabb9dd8').addOffsetToAsOopHandle(0))"
sun.jvm.hotspot.oops.Instance@eabb9dd8 
hsdb> jseval "bk = b.klass"
sun.jvm.hotspot.oops.InstanceKlass@0x00000007b9c60440 
hsdb> print 0x00000007b9c60440

public class Test$TestServiceImpl @0x00000007b9c60440



Super Class
public class java.lang.Object @0x00000007b9c00fc0
Interfaces
public abstract interface Test$TestService @0x00000007b9c60240

Methods
public void <init>() @0x0000000104fc2f20;
public java.util.List list(java.util.List) [signature (Ljava.util.List<Ljava.util.Map<Ljava.lang.String;Ljava.lang.Object;>;>;)Ljava.util.List<Ljava.util.Map<Ljava.lang.String;Ljava.lang.Object;>;>;]  @0x0000000104fc3040;

Constant Pool
Constant Pool of [public class Test$TestServiceImpl @0x00000007b9c60440] @0x0000000104fc2d20
hsdb> print 0x0000000104fc3e10

public final java.util.List list(java.util.List) @0x0000000104fc3e10

Holder Class
public final class com.sun.proxy.$Proxy0 @0x00000007b9c60a50
Bytecode
bci   bytecode 
0   aload_0 
1   getfield #16 [Field java.lang.reflect.InvocationHandler h] of public class java.lang.reflect.Proxy @0x00000007b9c4ad50 
4   aload_0 
5   getstatic #60 [Field java.lang.reflect.Method m3] of public final class com.sun.proxy.$Proxy0 @0x00000007b9c60a50 
8   iconst_1 
9   anewarray #22 [Class java.lang.Object] 
12   dup 
13   iconst_0 
14   aload_1 
15   aastore 
16   invokeinterface #28 [Method java.lang.Object invoke(java.lang.Object, java.lang.reflect.Method, java.lang.Object[])] 
21   checkcast #62 [Class java.util.List] 
24   areturn 
25   athrow 
26   astore_2 
27   new #42 [Class java.lang.reflect.UndeclaredThrowableException] 
30   dup 
31   aload_2 
32   invokespecial #45 [Method void <init>(java.lang.Throwable)] 
35   athrow 


Exception Table
start bci end bci handler bci catch type 
0 25 25 java.lang.Error 
0 25 25 java.lang.RuntimeException 
0 25 26 java.lang.Throwable 


Constant Pool
Constant Pool of [public final class com.sun.proxy.$Proxy0 @0x00000007b9c60a50] @0x0000000104fc36a0
hsdb> print 0x0000000104fc3040

public java.util.List list(java.util.List) [signature (Ljava.util.List<Ljava.util.Map<Ljava.lang.String;Ljava.lang.Object;>;>;)Ljava.util.List<Ljava.util.Map<Ljava.lang.String;Ljava.lang.Object;>;>;]  @0x0000000104fc3040

Holder Class
public class Test$TestServiceImpl @0x00000007b9c60440
Bytecode
line bci   bytecode 
16   0   aload_1 
16   1   areturn 


Constant Pool
Constant Pool of [public class Test$TestServiceImpl @0x00000007b9c60440] @0x0000000104fc2d20
hsdb> 

Test.java

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.Type;
import java.util.Arrays;
import java.util.List;
import java.util.Map;

public class Test {
    public interface TestService{
        List<Map<String,Object>>  list(List<Map<String,Object>> list);
    }

    public static class TestServiceImpl implements TestService {
        public List<Map<String, Object>> list(List<Map<String, Object>> list) {
            return list;
        }
    }

    public static TestService proxy(TestService bean) {
        return (TestService) Proxy.newProxyInstance(Test.class.getClassLoader(), new Class[]{TestService.class}, new InvocationHandler() {
            public Object invoke(Object proxy, Method method, Object[] args)
                    throws Throwable {
                return null;
            }
        });
    }

    static Type[] getGenericTypeParams(TestService bean) throws Exception{
        Class<?> clazz = bean.getClass();
        return clazz.getDeclaredMethod("list", List.class).getGenericParameterTypes();
    }

    public static void main(String[] args) throws Exception {
        TestService rawBean = new TestServiceImpl();
        TestService proxyBean = proxy(rawBean);
        System.out.println(Arrays.toString(getGenericTypeParams(rawBean)));
        System.out.println(Arrays.toString(getGenericTypeParams(proxyBean)));
        System.in.read();
    }
}