package org.jvminternals;
public class SimpleClass {
public void sayHello() {
System.out.println("Hello");
}
}
之后你可以通过运行如下的javap命令,获得字节码信息,如:
public class org.jvminternals.SimpleClass
SourceFile: "SimpleClass.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #6.#17 // java/lang/Object."":()V
#2 = Fieldref #18.#19 // java/lang/System.out:Ljava/io/PrintStream;
#3 = String #20 // "Hello"
#4 = Methodref #21.#22 // java/io/PrintStream.println:(Ljava/lang/String;)V
#5 = Class #23 // org/jvminternals/SimpleClass
#6 = Class #24 // java/lang/Object
#7 = Utf8
#8 = Utf8 ()V
#9 = Utf8 Code
#10 = Utf8 LineNumberTable
#11 = Utf8 LocalVariableTable
#12 = Utf8 this
#13 = Utf8 Lorg/jvminternals/SimpleClass;
#14 = Utf8 sayHello
#15 = Utf8 SourceFile
#16 = Utf8 SimpleClass.java
#17 = NameAndType #7:#8 // "":()V
#18 = Class #25 // java/lang/System
#19 = NameAndType #26:#27 // out:Ljava/io/PrintStream;
#20 = Utf8 Hello
#21 = Class #28 // java/io/PrintStream
#22 = NameAndType #29:#30 // println:(Ljava/lang/String;)V
#23 = Utf8 org/jvminternals/SimpleClass
#24 = Utf8 java/lang/Object
#25 = Utf8 java/lang/System
#26 = Utf8 out
#27 = Utf8 Ljava/io/PrintStream;
#28 = Utf8 java/io/PrintStream
#29 = Utf8 println
#30 = Utf8 (Ljava/lang/String;)V
{
public org.jvminternals.SimpleClass();
Signature: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 3: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lorg/jvminternals/SimpleClass;
public void sayHello();
Signature: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3 // String "Hello"
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 6: 0
line 7: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this Lorg/jvminternals/SimpleClass;
}
aload_0
|
此操作码是格式为aload_
对象栈中.
码用于加载数值,而非对象引用。如iload_
l是long,f是float,d是double类型。对于索引值超过3的局部变量,可以使用iload,lload,fload,dload进行
加载。这些操作码都只有一个操作数,用于指定要加载的局部变量的索引。
|
ldc
|
这个指令(操作码)用于将一个运行时常量池中的常量压入操作对象栈中
|
getstatic
|
此指令用于将运行时常量池的静态字段列表中的一个静态值压入到操作对象栈中。
|
invokespecial,
invokevirtual
|
这两个指令属于调用方法的指令组成员之一,调用方法的指令有:invokedynamic,invokestatic,
invokevirtual。在这个类文件中,invokevirtual用于基于对象的类进行方法调用,而invokespecial指令则用
于调用当前类的实例初始化方法,私有方法及父类方法。
|
return
|
此指令时返回型指令组成员之一,其它的还有ireturn,lreturn,freturn,dreturn,areturn及
return。每个指令都是一类返回不同类型数值的返回声明,i是int类型,l是long,f是float,d是
double,a则是对象引用。没有前置类型字符的return则是返回void类型。
|
Object foo = new Object();
0: new #2 // Class java/lang/Object
1: dup
2: invokespecial #3 // Method java/ lang/Object ""( ) V