Java BytecodeWhat is a .class file anyway?

Dan Fleck

George Mason University

Fall 2007

What is in a .class file?

Create source file Compile source file into .class file Run the .class file

So, what is in the .class file?So, what is in the .class file?

The way other languages work (C, ADA, etc…)

Source code is written Source code is ported to every different

platform Source code is compiled into platform

specific machine code (or binaries)machine code (or binaries) Binaries execute on a single platform

Source Code

Linux port Windows port Mac port

Linux Compile

Windows Compile

MacCompile

Linux binary Win binary Mac binary

Linux Machine

Windows Machine Mac Machine

Java: Write once, compile, run anywhere!

Java is compiled into machine independent bytecode class files

Bytecode is interpreted by the Java Virtual Machine (JVM)

JVM executes the step-by-step instructions given to it from the bytecode

JVM is specific to each computer architecture (Linux JVM, Windows JVM, Mac JVM).

Source Code

JavaCompile

Linux JVM Win JVM Mac JVM

Linux Machine

Windows Machine Mac Machine

How Java Does it

Advantages of Bytecode

Bytecode is architecture independent (and writing a VM is easier than rewriting a compiler for every architecture)

VMs can enforce different levels of security automatically (applets versus applications)

Just In-Time (JIT) compiling helps achieve same or better speed than traditional compiled code

Other languages that have an intermediate representation

C# Perl PHP Python Forth Tcl

Java Bytecode Stack-based virtual machine Small instruction set: 202 instructions (all are

1 byte opcode + operands) Intel x86: ~280 instructions (1 to 17 bytes long!)

Memory is typed Every Java class file begins with magic

number 3405691582 = 0xCAFEBABE

Referencing Memory

iload <varnum> Pushes (loads) the int in local variable <varnum>

(1 bytes) on the stack istore <varnum>

Pops the int on the top of the stack and stores it in local variable <varnum>

iload, fload, dload, aload (reference) istore, fstore, dstore, astore

Javap examples

public class Test1 {

public int add(int a, int b) { int c= a+b; return c; }}

…public int add(int, int); Code: 0: iload_1 1: iload_2 2: iadd 3: istore_3 4: iload_3 5: ireturn

javap -c Test1javac -g Test1.java

// add var 1 and 2// store as local var 3// store onto stack// return int

// push onto stack// push onto stack

Javap included with Java Development Kit (JDK)

JAD example

public class Test1 {

public int add(int a, int b) { int c= a+b; return c; }}

… public int add(int a, int b) { int c = a + b; // 0 0:iload_1 // 1 1:iload_2 // 2 2:iadd // 3 3:istore_3 return c; // 4 4:iload_3 // 5 5:ireturn }

jad -a Test1javac -g Test1.java

JAD is free, but not included with Java Development Kit (JDK)

Java Bytecode Explanation

Opcode Mnemonic Description

0 nop Does nothing

1 aconst_null Push null on the stack

3 iconst_0 Push int 0 on the stack

4 iconst_1 Push int 1 on the stack

…

Java Bytecode ExplanationOpcode Mnemonic Description

18 ldc <value> Push a one-word (4 bytes) constant onto the stack

ldc “Hello”ldc 201

Constant may be an int, float or String

The String is really a reference to an entry in the string constant table!

Java Bytecode Arithmetic

Opcode Mnemonic Description

96 iadd Pops two integers from the stack and pushes their sum

iconst_2iconst_3iadd

Java Bytecode Arithmetic

Opcode Mnemonic Description

96 iadd Pops two integers from the stack and pushes their sum

97 ladd Pops two long integers from the stack and pushes their sum

…

106 fmul Pops two floats from the stack and pushes their product

…

119 dneg Pops a double from the stack, and pushes its negation

Other types of Instructions

Control Flow (~20 instructions) if, goto, return

Method Calls (4 instructions) Loading and Storing Variables (65 instructions) Creating objects (1 instruction) Using object fields (4 instructions) Arrays (3 instructions)

Method Calls

invokevirtual <method> Invokes the method <method> on the parameters

and object on the top of the stack. Finds the appropriate method at run-time based

on the actual type of the this object.

invokevirtual <Method void println(java.lang.String)>

Control Flow ifeq <label>

Pop an int off the stack. If it is zero, jump to the label. Otherwise, continue normally.

if_icmple <label>Pop two ints off the stack. If the second one is <= the first one, jump to the label. Otherwise, continue normally.

Static Method Calls

invokestatic <method> Invokes a static (class) method <method> on the

parameters on the top of the stack. Finds the appropriate method at run-time based

on the actual type of the this object.

JAD example 2

public void count() { for (int i=0; i<10; i++) { System.out.println( "i is "+i); } }

javac -g Test1.java

VM Spec: http://java.sun.com/docs/books/jvms/second_edition/html/Overview.doc.html

JAD example

public void count() { for(int i = 0; i < 10; i++) //* 0 0:iconst_0 //* 1 1:istore_1 //* 2 2:iload_1 //* 3 3:bipush 10 //* 4 5:icmpge 39System.out.println((new StringBuilder()).append("i is ").append(i).toString()); // 5 8:getstatic #2 <Field PrintStream System.out> // 6 11:new #3 <Class StringBuilder> // 7 14:dup // 8 15:invokespecial #4 <Method void StringBuilder()> // 9 18:ldc1 #5 <String "i is "> // 10 20:invokevirtual #6 <Method StringBuilder StringBuilder.append(String)> // 11 23:iload_1 // 12 24:invokevirtual #7 <Method StringBuilder StringBuilder.append(int)> // 13 27:invokevirtual #8 <Method String StringBuilder.toString()> // 14 30:invokevirtual #9 <Method void PrintStream.println(String)>

// 15 33:iinc 1 1 //* 16 36:goto 2 // 17 39:return }

JAD example 3: Test2.java

// 0 0:iconst_5 // 1 1:istore_1 // 2 2:bipush 10 // 3 4:istore_2 // 4 5:iload_2 // 5 6:iload_1 // 6 7:isub // 7 8:istore_3 // 8 9:iload_3 // 9 10:ireturn

What does this method do?

JAD example 3: Test2.java

// 0 0:iconst_5 // 1 1:istore_1 // 2 2:bipush 10 // 3 4:istore_2 // 4 5:iload_2 // 5 6:iload_1 // 6 7:isub // 7 8:istore_3 // 8 9:iload_3 // 9 10:ireturn

What does this method do?Push 5 onto stackStore into local var 1

Store into local var 1

Store into local var 1

Load var 2 onto stack

Load var 3 onto stack

Load var 1 onto stackSubtract stack vars

Return (top val from stack)

Push 10 onto stack

JAD example 3: Test2.java public int subtract() { int a = 5; // 0 0:iconst_5 // 1 1:istore_1 int b = 10; // 2 2:bipush 10 // 3 4:istore_2 int c = b - a; // 4 5:iload_2 // 5 6:iload_1 // 6 7:isub // 7 8:istore_3 return c; // 8 9:iload_3 // 9 10:ireturn }

References http://ocw.mit.edu/NR/rdonlyres/Special-Programs/SP-772Spring-2005-Summer

-2005/3AFB411F-B5C0-4032-9964-BF733EEB3199/0/bytecode.pdf http://www.cs.virginia.edu/evans, Bytecode, Lecture 18