JVM探秘6--图解虚拟机栈的局部变量表和操作数栈工作流程

Posted 2021-02-16 jackion5

案例代码如下：

 1 public class JVMTest {
 2     public static Integer num = 10;
 3 
 4     public int add(int i){
 5         int j = 5;
 6         int k = i+j;
 7         j++;
 8         k = num + j;
 9         return k;
10     }
11 
12     public static void main(String[] args){
13         System.out.println(new JVMTest().add(4));
14     }
15 }

编译之后主要指令集如下：

 1 {
 2   public static java.lang.Integer num;
 3     descriptor: Ljava/lang/Integer;
 4     flags: ACC_PUBLIC, ACC_STATIC
 5 
 6   public com.lucky.grpc.JVMTest();
 7     descriptor: ()V
 8     flags: ACC_PUBLIC
 9     Code:
10       stack=1, locals=1, args_size=1
11          0: aload_0
12          1: invokespecial #1                  // Method java/lang/Object."<init>":()V
13          4: return
14       LineNumberTable:
15         line 3: 0
16 
17   public int add(int);
18     descriptor: (I)I
19     flags: ACC_PUBLIC
20     Code:
21       stack=2, locals=4, args_size=2
22          0: iconst_5
23          1: istore_2
24          2: iload_1
25          3: iload_2
26          4: iadd
27          5: istore_3
28          6: iinc          2, 1
29          9: getstatic     #2                  // Field num:Ljava/lang/Integer;
30         12: invokevirtual #3                  // Method java/lang/Integer.intValue:()I
31         15: iload_2
32         16: iadd
33         17: istore_3
34         18: iload_3
35         19: ireturn
36       LineNumberTable:
37         line 7: 0
38         line 8: 2
39         line 9: 6
40         line 10: 9
41         line 11: 18
42 
43   public static void main(java.lang.String[]);
44     descriptor: ([Ljava/lang/String;)V
45     flags: ACC_PUBLIC, ACC_STATIC
46     Code:
47       stack=3, locals=1, args_size=1
48          0: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
49          3: new           #5                  // class com/lucky/grpc/JVMTest
50          6: dup
51          7: invokespecial #6                  // Method "<init>":()V
52         10: iconst_4
53         11: invokevirtual #7                  // Method add:(I)I
54         14: invokevirtual #8                  // Method java/io/PrintStream.println:(I)V
55         17: return
56       LineNumberTable:
57         line 15: 0
58         line 16: 17
59 
60   static {};
61     descriptor: ()V
62     flags: ACC_STATIC
63     Code:
64       stack=1, locals=0, args_size=0
65          0: bipush        10
66          2: invokestatic  #9                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
67          5: putstatic     #2                  // Field num:Ljava/lang/Integer;
68          8: return
69       LineNumberTable:
70         line 4: 0
71 }

 

其中第17行到第41行是add方法的指令，接下来就一行一行图解虚拟机栈是如何执行的！

第一步：iconst_5  ：将int类型的常量5压入操作数栈

第二步：istore_2 ：将操作数栈顶中int类型数据放入局部常量表下标为2的位置，从0开始数也就是第三个位置

第三步：iload_1 ：

 

 

 

 

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