try{...}catch(){...}finally{...}语句你真的理解吗？-阿里云开发者社区

try{...}catch(){...}finally{...}语句你真的理解吗？

2023-01-06 103

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简介： try{...}catch(){...}finally{...}语句你真的理解吗？

/**
 * @desc: 探究try...catch...finally语句块
 * 六种模式：
 * （1）try{ ... } catch(){ ... }finally{ ... }  ... return;
 * （2）try{ ... return;}catch(){ ... } finally{ ... }  ... return;
 * （3）try{ ... } catch(){ ... return;} finally{ ... } ... return;
 * （4）try{ ... return;}catch(){ ... } finally{ ... return;}
 * （5）try{ ... } catch(){ ... return;}finally{ ... return;}
 * （6）try{ ... return;}catch(){ ... return;} finally{ ... return;}
 * @author: YanMingXin
 * @create: 2021/8/10-10:16
 **/
public class TryTest {
    /**
     * 常量1
     */
    public static final String s1 = "Hello";
    /**
     * 常量2
     */
    public static final String s2 = "World";
    /**
     * try{ ... } catch(){ ... }finally{ ... }  ... return;
     *
     * @return
     */
    public static String model1() {
        String s = new String();
        try {
            s = s1 + s2;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            System.out.println("finally " + s);
        }
        return s;
    }
    public static String model1HaveException() {
        String s = new String();
        try {
            s = s1 + s2;
            int i = 2 / 0;
        } catch (Exception e) {
            System.err.println("model1异常信息");
            e.printStackTrace();
        } finally {
            System.out.println("finally " + s);
        }
        return s;
    }
    /**
     * try{ ... return;}catch(){ ... } finally{ ... }  ... return;
     *
     * @return
     */
    public static String model2() {
        String s = new String();
        try {
            s = s1 + s2;
            return s;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            System.out.println("finally " + s);
        }
        return s;
    }
    public static String model2HaveException() {
        String s = new String();
        try {
            s = s1 + s2;
            int i = 2 / 0;
            return s;
        } catch (Exception e) {
            System.err.println("model2异常信息");
            e.printStackTrace();
        } finally {
            System.out.println("finally " + s);
        }
        return s;
    }
    /**
     * try{ ... } catch(){ ... return;} finally{ ... } ... return;
     *
     * @return
     */
    public static String model3() {
        String s = new String();
        try {
            s = s1 + s2;
            //return s;  与最后的return冲突
        } catch (Exception e) {
            e.printStackTrace();
            return s;
        } finally {
            System.out.println("finally " + s);
        }
        return s;
    }
    public static String model3HaveException() {
        String s = new String();
        try {
            s = s1 + s2;
            int i = 1 / 0;
            //return s;  与最后的return冲突
        } catch (Exception e) {
            System.err.println("model3异常信息");
            e.printStackTrace();
            return s;
        } finally {
            System.out.println("finally " + s);
        }
        return s;
    }
    /**
     * try{ ... return;}catch(){ ... } finally{ ... return;}
     *
     * @return
     */
    public static String model4() {
        String s = new String();
        try {
            s = s1 + s2;
            return s;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            System.out.println("finally " + s);
            return s;
        }
        //return s; 与finally的return冲突
    }
    public static String model4HaveException() {
        String s = new String();
        try {
            s = s1 + s2;
            int i = 5 / 0;
            return s;
        } catch (Exception e) {
            System.err.println("model4异常信息");
            e.printStackTrace();
        } finally {
            System.out.println("finally " + s);
            return s;
        }
        //return s; 与finally的return冲突
    }
    /**
     * try{ ... } catch(){ ... return;}finally{ ... return;}
     *
     * @return
     */
    public static String model5() {
        String s = new String();
        try {
            s = s1 + s2;
        } catch (Exception e) {
            e.printStackTrace();
            return s;
        } finally {
            System.out.println("finally " + s);
            return s;
        }
    }
    public static String model5HaveException() {
        String s = new String();
        try {
            s = s1 + s2;
            int i = 2 / 0;
        } catch (Exception e) {
            System.err.println("model5异常信息");
            e.printStackTrace();
            return s;
        } finally {
            System.out.println("finally " + s);
            return s;
        }
    }
    /**
     * try{ ... return;}catch(){ ... return;} finally{ ... return;}
     *
     * @return
     */
    public static String model6() {
        String s = new String();
        try {
            s = s1 + s2;
            return s;
        } catch (Exception e) {
            e.printStackTrace();
            return s;
        } finally {
            System.out.println("finally " + s);
            return s;
        }
    }
    public static String model6HaveException() {
        String s = new String();
        try {
            s = s1 + s2;
            int i = 2 / 0;
            return s;
        } catch (Exception e) {
            System.err.println("model6异常信息");
            e.printStackTrace();
            return s;
        } finally {
            System.out.println("finally " + s);
            return s;
        }
    }
    public static void main(String[] args) {
        System.out.println("model1无异常：");
        System.out.println(model1());
        System.out.println("model1有异常：");
        System.out.println(model1HaveException());
        System.out.println("model2无异常：");
        System.out.println(model2());
        System.out.println("model2有异常：");
        System.out.println(model2HaveException());
        System.out.println("model3无异常：");
        System.out.println(model3());
        System.out.println("model3有异常：");
        System.out.println(model3HaveException());
        System.out.println("model4无异常：");
        System.out.println(model4());
        System.out.println("model4有异常：");
        System.out.println(model4HaveException());
        System.out.println("model5无异常：");
        System.out.println(model5());
        System.out.println("model5有异常：");
        System.out.println(model5HaveException());
        System.out.println("model6无异常：");
        System.out.println(model6());
        System.out.println("model6有异常：");
        System.out.println(model6HaveException());
    }
}
复制代码

运行结果：

model1无异常：
finally HelloWorld
HelloWorld
model1有异常：
finally HelloWorld
HelloWorld
model2无异常：
finally HelloWorld
HelloWorld
model2有异常：
finally HelloWorld
HelloWorld
model3无异常：
finally HelloWorld
HelloWorld
model3有异常：
finally HelloWorld
HelloWorld
model4无异常：
finally HelloWorld
HelloWorld
model4有异常：
finally HelloWorld
HelloWorld
model5无异常：
finally HelloWorld
HelloWorld
model5有异常：
finally HelloWorld
HelloWorld
model6无异常：
finally HelloWorld
HelloWorld
model6有异常：
finally HelloWorld
HelloWorld
model1异常信息
java.lang.ArithmeticException: / by zero
  at org.ymx.test0810.TryTest.model1HaveException(TryTest.java:49)
  at org.ymx.test0810.TryTest.main(TryTest.java:234)
model2异常信息
java.lang.ArithmeticException: / by zero
  at org.ymx.test0810.TryTest.model2HaveException(TryTest.java:81)
  at org.ymx.test0810.TryTest.main(TryTest.java:239)
model3异常信息
java.lang.ArithmeticException: / by zero
  at org.ymx.test0810.TryTest.model3HaveException(TryTest.java:115)
  at org.ymx.test0810.TryTest.main(TryTest.java:244)
model4异常信息
java.lang.ArithmeticException: / by zero
  at org.ymx.test0810.TryTest.model4HaveException(TryTest.java:150)
  at org.ymx.test0810.TryTest.main(TryTest.java:249)
model5异常信息
java.lang.ArithmeticException: / by zero
  at org.ymx.test0810.TryTest.model5HaveException(TryTest.java:184)
  at org.ymx.test0810.TryTest.main(TryTest.java:254)
model6异常信息
java.lang.ArithmeticException: / by zero
  at org.ymx.test0810.TryTest.model6HaveException(TryTest.java:218)
  at org.ymx.test0810.TryTest.main(TryTest.java:259)
Process finished with exit code 0
复制代码

结论：

1、不管有没有出现异常，finally块中代码都会执行；

2、当try和catch中有return时，finally仍然会执行；

3、finally是在return后面的表达式运算后执行的（此时并没有返回运算后的值，而是先把要返回的值保存起来，管finally中的代码怎么样，返回的值都不会改变，任然是之前保存的值），所以函数返回值是在finally执行前确定的；

4、finally中最好不要包含return，否则程序会提前退出，返回值不是try或catch中保存的返回值。

任何执行try 或者catch中的return语句之前，都会先执行finally语句，如果finally存在的话。如果finally中有return语句，那么程序就return了，所以finally中的return是一定会被return的，编译器把finally中的return实现为一个warning。

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