字节码指令

灰太狼 2024-03-26 06:56 78阅读 0赞

**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** ###

![8d9873102086ac408144237bc2c76297.png][]

### **4）方法字节码载入方法区** ###

![8c092bde4ef3383bbd76e6d5b3df5dd1.png][]

### **5）main 线程开始运行，分配栈帧内存**  ###

（stack=2，locals=4）

![6b2b2e64245d68487ddec6ac55c0542f.png][]

### **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

![305255c8ddf17bbc636f3aee796ffc50.png][]

#### **istore\_1**  ####

将操作数栈顶数据弹出，存入局部变量表的 slot 1

![a749c8fb542986b2932b7ea5f9999cdc.png][]

#### **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

![4f8dbe6ecb8b7dc0abda1bf84d3ece00.png][]

#### **istore\_2** ####

![d89edf24900c8e9968cd4869c6964d49.png][]

**iload\_1**

![7aadf24e6f41bd5f4503bf1dbc0f5341.png][]

#### **iload\_2** ####

![380c5ba5f910eb506b850252551d5a98.png][]

#### **iadd** ####

![727fb35093847337e113934214282890.png][]

![775a653e4200ce775f83d539b8eb7037.png][]

#### **istore\_3** ####

![7e753a7796e79727057cec286404b3a1.png][]

#### **getstatic \#4** ####

![163f8691be15f1e7772b5d1ff2953dee.png][]

![ec4226afb3d1535526890c47f3ed124c.png][]

#### **iload\_3** ####

![c9d768648a5a73e7a113f3c6e850cfee.png][]

#### **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

![d5c7c0e5acbccdeee3fc8aaf108a6213.png][]

执行完毕，弹出栈帧

清除 main 操作数栈内容

![e57969b7682c415151fc9bdc1407a1cd.png][]

#### **return**  ####

完成 main 方法调用，弹出 main 栈帧

程序结束

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