【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
      - 2.2 执行javap
      - 2.3 jclasslib插件查看
    - 三、对slot的理解
    - - 3.1 说明
      - 3.2 slot索引图
      - 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
    - 四、slot的重复利用
    - - 4.1 说明
      - 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."":()V
   #2 = Class              #27            // com/learning/stack/local_variables/LocalVariableTest
   #3 = Methodref          #2.#26         // com/learning/stack/local_variables/LocalVariableTest."":()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               
  #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  服务器托管网             localVariableTest
  #21 = Utf8               num
  #22 = Utf8               I
  #23 = Utf8               print
服务器托管网  #24 = Utf8               SourceFile
  #25 = Utf8               LocalVariableTest.java
  #26 = NameAndType        #9:#10         // "":()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."":()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 "":()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"