Java Bytecode What is a .class file anyway?

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Java Bytecode
What is a .class file anyway?
Dan Fleck
George Mason University
Fall 2007
What is in a .class file?
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Create source file
Compile source file into .class file
Run the .class file
So, what is in the .class file?
The way other languages work
(C, ADA, etc…)
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Source code is written
Source code is ported to every different
platform
Source code is compiled into platform
specific machine code (or binaries)
Binaries execute on a single platform
Source Code
Linux port
Windows port
Mac port
Linux
Compile
Windows
Compile
Mac
Compile
Linux binary
Win binary
Mac binary
Linux
Machine
Windows
Machine
Mac Machine
Java: Write once, compile, run
anywhere!
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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).
How Java Does it
Source Code
Java
Compile
Linux JVM
Win JVM
Mac JVM
Linux
Machine
Windows
Machine
Mac Machine
Advantages of Bytecode
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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
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C#
Perl
PHP
Python
Forth
Tcl
Java Bytecode
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Stack-based virtual machine
Small instruction set: 202 instructions (all are
1 byte opcode + operands)
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Intel x86: ~280 instructions (1 to 17 bytes long!)
Memory is typed
Every Java class file begins with magic
number 3405691582 = 0xCAFEBABE
Referencing Memory
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iload <varnum>
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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
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Pushes (loads) the int in local variable <varnum>
(1 bytes) on the stack
Javap examples
javac -g Test1.java
public class Test1 {
public int add(int a, int b) {
int c= a+b;
return c;
}
}
javap -c Test1
…
public int add(int, int);
Code:
0: iload_1
// push onto stack
1: iload_2
// push onto stack
2: iadd
// add var 1 and 2
3: istore_3 // store as local var 3
4: iload_3
// store onto stack
5: ireturn
// return int
Javap included with Java Development Kit (JDK)
JAD example
javac -g Test1.java
public class Test1 {
public int add(int a, int b) {
int c= a+b;
return c;
}
}
jad -a Test1
…
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 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 Explanation
Opcode
18
Mnemonic
ldc <value>
Description
Push a one-word (4 bytes)
constant onto the stack
Constant may be an int, float or String
ldc “Hello”
ldc 201
The String is really a reference to an
entry in the string constant table!
Java Bytecode Arithmetic
Opcode
96
Mnemonic
iadd
Description
Pops two integers from the
stack and pushes their sum
iconst_2
iconst_3
iadd
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
fmul
Pops two floats from the stack and
pushes their product
dneg
Pops a double from the stack, and
pushes its negation
…
106
…
119
Other types of Instructions
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Control Flow (~20 instructions)
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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
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invokevirtual <method>
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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
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ifeq <label>
Pop an int off the stack. If it is zero, jump to the
label. Otherwise, continue normally.
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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
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invokestatic <method>
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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
javac -g Test1.java
public void count() {
for (int i=0; i<10; i++) {
System.out.println( "i is "+i);
}
}
VM Spec:
http://java.sun.com/docs/books/jvms/second_edition/html/Overview.doc.html
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
39
System.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)>
JAD example
JAD example 3: Test2.java
What does this method do?
//
//
//
//
//
//
//
//
//
//
0 0:iconst_5
1 1:istore_1
2 2:bipush
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
10
JAD example 3: Test2.java
What does this method do?
//
//
//
//
//
//
//
//
//
//
0 0:iconst_5
1 1:istore_1
2 2:bipush
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
Push 5 onto stack
Store into local var 1
10 Push 10 onto stack
Store into local var 1
Load var 2 onto stack
Load var 1 onto stack
Subtract stack vars
Store into local var 1
Load var 3 onto stack
Return (top val from 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
// 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
}
10
References
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http://ocw.mit.edu/NR/rdonlyres/Special-Programs/SP-772Spring-2005Summer-2005/3AFB411F-B5C0-4032-9964-BF733EEB3199/0/bytecode.pdf
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http://www.cs.virginia.edu/evans, Bytecode, Lecture 18
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