1. Java Decompiler

2. Decompiler Go from executable bytecode to the source code
Bytecode has an M->N correspondence
Partial structure destruction
Executable has worse!
Not all bytecode has corresponding source code

3. Class Parsing int flags = source.readUnsignedShort();
output.setFlags(flags);
int this_id = source.readUnsignedShort();
String name = output.getClassName(this_id);
output.setName(name);
int super_id = source.readUnsignedShort();
String superName = null;
if (super_id != 0)‏
superName = output.getClassName(super_id);
output.setSuperClass(superName);
int count = source.readUnsignedShort();
String interfaces[] = new String[count];
for (int i=0;i<count;i++) {
int id = source.readUnsignedShort();
interfaces[i] = output.getClassName(id); }
output.setInterfaces(interfaces);
current = ClassLevel.METADATA;

4. Signature Parsing private String FieldType() { switch (peek()) {
case 'B': case 'C': case 'D': case 'F': case 'I':
case 'S': case 'Z': return BaseType();
case 'L': return ClassType();
case '[': return ArrayType(); }
throw new ParseException();
private String ClassType() {
StringBuilder sb = new StringBuilder();
if (next() != 'L') throw new ParseException();
while (peek() != ';') {
char c = next();
sb.append(c == '/' ? '.' : c);
next(); // consume ;
return sb.toString();

5. Bytecode detection while (i < bytecode.length) { int index = i++;
BytecodeInstruction expr;
switch(bytecode[index] & 0xff) {
case 0x02: //iconst_m1
case 0x03: //iconst_0
expr = new LoadConstantInstruction(int.class, bytecode[index]-0x03);
break;
case 0x99: //ifeq
expr = new IfInstruction("eq",index+((bytecode[i++] << 8 & 0xff) | (bytecode[i++] & 0xff)), false);
case 0xb6: //invokevirtual
expr = new InvokeInstruction(info, (bytecode[i++] << 8 & 0xff) | (bytecode[i++] & 0xff), false, true);
default:
Logger.error("Illegal opcode: "+Integer.toHexString(bytecode[index] & 0xff));
expr = null; }
if (expr != null) {
CodeGraph.Node node = new Node(expr);
if (lastNode != null) {
if (last.usesNext())‏
lastNode.addChild(node);
lastNode = node;
last = expr;
map.put(index, node);

6. Runtime annotation processing
public final class OptClassLoader extends ClassLoader {
public final Class<?> loadClass(String name, boolean resolve) {
Annotation[] annotations = result.getAnnotations();
for (Annotation a : annotations) {
Class<? extends Annotation> c = a.annotationType();
if (c.getName().equals("util.Option")) {
addOption((Option)a);
} else if (c.getName().equals("util.Options")) {
Option[] defs = (Option[])c.getDeclaredMethod("value").invoke(a);
for (Option o : defs)‏
addOption(o); }
return result;
private void addOption(Option o) {
String name = o.name();
Class<?> type = o.type();
String def = o.def();
OptionManager.addOption(name,type,def);

7. Sample output
File:
abstract class Foobar implements java.io.Serializable {
public abstract <T, E extends Throwable> T foobar(T bar) throws E; }
Output:
/* FILE: test.java */
Foobar() {
: load java.lang.Object, 0-> { }
: invokenonvirtual java.lang.Object.<init>-> { }
: return (none)-> { }
public abstract <T extends java.lang.Object, E extends java.lang.Throwable>
T foobar(T {VAR0}) throws E;