Java Bytecode What is a .class file anyway? Dan Fleck George Mason University Fall 2007
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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
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