通过字节码分析switch语句的执行

public class Test {
    public static void main(String[] args) {
        int i = 7;
        switch (i) {
            case 1:
                System.out.println("1");
            default:
                System.out.println("2");
            case 3:
                System.out.println("3");
        }
    }
}

运行结果
2
3

Compiled from "Test.java"
public class com.rsms.iot.service.Test {
  public com.rsms.iot.service.Test();
    Code:
       0: aload_0                //加载一个对象引用到操作数栈，在这里第一个变量是this，所以是将this引用压到操作数栈
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V (构造方法、私有方法和父类中的方法)
       4: return
  public static void main(java.lang.String[]);
    Code:
       0: bipush        7    //将操作数7压到操作数栈中
       2: istore_1     //将整形数字存储到本地变量中
       3: iload_1      //i代表int，从局部变量数组中加载一个对象引用到操作数栈的栈顶
       4: lookupswitch  { // 2   
                     1: 32   //为1匹配32行
                     3: 48   //同上
               default: 40   //同上
          }
      32: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      35: ldc           #3                  // String 1
      37: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      40: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      43: ldc           #5                  // String 2
      45: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      48: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      51: ldc           #6                  // String 3
      53: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      56: return
}

int i = 7;
System.out.println(i);
System.out.println(i);