有如下代码
public class Test { public void work() { try { InputStream is = new FileInputStream("test.txt"); ServerSocket serverSocket = new ServerSocket(9999); serverSocket.accept(); } catch (FileNotFoundException e) { } catch (IOException e) { } catch (Exception e) { } finally { System.out.println("finally block"); } } }
javap -verbose -p Test 结果如下
D:\workspace-learn\common-learn\learn-classloader\target\classes\com\learn\jvm>javap -verbose -p Test
警告: 二进制文件Test包含com.learn.jvm.Test
Classfile /D:/workspace-learn/common-learn/learn-classloader/target/classes/com/learn/jvm/Test.class
Last modified 2019-9-5; size 1075 bytes
MD5 checksum 744e2b9632f4d7f2d38d6b77b4225cc2
Compiled from "Test.java"
public class com.learn.jvm.Test
SourceFile: "Test.java"
minor version: 0
major version: 51
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 block
#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/learn/jvm/Test
#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/learn/jvm/Test;
#23 = Utf8 work
#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 Test.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 block
#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/learn/jvm/Test
#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.learn.jvm.Test();
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 14: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/learn/jvm/Test;
public void work();
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 block
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 block
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 block
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 block
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 block
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
37 38 73 any
49 50 73 any
61 62 73 any
73 74 73 any
LineNumberTable:
line 17: 0
line 18: 10
line 19: 21
line 27: 26
line 28: 34
line 20: 37
line 27: 38
line 28: 46
line 22: 49
line 27: 50
line 28: 58
line 24: 61
line 27: 62
line 28: 70
line 27: 73
line 29: 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/learn/jvm/Test;
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 */
}
在上图中看到work方法的 stack=3, locals=4, args_size=1，其中size = 1表示参数个数为一个，即this，locals=4表示有4个局部变量，分别为 this，is，serverSocket，和 e，虽然有三个catch块，但是程序运行时只可能进入其中某一个，所以只有一个e
对异常表的解读
异常表的第一行为例，表示在[0,25]行这个范围内如果发生FileNotFoundException，将会跳转到字节码第37行，如下图
astore_1表示把一个引用赋值给局部变量e
Java字节码对于异常的处理方式：
- 统一采用异常表的方式来对异常进行处理
- 在jdk1.4.2之前的版本中，并不是使用异常表的方式来对异常进行处理的，而是采用特定的指令方式
- 当异常处理存在finally块时，现代化的jvm采取的处理方式是将finally语句块的字节码拼接到每一个catch块后面，换句话说，程序中存在多少个catch块，就会在每个catch块后面重复多少个finally语句块的字节码
最后，如果方法后面throws了异常，则字节码里方法会多出一个属性，是这样的：
很简单，知道即可