【jvm】虚拟机栈之局部变量表

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。10. 当一个实例方法被调用的时候，它的方法参数和方法体内部定义的局部变量将会按照顺序被复制到局部变量表中的每一个slot上。2.如果一个局部变量过了其作用域，在其作用域之后申明的新的局部变量有可能会复用过期的局部变量的槽位，

王佑辉

1117人浏览 · 2023-10-18 22:36:04

王佑辉 · 2023-10-18 22:36:04 发布

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"