通过字节码分析this关键字以及异常表的重要作用

在之前的字节码分析中缺少对异常的介绍，这次主要来对字节码异常表相关的东东进行一个学习，下面先来编写一个相关异常的小程序：

接着编译来看用javap -verbose来查看一下它的字节码信息：

xiongweideMacBook-Pro:classes xiongwei$ javap -verbose com/jvm/bytecode/MyTest3.class
Classfile /Users/xiongwei/Documents/workspace/IntelliJSpace/jvm_lectue/out/production/classes/com/jvm/bytecode/MyTest3.class
  Last modified Sep 26, 2018; size 1056 bytes
  MD5 checksum 67ac394f07ca1303eb0119b94486d428
  Compiled from "MyTest3.java"
public class com.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            // test.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/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/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               test.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/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.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/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 test.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 13: 0
        line 14: 10
        line 15: 21
        line 23: 26
        line 24: 34
        line 16: 37
        line 23: 38
        line 24: 46
        line 18: 49
        line 23: 50
        line 24: 58
        line 20: 61
        line 23: 62
        line 24: 70
        line 23: 73
        line 24: 82
        line 25: 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/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"

其中重点观注一下test()方法的信息：

其中stack表示这个方法运行的任何时刻所能达到的操作数栈的最大深度；local表示方法执行期间创建的局部变量的数目，包含用来表示传入的参数的局部变量。args_size表示方法的参数的总个数。
下面首先来解释一下为啥args_size等于1，很明显咱们这个方法是一个无参的定义，关于这个在之前的学习中已经解释过了，这里再巩固一下：对于Java类中的每一个实例方法（非static方法），其在编译后所生成的字节码当中，方法参数的数量总是会比源代码中方法的参数的数量多一个（this），它位于方法的第一个参数位置处；这样，我们就可以在Java的实例方法中使用this来去访问当前对象的属性以及其它方法。这个操作是在编译期间完成的，既由javac编译器在编译的时候将对this的访问转化为对一个普通实例方法参数的访问，接下来的运行期间，由JVM在调用实例方法时，自动向实例方法传入该this参数。所以，在实例方法的局部变量表中，至少会有一个指向当前对象的局部变量【也就是会存在于下面要分析的locals中】。

接着来看一下locals，为啥有4个局部变量呢？下面来分析一下：
1、当然就是隐含的this啦，上面分析stack中标明处有说明。

2、当然就是is喽，如下：

3、当然就是serverSocket喽：

4、那它倒底是啥呢？貌似咱们能见到局部变量就木有了，其实这个局部变量是位于catch当中的，如下：

也就是如果抛出了异常，那么最终这三个异常就会产生一个局部变量，如果不抛出异常，那么这个最多4个局部变量中最终只会使用3个局部变量。

最后再来看一下statck=3，也就是这个方法最多能往栈中压入3个元素，关于栈在之后还会详述的，这里有个基本印象。

接着来则到了该篇要讨论的核心话题：异常，对应方法的code属性中exception_table，这里存放的是处理异常的信息。每一个exception_table表项由start_pc、end_pc、handler_pc、catch_type组成，那这四个元素分别代表啥呢？下面来看一下：

• start_pc和end_pc表示在code数组中的从start_pc到end_pc处（包含start_pc，不包含end_pc）的指令抛出的异常会由这个表项来处理。
• handler_pc表示处理异常的代码的开始处。catch_type表示会被处理的异常类型，它指向常量池里的一个异常表。当catch_type为0时，表示处理所有的异常。

其实异常在字节码中是存在有goto语句的，也就是当发生异常则会goto到指定的位置进行异常处理，大至看一下：

具体异常相关的分析下次再继续。