字节码指令

目录

2.1 入门

2.2 javap 工具

2.3 图解方法执行流程

1）原始 java 代码

2）编译后的字节码文件

3）常量池载入运行时常量池

4）方法字节码载入方法区

5）main 线程开始运行，分配栈帧内存

6）执行引擎开始执行字节码

bipush 10

istore_1

ldc #3

istore_2

iload_2

iadd

istore_3

getstatic #4

iload_3

invokevirtual #5

return

---

2.1 入门

接着上一节，研究一下两组字节码指令，一个是

//构造方法的字节码指令

public cn.itcast.jvm.t5.HelloWorld();

2a b7 00 01 b1

1. 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
2. b7 => invokespecial 预备调用构造方法，哪个方法呢？
3. 00 01 引用常量池中 #1 项，即【 Method java/lang/Object."":()V 】
4. 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

1. b2 => getstatic 用来加载静态变量，哪个静态变量呢？
2. 00 02 引用常量池中 #2 项，即【Field java/lang/System.out:Ljava/io/PrintStream;】
3. 12 => ldc 加载参数，哪个参数呢？
4. 03 引用常量池中 #3 项，即 【String hello world】
5. b6 => invokevirtual 预备调用成员方法，哪个方法呢？
6. 00 04 引用常量池中 #4 项，即【Method java/io/PrintStream.println:(Ljava/lang/String;)V】
7. b1 表示返回

请参考

https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html#jvms-6.5

2.2 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."<init>":()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 <init>

#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 // "<init>":()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

}

2.3 图解方法执行流程

1）原始 java 代码

package cn.itcast.jvm.t3.bytecode;
/**
* 演示 字节码指令 和 操作数栈、常量池的关系
*/
public class Demo3_1 {public static void main(String[] args) {int a = 10;int b = Short.MAX_VALUE + 1;int c = a + b;System.out.println(c);}
}

2）编译后的字节码文件

[root@localhost ~]# javap -v Demo3_1.class

Classfile /root/Demo3_1.class

Last modified Jul 7, 2019; size 665 bytes

MD5 checksum a2c29a22421e218d4924d31e6990cfc5

Compiled from "Demo3_1.java"

public class cn.itcast.jvm.t3.bytecode.Demo3_1

minor version: 0

major version: 52

flags: ACC_PUBLIC, ACC_SUPER

Constant pool:

#1 = Methodref #7.#26 // java/lang/Object."<init>":()V

#2 = Class #27 // java/lang/Short

#3 = Integer 32768

#4 = Fieldref #28.#29 //

java/lang/System.out:Ljava/io/PrintStream;

#5 = Methodref #30.#31 // java/io/PrintStream.println:(I)V

#6 = Class #32 // cn/itcast/jvm/t3/bytecode/Demo3_1

#7 = Class #33 // java/lang/Object

#8 = Utf8 <init>

#9 = Utf8 ()V

#10 = Utf8 Code

#11 = Utf8 LineNumberTable

#12 = Utf8 LocalVariableTable

#13 = Utf8 this

#14 = Utf8 Lcn/itcast/jvm/t3/bytecode/Demo3_1;

#15 = Utf8 main

#16 = Utf8 ([Ljava/lang/String;)V

#17 = Utf8 args

#18 = Utf8 [Ljava/lang/String;

#19 = Utf8 a

#20 = Utf8 I

#21 = Utf8 b

#22 = Utf8 c

#23 = Utf8 MethodParameters

#24 = Utf8 SourceFile

#25 = Utf8 Demo3_1.java

#26 = NameAndType #8:#9 // "<init>":()V

#27 = Utf8 java/lang/Short

#28 = Class #34 // java/lang/System

#29 = NameAndType #35:#36 // out:Ljava/io/PrintStream;

#30 = Class #37 // java/io/PrintStream

#31 = NameAndType #38:#39 // println:(I)V

#32 = Utf8 cn/itcast/jvm/t3/bytecode/Demo3_1

#33 = Utf8 java/lang/Object

#34 = Utf8 java/lang/System

#35 = Utf8 out

#36 = Utf8 Ljava/io/PrintStream;

#37 = Utf8 java/io/PrintStream

#38 = Utf8 println

#39 = Utf8 (I)V

{

public cn.itcast.jvm.t3.bytecode.Demo3_1();

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 6: 0

LocalVariableTable:

Start Length Slot Name Signature

0 5 0 this Lcn/itcast/jvm/t3/bytecode/Demo3_1;

public static void main(java.lang.String[]);

descriptor: ([Ljava/lang/String;)V

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=4, args_size=1

0: bipush 10

2: istore_1

3: ldc #3 // int 32768

5: istore_2

6: iload_1

7: iload_2

8: iadd

9: istore_3

10: getstatic #4 // Field

java/lang/System.out:Ljava/io/PrintStream;

13: iload_3

14: invokevirtual #5 // Method

java/io/PrintStream.println:(I)V

17: return

LineNumberTable:

line 8: 0

line 9: 3

line 10: 6

line 11: 10

line 12: 17

LocalVariableTable:

Start Length Slot Name Signature

0 18 0 args [Ljava/lang/String;

3 15 1 a I

6 12 2 b I

10 8 3 c I

MethodParameters:

Name Flags

args

}

3）常量池载入运行时常量池

4）方法字节码载入方法区

5）main 线程开始运行，分配栈帧内存

（stack=2，locals=4）

6）执行引擎开始执行字节码

bipush 10

将一个 byte 压入操作数栈（其长度会补齐 4 个字节），类似的指令还有

sipush 将一个 short 压入操作数栈（其长度会补齐 4 个字节）

ldc 将一个 int 压入操作数栈

ldc2_w 将一个 long 压入操作数栈（分两次压入，因为 long 是 8 个字节）

这里小的数字都是和字节码指令存在一起，超过 short 范围的数字存入了常量池

istore_1

将操作数栈顶数据弹出，存入局部变量表的 slot 1

ldc #3

从常量池加载 #3 数据到操作数栈

注意 Short.MAX_VALUE 是 32767，所以 32768 = Short.MAX_VALUE + 1 实际是在编译期间计算

好的

istore_2

iload_1

iload_2

iadd

istore_3

getstatic #4

iload_3

invokevirtual #5

找到常量池 #5 项

定位到方法区 java/io/PrintStream.println:(I)V 方法

生成新的栈帧（分配 locals、stack等）

传递参数，执行新栈帧中的字节码

执行完毕，弹出栈帧

清除 main 操作数栈内容

return

完成 main 方法调用，弹出 main 栈帧

程序结束