当return遇到finally

冷不防 2022-04-11 09:12 197阅读 0赞

猜猜下面java程序分别会有什么样的输出？

程序：

public class Test {
        
        public static void main(String[] args) {
            
            System.out.println("=============test1==================");
            System.out.println(test1());
            System.out.println("===============================");
            System.out.println("=============test1_1==================");
            System.out.println(test1_1());
            System.out.println("===============================");
            System.out.println("\n============test2===================");
            System.out.println(test2());
            System.out.println("===============================");
            System.out.println("\n============test2_1===================");
            System.out.println(test2_1());
            System.out.println("===============================");
            System.out.println("\n============test3===================");
            System.out.println(test3());
            System.out.println("===============================");
            System.out.println("\n============test3_1===================");
            System.out.println(test3_1());
            System.out.println("===============================");
        }
    
        public static String test1() {
            String a = "in try";
            try {
                return a;
            } catch (Exception e) {
            } finally {
                a = "in finally";
                System.out.println("do finally");
            }
            return a;
        }
    
        public static String test1_1() {
            String a = "in try";
            try {
                return a;
            } catch (Exception e) {
            } finally {
                a = "in finally";
                System.out.println("do finally");
                return a;
            }
        }
    
        public static int test2() {
            int a = 1;
            try {
                return a;
            } catch (Exception e) {
            } finally {
                a = 2;
                System.out.println("do finally");
            }
            return a;
        }
    
        public static int test2_1() {
            int a = 1;
            try {
                return a;
            } catch (Exception e) {
            } finally {
                a = 2;
                System.out.println("do finally");
                return a;
            }
        }
    
        public static Helper test3() {
            Helper a = new Helper();
            a.a = 1;
            try {
                return a;
            } catch (Exception e) {
            } finally {
                a.a = 2;
                System.out.println("do finally");
            }
            return a;
        }
    
        public static Helper test3_1() {
            Helper a = new Helper();
            a.a = 1;
            try {
                return a;
            } catch (Exception e) {
            } finally {
                a.a = 2;
                System.out.println("do finally");
                return a;
            }
        }
    
        static class Helper {
            int a;
    
            public String toString() {
                return String.valueOf(a);
            }
        }
    }

运行结果：

=============test1==================
    do finally
    in try
    ===============================
    =============test1_1==================
    do finally
    in finally
    ===============================
    
    ============test2===================
    do finally
    1
    ===============================
    
    ============test2_1===================
    do finally
    2
    ===============================
    
    ============test3===================
    do finally
    2
    ===============================
    
    ============test3_1===================
    do finally
    2
    ===============================

结论：

1.在try catch块里return的时候，finally也会被执行。

2.return语句会把后面的值复制到一份用来返回，如果return的是基本类型的，finally里对变量的改动将不起效果，如果return 的是引用类型的，改动将可以起效果。

3.finally里的return语句会把try catch块里的return语句效果给覆盖掉。

看来return语句并不一定都是函数的出口，执行return时，只是把return后面的值复制了一份到返回值变量里去了。看来最佳实践是：

1.最好把return放到方法尾而不要在try cath 里return

2.不要在try catch块和finally块里都包含return

3.如果在try catch块里return, 则不要在finally块里操作被return的变量

文章来源：[http://sulong.me/2007/12/24/return\_in\_finally][http_sulong.me_2007_12_24_return_in_finally]

[http_sulong.me_2007_12_24_return_in_finally]: http://sulong.me/2007/12/24/return_in_finally