JVMJVM04(类加载与字节码-图解方法执行流程)

Posted 2022-02-16 温文艾尔

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1.字节码指令

1.1入门

字节码指令可参考官方文档介绍
官方文档地址

1.2图解方法执行流程

原始java代码

public class Demo02 
    public static void main(String[] args) 
        int a = 10;
        int b = Short.MAX_VALUE + 1;
        int c =a+b;
        System.out.println(c);

通过javap工具反编译后的字节码文件

javap -v Demo02.class

Classfile /F:/2021年12月之后的Idea项目/jvmProject/out/production/jvmProj
ect/com/wql/jvm/ClassInfo/Demo02.class
  Last modified 2022年1月22日; size 624 bytes
  SHA-256 checksum dc35d1e7095f253b0d5f0c8f3209953eba9fa5fc94f05edc6bc90
2db4f70f6ac
  Compiled from "Demo02.java"
public class com.wql.jvm.ClassInfo.Demo02
  minor version: 0
  major version: 52
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #6                          // com/wql/jvm/ClassInfo/Demo0
2
  super_class: #7                         // java/lang/Object
  interfaces: 0, fields: 0, methods: 2, attributes: 1
Constant pool:
   #1 = Methodref          #7.#25         // java/lang/Object."<init>":(
)V
   #2 = Class              #26            // java/lang/Short
   #3 = Integer            32768
   #4 = Fieldref           #27.#28        // java/lang/System.out:Ljava/
io/PrintStream;
   #5 = Methodref          #29.#30        // java/io/PrintStream.println
:(I)V
   #6 = Class              #31            // com/wql/jvm/ClassInfo/Demo0
2
   #7 = Class              #32            // java/lang/Object
   #8 = Utf8               <init>
   #9 = Utf8               ()V
  #10 = Utf8               Code
  #11 = Utf8               LineNumberTable
  #12 = Utf8               LocalVariableTable
  #13 = Utf8               this
  #14 = Utf8               Lcom/wql/jvm/ClassInfo/Demo02;
  #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               SourceFile
  #24 = Utf8               Demo02.java
  #25 = NameAndType        #8:#9          // "<init>":()V
  #26 = Utf8               java/lang/Short
  #27 = Class              #33            // java/lang/System
  #28 = NameAndType        #34:#35        // out:Ljava/io/PrintStream;
  #29 = Class              #36            // java/io/PrintStream
  #30 = NameAndType        #37:#38        // println:(I)V
  #31 = Utf8               com/wql/jvm/ClassInfo/Demo02
  #32 = Utf8               java/lang/Object
  #33 = Utf8               java/lang/System
  #34 = Utf8               out
  #35 = Utf8               Ljava/io/PrintStream;
  #36 = Utf8               java/io/PrintStream
  #37 = Utf8               println
  #38 = Utf8               (I)V

  public com.wql.jvm.ClassInfo.Demo02();
    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   Lcom/wql/jvm/ClassInfo/Demo02;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) 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/PrintStr
eam.println:(I)V
        17: return
      LineNumberTable:
        line 11: 0
        line 12: 3
        line 13: 6
        line 14: 10
        line 15: 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

SourceFile: "Demo02.java"

执行流程

1.2.1常量池载入运行时常量池

这里有一点误解，运行时常量池属于方法区，只是因为它相对比较特殊这里我们单独拎出来看，我们Class文件常量池中的数据，将来都要存储在运行时常量池

一些比较小的数字，比如int a=10;这些跟方法的字节码指令存在一起，而一旦数字的范围超过了整数最大值，他就会存储在常量池中，所以int b = Short.MAX_VALUE + 1;（32768）被存储在运行时常量池中