try return finally字节码解析

不念不忘少年蓝@ 2022-04-15 01:09 268阅读 0赞

我们经常会遇到一个问题，try 里面return了，finally还会执行吗？例如下面一个例子：

public class TryFinally {
        public static void main(String[] args) {
            System.out.println(f1());
        }
        public static String f1() {
            String str = "hello";
            try{
                return str;
            }
            finally{
                str = "world";
            }
        }
    }

输出是hello，而不是world。但是str="world"会执行。我们通过字节码来解释：

Classfile /F:/code/java/test/out/production/test/TryFinally.class
      Last modified Nov 19, 2018; size 788 bytes
      MD5 checksum 33506d15d7a020f1e9a7598ef4536991
      Compiled from "TryFinally.java"
    public class TryFinally
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #8.#29         // java/lang/Object."<init>":()V
       #2 = Fieldref           #30.#31        // java/lang/System.out:Ljava/io/PrintStream;
       #3 = Methodref          #7.#32         // TryFinally.f1:()Ljava/lang/String;
       #4 = Methodref          #33.#34        // java/io/PrintStream.println:(Ljava/lang/String;)V
       #5 = String             #35            // hello
       #6 = String             #36            // world
       #7 = Class              #37            // TryFinally
       #8 = Class              #38            // java/lang/Object
       #9 = Utf8               <init>
      #10 = Utf8               ()V
      #11 = Utf8               Code
      #12 = Utf8               LineNumberTable
      #13 = Utf8               LocalVariableTable
      #14 = Utf8               this
      #15 = Utf8               LTryFinally;
      #16 = Utf8               main
      #17 = Utf8               ([Ljava/lang/String;)V
      #18 = Utf8               args
      #19 = Utf8               [Ljava/lang/String;
      #20 = Utf8               f1
      #21 = Utf8               ()Ljava/lang/String;
      #22 = Utf8               str
      #23 = Utf8               Ljava/lang/String;
      #24 = Utf8               StackMapTable
      #25 = Class              #39            // java/lang/String
      #26 = Class              #40            // java/lang/Throwable
      #27 = Utf8               SourceFile
      #28 = Utf8               TryFinally.java
      #29 = NameAndType        #9:#10         // "<init>":()V
      #30 = Class              #41            // java/lang/System
      #31 = NameAndType        #42:#43        // out:Ljava/io/PrintStream;
      #32 = NameAndType        #20:#21        // f1:()Ljava/lang/String;
      #33 = Class              #44            // java/io/PrintStream
      #34 = NameAndType        #45:#46        // println:(Ljava/lang/String;)V
      #35 = Utf8               hello
      #36 = Utf8               world
      #37 = Utf8               TryFinally
      #38 = Utf8               java/lang/Object
      #39 = Utf8               java/lang/String
      #40 = Utf8               java/lang/Throwable
      #41 = Utf8               java/lang/System
      #42 = Utf8               out
      #43 = Utf8               Ljava/io/PrintStream;
      #44 = Utf8               java/io/PrintStream
      #45 = Utf8               println
      #46 = Utf8               (Ljava/lang/String;)V
    {
      public TryFinally();
        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 1: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   LTryFinally;
    
      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: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
             3: invokestatic  #3                  // Method f1:()Ljava/lang/String;
             6: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
             9: return
          LineNumberTable:
            line 3: 0
            line 4: 9
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      10     0  args   [Ljava/lang/String;
    
      public static java.lang.String f1();
        descriptor: ()Ljava/lang/String;
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=1, locals=3, args_size=0
             0: ldc           #5                  // String hello
             2: astore_0
             3: aload_0
             4: astore_1
             5: ldc           #6                  // String world
             7: astore_0
             8: aload_1
             9: areturn
            10: astore_2
            11: ldc           #6                  // String world
            13: astore_0
            14: aload_2
            15: athrow
          Exception table:
             from    to  target type
                 3     5    10   any
          LineNumberTable:
            line 6: 0
            line 8: 3
            line 11: 5
            line 8: 8
            line 11: 10
            line 12: 14
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                3      13     0   str   Ljava/lang/String;
          StackMapTable: number_of_entries = 1
            frame_type = 255 /* full_frame */
              offset_delta = 10
              locals = [ class java/lang/String ]
              stack = [ class java/lang/Throwable ]
    }
    SourceFile: "TryFinally.java"

关键的只有这段代码：

Code:
          stack=1, locals=3, args_size=0
             0: ldc           #5                  // String hello
             2: astore_0
             3: aload_0
             4: astore_1
             5: ldc           #6                  // String world
             7: astore_0
             8: aload_1
             9: areturn
            10: astore_2
            11: ldc           #6                  // String world
            13: astore_0
            14: aload_2
            15: athrow
          Exception table:
             from    to  target type
                 3     5    10   any

1、首先分析这段代码，有两个 ldc \#6和astore\_0，这句话的意思是把常量池里面world加到操作数栈里面然后将它放到本地变量表索引为0的位置，我们思考为什么会有两次？那是因为str="world"是在finally里面执行的，所以在返回之前一定会执行。也就是，如果不抛出异常正常就会执行，如果抛出异常也就是在areturn之后，也会执行。  
2、我们注意到，athrow也就是抛出异常之前，aload\_2，也就是把本地变量表里面索引为2的数据作为放到操作数栈里面然后抛出去，这个抛出数据就是异常，就是在astore\_2里面放进去的异常。我们从这里知道了，本地变量表里面索引为2的位置的内容就是异常。  
3、最后我们看areturn之前，把本地变量表里面索引为1的数据load进来操作数栈里面，也就是我们return的是本地变量表里面索引为1的数据，进过上面的分析，我们知道这个值是“hello”，而不是“world”。  
所以我们最终看到的是输出了“hello”而不是“world”，从字节码层面就这么分析出来。