JVM字节码（五）

这一节，我们来分析下异常在字节码文件中的表现，我们来看一下MyTest3：

package com.leolin.jvm.bytecode;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.net.ServerSocket;

public class MyTest3 {
    public void test() {
        try {
            InputStream is = new FileInputStream("text.txt");
            ServerSocket serverSocket = new ServerSocket(9999);
            serverSocket.accept();
        } catch (FileNotFoundException ex) {
        } catch (IOException ex) {
        } catch (Exception ex) {
        } finally {
            System.out.println("finally");
        }
    }
}

　　

上面的代码比较简单，就不解释了，我们来看下反编译之后的字节码：

D:Fjava_spacejvm-lecture	argetclasses>javap -verbose com.leolin.jvm.bytecode.MyTest3
Classfile /D:/F/java_space/jvm-lecture/target/classes/com/leolin/jvm/bytecode/MyTest3.class
  Last modified 2020-5-15; size 1066 bytes
  MD5 checksum 68544433621c8ea9d9ba3aa9638949a6
  Compiled from "MyTest3.java"
public class com.leolin.jvm.bytecode.MyTest3
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #15.#35        // java/lang/Object."<init>":()V
   #2 = Class              #36            // java/io/FileInputStream
   #3 = String             #37            // text.txt
   #4 = Methodref          #2.#38         // java/io/FileInputStream."<init>":(Ljava/lang/String;)V
   #5 = Class              #39            // java/net/ServerSocket
   #6 = Methodref          #5.#40         // java/net/ServerSocket."<init>":(I)V
   #7 = Methodref          #5.#41         // java/net/ServerSocket.accept:()Ljava/net/Socket;
   #8 = Fieldref           #42.#43        // java/lang/System.out:Ljava/io/PrintStream;
   #9 = String             #44            // finally
  #10 = Methodref          #45.#46        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #11 = Class              #47            // java/io/FileNotFoundException
  #12 = Class              #48            // java/io/IOException
  #13 = Class              #49            // java/lang/Exception
  #14 = Class              #50            // com/leolin/jvm/bytecode/MyTest3
  #15 = Class              #51            // java/lang/Object
  #16 = Utf8               <init>
  #17 = Utf8               ()V
  #18 = Utf8               Code
  #19 = Utf8               LineNumberTable
  #20 = Utf8               LocalVariableTable
  #21 = Utf8               this
  #22 = Utf8               Lcom/leolin/jvm/bytecode/MyTest3;
  #23 = Utf8               test
  #24 = Utf8               is
  #25 = Utf8               Ljava/io/InputStream;
  #26 = Utf8               serverSocket
  #27 = Utf8               Ljava/net/ServerSocket;
  #28 = Utf8               StackMapTable
  #29 = Class              #47            // java/io/FileNotFoundException
  #30 = Class              #48            // java/io/IOException
  #31 = Class              #49            // java/lang/Exception
  #32 = Class              #52            // java/lang/Throwable
  #33 = Utf8               SourceFile
  #34 = Utf8               MyTest3.java
  #35 = NameAndType        #16:#17        // "<init>":()V
  #36 = Utf8               java/io/FileInputStream
  #37 = Utf8               text.txt
  #38 = NameAndType        #16:#53        // "<init>":(Ljava/lang/String;)V
  #39 = Utf8               java/net/ServerSocket
  #40 = NameAndType        #16:#54        // "<init>":(I)V
  #41 = NameAndType        #55:#56        // accept:()Ljava/net/Socket;
  #42 = Class              #57            // java/lang/System
  #43 = NameAndType        #58:#59        // out:Ljava/io/PrintStream;
  #44 = Utf8               finally
  #45 = Class              #60            // java/io/PrintStream
  #46 = NameAndType        #61:#53        // println:(Ljava/lang/String;)V
  #47 = Utf8               java/io/FileNotFoundException
  #48 = Utf8               java/io/IOException
  #49 = Utf8               java/lang/Exception
  #50 = Utf8               com/leolin/jvm/bytecode/MyTest3
  #51 = Utf8               java/lang/Object
  #52 = Utf8               java/lang/Throwable
  #53 = Utf8               (Ljava/lang/String;)V
  #54 = Utf8               (I)V
  #55 = Utf8               accept
  #56 = Utf8               ()Ljava/net/Socket;
  #57 = Utf8               java/lang/System
  #58 = Utf8               out
  #59 = Utf8               Ljava/io/PrintStream;
  #60 = Utf8               java/io/PrintStream
  #61 = Utf8               println
{
  public com.leolin.jvm.bytecode.MyTest3();
    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 9: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/leolin/jvm/bytecode/MyTest3;

  public void test();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=4, args_size=1
         0: new           #2                  // class java/io/FileInputStream
         3: dup
         4: ldc           #3                  // String text.txt
         6: invokespecial #4                  // Method java/io/FileInputStream."<init>":(Ljava/lang/String;)V
         9: astore_1
        10: new           #5                  // class java/net/ServerSocket
        13: dup
        14: sipush        9999
        17: invokespecial #6                  // Method java/net/ServerSocket."<init>":(I)V
        20: astore_2
        21: aload_2
        22: invokevirtual #7                  // Method java/net/ServerSocket.accept:()Ljava/net/Socket;
        25: pop
        26: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
        29: ldc           #9                  // String finally
        31: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        34: goto          84
        37: astore_1
        38: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
        41: ldc           #9                  // String finally
        43: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        46: goto          84
        49: astore_1
        50: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
        53: ldc           #9                  // String finally
        55: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        58: goto          84
        61: astore_1
        62: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
        65: ldc           #9                  // String finally
        67: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        70: goto          84
        73: astore_3
        74: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
        77: ldc           #9                  // String finally
        79: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        82: aload_3
        83: athrow
        84: return
      Exception table:
         from    to  target type
             0    26    37   Class java/io/FileNotFoundException
             0    26    49   Class java/io/IOException
             0    26    61   Class java/lang/Exception
             0    26    73   any
      LineNumberTable:
        line 12: 0
        line 13: 10
        line 14: 21
        line 19: 26
        line 20: 34
        line 15: 37
        line 19: 38
        line 20: 46
        line 16: 49
        line 19: 50
        line 20: 58
        line 17: 61
        line 19: 62
        line 20: 70
        line 19: 73
        line 21: 84
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           10      16     1    is   Ljava/io/InputStream;
           21       5     2 serverSocket   Ljava/net/ServerSocket;
            0      85     0  this   Lcom/leolin/jvm/bytecode/MyTest3;
      StackMapTable: number_of_entries = 5
        frame_type = 101 /* same_locals_1_stack_item */
          stack = [ class java/io/FileNotFoundException ]
        frame_type = 75 /* same_locals_1_stack_item */
          stack = [ class java/io/IOException ]
        frame_type = 75 /* same_locals_1_stack_item */
          stack = [ class java/lang/Exception ]
        frame_type = 75 /* same_locals_1_stack_item */
          stack = [ class java/lang/Throwable ]
        frame_type = 10 /* same */
}
SourceFile: "MyTest3.java"

　　

我们专注在test()方法的Code部分，可以看到，局部变量有4个，而test()的局部变量只有3个。除了我们主动声明的is和serverSocket变量、Java帮我们隐式声明的this变量，这里还多出一个变量，是什么呢？不要忘了，我们还声明了异常对象ex，只是异常对象的具体类型只有在运行期才能确认，所以不在局部变量表中。

在偏移0到22，对应源代码的12到14行。由于这些指令之前都解释过，这里就不再解释了。在偏移22的指令执行完毕之后，原先serverSocket.accept()会将一个Socket对象压入栈中，由于在我们源代码里没有引用这个对象，所以又调用pop指令将这个对象弹出。之后偏移26到31行对应的是源代码的finally块，打印完finally之后就调用34行的goto语句，跳到84行return，整个程序结束。这是正常执行时候的逻辑，但是如果发生了异常呢？

我们来看异常表（Exception Table），这张表有四列：from、to、target和type，每一行的意思代表从偏移from到to（不包含to）的指令，有可能抛出对应type的异常，到异常发生时，需要跳转到偏移target的位置。以第一行为例：从偏移0到偏移26（不包含26），即0到25之间的指令，有可能抛出FileNotFoundException异常，当异常发生时，跳转到偏移37的位置处理异常，同理第二行和第三行的IOException和Exception异常。这里多出一个any，表示处理上面无法处理的异常，一般我们会人为Exception能处理所有的异常，但字节码并不这么认为，所以这里会多出一个any，这是编译器为我们自动生成。

如果我们从37、49、61的指令开始处，都是执行astore_1，这里是把异常对象赋值给局部变量表索引为1的位置，也许会有人疑问局部变量表索引1的位置不是存储is变量吗？其类型是InputStream。其实这种看法只是从Java语法层面上来看，异常对象和InputStream对象是两种不同的类型，但从字节码层面上来看，这两个对象都只是引用而已，而且一旦到达catch块，就意味着is变量的引用已经失效，所以我们可以复用索引1的位置，避免额外的空间开销。

如果我们从每个catch块对应的指令往下看，每个catch块都会跟着finally块中编译后的指令。这意味着在字节码层面，实现finally功能实际上就是在程序正常执行完毕，和在程序抛出异常处理异常之后，都会冗余一块finally代码块对应的指令，以实现不管程序是正常执行完毕还是抛出异常，finally块的代码始终都会执行的效果。

现在，我们不修改test方法内部的代码，不过我们在方法声明处抛出IOException、FileNotFoundException和NullPointerException这三种异常，NullPointerException是运行期异常，一般不需要特别声明，不过这里我们为了做实验特地声明一下：

public void test() throws IOException, FileNotFoundException, NullPointerException {
    ……
}

　　

我们重新编译MyTest3，看看反编译之后的test()方法：

public void test() throws java.io.IOException, java.io.FileNotFoundException, java.lang.NullPointerException
;
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      ……
    Exceptions:
      throws java.io.IOException, java.io.FileNotFoundException, java.lang.NullPointerException

　　

Code内容其实和之前一样，不过这里多了一个Exceptions，和Code平级，里面存储我们在test()方法声明处所抛出的异常。