方法区内存结构

不念不忘少年蓝@ 2021-07-24 20:58 465阅读 0赞

# 一 点睛 #

关于方法区的内部结构，先看看下图。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70][]

《深入理解Java虚拟机》书中对方法区（Method Area）存储内容描述如下：它用于存储已被虚拟机加载的类型信息、常量、静态变量、即时编译器编译后的代码缓存等。

![2021071114475752.png][]

# 二 类型信息——用于描述类型 #

对每个加载的类型（类 class、接口 interface、枚举 enum、注解 annotation），JVM 必须在方法区中存储以下类型信息：

*  这个类型的完整有效名称（全名=包名.类名）
 *  这个类型直接父类的完整有效名（对于 interface 或是 java.lang.object，都没有父类）
 *  这个类型的修饰符（public，abstract，final 的某个子集）
 *  这个类型直接接口的一个有序列表

# 三 域信息——用于描述类的字段 #

JVM 必须在方法区中保存类型的所有域的相关信息以及域的声明顺序。

域的相关信息包括：

*  域名称、
 *  域类型、
 *  域修饰符（public，private，protected，static，final，volatile，transient 的某个子集）。

# 四 方法（Method）信息——用于描述类中的方法 #

JVM 必须保存所有方法的以下信息，同域信息一样包括声明顺序。

*  方法名称
 *  方法的返回类型（或void）
 *  方法参数的数量和类型（按顺序）
 *  方法的修饰符（public，private，protected，static，final，synchronized，native，abstract 的一个子集）
 *  方法的字节码（bytecodes）、操作数栈、局部变量表及大小（abstract 和 native 方法除外）
 *  异常表（abstract 和 native 方法除外）。

> 每个异常处理的开始位置、结束位置、代码处理在程序计数器中的偏移地址、被捕获的异常类的常量池索引。

# 五 class 文件说明 #

## 1 代码 ##

/**
    * 测试方法区的内部构成
    */
    public class MethodInnerStrucTest extends Object implements Comparable<String>, Serializable {
        // 属性
        public int num = 10;
        private static String str = "测试方法的内部结构";
    
        // 构造器
        // 方法
        public void test1() {
            int count = 20;
            System.out.println("count = " + count);
        }
    
        public static int test2(int cal) {
            int result = 0;
            try {
                int value = 30;
                result = value / cal;
            } catch (Exception e) {
                e.printStackTrace();
            }
            return result;
        }
    
        @Override
        public int compareTo(String o) {
            return 0;
        }
    }

## 2 class文件解析说明 ##

Classfile /D:/workspace_idea5/JVMDemo/out/production/chapter09/com/atguigu/java/MethodInnerStrucTest.class
      Last modified 2020-4-22; size 1626 bytes
      MD5 checksum 69643a16925bb67a96f54050375c75d0
      Compiled from "MethodInnerStrucTest.java"
    // 类型信息
    public class com.atguigu.java.MethodInnerStrucTest extends java.lang.Object
    implements java.lang.Comparable<java.lang.String>, java.io.Serializable
    
    
      minor version: 0
      major version: 51
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #18.#52        // java/lang/Object."<init>":()V
       #2 = Fieldref           #17.#53        // com/atguigu/java/MethodInnerStrucTest.num:I
       #3 = Fieldref           #54.#55        // java/lang/System.out:Ljava/io/PrintStream;
       #4 = Class              #56            // java/lang/StringBuilder
       #5 = Methodref          #4.#52         // java/lang/StringBuilder."<init>":()V
       #6 = String             #57            // count =
       #7 = Methodref          #4.#58         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
       #8 = Methodref          #4.#59         // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
       #9 = Methodref          #4.#60         // java/lang/StringBuilder.toString:()Ljava/lang/String;
      #10 = Methodref          #61.#62        // java/io/PrintStream.println:(Ljava/lang/String;)V
      #11 = Class              #63            // java/lang/Exception
      #12 = Methodref          #11.#64        // java/lang/Exception.printStackTrace:()V
      #13 = Class              #65            // java/lang/String
      #14 = Methodref          #17.#66        // com/atguigu/java/MethodInnerStrucTest.compareTo:(Ljava/lang/String;)I
      #15 = String             #67            // 测试方法的内部结构
      #16 = Fieldref           #17.#68        // com/atguigu/java/MethodInnerStrucTest.str:Ljava/lang/String;
      #17 = Class              #69            // com/atguigu/java/MethodInnerStrucTest
      #18 = Class              #70            // java/lang/Object
      #19 = Class              #71            // java/lang/Comparable
      #20 = Class              #72            // java/io/Serializable
      #21 = Utf8               num
      #22 = Utf8               I
      #23 = Utf8               str
      #24 = Utf8               Ljava/lang/String;
      #25 = Utf8               <init>
      #26 = Utf8               ()V
      #27 = Utf8               Code
      #28 = Utf8               LineNumberTable
      #29 = Utf8               LocalVariableTable
      #30 = Utf8               this
      #31 = Utf8               Lcom/atguigu/java/MethodInnerStrucTest;
      #32 = Utf8               test1
      #33 = Utf8               count
      #34 = Utf8               test2
      #35 = Utf8               (I)I
      #36 = Utf8               value
      #37 = Utf8               e
      #38 = Utf8               Ljava/lang/Exception;
      #39 = Utf8               cal
      #40 = Utf8               result
      #41 = Utf8               StackMapTable
      #42 = Class              #63            // java/lang/Exception
      #43 = Utf8               compareTo
      #44 = Utf8               (Ljava/lang/String;)I
      #45 = Utf8               o
      #46 = Utf8               (Ljava/lang/Object;)I
      #47 = Utf8               <clinit>
      #48 = Utf8               Signature
      #49 = Utf8               Ljava/lang/Object;Ljava/lang/Comparable<Ljava/lang/String;>;Ljava/io/Serializable;
      #50 = Utf8               SourceFile
      #51 = Utf8               MethodInnerStrucTest.java
      #52 = NameAndType        #25:#26        // "<init>":()V
      #53 = NameAndType        #21:#22        // num:I
      #54 = Class              #73            // java/lang/System
      #55 = NameAndType        #74:#75        // out:Ljava/io/PrintStream;
      #56 = Utf8               java/lang/StringBuilder
      #57 = Utf8               count =
      #58 = NameAndType        #76:#77        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      #59 = NameAndType        #76:#78        // append:(I)Ljava/lang/StringBuilder;
      #60 = NameAndType        #79:#80        // toString:()Ljava/lang/String;
      #61 = Class              #81            // java/io/PrintStream
      #62 = NameAndType        #82:#83        // println:(Ljava/lang/String;)V
      #63 = Utf8               java/lang/Exception
      #64 = NameAndType        #84:#26        // printStackTrace:()V
      #65 = Utf8               java/lang/String
      #66 = NameAndType        #43:#44        // compareTo:(Ljava/lang/String;)I
      #67 = Utf8               测试方法的内部结构
      #68 = NameAndType        #23:#24        // str:Ljava/lang/String;
      #69 = Utf8               com/atguigu/java/MethodInnerStrucTest
      #70 = Utf8               java/lang/Object
      #71 = Utf8               java/lang/Comparable
      #72 = Utf8               java/io/Serializable
      #73 = Utf8               java/lang/System
      #74 = Utf8               out
      #75 = Utf8               Ljava/io/PrintStream;
      #76 = Utf8               append
      #77 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
      #78 = Utf8               (I)Ljava/lang/StringBuilder;
      #79 = Utf8               toString
      #80 = Utf8               ()Ljava/lang/String;
      #81 = Utf8               java/io/PrintStream
      #82 = Utf8               println
      #83 = Utf8               (Ljava/lang/String;)V
      #84 = Utf8               printStackTrace
    {
      // 域信息
      public int num;
        descriptor: I
        flags: ACC_PUBLIC
    
    
      private static java.lang.String str;
        descriptor: Ljava/lang/String;
        flags: ACC_PRIVATE, ACC_STATIC
    
    
      // 方法信息
      // 构造器，自动生成
      public com.atguigu.java.MethodInnerStrucTest();
        descriptor: ()V
        flags: ACC_PUBLIC
        Code:
          stack=2, locals=1, args_size=1
             0: aload_0
             1: invokespecial #1                  // Method java/lang/Object."<init>":()V
             4: aload_0
             5: bipush        10
             7: putfield      #2                  // Field num:I
            10: return
          LineNumberTable:
            line 10: 0
            line 12: 4
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      11     0  this   Lcom/atguigu/java/MethodInnerStrucTest;
    
    
      public void test1();
        descriptor: ()V
        flags: ACC_PUBLIC
        Code:
          stack=3, locals=2, args_size=1
             0: bipush        20
             2: istore_1
             3: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
             6: new           #4                  // class java/lang/StringBuilder
             9: dup
            10: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
            13: ldc           #6                  // String count =
            15: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
            18: iload_1
            19: invokevirtual #8                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
            22: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
            25: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
            28: return
          LineNumberTable:
            line 17: 0
            line 18: 3
            line 19: 28
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      29     0  this   Lcom/atguigu/java/MethodInnerStrucTest;
                3      26     1 count   I
    
    
      public static int test2(int);
        descriptor: (I)I
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=2, locals=3, args_size=1
             0: iconst_0
             1: istore_1
             2: bipush        30
             4: istore_2
             5: iload_2
             6: iload_0
             7: idiv
             8: istore_1
             9: goto          17
            12: astore_2
            13: aload_2
            14: invokevirtual #12                 // Method java/lang/Exception.printStackTrace:()V
            17: iload_1
            18: ireturn
          // 异常表
          Exception table:
             from    to  target type
                 2     9    12   Class java/lang/Exception
          LineNumberTable:
            line 21: 0
            line 23: 2
            line 24: 5
            line 27: 9
            line 25: 12
            line 26: 13
            line 28: 17
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                5       4     2 value   I
               13       4     2     e   Ljava/lang/Exception;
                0      19     0   cal   I
                2      17     1 result   I
          StackMapTable: number_of_entries = 2
            frame_type = 255 /* full_frame */
              offset_delta = 12
              locals = [ int, int ]
              stack = [ class java/lang/Exception ]
            frame_type = 4 /* same */
    
    
      public int compareTo(java.lang.String);
        descriptor: (Ljava/lang/String;)I
        flags: ACC_PUBLIC
        Code:
          stack=1, locals=2, args_size=2
             0: iconst_0
             1: ireturn
          LineNumberTable:
            line 33: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       2     0  this   Lcom/atguigu/java/MethodInnerStrucTest;
                0       2     1     o   Ljava/lang/String;
    
    
      public int compareTo(java.lang.Object);
        descriptor: (Ljava/lang/Object;)I
        flags: ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
        Code:
          stack=2, locals=2, args_size=2
             0: aload_0
             1: aload_1
             2: checkcast     #13                 // class java/lang/String
             5: invokevirtual #14                 // Method compareTo:(Ljava/lang/String;)I
             8: ireturn
          LineNumberTable:
            line 10: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       9     0  this   Lcom/atguigu/java/MethodInnerStrucTest;
    
    
      static {};
        descriptor: ()V
        flags: ACC_STATIC
        Code:
          stack=1, locals=0, args_size=0
             0: ldc           #15                 // String 测试方法的内部结构
             2: putstatic     #16                 // Field str:Ljava/lang/String;
             5: return
          LineNumberTable:
            line 13: 0
    }
    Signature: #49                          // Ljava/lang/Object;Ljava/lang/Comparable<Ljava/lang/String;>;Ljava/io/Serializable;
    SourceFile: "MethodInnerStrucTest.java"

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70]: /images/20210724/519e18c7bf074b989747a6429642ec2b.png
[2021071114475752.png]: /images/20210724/692922af10a04f778083d294d42085a9.png