1、try塊中沒有拋出異常,try、catch和finally塊中都有return語句
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public
static
int
NoException(){
int
i=
10
;
try
{
System.out.println(
"i in try block is:"
+i);
return
--i;
}
catch
(Exception e){
--i;
System.out.println(
"i in catch - form try block is:"
+i);
return
--i;
}
finally
{
System.out.println(
"i in finally - from try or catch block is:"
+i);
return
--i;
}
}
|
運行代碼:
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public
static
void
main(
String
[] args) {
System.out.println(
"=============NoException=================="
);
System.out.println(NoException());
System.out.println(
"==============================="
);
}
|
運行結果:
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=============NoException==================
i
in
try
block
is
:
10
i
in
finally
- from
try
or
catch
block
is
:
9
8
===============================
|
執行順序:
執行try塊,執行到return語句時,先執行return的語句,--i,但是不返回到main方法,執行finally塊,遇到finally塊中的return語句,執行--i,並將值返回到main方法,這里就不會再回去返回try塊中計算得到的值。
結論:try-catch-finally都有return語句時,沒有異常時,返回值是finally中的return返回的。從下面的字節碼可以看出,try里的return將被忽略。
2.try塊中沒有拋出異常,僅try和catch中有return語句
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public
static
int
NoException1(){
int
i=
10
;
try
{
System.out.println(
"i in try block is:"
+i);
return
--i;
}
catch
(Exception e){
--i;
System.out.println(
"i in catch - form try block is:"
+i);
return
--i;
}
finally
{
System.out.println(
"i in finally - from try or catch block is:"
+i);
--i;
System.out.println(
"i in finally block is:"
+i);
//return --i;
}
}
|
運行結果:
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=============NoException1==================
i
in
try
block
is
:
10
i
in
finally
- from
try
or
catch
block
is
:
9
i
in
finally
block
is
:
8
9
===============================
|
執行順序:
try中執行完return的語句后,不返回,執行finally塊,finally塊執行結束后,返回到try塊中,返回i在try塊中最后的值。
結論:try-catch都有return語句時,沒有異常時,返回值是try中的return返回的。
通過字節碼,我們發現,在try語句的return塊中,return 返回的引用變量(i 是引用類型)並不是try語句外定義的引用變量i,而是系統重新定義了一個局部引用i'(29行標紅色框的,系統在這里面用第三個變量保存當前的值),這個引用指向了引用i對應的值,也就是try ,即使在finally語句中把引用i指向了值減1,因為return的返回引用已經不是i 而是臨時的iload_3,所以引用i的對應的值和try語句中的返回值無關了。
3.try塊中拋出異常,try、catch和finally中都有return語句
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public
static
int
WithException(){
int
i=
10
;
try
{
System.out.println(
"i in try block is:"
+i);
i = i/
0
;
return
--i;
}
catch
(Exception e){
System.out.println(
"i in catch - form try block is:"
+i);
--i;
System.out.println(
"i in catch block is:"
+i);
return
--i;
}
finally
{
System.out.println(
"i in finally - from try or catch block is--"
+i);
--i;
System.out.println(
"i in finally block is--"
+i);
return
--i;
}
}
|
執行結果:
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=============WithException==================
i
in
try
block
is
:
10
i
in
catch
- form
try
block
is
:
10
i
in
catch
block
is
:
9
i
in
finally
- from
try
or
catch
block
is
--
8
i
in
finally
block
is
--
7
6
===============================
|
執行順序:
拋出異常后,執行catch塊,在catch塊的return的--i執行完后,並不直接返回而是執行finally,因finally中有return語句,所以,執行,返回結果6。
結論:
try塊中拋出異常,try、catch和finally中都有return語句,返回值是finally中的return。
4.try塊中拋出異常,try和catch中都有return語句
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|
public
static
int
WithException1(){
int
i=
10
;
try
{
System.out.println(
"i in try block is:"
+i);
i=i/
0
;
return
--i;
}
catch
(Exception e){
System.out.println(
"i in catch - form try block is:"
+i);
return
--i;
}
finally
{
System.out.println(
"i in finally - from try or catch block is:"
+i);
--i;
System.out.println(
"i in finally block is:"
+i);
//return i;
}
}
|
執行結果:
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|
=============WithException1==================
i
in
try
block
is
:
10
i
in
catch
- form
try
block
is
:
10
i
in
finally
- from
try
or
catch
block
is
:
9
i
in
finally
block
is
:
8
9
===============================
|
執行順序:
拋出異常后,執行catch塊,執行完finally語句后,依舊返回catch中的執行return語句后的值,而不是finally中修改的值。
結論:
返回的catch中return值。
5.try、catch中都出現異常,在finally中有返回
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public
static
int
WithException2(){
int
i=
10
;
try
{
System.out.println(
"i in try block is:"
+i);
i=i/
0
;
return
--i;
}
catch
(Exception e){
System.out.println(
"i in catch - form try block is:"
+i);
int
j = i/
0
;
return
--i;
}
finally
{
System.out.println(
"i in finally - from try or catch block is:"
+i);
--i;
--i;
System.out.println(
"i in finally block is:"
+i);
return
--i;
}
|
執行結果:
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=============WithException2==================
i
in
try block
is
:10
i
in
catch - form try block
is
:10
i
in
finally -
from
try
or
catch block
is
:10
i
in
finally block
is
:8
7
===============================
|
執行順序:
try塊中出現異常到catch,catch中出現異常到finally,finally中執行到return語句返回,不檢查異常。
結論:
返回finally中return值。
6、只在函數最后出現return語句
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public
static
int
WithException3(){
int
i=
10
;
try
{
System.out.println(
"i in try block is:"
+i);
i=i/
0
;
//return --i;
}
catch
(Exception e){
System.out.println(
"i in catch - form try block is:"
+i);
//int j = i/0;
//return --i;
}
finally
{
System.out.println(
"i in finally - from try or catch block is:"
+i);
--i;
--i;
System.out.println(
"i in finally block is:"
+i);
//return --i;
}
return
--i;
}
|
執行結果:
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=============WithException3==================
i
in
try
block
is
:
10
i
in
catch
- form
try
block
is
:
10
i
in
finally
- from
try
or
catch
block
is
:
10
i
in
finally
block
is
:
8
7
===============================
|
總體結論:
結論一:
(1)return語句並不是函數的最終出口,如果有finally語句,這在return之后還會執行finally(return的值會暫存在棧里面,等待finally執行后再返回)
(2)try、catch、finally語句中,在如果try語句有return語句,則返回的之后當前try中變量此時對應的值,此后對變量做任何的修改,都不影響try中return的返回值
(3)如果finally塊中有return 語句,則返回try或catch中的返回語句忽略。
(4)如果finally塊中拋出異常,則整個try、catch、finally塊中拋出異常
結論二:
finally里面不建議放return語句,根據需要,return語句可以放在try和catch里面和函數的最后。可行的做法有四:
(1)return語句只在函數最后出現一次。
(2)return語句僅在try和catch里面都出現。
(3)return語句僅在try和函數的最后都出現。
(4)return語句僅在catch和函數的最后都出現。
注意,除此之外的其他做法都是不可行的,編譯器會報錯。
注意:
(1)盡量在try或者catch中使用return語句。通過finally塊中達到對try或者catch返回值修改是不可行的。
(2)finally塊中避免使用return語句,因為finally塊中如果使用return語句,會顯示的消化掉try、catch塊中的異常信息,屏蔽了錯誤的發生
(3)finally塊中避免再次拋出異常,否則整個包含try語句塊的方法回拋出異常,並且會消化掉try、catch塊中的異常
Reference
[1] http://qing0991.blog.51cto.com/1640542/1387200
[2] https://mp.weixin.qq.com/s?__biz=MzIxNjA5MTM2MA==&mid=2652434506&idx=1&sn=f62a0cc1a18c628e1a52979921184af4&chksm=8c620fc5bb1586d3fe093e03749d9879dbbc4d540f3a68eb18b16d97d973a80a292ba6f7a0c3&mpshare=1&scene=23&srcid=0304GBqHaOULDvnnqLVyCoCk#rd