'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