'try(A a = new A())' VS 'try finally'
- 2019 年 11 月 23 日
- 筆記
实现了AutoCloseable接口的类，可以在try的时候直接实例化对象。try代码块完成之后，自动调用close方法，相当于在finally里主动调用。但是出现异常后的流程和try finally有什么不同呢？ 下面写代码测试一下。
首先定义一个类Cat，实现AutoCloseable接口
class Cat implements AutoCloseable{ void sayHello() throws Exception { Utils.println("calling sayHello(), I will throw an exception"); throw new Exception("Exception in sayHello() "); } @Override public void close() throws Exception { Utils.println("I'm closing, I will throw an exception"); throw new Exception("Exception in close()"); } }
我们的这个Cat有以下特点：
- sayHello方法会抛出异常
- close方法也会抛出异常
test v1: 'try(Cat cat = new Cat())' VS 'try finally'
没有catch（不要被外层的catch迷惑，那只是为了打印异常）
static void testV1(){ Utils.println("----try(Cat cat = new Cat())-----"); try{ try(Cat cat = new Cat()){ cat.sayHello(); } }catch (Exception e){ Utils.println("cache error in main (" + e + "), let's see its stack trace"); Utils.printStackTrace(e); } Utils.println("--------------"); Utils.println("----try finally-----"); try{ Cat cat = null; try{ cat = new Cat(); cat.sayHello(); }finally { if(cat != null){ cat.close(); } } }catch (Exception e){ Utils.println("cache error in main (" + e + "), let's see its stack trace"); Utils.printStackTrace(e); } Utils.println("--------------"); }
结果输出：
----test v1---------------------------------------- ----try(Cat cat = new Cat())----- calling sayHello(), I will throw an exception I'm closing, I will throw an exception cache error in main (java.lang.Exception: Exception in sayHello() ), let's see its stack trace java.lang.Exception: Exception in sayHello() at Cat.sayHello(Cat.java:4) at Test.testV1(Test.java:16) at Test.main(Test.java:4) Suppressed: java.lang.Exception: Exception in close() at Cat.close(Cat.java:10) at Test.testV1(Test.java:17) ... 1 more -------------- ----try finally----- calling sayHello(), I will throw an exception I'm closing, I will throw an exception cache error in main (java.lang.Exception: Exception in close()), let's see its stack trace java.lang.Exception: Exception in close() at Cat.close(Cat.java:10) at Test.testV1(Test.java:33) at Test.main(Test.java:4) --------------
结论
- try(Cat cat = new Cat())
- try代码块完成之后会自动调用close
- close抛出的异常，被Suppressed了，外层捕获的只有sayHello的异常，但通过堆栈可以找到这个Suppressed的异常
- try finally
- 外层捕获的是在finally执行close时抛出的异常，sayHello的异常完全不见了。
test v2: 'try(Cat cat = new Cat()) catch{}' VS 'try catch finally'
有catch，并且catch里再抛出异常
static void testV2(){ Utils.println("----try(Cat cat = new Cat()) catch-----"); try{ try(Cat cat = new Cat()){ cat.sayHello(); } catch (Exception e) { Utils.println("cached err (" + e.getMessage() + "), I will throw an exception again"); throw new Exception("Exception in catch", e); } }catch (Exception e){ Utils.println("cache error in main (" + e + "), let's see its stack trace"); Utils.printStackTrace(e); } Utils.println("-----------------------------------------"); Utils.println("----try catch finally--------------------"); try{ Cat cat = null; try{ cat = new Cat(); cat.sayHello(); } catch (Exception e) { Utils.println("cached err (" + e.getMessage() + "), I will throw an exception again"); throw new Exception("Exception in catch", e); }finally { if(cat != null){ cat.close(); } } }catch (Exception e){ Utils.println("cache error in main (" + e + "), let's see its stack trace"); Utils.printStackTrace(e); } Utils.println("-------------------------------------------"); }
结果输出
----test v2------ ----try(Cat cat = new Cat()){} catch{}----- calling sayHello(), I will throw an exception I'm closing, I will throw an exception cached err (Exception in sayHello() ), I will throw an exception again cache error in main (java.lang.Exception: Exception in catch), let's see its stack trace java.lang.Exception: Exception in catch at Test.testV2(Test.java:50) at Test.main(Test.java:8) ----------------------------------------- ----try catch finally-------------------- calling sayHello(), I will throw an exception cached err (Exception in sayHello() ), I will throw an exception again I'm closing, I will throw an exception cache error in main (java.lang.Exception: Exception in close()), let's see its stack trace java.lang.Exception: Exception in close() at Cat.close(Cat.java:10) at Test.testV2(Test.java:70) at Test.main(Test.java:8) ------------------------------------------- ---------------------------------------------------------------------
结论
- try(Cat cat = new Cat()) catch
- catch之前就调用了close（符合try代码块完成之后会自动调用close这个结论）
- catch到的是sayHello的异常，close抛出的异常依然被Suppressed了
- catch中再次抛出的异常被外层捕获
- try catch finally
- 先走catch，再走finally，所以catch捕获的是sayHello的异常
- catch中再次抛出的异常不见了，外层捕获的是在finally执行close时抛出的异常。
测试代码地址：https://github.com/kongxiangxin/pine/tree/master/auto-closeable
