Final rethrow in Java: Exception Handling

Question

I was going through some Exception Handling concepts in Java and came across the concept of final rethrow. I read the following article-
https://baptiste-wicht.com/posts/2010/05/better-exception-handling-in-java-7-multicatch-and-final-rethrow.html
Here, its written-

And the second improvement is a little more complicated. Imagine that you want to catch all exceptions, make several operations and then rethrow it. The code isn't hard to make, but the big problem is that you must add a throws clause to your method signature to manage the new exception launched by your code and this is not the objective. Now, you can do that without adding an exception throws clause :

     // some code
} catch (final Throwable ex) {
     // some more code
    throw ex;
}

Using the final keyword it allows you to throw an exception of the exact dynamic type that will be throwed. So if an IOException occurs, an IOException will be throwed. Of course, you have to declare the exceptions not caught. You throws clauses will exactly the same if you use the code (in //some code) without catching anything but now you can do something if that happens.

Can somebody please explain what exactly does this mean and what purpose does it solve?

That explanation is just bad. The final keyword has no direct impact on exceptions or throwing them. It simply means that you will get a compile error if you try to assign anything to ex, so the code before throw ex cannot do something like ex = new WhateverException(). Thus it guarantees that whatever is in ex is still the same at the end of the catch block, but that's it - if you omit the final and simply write code that doesn't assign ex, you will get identical behaviour. Also, final has nothing to do with the finally keyword if that's a reason for your confusion. — l4mpi
– l4mpi, Commented Jul 6, 2023 at 14:15

assylias · Accepted Answer · 2023-07-06 14:18:11Z

Imagine this code:

void m() throws IOException {
  try {
    throw new IOException();
  } catch (Exception e) {
    //do something when an exception is thrown...
    throw e;
  }
}

In Java 6, it did not compile and you had to add Exception to the method signature:

void m() throws IOException, Exception { ... }

which can be replaced by

void m() throws Exception { ... }

In other words, if you wanted to catch all exceptions, the compiler forced you to change the method signature, although in this code it is clear that only IOException can be thrown.

The article you link to explains that, in Java 7, you can do a catch-all without changing the method signature.

More information in the "Rethrowing Exceptions" section of this article: https://www.oracle.com/technical-resources/articles/java/java7exceptions.html.

ps: the explanation around the final keyword is wrong, it makes no real difference in this example.

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