方法区使用举例

水深无声 2021-07-24 17:13 602阅读 0赞

# 一 代码 #

public class MethodAreaDemo {
        public static void main(String args[]) {
            int x = 500;
            int y = 100;
            int a = x / y;
            int b = 50;
            System.out.println(a+b);
        }
    }

# 二 对应常量池 #

#1 = Methodref          #5.#24         // java/lang/Object."<init>":()V
    #2 = Fieldref           #25.#26        // java/lang/System.out:Ljava/io/PrintStream;
    #3 = Methodref          #27.#28        // java/io/PrintStream.println:(I)V
    #4 = Class              #29            // com/atguigu/java1/MethodAreaDemo
    #5 = Class              #30            // java/lang/Object
    #6 = Utf8               <init>
    #7 = Utf8               ()V
    #8 = Utf8               Code
    #9 = Utf8               LineNumberTable
    #10 = Utf8               LocalVariableTable
    #11 = Utf8               this
    #12 = Utf8               Lcom/atguigu/java1/MethodAreaDemo;
    #13 = Utf8               main
    #14 = Utf8               ([Ljava/lang/String;)V
    #15 = Utf8               args
    #16 = Utf8               [Ljava/lang/String;
    #17 = Utf8               x
    #18 = Utf8               I
    #19 = Utf8               y
    #20 = Utf8               a
    #21 = Utf8               b
    #22 = Utf8               SourceFile
    #23 = Utf8               MethodAreaDemo.java
    #24 = NameAndType        #6:#7          // "<init>":()V
    #25 = Class              #31            // java/lang/System
    #26 = NameAndType        #32:#33        // out:Ljava/io/PrintStream;
    #27 = Class              #34            // java/io/PrintStream
    #28 = NameAndType        #35:#36        // println:(I)V
    #29 = Utf8               com/atguigu/java1/MethodAreaDemo
    #30 = Utf8               java/lang/Object
    #31 = Utf8               java/lang/System
    #32 = Utf8               out
    #33 = Utf8               Ljava/io/PrintStream;
    #34 = Utf8               java/io/PrintStream
    #35 = Utf8               println
    #36 = Utf8               (I)V

# 三 字节码执行过程展示 #

**1 执行前，方法区、虚拟机栈以及程序计数器的样子**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70][]

**2 将操作数 500 放入到操作数栈中。**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 1][]

**3 弹出操作数栈栈顶 500，保存到本地变量表1。**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 2][]

**4 把 100 这个数值压入操作数栈**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 3][]

**5 弹出操作数栈栈顶 100，保存到本地变量表2。**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 4][]

**6 读取本地变量1压入操作数栈**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 5][]

**7 读取本地变量2压入操作数栈**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 6][]

**8 将栈顶两个 int 类型取出并相除，结果5入栈。**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 7][]

**9 将5保存到本地变量3，然后将50压入操作数栈**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 8][]

**10 将栈顶 int 类型值保存到局部变量表4中。**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 9][]

**11 获取类或接口字段的值并将其压入操作数栈**

\#2 在常量池经过一番拼接，最终拼接成： java/lang/System.out:Ljava/io/PrintStream，它是Fieldref 类型。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 10][]

**12 将本地变量3取出，压入操作数栈。**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 11][]

**13 将本地变量4取出，压入操作数栈。**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 12][]

**14 将栈顶两个 int 类型取出并相加，结果55入栈。**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 13][]

**15 调用 invokevirtual \#3**

\#3 = Methodref          \#27.\#28        // java/io/PrintStream.println:(I)V

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 14][]

**16 main方法执行结束，进行 return**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 15][]

整个过程中，程序计数器始终计算的都是当前代码运行的位置，目的是为了方便记录方法调用后能够正常返回，或者是进行了 CPU 切换后，也能回来到原来的代码进行 执行。

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