一起养成写作习惯！这是我参与「掘金日新计划 · 4 月更文挑战」的第14天，点击查看活动详情。
字节码指令
（1）入门
两组字节码指令，一个是
public cn.itcast.jvm.t5.HelloWorld(); //构造方法的字节码指令
2a b7 00 01 b1
- 2a => aload_0 加载 slot 0 的局部变量，即 this，做为下面的 invokespecial 构造方法调用的参数
- b7 => invokespecial 预备调用构造方法，哪个方法呢？
- 00 01 引用常量池中 #1 项，即【 Method java/lang/Object."":()V 】
- b1 表示返回
另一个是
public static void main(java.lang.String[]); //主方法的字节码指令
b2 00 02 12 03 b6 00 04 b1
- b2 => getstatic 用来加载静态变量，哪个静态变量呢？
- 00 02 引用常量池中 #2 项，即【Field java/lang/System.out:Ljava/io/PrintStream;】
- 12 => ldc 加载参数，哪个参数呢？
- 03 引用常量池中 #3 项，即 【String hello world】
- b6 => invokevirtual 预备调用成员方法，哪个方法呢？
- 00 04 引用常量池中 #4 项，即【Method java/io/PrintStream.println:(Ljava/lang/String;)V】
- b1 表示返回
请参考 docs.oracle.com/javase/spec…
（2）javap工具
自己分析类文件结构太麻烦了，Oracle 提供了 javap 工具来反编译 class 文件
[root@localhost ~]# javap -v HelloWorld.class
Classfile /root/HelloWorld.class
Last modified Jul 7, 2019; size 597 bytes
MD5 checksum 361dca1c3f4ae38644a9cd5060ac6dbc
Compiled from "HelloWorld.java"
public class cn.itcast.jvm.t5.HelloWorld
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
- #1 = Methodref #6.#21 // java/lang/Object."":()V
- #2 = Fieldref #22.#23 //java/lang/System.out:Ljava/io/PrintStream;
- #3 = String #24 // hello world
- #4 = Methodref #25.#26 // java/io/PrintStream.println:(Ljava/lang/String;)V
- #5 = Class #27 // cn/itcast/jvm/t5/HelloWorld
- #6 = Class #28 // java/lang/Object
- #7 = Utf8
- #8 = Utf8 ()V
- #9 = Utf8 Code
- #10 = Utf8 LineNumberTable
- #11 = Utf8 LocalVariableTable
- #12 = Utf8 this
- #13 = Utf8 Lcn/itcast/jvm/t5/HelloWorld;
- #14 = Utf8 main
- #15 = Utf8 ([Ljava/lang/String;)V
- #16 = Utf8 args
- #17 = Utf8 [Ljava/lang/String;
- #18 = Utf8 MethodParameters
- #19 = Utf8 SourceFile
- #20 = Utf8 HelloWorld.java
- #21 = NameAndType #7:#8 // "":()V
- #22 = Class #29 // java/lang/System
- #23 = NameAndType #30:#31 // out:Ljava/io/PrintStream;
- #24 = Utf8 hello world
- #25 = Class #32 // java/io/PrintStream
- #26 = NameAndType #33:#34 // println:(Ljava/lang/String;)V
- #27 = Utf8 cn/itcast/jvm/t5/HelloWorld
- #28 = Utf8 java/lang/Object
- #29 = Utf8 java/lang/System
- #30 = Utf8 out
- #31 = Utf8 Ljava/io/PrintStream;
- #32 = Utf8 java/io/PrintStream
- #33 = Utf8 println
- #34 = Utf8 (Ljava/lang/String;)V
{
public cn.itcast.jvm.t5.HelloWorld();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."
<init>":()V
4: return
LineNumberTable:
line 4: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcn/itcast/jvm/t5/HelloWorld;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #2 // Field
java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3 // String hello world
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 args [Ljava/lang/String;
MethodParameters:
Name Flags
args
}
