[JVM]了断局: 局部变量表和操作数栈实例分析

Dear 丶 2022-12-01 11:46 192阅读 0赞

# 一.前言 #

本文以两段代码示例来解释说明,JVM在执行类中的方法时,\[局部变量表\]和\[操作数栈\]是如何配合工作的.

# 示例一:  #

## 1.代码 ##

package com.classloading;
    
    public class Circumference {
    
        public static void main(String[] args) {
            Circumference.circumference(10) ;
        }
    
        public static double circumference(float r) {
            float pi = 3.14f;
            float area = 2 * pi * r;
            return area;
        }
    }

## 2.字节码 ##

## 3.解析. ##

我们主要关注circumference 方法,该方法对应的字节码片段如下:

bogon:classloading sysadmin$ javap -v Circumference.class 
    Classfile /workspace/java-basis/target/classes/com/classloading/Circumference.class
      Last modified 2020-8-26; size 585 bytes
      MD5 checksum 848df06176b0272e2c0fd4426a8135f5
      Compiled from "Circumference.java"
    public class com.classloading.Circumference
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #6.#26         // java/lang/Object."<init>":()V
       #2 = Float              10.0f
       #3 = Methodref          #5.#27         // com/classloading/Circumference.circumference:(F)D
       #4 = Float              3.14f
       #5 = Class              #28            // com/classloading/Circumference
       #6 = Class              #29            // java/lang/Object
       #7 = Utf8               <init>
       #8 = Utf8               ()V
       #9 = Utf8               Code
      #10 = Utf8               LineNumberTable
      #11 = Utf8               LocalVariableTable
      #12 = Utf8               this
      #13 = Utf8               Lcom/classloading/Circumference;
      #14 = Utf8               main
      #15 = Utf8               ([Ljava/lang/String;)V
      #16 = Utf8               args
      #17 = Utf8               [Ljava/lang/String;
      #18 = Utf8               circumference
      #19 = Utf8               (F)D
      #20 = Utf8               r
      #21 = Utf8               F
      #22 = Utf8               pi
      #23 = Utf8               area
      #24 = Utf8               SourceFile
      #25 = Utf8               Circumference.java
      #26 = NameAndType        #7:#8          // "<init>":()V
      #27 = NameAndType        #18:#19        // circumference:(F)D
      #28 = Utf8               com/classloading/Circumference
      #29 = Utf8               java/lang/Object
    {
      public com.classloading.Circumference();
        #描述符: 无入参, 返回值为空
        descriptor: ()V
        #访问权限:  public
        flags: ACC_PUBLIC
        #代码
        Code:
          #栈深度为1, 本地变量为1个, 入参为1个.
          stack=1, locals=1, args_size=1
             # 将第一个引用类型本地变量推送至栈顶
             0: aload_0
             #调用超类构建方法, 实例初始化方法, 私有方法
             1: invokespecial #1                  // Method java/lang/Object."<init>":()V
             # 从当前方法返回void
             4: return
          LineNumberTable:
            line 3: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   Lcom/classloading/Circumference;
      
      # main 方法入口
      public static void main(java.lang.String[]);
        #描述符: 方法入参是一个 String类型的数组, 返回值为空
        descriptor: ([Ljava/lang/String;)V
        #访问权限:  public  static 方法
        flags: ACC_PUBLIC, ACC_STATIC
        #代码
        Code:
          #栈深度为2, 本地变量为1个, 入参为1个.
          stack=2, locals=1, args_size=1
             # 将float类型常量值[坐标 #2 10.0f]从常量池中推送至栈顶
             0: ldc           #2                  // float 10.0f
             # 调用静态方法 [ 常量池 坐标 #3 Method circumference:(F)D  ]
             2: invokestatic  #3                  // Method circumference:(F)D
             # 将栈顶的一个(对于非long或double类型)或两个数值(对于非long或double的其他类型)弹出
             5: pop2
             # 从当前方法返回void
             6: return
          LineNumberTable:
            line 6: 0
            line 7: 6
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       7     0  args   [Ljava/lang/String;
    
      #circumference 方法
      public static double circumference(float);
        #描述符 : 入参为 float 类型, 返回值为Double类型
        descriptor: (F)D
        #访问权限:  public  static 方法
        flags: ACC_PUBLIC, ACC_STATIC
        #代码
        Code:
          #栈深度为2, 本地变量有三个, 输入参数1个
          stack=2, locals=3, args_size=1
             # 将float类型常量值[坐标  #4  3.14f ]从常量池中推送至栈顶.
             0: ldc           #4                  // float 3.14f
             # 将栈顶float型数值存入第二个本地变量
             2: fstore_1
             # 将float型2推送至栈顶
             3: fconst_2 
             # 将第二个float型本地变量推送至栈顶
             4: fload_1
             # 将栈顶两float型数值相乘并将结果压入栈顶
             5: fmul
             # 将第一个float型本地变量推送至栈顶
             6: fload_0
             # 将栈顶两float型数值相乘并将结果压入栈顶
             7: fmul
             # 将栈顶float型数值存入第三个本地变量
             8: fstore_2
             # 将第三个float型本地变量推送至栈顶
             9: fload_2
             # 将栈顶float型数值强制转换为double型数值并将结果压入栈顶
            10: f2d
            #  从当前方法返回double
            11: dreturn
          LineNumberTable:
            line 10: 0
            line 11: 3
            line 12: 9
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      12     0     r   F
                3       9     1    pi   F
                9       3     2  area   F

指令含义可以通过这篇文章来进行查询: [\[JVM\]了断局: 虚拟机字节码指令表速查][JVM_]

## 4.画图演示 ##

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3poYW5nbG9uZ180NDQ0_size_16_color_FFFFFF_t_70][]

# 示例二:   1.代码 #

package com.classloading;
    
    public class Calc {
    
        public static void main(String[] args) {
            new Calc().calc();
        }
    
        public int calc() {
            int a = 100;
            int b = 200;
            int c = 300;
            return (a + b) * c;
        }
    
    }

##    2.字节码 ##

##    3.解析     ##

bogon:classloading sysadmin$ javap -v Calc.class 
    Classfile /workspace/java-basis/target/classes/com/classloading/Calc.class
      Last modified 2020-8-26; size 563 bytes
      MD5 checksum bfe5bf36c453f085c6dd0a44c7d7e2ee
      Compiled from "Calc.java"
    public class com.classloading.Calc
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #5.#25         // java/lang/Object."<init>":()V
       #2 = Class              #26            // com/classloading/Calc
       #3 = Methodref          #2.#25         // com/classloading/Calc."<init>":()V
       #4 = Methodref          #2.#27         // com/classloading/Calc.calc:()I
       #5 = Class              #28            // java/lang/Object
       #6 = Utf8               <init>
       #7 = Utf8               ()V
       #8 = Utf8               Code
       #9 = Utf8               LineNumberTable
      #10 = Utf8               LocalVariableTable
      #11 = Utf8               this
      #12 = Utf8               Lcom/classloading/Calc;
      #13 = Utf8               main
      #14 = Utf8               ([Ljava/lang/String;)V
      #15 = Utf8               args
      #16 = Utf8               [Ljava/lang/String;
      #17 = Utf8               calc
      #18 = Utf8               ()I
      #19 = Utf8               a
      #20 = Utf8               I
      #21 = Utf8               b
      #22 = Utf8               c
      #23 = Utf8               SourceFile
      #24 = Utf8               Calc.java
      #25 = NameAndType        #6:#7          // "<init>":()V
      #26 = Utf8               com/classloading/Calc
      #27 = NameAndType        #17:#18        // calc:()I
      #28 = Utf8               java/lang/Object
    {
      public com.classloading.Calc();
        #描述符: 无入参, 返回值为空
        descriptor: ()V
        #访问权限:  public
        flags: ACC_PUBLIC
        #代码
        Code:
          #栈深度为1, 本地变量为1个, 入参为1个.
          stack=1, locals=1, args_size=1
             # 将第一个引用类型本地变量推送至栈顶
             0: aload_0
             #调用超类构建方法, 实例初始化方法, 私有方法
             1: invokespecial #1                  // Method java/lang/Object."<init>":()V
             # 从当前方法返回void
             4: return
          LineNumberTable:
            line 3: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   Lcom/classloading/Calc;
      # main 方法入口
      public static void main(java.lang.String[]);
        描述符: 方法入参是一个 String类型的数组, 返回值为空
        descriptor: ([Ljava/lang/String;)V
        #访问权限:  public  static 方法
        flags: ACC_PUBLIC, ACC_STATIC
        #代码
        Code:
          #栈深度为2, 本地变量为1个, 入参为1个.
          stack=2, locals=1, args_size=1
             # 创建一个对象, 并将其引用引用值压入栈顶
             0: new           #2                  // class com/classloading/Calc
             # 复制栈顶数值并将复制值压入栈顶
             3: dup
             #调用超类构建方法, 实例初始化方法, 私有方法
             4: invokespecial #3                  // Method "<init>":()V
             #调用实例方法
             7: invokevirtual #4                  // Method calc:()I
             # 将栈顶数值弹出(数值不能是long或double类型的)
            10: pop
            #从当前方法返回void
            11: return
          LineNumberTable:
            line 6: 0
            line 7: 11
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      12     0  args   [Ljava/lang/String;
    
      public int calc();
        描述符: 无入参, 返回值为Int类型
        descriptor: ()I
        #访问权限:  public
        flags: ACC_PUBLIC
        #代码
        Code:
          #栈深度为2, 本地变量有四个, 输入参数1个
          stack=2, locals=4, args_size=1
             # 将单字节的常量值 [100] 推送至栈顶
             0: bipush        100
             # 将栈顶int型数值存入第二个本地变量
             2: istore_1
             # 将一个短整型常量200推送至栈顶
             3: sipush        200
             # 将栈顶int型数值存入第三个本地变量
             6: istore_2
             # 将一个短整型常量300推送至栈顶
             7: sipush        300
             # 将栈顶int型数值存入第四个本地变量
            10: istore_3
            # 将第二个int型本地变量推送至栈顶
            11: iload_1
            # 将第三个int型本地变量推送至栈顶
            12: iload_2
            # 将栈顶两int型数值相加并将结果压入栈顶
            13: iadd
            # 将第四个int型本地变量推送至栈顶
            14: iload_3
            # 将栈顶两int型数值相乘并将结果压入栈顶
            15: imul
            # 从当前方法返回int
            16: ireturn
          LineNumberTable:
            line 10: 0
            line 11: 3
            line 12: 7
            line 13: 11
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      17     0  this   Lcom/classloading/Calc;
                3      14     1     a   I
                7      10     2     b   I
               11       6     3     c   I
    }
    SourceFile: "Calc.java"
    bogon:classloading sysadmin$

##    4.画图演示 ##

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3poYW5nbG9uZ180NDQ0_size_16_color_FFFFFF_t_70 1][]

[JVM_]: https://zhangboyi.blog.csdn.net/article/details/91434714
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3poYW5nbG9uZ180NDQ0_size_16_color_FFFFFF_t_70]: /images/20221123/e7d1960a02be4fe4b3f5754bc18088a9.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3poYW5nbG9uZ180NDQ0_size_16_color_FFFFFF_t_70 1]: /images/20221123/18c1e5309c094615837ee5f9f37300b3.png