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

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