Java中的try finally 的原理
情况一： doTryFinally1() 就是try 中修改值并且return
Java 代码：
public static String doTryFinally1 (){
String name = null;
try {
name= "zhuang";
return name;
} finally {
name = "zhuang111111";
}
}
字节码： (加括号是自己的理解)
public static java.lang.String doTryFinally1();
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=3, args_size=0
0: aconst_null (初始化null值，进栈)
1: astore_0
2: ldc #2 // String zhuang
(int, float或String型常量值从常量池中推送至栈顶)
4: astore_0
(将栈顶引用型数值存入第一个本地变量)
5: aload_0
(将一个局部变量加载到操作数栈的指令)
6: astore_1
(将栈顶引用型数值存入第二个本地变量 这里等于又存了个要 return 的值到本地变量表)
7: ldc #3 // String zhuang111111
9: astore_0
(给name 赋值，也是 finally 块里的代码)
10: aload_1
(将第二个局部变量加载到操作数栈的指令 也就是 name = "zhuang" 的值, 所以finally 中对name 的地址进行操作对返回值没有效果)
11: areturn
12: astore_2
13: ldc #3 // String zhuang111111
15: astore_0
16: aload_2
17: athrow
Exception table:
from to target type
2 7 12 any
(finally 的另一个作用是当抛异常时，finally 中的代码块还依然执行，所以在exception table 中，又定义了一个 any 的异常处理如果抛异常了，程序也依然执行target 下的代码，最后返回的值也是不变的 )
LineNumberTable:
line 26: 0
line 28: 2
line 29: 5
line 31: 7
line 29: 10
line 31: 12
LocalVariableTable:
Start Length Slot Name Signature
2 16 0 name Ljava/lang/String;
StackMapTable: number_of_entries = 1
frame_type = 255 /* full_frame */
offset_delta = 12
locals = [ class java/lang/String ]
stack = [ class java/lang/Throwable ]
结论：
finally 的代码块编译后都会接到 try 代码块之后
1.如果 try 代码块中return ,就return 了 2.如果 try 代码块后还有代码继续执行，则会出现 goto 指令，跳转到下段指令 然后在 exception table 中注册了 any 异常 如果在 try 内抛了异常，就会去异常表找到 any 然后，跳转到对应的 target 代码段继续执行
测试的Java 代码
/**
* Created by zhuangjiesen on 2017/12/9.
*/
public class TryFinallyTest {
public static void main(String[] args) {
try {
String str = "zhuang";
} finally {
String name = "zhuang111111";
}
System.out.println("doTryFinally1 : " + doTryFinally1());
System.out.println("doTryFinally2 : " + doTryFinally2());
System.out.println("doTryFinally3 : " + doTryFinally3());
System.out.println("doTryFinally4 : " + doTryFinally4());
}
public static String doTryFinally1 (){
String name = null;
try {
name= "zhuang";
return name;
} finally {
name = "zhuang111111";
}
}
public static String doTryFinally2 (){
String name = null;
try {
name= "zhuang";
return name;
} catch (Exception e) {
e.printStackTrace();
} finally {
name = "zhuang111111";
return name;
}
}
public static String doTryFinally3 (){
String name = null;
try {
name= "zhuang";
} catch (Exception e) {
e.printStackTrace();
} finally {
name = "zhuang111111";
return name;
}
}
public static int doTryFinally4 (){
int a = 0;
try {
a = 10;
return a;
} catch (Exception e) {
e.printStackTrace();
} finally {
a++;
return a;
}
}
public static String doTryFinally5 (){
String name = null;
try {
name= "zhuang";
} catch (Exception e) {
e.printStackTrace();
} finally {
name = "zhuang111111";
}
return name;
}
public static String doTryFinally6 (){
String name = null;
try {
name= "zhuang";
} catch (Exception e) {
name= "444444";
return name;
} finally {
name = "zhuang111111";
}
return name;
}
}
字节码 (javap -verbose TryFinallyTest)
Classfile /C:/Users/zhuangjiesen/IdeaProjects/HelloWorld/target/classes/com/java/TryFinallyTest.class
Last modified 2017-12-9; size 2387 bytes
MD5 checksum 1d97c2b783088a05dc7a89a3066f1fcc
Compiled from "TryFinallyTest.java"
public class com.java.TryFinallyTest
SourceFile: "TryFinallyTest.java"
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #23.#55 // java/lang/Object."<init>":()V
#2 = String #56 // zhuang
#3 = String #57 // zhuang111111
#4 = Fieldref #58.#59 // java/lang/System.out:Ljava/io/PrintStream;
#5 = Class #60 // java/lang/StringBuilder
#6 = Methodref #5.#55 // java/lang/StringBuilder."<init>":()V
#7 = String #61 // doTryFinally1 :
#8 = Methodref #5.#62 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#9 = Methodref #22.#63 // com/java/TryFinallyTest.doTryFinally1:()Ljava/lang/String;
#10 = Methodref #5.#64 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#11 = Methodref #65.#66 // java/io/PrintStream.println:(Ljava/lang/String;)V
#12 = String #67 // doTryFinally2 :
#13 = Methodref #22.#68 // com/java/TryFinallyTest.doTryFinally2:()Ljava/lang/String;
#14 = String #69 // doTryFinally3 :
#15 = Methodref #22.#70 // com/java/TryFinallyTest.doTryFinally3:()Ljava/lang/String;
#16 = String #71 // doTryFinally4 :
#17 = Methodref #22.#72 // com/java/TryFinallyTest.doTryFinally4:()I
#18 = Methodref #5.#73 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#19 = Class #74 // java/lang/Exception
#20 = Methodref #19.#75 // java/lang/Exception.printStackTrace:()V
#21 = String #76 // 444444
#22 = Class #77 // com/java/TryFinallyTest
#23 = Class #78 // java/lang/Object
#24 = Utf8 <init>
#25 = Utf8 ()V
#26 = Utf8 Code
#27 = Utf8 LineNumberTable
#28 = Utf8 LocalVariableTable
#29 = Utf8 this
#30 = Utf8 Lcom/java/TryFinallyTest;
#31 = Utf8 main
#32 = Utf8 ([Ljava/lang/String;)V
#33 = Utf8 args
#34 = Utf8 [Ljava/lang/String;
#35 = Utf8 StackMapTable
#36 = Class #79 // java/lang/Throwable
#37 = Utf8 doTryFinally1
#38 = Utf8 ()Ljava/lang/String;
#39 = Utf8 name
#40 = Utf8 Ljava/lang/String;
#41 = Class #80 // java/lang/String
#42 = Utf8 doTryFinally2
#43 = Utf8 e
#44 = Utf8 Ljava/lang/Exception;
#45 = Class #74 // java/lang/Exception
#46 = Utf8 doTryFinally3
#47 = Utf8 doTryFinally4
#48 = Utf8 ()I
#49 = Utf8 a
#50 = Utf8 I
#51 = Utf8 doTryFinally5
#52 = Utf8 doTryFinally6
#53 = Utf8 SourceFile
#54 = Utf8 TryFinallyTest.java
#55 = NameAndType #24:#25 // "<init>":()V
#56 = Utf8 zhuang
#57 = Utf8 zhuang111111
#58 = Class #81 // java/lang/System
#59 = NameAndType #82:#83 // out:Ljava/io/PrintStream;
#60 = Utf8 java/lang/StringBuilder
#61 = Utf8 doTryFinally1 :
#62 = NameAndType #84:#85 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#63 = NameAndType #37:#38 // doTryFinally1:()Ljava/lang/String;
#64 = NameAndType #86:#38 // toString:()Ljava/lang/String;
#65 = Class #87 // java/io/PrintStream
#66 = NameAndType #88:#89 // println:(Ljava/lang/String;)V
#67 = Utf8 doTryFinally2 :
#68 = NameAndType #42:#38 // doTryFinally2:()Ljava/lang/String;
#69 = Utf8 doTryFinally3 :
#70 = NameAndType #46:#38 // doTryFinally3:()Ljava/lang/String;
#71 = Utf8 doTryFinally4 :
#72 = NameAndType #47:#48 // doTryFinally4:()I
#73 = NameAndType #84:#90 // append:(I)Ljava/lang/StringBuilder;
#74 = Utf8 java/lang/Exception
#75 = NameAndType #91:#25 // printStackTrace:()V
#76 = Utf8 444444
#77 = Utf8 com/java/TryFinallyTest
#78 = Utf8 java/lang/Object
#79 = Utf8 java/lang/Throwable
#80 = Utf8 java/lang/String
#81 = Utf8 java/lang/System
#82 = Utf8 out
#83 = Utf8 Ljava/io/PrintStream;
#84 = Utf8 append
#85 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#86 = Utf8 toString
#87 = Utf8 java/io/PrintStream
#88 = Utf8 println
#89 = Utf8 (Ljava/lang/String;)V
#90 = Utf8 (I)Ljava/lang/StringBuilder;
#91 = Utf8 printStackTrace
{
public com.java.TryFinallyTest();
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 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/java/TryFinallyTest;
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=4, args_size=1
0: ldc #2 // String zhuang
2: astore_1
3: ldc #3 // String zhuang111111
5: astore_1
6: goto 15
9: astore_2
10: ldc #3 // String zhuang111111
12: astore_3
13: aload_2
14: athrow
15: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
18: new #5 // class java/lang/StringBuilder
21: dup
22: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
25: ldc #7 // String doTryFinally1 :
27: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
30: invokestatic #9 // Method doTryFinally1:()Ljava/lang/String;
33: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
36: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
39: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
42: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
45: new #5 // class java/lang/StringBuilder
48: dup
49: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
52: ldc #12 // String doTryFinally2 :
54: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
57: invokestatic #13 // Method doTryFinally2:()Ljava/lang/String;
60: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
63: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
66: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
69: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
72: new #5 // class java/lang/StringBuilder
75: dup
76: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
79: ldc #14 // String doTryFinally3 :
81: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
84: invokestatic #15 // Method doTryFinally3:()Ljava/lang/String;
87: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
90: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
93: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
96: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
99: new #5 // class java/lang/StringBuilder
102: dup
103: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
106: ldc #16 // String doTryFinally4 :
108: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
111: invokestatic #17 // Method doTryFinally4:()I
114: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
117: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
120: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
123: return
Exception table:
from to target type
0 3 9 any
LineNumberTable:
line 11: 0
line 13: 3
line 14: 6
line 13: 9
line 14: 13
line 17: 15
line 18: 42
line 19: 69
line 20: 96
line 21: 123
LocalVariableTable:
Start Length Slot Name Signature
0 124 0 args [Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 73 /* same_locals_1_stack_item */
stack = [ class java/lang/Throwable ]
frame_type = 5 /* same */
public static java.lang.String doTryFinally1();
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=3, args_size=0
0: aconst_null
1: astore_0
2: ldc #2 // String zhuang
4: astore_0
5: aload_0
6: astore_1
7: ldc #3 // String zhuang111111
9: astore_0
10: aload_1
11: areturn
12: astore_2
13: ldc #3 // String zhuang111111
15: astore_0
16: aload_2
17: athrow
Exception table:
from to target type
2 7 12 any
LineNumberTable:
line 26: 0
line 28: 2
line 29: 5
line 31: 7
line 29: 10
line 31: 12
LocalVariableTable:
Start Length Slot Name Signature
2 16 0 name Ljava/lang/String;
StackMapTable: number_of_entries = 1
frame_type = 255 /* full_frame */
offset_delta = 12
locals = [ class java/lang/String ]
stack = [ class java/lang/Throwable ]
public static java.lang.String doTryFinally2();
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=3, args_size=0
0: aconst_null
1: astore_0
2: ldc #2 // String zhuang
4: astore_0
5: aload_0
6: astore_1
7: ldc #3 // String zhuang111111
9: astore_0
10: aload_0
11: areturn
12: astore_1
13: aload_1
14: invokevirtual #20 // Method java/lang/Exception.printStackTrace:()V
17: ldc #3 // String zhuang111111
19: astore_0
20: aload_0
21: areturn
22: astore_2
23: ldc #3 // String zhuang111111
25: astore_0
26: aload_0
27: areturn
Exception table:
from to target type
2 7 12 Class java/lang/Exception
2 7 22 any
12 17 22 any
LineNumberTable:
line 39: 0
line 41: 2
line 42: 5
line 46: 7
line 47: 10
line 43: 12
line 44: 13
line 46: 17
line 47: 20
line 46: 22
line 47: 26
LocalVariableTable:
Start Length Slot Name Signature
13 4 1 e Ljava/lang/Exception;
2 26 0 name Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 12
locals = [ class java/lang/String ]
stack = [ class java/lang/Exception ]
frame_type = 73 /* same_locals_1_stack_item */
stack = [ class java/lang/Throwable ]
public static java.lang.String doTryFinally3();
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=3, args_size=0
0: aconst_null
1: astore_0
2: ldc #2 // String zhuang
4: astore_0
5: ldc #3 // String zhuang111111
7: astore_0
8: aload_0
9: areturn
10: astore_1
11: aload_1
12: invokevirtual #20 // Method java/lang/Exception.printStackTrace:()V
15: ldc #3 // String zhuang111111
17: astore_0
18: aload_0
19: areturn
20: astore_2
21: ldc #3 // String zhuang111111
23: astore_0
24: aload_0
25: areturn
Exception table:
from to target type
2 5 10 Class java/lang/Exception
2 5 20 any
10 15 20 any
LineNumberTable:
line 56: 0
line 58: 2
line 62: 5
line 63: 8
line 59: 10
line 60: 11
line 62: 15
line 63: 18
line 62: 20
line 63: 24
LocalVariableTable:
Start Length Slot Name Signature
11 4 1 e Ljava/lang/Exception;
2 24 0 name Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 10
locals = [ class java/lang/String ]
stack = [ class java/lang/Exception ]
frame_type = 73 /* same_locals_1_stack_item */
stack = [ class java/lang/Throwable ]
public static int doTryFinally4();
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=3, args_size=0
0: iconst_0
1: istore_0
2: bipush 10
4: istore_0
5: iload_0
6: istore_1
7: iinc 0, 1
10: iload_0
11: ireturn
12: astore_1
13: aload_1
14: invokevirtual #20 // Method java/lang/Exception.printStackTrace:()V
17: iinc 0, 1
20: iload_0
21: ireturn
22: astore_2
23: iinc 0, 1
26: iload_0
27: ireturn
Exception table:
from to target type
2 7 12 Class java/lang/Exception
2 7 22 any
12 17 22 any
LineNumberTable:
line 70: 0
line 72: 2
line 73: 5
line 77: 7
line 78: 10
line 74: 12
line 75: 13
line 77: 17
line 78: 20
line 77: 22
line 78: 26
LocalVariableTable:
Start Length Slot Name Signature
13 4 1 e Ljava/lang/Exception;
2 26 0 a I
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 12
locals = [ int ]
stack = [ class java/lang/Exception ]
frame_type = 73 /* same_locals_1_stack_item */
stack = [ class java/lang/Throwable ]
public static java.lang.String doTryFinally5();
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=3, args_size=0
0: aconst_null
1: astore_0
2: ldc #2 // String zhuang
4: astore_0
5: ldc #3 // String zhuang111111
7: astore_0
8: goto 28
11: astore_1
12: aload_1
13: invokevirtual #20 // Method java/lang/Exception.printStackTrace:()V
16: ldc #3 // String zhuang111111
18: astore_0
19: goto 28
22: astore_2
23: ldc #3 // String zhuang111111
25: astore_0
26: aload_2
27: athrow
28: aload_0
29: areturn
Exception table:
from to target type
2 5 11 Class java/lang/Exception
2 5 22 any
11 16 22 any
LineNumberTable:
line 85: 0
line 87: 2
line 91: 5
line 92: 8
line 88: 11
line 89: 12
line 91: 16
line 92: 19
line 91: 22
line 93: 28
LocalVariableTable:
Start Length Slot Name Signature
12 4 1 e Ljava/lang/Exception;
2 28 0 name Ljava/lang/String;
StackMapTable: number_of_entries = 3
frame_type = 255 /* full_frame */
offset_delta = 11
locals = [ class java/lang/String ]
stack = [ class java/lang/Exception ]
frame_type = 74 /* same_locals_1_stack_item */
stack = [ class java/lang/Throwable ]
frame_type = 5 /* same */
public static java.lang.String doTryFinally6();
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=4, args_size=0
0: aconst_null
1: astore_0
2: ldc #2 // String zhuang
4: astore_0
5: ldc #3 // String zhuang111111
7: astore_0
8: goto 28
11: astore_1
12: ldc #21 // String 444444
14: astore_0
15: aload_0
16: astore_2
17: ldc #3 // String zhuang111111
19: astore_0
20: aload_2
21: areturn
22: astore_3
23: ldc #3 // String zhuang111111
25: astore_0
26: aload_3
27: athrow
28: aload_0
29: areturn
Exception table:
from to target type
2 5 11 Class java/lang/Exception
2 5 22 any
11 17 22 any
LineNumberTable:
line 98: 0
line 100: 2
line 105: 5
line 106: 8
line 101: 11
line 102: 12
line 103: 15
line 105: 17
line 103: 20
line 105: 22
line 107: 28
LocalVariableTable:
Start Length Slot Name Signature
12 10 1 e Ljava/lang/Exception;
2 28 0 name Ljava/lang/String;
StackMapTable: number_of_entries = 3
frame_type = 255 /* full_frame */
offset_delta = 11
locals = [ class java/lang/String ]
stack = [ class java/lang/Exception ]
frame_type = 74 /* same_locals_1_stack_item */
stack = [ class java/lang/Throwable ]
frame_type = 5 /* same */
}
