1. Main sponsor
Mastering Java Bytecode
With ASM
Anton Arhipov

2. Me
Anton Arhipov
JRebel @
Product manager by
day, coding monkey by
night
anton@zeroturnaround.com
@antonarhipov

3. Bytecode
• One-byte instructions
• 256 possible opcodes (200+ in use)
• Stack based (pop, push)

4. ASM
• Low-level API for bytecode crunching
• Core API – visitors
• Tree API – nodes
• Analysis package
• http://asm.ow2.org

5. Why?
• Compilers for JVM
• Programming model (AOP, ORM)
• Awesome tools for JVM

7. public class Items implements Serializable {
private List<Integer> ids
= new ArrayList<Integer>();
{
ids.add(1);
ids.add(100);
ids.add(100000);
}
public int getId(int i) {
return ids.get(i);
}
}

8. public class Items implements Serializable {
private List<Integer> ids
= new ArrayList<Integer>();
{
ids.add(1);
ids.add(100);
ids.add(100000);
}
public int getId(int i) {
return ids.get(i);
}
}

9. public class Items implements Serializable {
private List<Integer> ids
= new ArrayList<Integer>();
{
ids.add(1);
ids.add(100);
ids.add(100000);
}
public int getId(int i) {
return ids.get(i);
}
}

10. public class Items implements Serializable {
private List<Integer> ids
= new ArrayList<Integer>();
{
ids.add(1);
ids.add(100);
ids.add(100000);
}
public int getId(int i) {
return ids.get(i);
}
}

11. public class Items implements Serializable {
private List<Integer> ids
= new ArrayList<Integer>();
{
ids.add(1);
ids.add(100);
ids.add(100000);
}
public int getId(int i) {
return ids.get(i);
}
}

12. Basic Process
• Construct ClassWriter
• Stack up the visitors for:
• annotations, methods, fields, etc
• Write out bytes

13. javap

14. ClassWriter
ClassWriter cw = new ClassWriter(
ClassWriter.COMPUTE_MAXS |
ClassWriter.COMPUTE_FRAMES);

15. Visit Class
cw.visit(
Opcodes.V1_6,
Opcodes.ACC_PUBLIC,
"zt/asm/Items",
null,
"java/lang/Object",
new String[]
{"java/io/Serializable"});

16. Visit Class
cw.visit(
Opcodes.V1_6,
Opcodes.ACC_PUBLIC,
"zt/asm/Items",
null,
"java/lang/Object",
new String[]
{"java/io/Serializable"});

17. Visit Class
cw.visit(
Opcodes.V1_6,
Opcodes.ACC_PUBLIC,
"zt/asm/Items",
null,
"java/lang/Object",
new String[]
{"java/io/Serializable"});

18. Visit Class
cw.visit(
Opcodes.V1_6,
Opcodes.ACC_PUBLIC,
"zt/asm/Items",
null,
"java/lang/Object",
new String[]
{"java/io/Serializable"});

19. Visit Class
cw.visit(
Opcodes.V1_6,
Opcodes.ACC_PUBLIC,
"zt/asm/Items",
null,
"java/lang/Object",
new String[]
{"java/io/Serializable"});

20. Field
private List<Integer> ids
= new ArrayList<Integer>();
private List<Integer> ids;
public Items() {
ids = new ArrayList<Integer>();
}

21. Visit Field
FieldVisitor fv = cw.visitField(
Opcodes.ACC_PRIVATE,
"ids",
"Ljava/util/List;",
"Ljava/util/List<Lj/l/Integer;>;",
null);

22. Visit Field
FieldVisitor fv = cw.visitField(
Opcodes.ACC_PRIVATE,
"ids",
"Ljava/util/List;",
"Ljava/util/List<Lj/l/Integer;>;",
null);

23. Descriptor
• Primitives
• B, C, S, I, J, F, D, Z, V
• References
• Ljava/lang/Object;
• Arrays
• Prefixed with [, i.e. [Ljava/lang/Object;

24. Descriptor
Ljava/util/List;
([Ljava/lang/String;)V

25. org.objectweb.asm.Type
Type.getObjectType("java/lang/String")
Ljava/lang/String;

26. Visit Method
MethodVisitor constructor =
cw.visitMethod(
Opcodes.ACC_PUBLIC,
"<init>",
"()V",
null,
null);

27. Visit Method
MethodVisitor get =
cw.visitMethod(
Opcodes.ACC_PUBLIC,
"get",
"(I)I",
null,
null);

28. public class Items implements Serializable {
private List<Integer> ids
= new ArrayList<Integer>();
{
ids.add(1);
ids.add(100);
ids.add(100000);
}
public int getId(int i) {
return ids.get(i);
}
}

29. public class Items implements Serializable {
private List<Integer> ids
= new ArrayList<Integer>();
{
ids.add(1);
ids.add(100);
ids.add(100000);
}
public int getId(int i) {
return ids.get(i);
}
}

30. public Items() {
super();
ids = new ArrayList<Integer>();
ids.add(1);
ids.add(100);
ids.add(100000);
}

31. public Items() {
super();
ids = new ArrayList<Integer>();
ids.add(1);
ids.add(100);
ids.add(100000);
}

32. super()
0: aload_0
1: invokespecial #1 // Object.<init>
MethodVisitor mv = …
mv.visitVarInsn(ALOAD, 0);
mv.visitMethodInsn(INVOKESPECIAL,
"java/lang/Object", "<init>", "()V");

33. public Items() {
super();
ids = new ArrayList<Integer>();
ids.add(1);
ids.add(100);
ids.add(100000);
}

34. public Items() {
super();
ids = new ArrayList<Integer>();
ids.add(1);
ids.add(100);
ids.add(100000);
}

35. Assignment
4: aload_0
5: new #2 // ArrayList
8: dup
9: invokespecial #3 // <init>
12: putfield #4 // ids

36. Assignment
Create instance
4: aload_0
5: new #2 // ArrayList
8: dup
9: invokespecial #3 // <init>
12: putfield #4 // ids

37. Assignment
4: aload_0
5: new #2 // ArrayList
8: dup
9: invokespecial #3 // <init>
12: putfield #4 // ids
Assign to a field

38. Assignment
5: new #2 // ArrayList
8: dup
9: invokespecial #3 // <init>
12: astore_1 #4 // ids
Assign to local variable

39. Assignment
mv.visitVarInsn(ALOAD, 0);
mv.visitTypeInsn(NEW, "java/util/ArrayList");
mv.visitInsn(DUP);
mv.visitMethodInsn(INVOKESPECIAL,
"java/util/ArrayList", "<init>", "()V");
mv.visitFieldInsn(PUTFIELD,
"zt/asm/Items", "ids", "Ljava/util/List;");

40. Assignment
mv.visitVarInsn(ALOAD, 0);
mv.visitTypeInsn(NEW, "java/util/ArrayList");
mv.visitInsn(DUP);
mv.visitMethodInsn(INVOKESPECIAL,
"java/util/ArrayList", "<init>", "()V");
mv.visitFieldInsn(PUTFIELD,
"zt/asm/Items", "ids", "Ljava/util/List;");

41. public Items() {
super();
ids = new ArrayList<Integer>();
ids.add(1);
ids.add(100);
ids.add(100000);
}

42. public Items() {
super();
ids = new ArrayList<Integer>();
ids.add(1);
ids.add(100);
ids.add(100000);
}

43. ids.add(1)
15: aload_0
16: getfield #4 // ids
19: iconst_1
20: invokestatic #5 // Integer.valueOf
23: invokeinterface #6, 2 // List.add
28: pop

44. ids.add(1)
15: aload_0
16: getfield #4 // ids
19: iconst_1
20: invokestatic #5 // Integer.valueOf
23: invokeinterface #6, 2 // List.add
28: pop

45. ids.add(1)
15: aload_0
16: getfield #4 // ids
19: iconst_1
20: invokestatic #5 // Integer.valueOf
23: invokeinterface #6, 2 // List.add
28: pop

46. ids.add(1)
15: aload_0
16: getfield #4 // ids
19: iconst_1
20: invokestatic #5 // Integer.valueOf
23: invokeinterface #6, 2 // List.add
28: pop

47. ids.add(1)
15: aload_0
16: getfield #4 // ids
19: iconst_1
20: invokestatic #5 // Integer.valueOf
23: invokeinterface #6, 2 // List.add
28: pop

48. ids.add(1)
15: aload_0
16: getfield #4 // ids
19: iconst_1
20: invokestatic #5 // Integer.valueOf
23: invokeinterface #6, 2 // List.add
28: pop

49. ids.add(100)
15: aload_0
16: getfield #4 // ids
19: bipush 100
20: invokestatic #5 // Integer.valueOf
23: invokeinterface #6, 2 // List.add
28: pop

50. ids.add(100_000)
15: aload_0
16: getfield #4 // ids
19: ldc #7 // int 100000
20: invokestatic #5 // Integer.valueOf
23: invokeinterface #6, 2 // List.add
28: pop

51. mv.visitVarInsn(ALOAD, 0);
mv.visitFieldInsn(GETFIELD,
"zt/asm/Items", "ids",
"Ljava/util/List;");
mv.visitInsn(ICONST_1);
mv.visitMethodInsn(INVOKESTATIC, "java/lang
/Integer", "valueOf", "(I)Ljava/lang/Intege
r;");
mv.visitMethodInsn(INVOKEINTERFACE, "java/u
til/List", "add", "(Ljava/lang/Object;)Z");
mv.visitInsn(POP);

52. public class Items implements Serializable {
private List<Integer> ids
= new ArrayList<Integer>();
{
ids.add(1);
ids.add(100);
ids.add(100000);
}
public int getId(int i) {
return ids.get(i);
}
}

53. ASMified
mv = cw.visitMethod(ACC_PUBLIC, "getId", "(I)I", null, null);
mv.visitCode();
Label l0 = new Label();
mv.visitLabel(l0);
mv.visitLineNumber(16, l0);
mv.visitVarInsn(ALOAD, 0);
mv.visitFieldInsn(GETFIELD, "zt/asm/Items", "ids", "Ljava/util/List;");
mv.visitVarInsn(ILOAD, 1);
mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List",
"get", "(I)Ljava/lang/Object;");
mv.visitTypeInsn(CHECKCAST, "java/lang/Integer");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Integer",
"intValue", "()I");
mv.visitInsn(IRETURN);
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitLocalVariable("this", "Lzt/asm/Items;", null, l0, l1, 0);
mv.visitLocalVariable("i", "I", null, l0, l1, 1);
mv.visitMaxs(2, 2);
mv.visitEnd();

54. ASMified
mv = cw.visitMethod(ACC_PUBLIC, "getId", "(I)I", null, null);
mv.visitCode();
Label l0 = new Label();
java -cp asm-all-3.3.1.jar:asm-util-3.3.1.jar
mv.visitLabel(l0);
mv.visitLineNumber(16, l0);
org.objectweb.asm.util.ASMifierClassVisitor
mv.visitVarInsn(ALOAD, 0);
Items.class
mv.visitFieldInsn(GETFIELD, "zt/asm/Items", "ids", "Ljava/util/List;");
mv.visitVarInsn(ILOAD, 1);
mv.visitMethodInsn(INVOKEINTERFACE, "java/util/List",
"get", "(I)Ljava/lang/Object;");
mv.visitTypeInsn(CHECKCAST, "java/lang/Integer");
mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Integer",
"intValue", "()I");
mv.visitInsn(IRETURN);
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitLocalVariable("this", "Lzt/asm/Items;", null, l0, l1, 0);
mv.visitLocalVariable("i", "I", null, l0, l1, 1);
mv.visitMaxs(2, 2);
mv.visitEnd();

55. javap
public class zt.asm.deg.Items {
public java.util.List<java.lang.Integer> ids;
public int getId(int);
Code:
0: aload_0
1: getfield #4
4: iload_1
5: invokeinterface #8, 2
10: checkcast #9
13: invokevirtual #10
16: ireturn
}

56. Groovyfied
public class zt/asm/Items {
public Ljava/util/List; ids
@groovyx.ast.bytecode.Bytecode
public int getId(int a) {
aload 0
getfield zt.asm.Items.ids >> List
iload 1
invokeinterface List.get(int) >> Object
checkcast Integer
invokevirtual Integer.intValue() >> int
ireturn
}
}
https://github.com/melix/groovy-bytecode-ast

57. Generating bytecode
from scratch
is too simple …
Transforming the bytecode is
much more fun! 

58. Instrument
some bytecode

59. Ninja.class Ninja.class’
10101010101 10101010101
11000101010 11100001010
10101010001 10101010001
00010001110 00010001110
11011101011 11011101110

60. How?
• Add –javaagent to hook into class loading
process
• Implement ClassFileTransformer
• Use bytecode manipulation libraries
(Javassist, cglib, asm) to add any custom logic
java.lang.instrument

61. How ? (2)
• Use custom ClassLoader
– Override ClassLoader#findClass
– Use ClassReader(String) to read the class
in and transform it via visitor chain
– Call ClassLoader#defineClass explicitly
with the result from the transformation
step

62. java.lang.instrument
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;
public class Agent {
public static void premain(String args, Instrumentation inst)
{ inst.addTransformer(new ClassFileTransformer(), true); }
public static void agentmain(String args, Instrumentation inst)
{ premain(args,inst); }
}

63. java.lang.instrument
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;
public class Agent {
public static void premain(String args, Instrumentation inst)
{ inst.addTransformer(new ClassFileTransformer(), true); }
public static void agentmain(String args, Instrumentation inst)
{ premain(args,inst); }
}

64. java.lang.instrument
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;
public class Agent {
public static void premain(String args, Instrumentation inst)
{ inst.addTransformer(new ClassFileTransformer(), true); }
public static void agentmain(String args, Instrumentation inst)
{ premain(args,inst); }
}
META-INF/MANIFEST.MF
Premain-Class: Agent
java –javaagent:agent.jar …
Agent-Class: Agent

65. j.l.instrument.ClassFileTransformer
new ClassFileTransformer() {
public byte[] transform(ClassLoader loader, String className,
Class<?>classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer){
ClassReader cr = new ClassReader(classfileBuffer);
ClassWriter cw = new ClassWriter(cr,
ClassWriter.COMPUTE_MAXS |
ClassWriter.COMPUTE_FRAMES);
MyAdapter ca = new MyAdapter(cw);
cr.accept(ca, ClassReader.EXPAND_FRAMES);
return cw.toByteArray();
}

66. j.l.instrument.ClassFileTransformer
new ClassFileTransformer() {
public byte[] transform(ClassLoader loader, String className,
Class<?>classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer){
ClassReader cr = new ClassReader(classfileBuffer);
ClassWriter cw = new ClassWriter(cr,
ClassWriter.COMPUTE_MAXS |
ClassWriter.COMPUTE_FRAMES);
MyAdapter ca = new MyAdapter(cw);
cr.accept(ca, ClassReader.EXPAND_FRAMES);
return cw.toByteArray();
}

67. j.l.instrument.ClassFileTransformer
new ClassFileTransformer() {
public byte[] transform(ClassLoader loader, String className,
Class<?>classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer){
ClassReader cr = new ClassReader(classfileBuffer);
ClassWriter cw = new ClassWriter(cr,
ClassWriter.COMPUTE_MAXS |
ClassWriter.COMPUTE_FRAMES);
MyAdapter ca = new MyAdapter(cw);
cr.accept(ca, ClassReader.EXPAND_FRAMES);
return cw.toByteArray();
}

68. public class MyClassLoader extends ClassLoader {
protected Class findClass(String name)
throws ClassNotFoundException {
ClassReader cr = new ClassReader(name);
ClassWriter cw = new ClassWriter(cr,
ClassWriter.COMPUTE_MAXS |
ClassWriter.COMPUTE_FRAMES);
MyClassAdapter ca =
new MyClassAdapter(cw);
cr.accept(ca, ClassReader.EXPAND_FRAMES);
byte b[] = cw.toByteArray();
return defineClass(name, b, 0, b.length);
}

69. SLIDES
GOTO IDE
SLIDES
IDE: DEMO

70. @antonarhipov
anton@zeroturnaround.com