jvm 字节码的执行

javap

简单的字节码执行过程

常用的字节码

使用 ASM 生成 Java 字节码

JIT(及时编译) 及其相关参数

javap class文件的反汇编工具

package javap;

/**
 * @Package Name : ${PACKAG_NAME}
 * @Author : [email protected]
 * @Creation Date : 2018年06月30日下午5:55
 * @Function : todo
 */
public class Calc {
    public int calc() {
        int a = 500;
        int b = 200;
        int c = 50;
        return (a + b) / c;
    }

}

javap –verbose Calc

➜ javap javap -verbose Calc
Warning: File ./Calc.class does not contain class Calc
Classfile /Users/dongfucai/dongpractice/target/classes/javap/Calc.class
Last modified 2018年6月30日; size 393 bytes
MD5 checksum b24b3aa73990f1c1ef0c28bd1643c9c0
Compiled from "Calc.java"
public class javap.Calc
minor version: 0
major version: 52
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #2 // javap/Calc
super_class: #3 // java/lang/Object
interfaces: 0, fields: 0, methods: 2, attributes: 1
Constant pool:
#1 = Methodref #3.#19 // java/lang/Object."<init>":()V
#2 = Class #20 // javap/Calc
#3 = Class #21 // java/lang/Object
#4 = Utf8 <init>
#5 = Utf8 ()V
#6 = Utf8 Code
#7 = Utf8 LineNumberTable
#8 = Utf8 LocalVariableTable
#9 = Utf8 this
#10 = Utf8 Ljavap/Calc;
#11 = Utf8 calc
#12 = Utf8 ()I
#13 = Utf8 a
#14 = Utf8 I
#15 = Utf8 b
#16 = Utf8 c
#17 = Utf8 SourceFile
#18 = Utf8 Calc.java
#19 = NameAndType #4:#5 // "<init>":()V
#20 = Utf8 javap/Calc
#21 = Utf8 java/lang/Object
{
public javap.Calc();
descriptor: ()V
flags: (0x0001) 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 9: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Ljavap/Calc;

public int calc();
descriptor: ()I
flags: (0x0001) ACC_PUBLIC
Code:
stack=2, locals=4, args_size=1
0: sipush 500
3: istore_1
4: sipush 200
7: istore_2
8: bipush 50
10: istore_3
11: iload_1
12: iload_2
13: iadd
14: iload_3
15: idiv
16: ireturn
LineNumberTable:
line 11: 0
line 12: 4
line 13: 8
line 14: 11
LocalVariableTable:
Start Length Slot Name Signature
0 17 0 this Ljavap/Calc;
4 13 1 a I
8 9 2 b I
11 6 3 c I
}
SourceFile: "Calc.java"

------------------

Code:
stack=2, locals=4, args_size=1
0: sipush 500
3: istore_1
4: sipush 200
7: istore_2
8: bipush 50
10: istore_3
11: iload_1
12: iload_2
13: iadd
14: iload_3
15: idiv