JAVA高级面试进阶训练营视频教程

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java异常语句中的finally块通常用来做资源释放操作，如关闭文件、关闭网络连接、关闭数据库连接等。正常情况下finally语句中不应该使用return语句也不应该抛出异常，以下讨论仅限于java语言设计本身，正常编码时应避免。

finally块中使用return会覆盖method的返回值

以下代码的返回值为：1

public static int div(){
        try {
            return 3;
        }catch (ArithmeticException e){
            System.out.println("catch in div");
            return 2;
        }
        finally {
            System.out.println("finally in div");
            return 1;
        }
    }

以下代码的返回值同样是：1

 public static int div(){
        try {
            return 3/0;
        }catch (ArithmeticException e){
            System.out.println("catch in div");
            return 2;
        }
        finally {
            System.out.println("finally in div");
            return 1;
        }
    }

 

finally块中使用return会抑制异常的冒泡传输

即：只要finally中使用了return语句，调用者便认为该方法正常返回

以下代码的输出为

catch in div
finally in div
catch in adapter
finally in adapter

/**
 * Created by Administrator on 2017/11/27.
 */

public class Test {

    public static void main(String[] args) {
        adapter();
    }

    public static void adapter() {
        try {
            div();
        } catch (ArithmeticException e) {
            System.out.println("catch in adapter");
        } finally {
            System.out.println("finally in adapter");
        }
    }

    public static int div() {
        try {
            int a = 5 / 0;
            return a;
        } catch (ArithmeticException e) {
            System.out.println("catch in div");
            throw e;  // 重新将异常抛出给调用者
        } finally {
            System.out.println("finally in div");
        }
    }
}

 

 但如果在 div 的finally块中添加了return语句

    public static int div(){
        try {
            int a =  5/0;
            return a;
        }catch (ArithmeticException e){
            System.out.println("catch in div");
            throw  e; // 重新将异常抛出给调用者，但是抛出会被忽略
        }
        finally {
            System.out.println("finally in div");
            return 1;
        }
    }

则代码的输出为

catch in div
finally in div
finally in adapter

即：finally块中的return语句会阻止异常的栈调用传输，使caller认为该方法已经正常返回

 

finally块中的throw语句会覆盖try和catch语句中的异常

以下代码

/**
 * Created by Administrator on 2017/11/27.
 */

public class Test {

    public static void main(String[] args) {
        adapter();
    }

    public static void adapter() {
        try {
            div();
        } catch (Exception e) {
            System.out.println(String.format("catch in adapter: %s",e.getMessage()));
        } finally {
            System.out.println("finally in adapter");
        }
    }

    public static int div() throws Exception{
        try {
            int a = 5 / 0;
            return a;
        } catch (ArithmeticException e) {
            System.out.println("catch in div");
            throw new Exception("Exception in div"); // 抛出新的异常
        } finally {
            System.out.println("finally in div");
            throw new Exception("Exception in Finally");  // 抛出新的异常
        }
    }
}

输出是：

catch in div
finally in div
catch in adapter: Exception in Finally
finally in adapter

即，catch块中抛出的异常北finally块抛出的异常替换了

 

修改div方法为

 public static int div() throws Exception{
        try {
            int a = 5 / 0;
            return a;
        }  finally {
            System.out.println("finally in div");
            throw new Exception("Exception in Finally");  // 抛出新的异常
        }
    }

输出为：

finally in div
catch in adapter: Exception in Finally
finally in adapter

即，try块中捕获的异常北finally块抛出的异常替换

 

finally块和普通代码块一样，无法同时使用return语句和throw语句，因为无法通过编译