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  1. import java.io.IOException;
  2. import java.lang.invoke.MethodHandles;
  3. import java.nio.ByteBuffer;
  4. import java.nio.charset.StandardCharsets;
  5.  
  6. public class Main {
  7.     interface Foo {
  8.         public void bar();
  9.     }
  10.     enum MyEnum {
  11.         A {
  12.             public void bar() {
  13.                 System.out.println("Hello!");
  14.             }
  15.         },
  16.         B,
  17.         C;
  18.     }
  19.     public static void main(String[] arg) {
  20.         try {
  21.             patch();
  22.         } catch(IOException|IllegalAccessException ex) {
  23.             System.err.println("patching failed: "+ex);
  24.             return;
  25.         }
  26.         test();
  27.     }
  28.     static void test() {
  29.         for(MyEnum e: MyEnum.values()) {
  30.             System.out.println(e.name());
  31.             if (e instanceof Foo) {
  32.                 System.out.println("\timplements Foo");
  33.                 ((Foo)e).bar();
  34.             }
  35.         }
  36.     }
  37.     private static void patch() throws IOException, IllegalAccessException {
  38.         ByteBuffer bb = ByteBuffer.wrap(Main.class
  39.             .getResourceAsStream("Main$MyEnum$1.class").readAllBytes());
  40.         int poolSize = bb.position(8).getChar();
  41.         for(int ix = 1; ix < poolSize; ix++) {
  42.             byte tag = bb.get();
  43.             switch(tag) {
  44.                 default: bb.position(bb.position() + 4); break;
  45.                 case 1: bb.position(bb.getChar() + bb.position()); break;
  46.                 case 7: case 8: case 16: case 19: case 20:
  47.                     bb.position(bb.position() + 2); break;
  48.                 case 5: case 6: bb.position(bb.position() + 8); ix++; break;
  49.                 case 15: bb.position(bb.position() + 3);
  50.             }
  51.         }
  52.         ByteBuffer newBB = ByteBuffer.allocate(bb.capacity() + 16);
  53.         newBB.put(bb.duplicate().flip());
  54.         int ifType = poolSize;
  55.         newBB.putShort(8, (short)(poolSize + 2));
  56.         newBB.put((byte)7).putShort((short)(ifType + 1));
  57.         byte[] ifName = "Main$Foo".getBytes(StandardCharsets.UTF_8);
  58.         newBB.put((byte)1).putShort((short)ifName.length).put(ifName);
  59.         bb.limit(bb.position() + 6);
  60.         newBB.put(bb);
  61.         bb.limit(bb.capacity());
  62.         newBB.putShort((short)(bb.getShort() + 1)).putShort((short)ifType);
  63.         newBB.put(bb);
  64.         MethodHandles.lookup().defineClass(newBB.array());
  65.     }
  66. }
Success #stdin #stdout 0.08s 33760KB
comments (?)
stdin
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Standard input is empty
stdout
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A
	implements Foo
Hello!
B
C
https://ideone.com/xPej5O
language:
Java (HotSpot 12)
created:
5 years ago
visibility:
public

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