java异常处理-finally中使用return和throw语句

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;
}
}

即：只要finally中使用了return语句，调用者便认为该方法正常返回

finally块中使用return会抑制异常的冒泡传输

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