Try catch finally 與 return 結合使用執行順序所有情況分析

首先對按鈕添加點擊事件，事件代碼爲：System.out.println(test());

一、無異常時

1、			
private String test(){
        try {
            System.out.println("try...");   
        } catch (Exception e) {
            e.getMessage();
            System.out.println("catch..."); 
        } finally {
            System.out.println("finally..."); 
        }
        return "return at last...";
    }
運行結果：
2019-07-15 10:21:52.049 22618-22618/com.example.mymvp I/System.out: try...
2019-07-15 10:21:52.049 22618-22618/com.example.mymvp I/System.out: finally...
2019-07-15 10:21:52.049 22618-22618/com.example.mymvp I/System.out: return at last...

2、
private String test(){
        try {
            System.out.println("try...");
            return "return at try..."; 
        } catch (Exception e) {
            e.getMessage();
            System.out.println("catch...");
        } finally {
            System.out.println("finally...");
        }
        return "return at last...";
    }
運行結果：
2019-07-15 10:25:13.605 22816-22816/com.example.mymvp I/System.out: try...
2019-07-15 10:25:13.605 22816-22816/com.example.mymvp I/System.out: finally...
2019-07-15 10:25:13.605 22816-22816/com.example.mymvp I/System.out: return at try...

3、
private String test(){
        try {
            System.out.println("try...");
            return "return at try...";
        } catch (Exception e) {
            e.getMessage();
            System.out.println("catch...");
        } finally {
            System.out.println("finally...");
            return "return at finally...";
        }
    }
運行結果：
2019-07-15 10:26:35.815 23033-23033/com.example.mymvp I/System.out: try...
2019-07-15 10:26:35.815 23033-23033/com.example.mymvp I/System.out: finally...
2019-07-15 10:26:35.815 23033-23033/com.example.mymvp I/System.out: return at finally...

結論：無異常時，首先執行try代碼塊，如果try代碼塊有return則在執行return語句前跳轉到finally代碼塊執行，如果無return，則執行完整個try代碼塊後跳轉到finally代碼塊，執行finally代碼塊，如果finally代碼塊有return，則執行return返回，如果沒有則執行完finally代碼塊後回到try的return語句處執行並返回。

二、有異常時

1、
   private String test(){
        try {
            System.out.println("try...");
            System.out.println("exception before...");
            int i = 1/0;
            System.out.println("exception after...");
        } catch (Exception e) {
            e.getMessage();
            System.out.println("catch...");
        } finally {
            System.out.println("finally...");
        }
        return "return at last...";
}
執行結果
2019-07-15 10:29:27.109 23259-23259/com.example.mymvp I/System.out: try...
2019-07-15 10:29:27.109 23259-23259/com.example.mymvp I/System.out: exception before...
2019-07-15 10:29:27.110 23259-23259/com.example.mymvp I/System.out: catch...
2019-07-15 10:29:27.110 23259-23259/com.example.mymvp I/System.out: finally...
2019-07-15 10:29:27.110 23259-23259/com.example.mymvp I/System.out: return at last...

2、
    private String test(){
        try {
            System.out.println("try...");
            System.out.println("exception before...");
            int i = 1/0;
            System.out.println("exception after...");
            return "return at try...";
        } catch (Exception e) {
            e.getMessage();
            System.out.println("catch...");
            return "return at catch...";
        } finally {
            System.out.println("finally...");
        }
    }
執行結果
2019-07-15 10:31:12.413 23600-23600/com.example.mymvp I/System.out: try...
2019-07-15 10:31:12.413 23600-23600/com.example.mymvp I/System.out: exception before...
2019-07-15 10:31:12.413 23600-23600/com.example.mymvp I/System.out: catch...
2019-07-15 10:31:12.413 23600-23600/com.example.mymvp I/System.out: finally...
2019-07-15 10:31:12.413 23600-23600/com.example.mymvp I/System.out: return at catch...

3、
   private String test(){
        try {
            System.out.println("try...");
            System.out.println("exception before...");
            int i = 1/0;
            System.out.println("exception after...");
            return "return at try...";
        } catch (Exception e) {
            e.getMessage();
            System.out.println("catch...");
            return "return at catch...";
        } finally {
            System.out.println("finally...");
            return "return at finally...";
        }
}
執行結果
2019-07-15 10:32:32.494 23784-23784/com.example.mymvp I/System.out: try...
2019-07-15 10:32:32.494 23784-23784/com.example.mymvp I/System.out: exception before...
2019-07-15 10:32:32.494 23784-23784/com.example.mymvp I/System.out: catch...
2019-07-15 10:32:32.494 23784-23784/com.example.mymvp I/System.out: finally...
2019-07-15 10:32:32.494 23784-23784/com.example.mymvp I/System.out: return at finally...

4、
private String test(){
        try {
            System.out.println("try...");
            System.out.println("exception before...");
            int i = 1/0;
            System.out.println("exception after...");
            return "return at try...";
        } catch (Exception e) {
            e.getMessage();
            System.out.println("catch...");
        } finally {
            System.out.println("finally...");
        }
        return "return at last...";
}
結果：
2019-07-22 15:18:17.325 7549-7549/com.example.mymvp I/System.out: try...
2019-07-22 15:18:17.325 7549-7549/com.example.mymvp I/System.out: exception before...
2019-07-22 15:18:17.326 7549-7549/com.example.mymvp I/System.out: catch...
2019-07-22 15:18:17.326 7549-7549/com.example.mymvp I/System.out: finally...
2019-07-22 15:18:17.326 7549-7549/com.example.mymvp I/System.out: return at last...
如果catch和finally中沒有return,而在try發生異常的後面有return時，try中的return並不執行，切記。

總結：如果有異常時，首先執行try代碼塊到發生異常的地方，然後執行catch代碼塊到return語句的前一行代碼處（如果return存在），然後再執行finally代碼塊，如果finally中有return，則執行並返回，如果沒有，再檢查catch中是否有return，有則執行並返回，無則跳出try catch finally代碼塊。

 

總結：

整個try catch finally 與 return 結合使用的執行，首先按照try catch finally的順序正常執行代碼，這個過程中不執行return（如果存在return的話） ，執行完成後，再反向finaly catch try 的順序去查找return，執行遇到的第一個return，然後返回，代碼執行完成。

 

但是，依然有一個需要特別注意的地方，如下：

1、
public static String test() {
        String str = "A";
        try {
            str = "B";
            return "the return is: " + str;
        } finally {
            System.out.println("finally change return string to C");
            str = "C";
            //return "the return is" + str;
        }
}
執行結果：
    finally change return string to C
    the return is: B
表示finally中的代碼str = "C";並沒有生效。

如果：
2、
public static String test() {
        String str = "A";
        try {
            str = "B";
            //return "the return is: " + str;
        } finally {
            System.out.println("finally change return string to C");
            str = "C";
            return "the return is: " + str;
        }
}
執行結果：
    finally change return string to C
    the return is: C
表示finally中的代碼修改str = "C";生效了

表示，如果finally中修改某變量，如果return在finally中，則該修改生效，如果return在try中，則該修改並不生效，需要特別注意。