带你手撕Java虚拟机栈（手把手实战）

Java虚拟机栈

Java方法执行的内存模型

一个栈帧就是对应一个方法被执行的过程，入栈、出栈分别对应方法加载、执行结束

让我们来个????
首先，创建一个类

把它编译为class，再把它javap反编译回来，脱裤子放屁就为了看两行字节码，也是不容易

qiyankaideMacBook-Pro:study qiyankai$ javap -verbose Demo.class 
Classfile /Users/qiyankai/Desktop/project/foodie-dev/foodie-dev-study/target/classes/study/Demo.class
  Last modified 2020-5-26; size 405 bytes
  MD5 checksum e3064bf4c80f943fa3031fe07f62d00e
  Compiled from "Demo.java"
public class study.Demo
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #3.#20         // java/lang/Object."<init>":()V
   #2 = Class              #21            // study/Demo
   #3 = Class              #22            // java/lang/Object
   #4 = Utf8               <init>
   #5 = Utf8               ()V
   #6 = Utf8               Code
   #7 = Utf8               LineNumberTable
   #8 = Utf8               LocalVariableTable
   #9 = Utf8               this
  #10 = Utf8               Lstudy/Demo;
  #11 = Utf8               add
  #12 = Utf8               (II)I
  #13 = Utf8               a
  #14 = Utf8               I
  #15 = Utf8               b
  #16 = Utf8               c
  #17 = Utf8               MethodParameters
  #18 = Utf8               SourceFile
  #19 = Utf8               Demo.java
  #20 = NameAndType        #4:#5          // "<init>":()V
  #21 = Utf8               study/Demo
  #22 = Utf8               java/lang/Object
{
  public study.Demo();
    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 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lstudy/Demo;

  public static int add(int, int);
    descriptor: (II)I
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=2
         0: iconst_0
         1: istore_2
         2: iload_0
         3: iload_1
         4: iadd
         5: istore_2
         6: iload_2
         7: ireturn
      LineNumberTable:
        line 5: 0
        line 6: 2
        line 7: 6
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       8     0     a   I
            0       8     1     b   I
            2       6     2     c   I
    MethodParameters:
      Name                           Flags
      a
      b
}
SourceFile: "Demo.java"

javap -verbose 口语化反编译

  public static int add(int, int);
    descriptor: (II)I
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=2
         0: iconst_0
         1: istore_2
         2: iload_0
         3: iload_1
         4: iadd
         5: istore_2
         6: iload_2
         7: ireturn
      LineNumberTable:
        line 5: 0
        line 6: 2
        line 7: 6
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       8     0     a   I
            0       8     1     b   I
            2       6     2     c   I
    MethodParameters:
      Name                           Flags
      a
      b

执行add(1,2)
我的图有点模糊，凑活看吧

1. iconst_0：1,2进入局部变量表，操作数栈0，程序计数器0
2. istore_2：pop操作数栈，放到局部变量表第2位（从0开始算），程序计数器1
3. iload_0：加载局部变量表0位到，操作数栈
4. iload_1：加载局部变量表1位到，操作数栈
5. iadd：操作数栈值相加
6. istore_2：pop操作数栈，放到局部变量表第2位
7. iload_2：加载局部变量表2位到，操作数栈
8. ireturn：返回操作数栈的值

我真的尽力了，我觉得聪明的大家肯定能听懂！

下来我们讲讲递归吧

递归为什么会引发java.lang.*Error异常

因为执行一个方法，会增加一个栈帧，保存当前栈帧，一致一直增加
压栈、压栈、压栈、压栈、boom！