深入理解JVM之--字节码

素颜马尾好姑娘i 2023-07-11 06:20 23阅读 0赞

**       上篇讲到栈帧，现在我们需要了解栈帧的执行过程，那就涉及到反编译字节码。**

## 一. 反编译代码 ##

(1)源代码

/**
     * 编译：javac com\jvm\StackFrame.java
     * 反编译：javap -p -v com\jvm\StackFrame.class
     */
    public class StackFrame {
        public static void main(String[] args) {
            add(1, 2);
        }
    
        private static int add(int a, int b) {
            int c = 0;
            c = a + b;
            return c;
        }
    }

(2)反编译后的字节码，不懂没关系，接下来进行字节码解释。

{
      public com.jvm.StackFrame();
        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
    
      public static void main(java.lang.String[]);
        descriptor: ([Ljava/lang/String;)V
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=2, locals=1, args_size=1
             0: iconst_1
             1: iconst_2
             2: invokestatic  #2                  // Method add:(II)I
             5: pop
             6: return
          LineNumberTable:
            line 9: 0
            line 10: 6
    
      private static int add(int, int);
        descriptor: (II)I
        flags: ACC_PRIVATE, ACC_STATIC
        Code:
          stack=2, locals=3, args_size=2
             0: iconst_0
             1: istore_2
             2: iload_0
             3: iload_1
             4: iadd
             5: istore_2
             6: iload_2
             7: ireturn
          LineNumberTable:
            line 13: 0
            line 14: 2
            line 15: 6
    }

## **2. 字节码解释** ##

（1）\# 方法描述  
\# 括号内为入数类型，这里为两个 int 型入参  
\# 括号外为返回类型，这里为返回 int 型  
descriptor: (II)I

\# 方法类型，这里为私有的静态方法  
flags: ACC\_PRIVATE, ACC\_STATIC

\# 操作数栈为 2  
\# 本地变量容量为 3  
\# 入参个数为 2  
stack=2, locals=3, args\_size=2

### 执行 add(1,2) 的过程，最后 ireturn 会将操作数栈栈顶的值返回给调用者 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3k1MzI3OTgxMTM_size_16_color_FFFFFF_t_70][]

### 代码行号与字节码行号的对应关系 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3k1MzI3OTgxMTM_size_16_color_FFFFFF_t_70 1][]

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3k1MzI3OTgxMTM_size_16_color_FFFFFF_t_70 1]: https://img-blog.csdnimg.cn/2020030417183384.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3k1MzI3OTgxMTM=,size_16,color_FFFFFF,t_70