First define a class Cat to implement the AutoCloseable interface

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()");
    }
}

Our Cat has the following features:

• The sayHello method throws an exception
• The close method also throws an exception

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

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

Output of results:

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

conclusion

• try(Cat cat = new Cat())
  • close is called automatically when the try block is complete
  • close throws an exception, which is Suppresseded. The outer layer only catches the exception of sayHello, but this Suppresseded exception can be found on the stack
• try finally
  • The outer layer captures the exception thrown when finally close s, and the sayHello exception is completely missing.

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

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

Output of Results

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

conclusion

• try(Cat cat = new Cat()) catch
  • Close was called before catch (the conclusion that close is automatically called after a try-compliant block is completed)
  • catch gets an exception for sayHello, and close throws an exception that is still Suppressed ed
  • An exception thrown again in the catch is caught by the outer layer
• try catch finally
  • Catch first, then finally, so catch catches the exception of sayHello
  • The exception thrown again in the catch disappears, and the outer layer captures the exception thrown when finally close s.

Test code address: https://github.com/kongxiangxin/pine/tree/master/auto-closeable