Exception thrown in catch and finally clause

Question

On a question for Java at the university  there was this snippet of code   class MyExc1 extends Exception    class MyExc2 extends Exception    class MyExc3 extends MyExc2     public class C1       public static void main String   args  throws Exception           try               System out print 1               q                      catch  Exception i                throw new MyExc2                      finally               System out print 2               throw new MyExc1                         static void q   throws Exception           try               throw new MyExc1                      catch  Exception y                      finally               System out print 3               throw new Exception                        I was asked to give its output   I answered 13Exception in thread main MyExc2  but the correct answer is 132Exception in thread main MyExc1  Why is it that  I just can t understand where does MyExc2 go

User · Answer

This is what Wikipedia says about finally clause:

More common is a related clause (finally, or ensure) that is executed whether an exception occurred or not, typically to release resources acquired within the body of the exception-handling block.

Let's dissect your program.

try {
    System.out.print(1);
    q();
}

So, 1 will be output into the screen, then q() is called. In q(), an exception is thrown. The exception is then caught by Exception y but it does nothing. A finally clause is then executed (it has to), so, 3 will be printed to screen. Because (in method q() there's an exception thrown in the finally clause, also q() method passes the exception to the parent stack (by the throws Exception in the method declaration) new Exception() will be thrown and caught by catch ( Exception i ), MyExc2 exception will be thrown (for now add it to the exception stack), but a finally in the main block will be executed first.

So in,

catch ( Exception i ) {
    throw( new MyExc2() );
} 
finally {
    System.out.print(2);
    throw( new MyExc1() );
}

A finally clause is called...(remember, we've just caught Exception i and thrown MyExc2) in essence, 2 is printed on screen...and after the 2 is printed on screen, a MyExc1 exception is thrown. MyExc1 is handled by the public static void main(...) method.

Output:

"132Exception in thread main MyExc1"

Lecturer is correct! :-)

In essence, if you have a finally in a try/catch clause, a finally will be executed (after catching the exception before throwing the caught exception out)

User · Answer

A method can t throw two exceptions at the same time  It will always throw the last thrown exception  which in this case it will be always the one from the finally block   When the first exception from method q   is thrown  it will catch ed and then swallowed by the finally block thrown exception   q   -  thrown new Exception -  main catch Exception -  throw new Exception -  finally throw a new exception  and the one from the catch is  lost

User · Answer

Based on reading your answer and seeing how you likely came up with it  I believe you think an  exception-in-progress  has  precedence    Keep in mind   When an new exception is thrown in a catch block or finally block that will propagate out of that block  then the current exception will be aborted  and forgotten  as the new exception is propagated outward   The new exception starts unwinding up the stack just like any other exception  aborting out of the current block  the catch or finally block  and subject to any applicable catch or finally blocks along the way   Note that applicable catch or finally blocks includes   When a new exception is thrown in a catch block  the new exception is still subject to that catch s finally block  if any   Now retrace the execution remembering that  whenever you hit throw  you should abort tracing the current exception and start tracing the new exception

User · Answer

Finally clause is executed even when exception is thrown from anywhere in try catch block   Because it s the last to be executed in the main and it throws an exception  that s the exception that the callers see   Hence the importance of making sure that the finally clause does not throw anything  because it can swallow exceptions from the try block

User · Answer

Exceptions in the finally block supersede exceptions in the catch block  Java Language Specification   If the catch block completes abruptly for reason R  then the finally block is executed  Then there is a choice   If the finally block completes normally  then the try statement completes abruptly for reason R   If the finally block completes abruptly for reason S  then the try statement completes abruptly for reason S  and reason R is discarded

User · Answer

The easiest way to think of this is imagine that there is a variable global to the entire application that is holding the current exception   Exception currentException   null    As each exception is thrown   currentException  is set to that exception  When the application ends  if currentException is    null  then the runtime reports the error   Also  the finally blocks always run before the method exits  You could then requite the code snippet to   public class C1        public static void main String    argv  throws Exception           try               System out print 1               q                       catch   Exception i                     lt -- currentException   Exception  as thrown by q   s finally block             throw  new MyExc2          lt -- currentException   MyExc2                   finally                    lt -- currentException   MyExc2  thrown from main   s catch block             System out print 2               throw  new MyExc1          lt -- currentException   MyExc1                       lt -- At application exit  currentException   MyExc1  from main   s finally block  Java now dumps that to the console       static void q   throws Exception           try               throw  new MyExc1          lt -- currentException   MyExc1                   catch  Exception y                    lt -- currentException   null  because the exception is caught and not rethrown                   finally               System out print 3               throw  new Exception          lt -- currentException   Exception                     The order in which the application executes is   main       try     q               try       catch       finally         catch   finally

User · Answer

class MyExc1 extends Exception    class MyExc2 extends Exception    class MyExc3 extends MyExc2     public class C1       public static void main String   args  throws Exception           try               System out print  TryA L1 n                q                System out print  TryB L1 n                      catch  Exception i                System out print  Catch L1 n                                      finally               System out print  Finally L1 n                throw new MyExc1                         static void q   throws Exception           try               System out print  TryA L2 n                q2                System out print  TryB L2 n                      catch  Exception y                System out print  Catch L2 n                throw new MyExc2                        finally               System out print  Finally L2 n                throw new Exception                         static void q2   throws Exception           throw new MyExc1              Order   TryA L1 TryA L2 Catch L2 Finally L2 Catch L1 Finally L1         Exception in thread  main  MyExc1 at C1 main C1 java 30    https   www compilejava net

User · Answer

The logic is clear till finish printing out 13  Then the exception thrown in q   is caught by catch  Exception i  in main   and a new MyEx2   is ready to be thrown  However  before throwing the exception  the finally block have to be executed first  Then the output becomes 132 and finally asks to thrown another exception new MyEx1       As a method cannot throw more than one Exception  it will always throw the latest Exception  In other words  if both catch and finally blocks try to throw Exception  then the Exception in catch is swallowed and only the exception in finally will be thrown     Thus  in this program  Exception MyEx2 is swallowed and MyEx1 is thrown  This Exception is thrown out of main   and no longer caught  thus JVM stops and the final output is 132Exception in thread main MyExc1     In essence  if you have a finally in a try catch clause  a finally will be executed AFTER catching the exception  but BEFORE throwing any caught exception  and ONLY the lastest exception would be thrown in the end

User · Answer

I think this solve the problem    boolean allOk   false  try    q      allOk   true    finally     try        is close        catch  Exception e         if allOk           throw new SomeException e

User · Answer

To handle this kind of situation i e  handling the exception raised by finally block  You can surround the finally block by try block  Look at the below example in python   try     fh   open  testfile    w      try        fh write  This is my test file for exception handling        finally        print  Going to close the file        fh close   except IOError     print  Error  can  t find file or read data

User · Answer

I think you just have to walk the finally blocks    Print  1   finally in q print  3   finally in main print  2

User · Answer

It is well known that the finally block is executed after the the try and catch and is always executed     But as you saw it s a little bit tricky sometimes check out those code snippet below and you will that the return and throw statements don t always do what they should do in the order that we expect theme to   Cheers                Return dont always return         try       return  In Try       finally       return  In Finally                                                                                                       while true          try            return  In try              finally           break                                        return  Out of try                                                                                                            while  true              try                        return  In try                     finally                continue                                                                                                            Throw dont always throw           try        throw new RuntimeException        finally        return  Ouuuups no throw

[java] Exception thrown in catch and finally clause

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