'try(A a = new A())' VS 'try finally'

實現了AutoCloseable接口的類，可以在try的時候直接實例化對象。try代碼塊完成之後，自動調用close方法，相當於在finally裏主動調用。但是出現異常後的流程和try finally有什麼不同呢？ 下面寫代碼測試一下。

首先定義一個類Cat，實現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()");
    }
}

我們的這個Cat有以下特點：

• sayHello方法會拋出異常
• close方法也會拋出異常

test v1: 'try(Cat cat = new Cat())' VS 'try finally'

沒有catch（不要被外層的catch迷惑，那只是爲了打印異常）

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("--------------");
}

結果輸出：

----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)
--------------

結論

• try(Cat cat = new Cat())
  • try代碼塊完成之後會自動調用close
  • close拋出的異常，被Suppressed了，外層捕獲的只有sayHello的異常，但通過堆棧可以找到這個Suppressed的異常
• try finally
  • 外層捕獲的是在finally執行close時拋出的異常，sayHello的異常完全不見了。

test v2: 'try(Cat cat = new Cat()) catch{}' VS 'try catch finally'

有catch，並且catch裏再拋出異常

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("-------------------------------------------");
}

結果輸出

----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)
-------------------------------------------
---------------------------------------------------------------------

結論

• try(Cat cat = new Cat()) catch
  • catch之前就調用了close（符合try代碼塊完成之後會自動調用close這個結論）
  • catch到的是sayHello的異常，close拋出的異常依然被Suppressed了
  • catch中再次拋出的異常被外層捕獲
• try catch finally
  • 先走catch，再走finally，所以catch捕獲的是sayHello的異常
  • catch中再次拋出的異常不見了，外層捕獲的是在finally執行close時拋出的異常。

測試代碼地址：https://github.com/kongxiangxin/pine/tree/master/auto-closeable