jvm 汇编码

flamezealot

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public class TestTryCatch {
    public static void main(String[] args) {
        TestTryCatch test = new TestTryCatch();
        System.out.println(test.fun());
        System.out.println(test.fun2());
    }

    public int fun() {
        int i = 10;
        try {
            return i;
        }  finally {
            i = 20;
        }
    }

    public StringBuilder fun2() {
        StringBuilder s = new StringBuilder("Hello");
        try {
            //doing something
            s.append("Word");
            return s;
        } catch (Exception e) {
            return s;
        } finally {
            s.append("finally");
        }
    }
}

javap -c TestTryCatch.class 之后，就可以看见了

Compiled from "TestTryCatch.java"
public class cn.com.flamezealot.base.jvm.trycatch.TestTryCatch {
  public cn.com.flamezealot.base.jvm.trycatch.TestTryCatch();
    Code:
       0: aload_0
       1: invokespecial #8                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #1                  // class cn/com/flamezealot/base/jvm/trycatch/TestTryCatch
       3: dup
       4: invokespecial #16                 // Method "<init>":()V
       7: astore_1
       8: getstatic     #17                 // Field java/lang/System.out:Ljava/io/PrintStream;
      11: aload_1
      12: invokevirtual #23                 // Method fun:()I
      15: invokevirtual #27                 // Method java/io/PrintStream.println:(I)V
      18: getstatic     #17                 // Field java/lang/System.out:Ljava/io/PrintStream;
      21: aload_1
      22: invokevirtual #33                 // Method fun2:()Ljava/lang/StringBuilder;
      25: invokevirtual #37                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      28: return

  public int fun();
    Code:
       0: bipush        10   10入栈顶
       2: istore_1           栈顶数据保存到局部变量数组第2个位置
       3: iload_1            取出第2个局部变量入栈顶
       4: istore        4    栈顶数据保存到第5个局部变量
       6: bipush        20   20入栈顶
       8: istore_1           栈顶数据保存到第2个局部变量
       9: iload         4    第5个局部变量入栈顶
      11: ireturn            返回
      12: astore_2           栈顶数据放到局部变量数组第3个位置
      13: iload_1            第2个局部变量的值放入栈顶
      14: istore        4    栈顶数据保存到第5个局部变量
      16: bipush        20   20入栈顶
      18: istore_1           保存到第2个局部变量
      19: iload         4    第5个局部变量值入栈顶
      21: ireturn            返回
      22: astore_3           栈顶数据放到局部变量数组第4个位置
      23: bipush        20   20入栈顶
      25: istore_1           保存到第2个局部变量
      26: aload_3            局部变量数组第4个位置数据入栈顶
      27: athrow             将栈顶的数值作为异常或错误抛出
    Exception table:
       from    to  target type
           3     6    12   Class java/lang/Exception
           3     6    22   any
          12    16    22   any

  public java.lang.StringBuilder fun2();
    Code:
       0: new           #52                 // class java/lang/StringBuilder
       3: dup                               复制一份对象引用，因为接下来要先初始化，再保存到变量，所以这里复制了一份
       4: ldc           #54                 // String Hello
       6: invokespecial #56                 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V   类初始化会弹出一个引用
       9: astore_1                          另一个引用存入局部变量表2号位置
      10: aload_1
      11: ldc           #59                 // String Word
      13: invokevirtual #61                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      16: pop                               这里返回值没有赋给任何变量，所以pop出栈
      17: aload_1
      18: astore        4
      20: aload_1
      21: ldc           #65                 // String finally
      23: invokevirtual #61                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      26: pop
      27: aload         4                    sotre、load的都是引用
      29: areturn
      30: astore_2
      31: aload_1
      32: astore        4
      34: aload_1
      35: ldc           #65                 // String finally
      37: invokevirtual #61                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      40: pop
      41: aload         4
      43: areturn                           每次return前都会执行fianlly里的方法
      44: astore_3
      45: aload_1
      46: ldc           #65                 // String finally
      48: invokevirtual #61                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      51: pop
      52: aload_3
      53: athrow                            异常处理前也会执行finally里的方法
    Exception table:
       from    to  target type
          10    20    30   Class java/lang/Exception
          10    20    44   any
          30    34    44   any
}

jvm的指令集说明可以看这里

https://docs.oracle.com/javase/specs/jvms/se8/jvms8.pdf

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