【jvm】虚拟机栈之局部变量表

- - - 一、说明
    - 二、代码分析
    - - [2.1 代码示例](#2.1 代码示例)
      - [2.2 执行javap](#2.2 执行javap)
      - [2.3 jclasslib插件查看](#2.3 jclasslib插件查看)
    - 三、对slot的理解
    - - [3.1 说明](#3.1 说明)
      - [3.2 slot索引图](#3.2 slot索引图)
      - [3.3 实例方法的局部变量表](#3.3 实例方法的局部变量表)
      - [3.4 long和double类型变量占2个slot](#3.4 long和double类型变量占2个slot)
    - 四、slot的重复利用
    - - [4.1 说明](#4.1 说明)
      - [4.2 变量c复用变量b的槽位](#4.2 变量c复用变量b的槽位)
    - 五、静态变量与局部变量对比

一、说明

1.Local Variables
2.局部变量表也被称为局部变量组或本地变量表
3.定义为一个数字数组，主要用于存储方法参数和定义在方法体内的局部变量，这些数据类型包括各类基本数据类型、对象引用（reference）,以及returnAddress类型
4.由于局部变量表是建立在线程的栈上，是线程的私有数据，因此不存在数据安全问题
5.局部变量表所需的容量大小是在编译期确定下来的，并保存在方法的Code属性的maximum local variables数据项中。在方法运行期间是不会改边局部变量表的大小的，因为在java代码编译后就已经确定了
6.方法嵌套调用的次数由栈的大小决定。一般来说，栈越大，方法嵌套调用次数越多。对一个函数而言，它的参数和局部变量越多，使得局部变量表膨胀，它的栈帧就越大，以满足方法调用所需传递的信息增大的需求，函数调用就会占用更多的栈空间，导致其嵌套调用次数就会减少
7.局部变量表中的变量只在当前方法调用中有效。在方法执行时，虚拟机通过使用局部变量表完成参数值到参数变量列表的传递过程。当方法调用结束后，随着方法栈帧的销毁，局部变量表也随之销毁

二、代码分析

2.1 代码示例

package com.learning.stack.local_variables;

/**
 * @Author wangyouhui
 * @Description 局部变量表测试
 **/
public class LocalVariableTest {
    public static void main(String[] args) {
        LocalVariableTest localVariableTest = new LocalVariableTest();
        int num = 10;
        localVariableTest.print();
    }

    private void print() {
        System.out.println("局部变量表");
    }
}

2.2 执行javap

1.执行javap -v StackOverflowErrorLearning.class

Classfile /F:/jdk-learning/jvm/target/classes/com/learning/stack/local_variables/LocalVariableTest.class
Last modified 2023-10-16; size 785 bytes
MD5 checksum e16aaaceec49515090d5cb15e4c6467a
Compiled from "LocalVariableTest.java"
public class com.learning.stack.local_variables.LocalVariableTest
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #8.#26 // java/lang/Object."<init>":()V
#2 = Class #27 // com/learning/stack/local_variables/LocalVariableTest
#3 = Methodref #2.#26 // com/learning/stack/local_variables/LocalVariableTest."<init>":()V
#4 = Methodref #2.#28 // com/learning/stack/local_variables/LocalVariableTest.print:()V
#5 = Fieldref #29.#30 // java/lang/System.out:Ljava/io/PrintStream;
#6 = String #31 // 局部变量表
#7 = Methodref #32.#33 // java/io/PrintStream.println:(Ljava/lang/String;)V
#8 = Class #34 // java/lang/Object
#9 = Utf8 <init>
#10 = Utf8 ()V
#11 = Utf8 Code
#12 = Utf8 LineNumberTable
#13 = Utf8 LocalVariableTable
#14 = Utf8 this
#15 = Utf8 Lcom/learning/stack/local_variables/LocalVariableTest;
#16 = Utf8 main
#17 = Utf8 ([Ljava/lang/String;)V
#18 = Utf8 args
#19 = Utf8 [Ljava/lang/String;
#20 = Utf8 localVariableTest
#21 = Utf8 num
#22 = Utf8 I
#23 = Utf8 print
#24 = Utf8 SourceFile
#25 = Utf8 LocalVariableTest.java
#26 = NameAndType #9:#10 // "<init>":()V
#27 = Utf8 com/learning/stack/local_variables/LocalVariableTest
#28 = NameAndType #23:#10 // print:()V
#29 = Class #35 // java/lang/System
#30 = NameAndType #36:#37 // out:Ljava/io/PrintStream;
#31 = Utf8 局部变量表
#32 = Class #38 // java/io/PrintStream
#33 = NameAndType #39:#40 // println:(Ljava/lang/String;)V
#34 = Utf8 java/lang/Object
#35 = Utf8 java/lang/System
#36 = Utf8 out
#37 = Utf8 Ljava/io/PrintStream;
#38 = Utf8 java/io/PrintStream
#39 = Utf8 println
#40 = Utf8 (Ljava/lang/String;)V
{
public com.learning.stack.local_variables.LocalVariableTest();
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 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/learning/stack/local_variables/LocalVariableTest;
```
public static void main(java.lang.String[]);
  descriptor: ([Ljava/lang/String;)V
  flags: ACC_PUBLIC, ACC_STATIC
  Code:
    stack=2, locals=3, args_size=1
       0: new           #2                  // class com/learning/stack/local_variables/LocalVariableTest
       3: dup
       4: invokespecial #3                  // Method "<init>":()V
       7: astore_1
       8: bipush        10
      10: istore_2
      11: aload_1
      12: invokespecial #4                  // Method print:()V
      15: return
    LineNumberTable:
      line 9: 0
      line 10: 8
      line 11: 11
      line 12: 15
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      16     0  args   [Ljava/lang/String;
          8       8     1 localVariableTest   Lcom/learning/stack/local_variables/LocalVariableTest;
         11       5     2   num   I
```
}
SourceFile: "LocalVariableTest.java"
2.图示说明