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

案例代码如下：

 public class JVMTest {
     public static Integer num = 10;

     public int add(int i){
         int j = 5;
         int k = i+j;
         j++;
         k = num + j;
         return k;
     }

     public static void main(String[] args){
         System.out.println(new JVMTest().add(4));
     }
 }

编译之后主要指令集如下：

 {
   public static java.lang.Integer num;
     descriptor: Ljava/lang/Integer;
     flags: ACC_PUBLIC, ACC_STATIC

   public com.lucky.grpc.JVMTest();
     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 3: 0

   public int add(int);
     descriptor: (I)I
     flags: ACC_PUBLIC
     Code:
       stack=2, locals=4, args_size=2
          0: iconst_5
          1: istore_2
          2: iload_1
          3: iload_2
          4: iadd
          5: istore_3
          6: iinc          2, 1
          9: getstatic     #2                  // Field num:Ljava/lang/Integer;
         12: invokevirtual #3                  // Method java/lang/Integer.intValue:()I
         15: iload_2
         16: iadd
         17: istore_3
         18: iload_3
         19: ireturn
       LineNumberTable:
         line 7: 0
         line 8: 2
         line 9: 6
         line 10: 9
         line 11: 18

   public static void main(java.lang.String[]);
     descriptor: ([Ljava/lang/String;)V
     flags: ACC_PUBLIC, ACC_STATIC
     Code:
       stack=3, locals=1, args_size=1
          0: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
          3: new           #5                  // class com/lucky/grpc/JVMTest
          6: dup
          7: invokespecial #6                  // Method "<init>":()V
         10: iconst_4
         11: invokevirtual #7                  // Method add:(I)I
         14: invokevirtual #8                  // Method java/io/PrintStream.println:(I)V
         17: return
       LineNumberTable:
         line 15: 0
         line 16: 17

   static {};
     descriptor: ()V
     flags: ACC_STATIC
     Code:
       stack=1, locals=0, args_size=0
          0: bipush        10
          2: invokestatic  #9                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
          5: putstatic     #2                  // Field num:Ljava/lang/Integer;
          8: return
       LineNumberTable:
         line 4: 0
 }

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 ：

是