Understanding Try/Finally Suppressed Exceptions-CodePudding

I'm learning Exceptions and I found out that you can have suppressed ones. I read many examples on stackoverflow but still, they don't work as they are supposed to in the "try/finally" case:

public class MultipleExceptionsExample {

   static class IOManip implements Closeable{
       @Override
       public void close() {
           throw new RuntimeException("from IOManip.close");
       }
   }

   public static void main(String[] args) {
       try(IOManip ioManip = new IOManip()){
           throw new RuntimeException("from try!");
       }catch(Exception e){
           throw new RuntimeException("from catch!");
       }finally{
           throw new RuntimeException("from finally!");
       }
   }
}

As many explain, with all lines I should get: "java.lang.RuntimeException: from finally!" (Yes I do)

Removing finally block I should get: "java.lang.RuntimeException: from catch!" (Yes I do)

Removing catch block I should get:

Exception in thread "main" java.lang.RuntimeException: from try!
    Suppressed: java.lang.RuntimeException: from IOManip.close

And I never do! Why? What am I missing?

By removing the catch block I should see the try message, but I get this instead:

Exception in thread "main" java.lang.RuntimeException: from finally!
    at it.core.MultipleExceptionsExample.main(MultipleExceptionsExample.java:18)

Thanks a lot.

CodePudding user response：

A return or throw in a finally block causes any other returned (or thrown) result to be ignored.

Don't return or throw in a finally block, unless you really know what you're doing.

It should be noted that Java Throwables have a notion of "suppressed" exceptions: you can invoke addSuppressed on a Throwable that you have caught; this is done automatically with a try-with-resources block, if an exception occurs in the body and whilst closing the resource; but this does not happen for a throw in a finally block.

CodePudding user response：

It is because finally block would always execute. It doesn't matter if you catch the exception or not. To understand it better try the code bellow:

public static void main(String[] args) {
    try (IOManip ioManip = new IOManip()) {
        System.out.println("Try!");
        throw new RuntimeException("from try!");
    } finally {
        System.out.println("Finally!");
    }
}

Remember that it is not a good practice to return or throw from the finally block, and you should avoid doing that.