紅豆生南國，春來發幾枝。願君多采擷，此物最相思。
（原創不容易，引用請標記，謝謝）
try catch爲一般用於java中的異常處理，
1：try()的用法：
try（）是1.7新增的用法，用來自動關閉括號內能關閉的資源，而不必要手動關閉，其必須實現autocloseable接口，
such as下列代碼，MySource是資源類，繼承了AutoCloseable接口
public class Myresource implements AutoCloseable{
@Override
public void close() throws Exception {
System.out.println("正在執行關閉方法");
}
public void throwTest(int k) throws Exception{
System.out.println("正在執行方法throwTest");
if(k>5){
throw new Exception();
}
}
}而我測試類這樣寫時：
public class TryTest {
public static void main(String[] args) {
try{
Myresource myresource = new Myresource();
myresource.throwTest(4);
}catch (Exception e) {
e.printStackTrace();
}
}
}執行結果爲：
但是當我測試類這樣寫時：
public static void main(String[] args) {
try (Myresource myresource = new Myresource()) {
myresource.throwTest(4);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}運行結果爲：
將自動關閉資源，測試了下上面資源類繼承closeable接口時，採用下面的方法，運行結果也會自動關閉資源，
2：try的返回值問題
當try catch final 中都有return的時候怎麼返回呢，寫demo
public class TryTest {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 執行");
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 執行");
return string;
}finally {
string = "finally model";
System.out.println("finally 執行");
}
}
}執行結果爲：
預料中執行finally已經把值改變爲finally 了 但是返回的確實try
javap -verbose一下：
Code:
stack=2, locals=4, args_size=0
0: ldc #34 // String
2: astore_0
3: ldc #36 // String try model
5: astore_0
6: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
9: ldc #38 // String try 執行
11: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
14: aload_0
15: astore_3
16: ldc #40 // String finally model
18: astore_0
19: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
22: ldc #42 // String finally 執行
24: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
27: aload_3
28: areturn
29: astore_1
30: ldc #44 // String catch model
32: astore_0
33: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
36: ldc #46 // String catch 執行
38: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
41: aload_0
42: astore_3
43: ldc #40 // String finally model
45: astore_0
46: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
49: ldc #42 // String finally 執行
51: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
54: aload_3
55: areturn
56: astore_2
57: ldc #40 // String finally model
59: astore_0
60: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
63: ldc #42 // String finally 執行
65: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
68: aload_2
69: athrow第0-2給string0賦值“”,並儲存
第3-5 給string0賦值“try model“，並儲存
第6-11 調用靜態方法print並打印“try 執行”
第14-15 把string0的值放進另一個string3
第16-18 給string0賦值“finally model”
第19-24 打印“String finally 執行”
第27-28 加載string3(當前是以前的string0即“try model”)，然後返回
由此可知，實際返回的是try model，在執行try之前會有一個臨時變量String3將string0裝進去，然後返回。
當在finally中返回的時候呢?代碼如下：
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 執行");
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 執行");
return string;
}finally {
string = "finally model";
System.out.println("finally 執行");
return string;
}
}
}得到的結果爲：
同理分析javap 皮一下
Code:
stack=2, locals=2, args_size=0
0: ldc #34 // String
2: astore_0
3: ldc #36 // String try model
5: astore_0
6: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
9: ldc #38 // String try 執行
11: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
14: goto 33
17: astore_1
18: ldc #40 // String catch model
20: astore_0
21: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
24: ldc #42 // String catch 執行
26: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
29: goto 33
32: pop
33: ldc #44 // String finally model
35: astore_0
36: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
39: ldc #46 // String finally 執行
41: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
44: aload_0
45: areturn同上面的分析，不同的是14行的時候就goto到33,33-35直接把“finally model”裝進string0
然後打印“finally 執行” 最後aload加載string0 然後返回，即返回的是“finally model”
萬一遇見個錯誤呢怎麼返回，代碼一下：
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 執行");
int k = 5/0;
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 執行");
return string;
}finally {
string = "finally model";
System.out.println("finally 執行");
}
}
}返回的結果爲：
皮一下：
Code:
stack=2, locals=4, args_size=0
0: ldc #34 // String
2: astore_0
3: ldc #36 // String try model
5: astore_0
6: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
9: ldc #38 // String try 執行
11: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
14: iconst_5
15: iconst_0
16: idiv
17: istore_1
18: aload_0
19: astore_3
20: ldc #40 // String finally model
22: astore_0
23: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
26: ldc #42 // String finally 執行
28: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
31: aload_3
32: areturn
33: astore_1
34: ldc #44 // String catch model
36: astore_0
37: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
40: ldc #46 // String catch 執行
42: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
45: aload_0
46: astore_3
47: ldc #40 // String finally model
49: astore_0
50: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
53: ldc #42 // String finally 執行
55: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
58: aload_3
59: areturn
60: astore_2
61: ldc #40 // String finally model
63: astore_0
64: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
67: ldc #42 // String finally 執行
69: invokevirtual #26 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
72: aload_2
73: athrow分析分析，前面講過的不說，第14-16行除法，存進寄存器1中，遇見異常還是直接跳過第一個return還是咋的
跳過第一個return不說（具體原因我也不清楚），34-46 將string0標記爲“catch model”然後打印“catch 執行”然後裝進寄存器3中，其他同上面1的裏，然後的還是寄存器3裏面的值。
在異常的時候 finally加return
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 執行");
int k = 5/0;
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 執行");
return string;
}finally {
string = "finally model";
System.out.println("finally 執行");
return string;
}
}
}這個就不說了 和第二個類似 返回finally model
在catch裏面遇見異常呢：
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 執行");
int k = 5/0;
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 執行");
int k = 5/0;
return string;
}finally {
string = "finally model";
System.out.println("finally 執行");
}
}
}直接返回異常
在finally的時候返回呢？
public class TryTest2 {
public static void main(String[] args) {
System.out.println(getRet());
}
public static String getRet(){
String string = "";
try{
string = "try model";
System.out.println("try 執行");
int k = 5/0;
return string;
} catch (Exception e) {
string = "catch model";
System.out.println("catch 執行");
int k = 5/0;
return string;
}finally {
string = "finally model";
System.out.println("finally 執行");
return string;
}
}
}運行結果：
總結總結：在finally中返回，必爲返回值finally
如果在catch中抓取的不是那個異常，或者catch中遇見異常，或者finally中遇見異常直接拋出異常
try中異常 返回的是try中的值
so：千萬不要在finally中return