Java Bytecodes by Example
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This document discusses Java bytecodes and provides examples of evaluating expressions and decompiling Java bytecode. It begins with an introduction to Java bytecodes and shows the bytecode generated from a Java source code example. Stack-based evaluation of an expression using bytecode operations is demonstrated. The document also shows using javap to disassemble a Java class file and decompiling bytecode back into Java source code.
![$ cat Expr.java
class Expr {
public static void main(String []args) {
int a = 1, b = 2, c = 3, d = 4, e = 5, f = 6;
int r = (a - (b / c)) + ((d % e) * f);
System.out.println("" + r);
}
}
$ javac Expr.java
$ java Expr
25
$ javap -c Expr.class
Compiled from "Expr.java"
class Expr {
Expr();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_1
1: istore_1
...
Java
compiler
JavaVM
Java
disassembler
Use java tool for
disassembling](https://image.slidesharecdn.com/javabytecodesbyexample-160428003107/85/Java-Bytecodes-by-Example-27-320.jpg)
![public static void
main(java.lang.String[]);
descriptor: ??
flags: ??, ??
Code:
stack=??, locals=??, args_size=??
Pop
Quiz
public static void main(String []args) {
int a = 1, b = 2, c = 3, d = 4, e = 5, f = 6;
int r = (a - (b / c)) + ((d % e) * f);
System.out.println("" + r);
}](https://image.slidesharecdn.com/javabytecodesbyexample-160428003107/85/Java-Bytecodes-by-Example-31-320.jpg)
![public static void
main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=8, args_size=1
Answer](https://image.slidesharecdn.com/javabytecodesbyexample-160428003107/85/Java-Bytecodes-by-Example-32-320.jpg)
![Answer
public static void main(String []args) {
int sum = 0;
for(int i = 0; i < 10; i++) {
sum += i;
}
System.out.println(sum);
}
0: iconst_0
1: istore_1
2: iconst_0
3: istore_2
4: iload_2
5: bipush 10
7: if_icmpge 20
10: iload_1
11: iload_2
12: iadd
13: istore_1
14: iinc 2, 1
17: goto 4
20: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
23: iload_1
24: invokevirtual #3 // Method java/io/PrintStream.println:(I)V
27: return](https://image.slidesharecdn.com/javabytecodesbyexample-160428003107/85/Java-Bytecodes-by-Example-37-320.jpg)
Java Bytecodes by Example
- 1. JAVA BYTECODES BY EXAMPLE Ganesh Samarthyam www.designsmells.com
- 2. Let’s jump into the rabbit hole and explore a new world!
- 3. Java Bytecodes By Example But this low level stuff is scary - I don't want to learn it
- 6. Perform post-order traversal of the tree
- 7. 1 2 3 / - 4 5 % 6 * + post-order traversal result
- 8. Use a stack for evaluating this postfix expression 1 2 3 / - 4 5 % 6 * +
- 9. 1 2 3 / - 4 5 % 6 * + 1 1 2 1 2 3 1 0 Initial empty push 1 push 2 push 3 pop 3 pop 2 push 2 / 3 1 pop 0 pop 1 push 1 - 0 1 push 4 4 1 push 5 4 5 1 pop 5 pop 4 push 4 % 5 4 1 push 6 4 6 1 pop 6 pop 4 push 6 * 4 24 25 pop 24 pop 1 push 24 + 1
- 11. 1 2 3 / - 4 5 % 6 * + Initial empty
- 12. 1 2 3 / - 4 5 % 6 * + 1 push 1
- 13. 1 2 3 / - 4 5 % 6 * + 1 2 push 2
- 14. 1 2 3 / - 4 5 % 6 * + 1 2 3 push 3
- 15. 1 2 3 / - 4 5 % 6 * + 1 0 pop 3 pop 2 push 2 / 3
- 16. 1 2 3 / - 4 5 % 6 * + 1 pop 0 pop 1 push 1 - 0
- 17. 1 2 3 / - 4 5 % 6 * + 1 push 4 4
- 18. 1 2 3 / - 4 5 % 6 * + 1 push 5 4 5
- 19. 1 2 3 / - 4 5 % 6 * + 1 pop 5 pop 4 push 4 % 5 4
- 20. 1 2 3 / - 4 5 % 6 * + 1 push 6 4 6
- 21. 1 2 3 / - 4 5 % 6 * + 1 pop 6 pop 4 push 6 * 4 24
- 22. 1 2 3 / - 4 5 % 6 * + 25 pop 24 pop 1 push 24 + 1
- 23. 1 2 3 / - 4 5 % 6 * + Let us give names to these operations push 1 push 2 push 3 div sub push 4 push 5 mod push 6 mul add
- 24. int a = 1, b = 2, c = 3, d = 4, e = 5, f = 6; int r = (a - (b / c)) + ((d % e) * f); This is what a Java compiler generates iload_1 iload_2 iload_3 idiv isub iload 4 iload 5 irem iload 6 imul iadd istore 7 push 1 push 2 push 3 div sub push 4 push 5 mod push 6 mul add ourbytecode Javabytecodes
- 25. (1 - (2 / 3)) + ((4 % 5) * 6)Source code Java Compiler JavaBytecode JVM iload_1 iload_2 iload_3 idiv isub iload 4 iload 5 irem iload 6 imul iadd istore 7
- 26. Java bytecodes supports object oriented programming Typed intermediate language Supports primitive types (int, float, double, …) and reference types (arrays, strings, objects, …) Instructions can be classified into various types such as: loading (*load*) storing (*store*) method invocation arithmetic operations logical operations control flow memory allocation exception handling …
- 27. $ cat Expr.java class Expr { public static void main(String []args) { int a = 1, b = 2, c = 3, d = 4, e = 5, f = 6; int r = (a - (b / c)) + ((d % e) * f); System.out.println("" + r); } } $ javac Expr.java $ java Expr 25 $ javap -c Expr.class Compiled from "Expr.java" class Expr { Expr(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return public static void main(java.lang.String[]); Code: 0: iconst_1 1: istore_1 ... Java compiler JavaVM Java disassembler Use java tool for disassembling
- 28. System.out.println(“Hello World"); Java bytecodes // disassembled code using javap tool 0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3 // String Hello World 5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
- 29. int i = 10; if(i != 20) i = i*20; System.out.println(i); javap -c 0: bipush 10 2: istore_1 3: iload_1 4: bipush 20 6: if_icmpeq 14 9: iload_1 10: bipush 20 12: imul 13: istore_1 14: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 17: iload_1 18: invokevirtual #3 // Method java/io/PrintStream.println:(I)V 21: return
- 31. public static void main(java.lang.String[]); descriptor: ?? flags: ??, ?? Code: stack=??, locals=??, args_size=?? Pop Quiz public static void main(String []args) { int a = 1, b = 2, c = 3, d = 4, e = 5, f = 6; int r = (a - (b / c)) + ((d % e) * f); System.out.println("" + r); }
- 32. public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=3, locals=8, args_size=1 Answer
- 33. 1 2 3 / - 4 5 % 6 * + 1 1 2 1 2 3 1 0 Initial empty push 1 push 2 push 3 pop 3 pop 2 push 2 / 3 1 pop 0 pop 1 push 1 - 0 1 push 4 4 1 push 5 4 5 1 pop 5 pop 4 push 4 % 5 4 1 push 6 4 6 1 pop 6 pop 4 push 6 * 4 24 25 pop 24 pop 1 push 24 + 1 Answer: max stack value is 3
- 34. Supplier<String> s = () -> "hello world"; System.out.println(s.get()); Pop Quiz What bytecode instruction would s.get() generate?
- 36. Pop Quiz 0: iconst_0 1: istore_1 2: iconst_0 3: istore_2 4: iload_2 5: bipush 10 7: if_icmpge 20 10: iload_1 11: iload_2 12: iadd 13: istore_1 14: iinc 2, 1 17: goto 4 20: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 23: iload_1 24: invokevirtual #3 // Method java/io/PrintStream.println:(I)V 27: return Decompile this assembly code
- 37. Answer public static void main(String []args) { int sum = 0; for(int i = 0; i < 10; i++) { sum += i; } System.out.println(sum); } 0: iconst_0 1: istore_1 2: iconst_0 3: istore_2 4: iload_2 5: bipush 10 7: if_icmpge 20 10: iload_1 11: iload_2 12: iadd 13: istore_1 14: iinc 2, 1 17: goto 4 20: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 23: iload_1 24: invokevirtual #3 // Method java/io/PrintStream.println:(I)V 27: return
- 38. PROJECT WORK The best way to learn Java bytecodes is to implement a Java disassembler on your own! For implementation, read the documentation of Java bytecodes (in the JVM specification) and use javap tool as the reference implementation.
- 39. BOOKSTO READ Free download here: https://docs.oracle.com/javase/specs/jvms/se8/jvms8.pdf
- 40. BOOKSTO READ
- 41. BOOKSTO READ
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