try-catch-finally-return的执行顺序
总结如下几条情况
-
·try-catch-finally都有return语句时,没有异常时,返回值是finally中的return返回的·。
- 执行try块,执行到return语句时,先执行return的语句
- 但是不返回到main方法,执行finally块,遇到finally块中的return语句
- 并将值返回到main方法,这里就不会再回去返回try块中计算得到的值
- 所以最终的值是finally的返回
- 举例如下
public static void main(String[] args) throws ParseException { int i = NoException(); System.out.println("最终值: " + i); } public static int NoException() { int i = 10; try { System.out.println("i in try block is:" + i); return --i; } catch (Exception e) { --i; System.out.println("i in catch - form try block is:" + i); return --i; } finally { System.out.println("i in finally - from try or catch block is:" + i); return --i; } } /* i in try block is:10 i in finally - from try or catch block is:9 最终值: 8 */
-
try-catch都有return语句时,没有异常时,返回值是try中的return返回的
- try中执行完return的语句后,不返回,执行finally块
- finally块执行结束后,返回到try块中
- 最后,返回在try块中最后的值
- 举例如下
public static void main(String[] args) throws ParseException { int i = NoException(); System.out.println("最终值: " + i); } public static int NoException() { int i=10; try{ System.out.println("i in try block is:"+i); return --i; } catch(Exception e){ --i; System.out.println("i in catch - form try block is:"+i); return --i; } finally{ System.out.println("i in finally - from try or catch block is:"+i); --i; System.out.println("i in finally block is:"+i); //return --i; } } /* i in try block is:10 i in finally - from try or catch block is:9 i in finally block is:8 最终值: 9 */
-
try块中抛出异常,try、catch和finally中都有return语句,返回值是finally中的return。
- 抛出异常后,执行catch块,在catch块的return的执行完后
- 并不直接返回而是执行finally,因finally中有return语句,所以,执行finally里面的语句
- 最终返回finally的return
- 举例如下
public static void main(String[] args) throws ParseException { int i = NoException(); System.out.println("最终值: " + i); } public static int NoException() { int i = 10; try { System.out.println("i in try block is:" + i); i = i / 0; return --i; } catch (Exception e) { System.out.println("i in catch - form try block is:" + i); --i; System.out.println("i in catch block is:" + i); return --i; } finally { System.out.println("i in finally - from try or catch block is--" + i); --i; System.out.println("i in finally block is--" + i); return --i; } } /* i in try block is:10 i in catch - form try block is:10 i in catch block is:9 i in finally - from try or catch block is--8 i in finally block is--7 最终值: 6 */
-
try块中抛出异常,try和catch中都有return语句,返回的catch中return值。
- 抛出异常后,执行catch块
- 执行完finally语句后,依旧返回catch中的执行return语句后的值,而不是finally中修改的值。
- 举例如下
public static void main(String[] args) throws ParseException { int i = NoException(); System.out.println("最终值: " + i); } public static int NoException() { int i = 10; try { System.out.println("i in try block is:" + i); i = i / 0; return --i; } catch (Exception e) { System.out.println("i in catch - form try block is:" + i); return --i; } finally { System.out.println("i in finally - from try or catch block is:" + i); --i; System.out.println("i in finally block is:" + i); //return i; } } /* i in try block is:10 i in catch - form try block is:10 i in finally - from try or catch block is:9 i in finally block is:8 最终值: 9 */
-
try、catch中都出现异常,在finally中有返回,返回finally中return值
- try块中出现异常到catch,catch中出现异常到finally
- finally中执行到return语句返回,不检查异常。
- 举例如下
public static void main(String[] args) throws ParseException { int i = NoException(); System.out.println("最终值: " + i); } public static int NoException() { int i = 10; try { System.out.println("i in try block is:" + i); i = i / 0; return --i; } catch (Exception e) { System.out.println("i in catch - form try block is:" + i); int j = i / 0; return --i; } finally { System.out.println("i in finally - from try or catch block is:" + i); --i; System.out.println("i in finally block is:" + i); return --i; } } /* i in try block is:10 i in catch - form try block is:10 i in finally - from try or catch block is:10 i in finally block is:9 最终值: 8 */
try-with-resource语法糖
JDK7之后,Java多了个新的语法:try-with-resources语句,该语句确保在语句执行完毕后,每个资源都被自动关闭 。
可以理解为是一个声明一个或多个资源的 try语句(用分号隔开),一个资源作为一个对象,并且这个资源必须要在执行完关闭的,
其实之前的手动关闭,在这个语法糖里面并没有被取消,而是因为编译器,编译器自动帮我们生成了finally块,并且在里面调用了资源的close方法,所以例子中的close方法会在运行的时候被执行。
原理
传统写法
public void readFile() throws FileNotFoundException {
FileReader fr = null;
BufferedReader br = null;
try{
fr = new FileReader("d:/test.txt");
br = new BufferedReader(fr);
String s = "";
while((s = br.readLine()) != null){
System.out.println(s);
}
} catch (IOException e) {
e.printStackTrace();
}finally {
try {
br.close();
fr.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
使用当前语法
public void readFile() throws FileNotFoundException {
try(
FileReader fr = new FileReader("d:/test.txt");
BufferedReader br = new BufferedReader(fr)
){
String s = "";
while((s = br.readLine()) != null){
System.out.println(s);
}
} catch (IOException e) {
e.printStackTrace();
}
}