Java Bytecode What 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?

The way other languages work (C, ADA, etc…) Source code is written Source code is ported to every different platform machine code (or binaries) Source code is compiled into platform specific machine code (or binaries) Binaries execute on a single platform

Source Code Linux portWindows portMac port Linux Compile Windows Compile Mac Compile Linux binaryWin binaryMac 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 Java Compile Linux JVMWin JVMMac 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 = 0xCAFEBABE

Referencing Memory iload Pushes (loads) the int in local variable (1 bytes) on the stack istore Pops the int on the top of the stack and stores it in local variable 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 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 OpcodeMnemonicDescription 0nopDoes nothing 1aconst_null Push null on the stack 3iconst_0Push int 0 on the stack 4iconst_1Push int 1 on the stack …

Java Bytecode Explanation OpcodeMnemonicDescription 18ldc 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 OpcodeMnemonicDescription 96iaddPops two integers from the stack and pushes their sum iconst_2 iconst_3 iadd

Java Bytecode Arithmetic OpcodeMnemonicDescription 96iadd Pops two integers from the stack and pushes their sum 97ladd Pops two long integers from the stack and pushes their sum … 106fmul Pops two floats from the stack and pushes their product … 119dneg 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 Invokes the 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

Control Flow ifeq Pop an int off the stack. If it is zero, jump to the label. Otherwise, continue normally. if_icmple 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 Invokes a static (class) 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:

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 39 System.out.println((new StringBuilder()).append("i is ").append(i).toString()); // 5 8:getstatic #2 // 6 11:new #3 // 7 14:dup // 8 15:invokespecial #4 // 9 18:ldc1 #5 // 10 20:invokevirtual #6 // 11 23:iload_1 // 12 24:invokevirtual #7 // 13 27:invokevirtual #8 // 14 30:invokevirtual #9 // 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 stack Store into local var 1 Load var 2 onto stack Load var 3 onto stack Load var 1 onto stack Subtract 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 Summer-2005/3AFB411F-B5C BF733EEB3199/0/bytecode.pdf Summer-2005/3AFB411F-B5C BF733EEB3199/0/bytecode.pdf Bytecode, Lecture 18