Java ByteCode

Java ByteCode1，什么是Bytecode C/C++编译器把源代码编译成汇编代码，Java编译器把Java源代码编译成字节码bytecode。 Java跨平台其实就是基于相同的bytecode规范做不同平台的虚拟机，我们的Java程序编译成bytecode后就可以在不同平台跑了。 .net框架有IL(intermediate language)，汇编是C/C++程序的中间表达方式，

liyangbing315

6749人浏览 · 2010-04-11 15:36:00

liyangbing315 · 2010-04-11 15:36:00 发布

Java ByteCode

1，什么是Bytecode
C/C++编译器把源代码编译成汇编代码，Java编译器把Java源代码编译成字节码bytecode。
Java跨平台其实就是基于相同的bytecode规范做不同平台的虚拟机，我们的Java程序编译成bytecode后就可以在不同平台跑了。
.net框架有IL(intermediate language)，汇编是C/C++程序的中间表达方式，而bytecode可以说是Java平台的中间语言。
了解Java字节码知识对debugging、performance tuning以及做一些高级语言扩展或框架很有帮助。

2，使用javap生成Bytecode
JDK自带的javap.exe文件可以反汇编Bytecode，让我们看个例子:
Test.java:

Java代码

public class Test {
public static void main(String[] args) {
int i = 10000;
System.out.println("Hello Bytecode! Number = " + i);
}
}

public class Test {

int i = 10000;

}

编译后的Test.class:

Java代码

漱壕 1 +
<init> ()V Code LineNumberTable main ([Ljava/lang/String;)V
SourceFile Test.java
! " java/lang/StringBuilder Hello Bytecode! Number = # $ # % & ' ( ) * Test java/lang/Object java/lang/System out Ljava/io/PrintStream; append -(Ljava/lang/String;)Ljava/lang/StringBuilder; (I)Ljava/lang/StringBuilder; toString ()Ljava/lang/String; java/io/PrintStream println (Ljava/lang/String;)V !
* > ' < Y

漱壕 1 +

SourceFile Test.java

使用javap -c Test > Test.bytecode生成的Test.bytecode:

Java代码

Compiled from "Test.java"
public class Test extends java.lang.Object{
public Test();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: sipush 10000
3: istore_1
4: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
7: new #3; //class java/lang/StringBuilder
10: dup
11: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
14: ldc #5; //String Hello Bytecode! Number =
16: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: iload_1
20: invokevirtual #7; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
23: invokevirtual #8; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
26: invokevirtual #9; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
29: return
}

Compiled from "Test.java"

public Test();

Code:

0: aload_0

4: return

Code:

0: sipush 10000

3: istore_1

10: dup

19: iload_1

29: return

}

JVM就是一个基于stack的机器，每个thread拥有一个存储着一些frames的JVM stack，每次调用一个方法时生成一个frame。
一个frame包括一个local variables数组(本地变量表)，一个Operand LIFO stack和运行时常量池的一个引用。

我们来简单分析一下生成的字节码指令:
aload和iload指令的“a”前缀和“i”分别表示对象引用和int类型，其他还有“b”表示byte，“c”表示char，“d”表示double等等
我们这里的aload_0表示将把local variable table中index 0的值push到Operand stack，iload_1类似
invokespecial表示初始化对象，return表示返回
sipush表示把10000这个int值push到Operand stack
getstatic表示取静态域
invokevirtual表示调用一些实例方法
这些指令又称为opcode，Java一直以来只有约202個Opcode，具体请参考Java Bytecode规范。

我们看到Test.class文件不全是二进制的指令，有些是我们可以识别的字符，这是因为有些包名、类名和常量字符串没有编译成二进制Bytecode指令。

3，体验字节码增强的魔力
我们J2EE常用的Hibernate、Spring都用到了动态字节码修改来改变类的行为。
让我们通过看看ASM的org.objectweb.asm.MethodWriter类的部分方法来理解ASM是如何修改字节码的:

Java代码

class MethodWriter implements MethodVisitor {
private ByteVector code = new ByteVector();
public void visitIntInsn(final int opcode, final int operand) {
// Label currentBlock = this.currentBlock;
if (currentBlock != null) {
if (compute == FRAMES) {
currentBlock.frame.execute(opcode, operand, null, null);
} else if (opcode != Opcodes.NEWARRAY) {
// updates current and max stack sizes only for NEWARRAY
// (stack size variation = 0 for BIPUSH or SIPUSH)
int size = stackSize + 1;
if (size > maxStackSize) {
maxStackSize = size;
}
stackSize = size;
}
}
// adds the instruction to the bytecode of the method
if (opcode == Opcodes.SIPUSH) {
code.put12(opcode, operand);
} else { // BIPUSH or NEWARRAY
code.put11(opcode, operand);
}
}
public void visitMethodInsn(
final int opcode,
final String owner,
final String name,
final String desc)
{
boolean itf = opcode == Opcodes.INVOKEINTERFACE;
Item i = cw.newMethodItem(owner, name, desc, itf);
int argSize = i.intVal;
// Label currentBlock = this.currentBlock;
if (currentBlock != null) {
if (compute == FRAMES) {
currentBlock.frame.execute(opcode, 0, cw, i);
} else {
/*
* computes the stack size variation. In order not to recompute
* several times this variation for the same Item, we use the
* intVal field of this item to store this variation, once it
* has been computed. More precisely this intVal field stores
* the sizes of the arguments and of the return value
* corresponding to desc.
*/
if (argSize == 0) {
// the above sizes have not been computed yet,
// so we compute them...
argSize = getArgumentsAndReturnSizes(desc);
// ... and we save them in order
// not to recompute them in the future
i.intVal = argSize;
}
int size;
if (opcode == Opcodes.INVOKESTATIC) {
size = stackSize - (argSize >> 2) + (argSize & 0x03) + 1;
} else {
size = stackSize - (argSize >> 2) + (argSize & 0x03);
}
// updates current and max stack sizes
if (size > maxStackSize) {
maxStackSize = size;
}
stackSize = size;
}
}
// adds the instruction to the bytecode of the method
if (itf) {
if (argSize == 0) {
argSize = getArgumentsAndReturnSizes(desc);
i.intVal = argSize;
}
code.put12(Opcodes.INVOKEINTERFACE, i.index).put11(argSize >> 2, 0);
} else {
code.put12(opcode, i.index);
}
}
}