Re: Throwing Constructor Exceptions and cleaning up

Joshua Cranmer <Pidgeot18@verizon.invalid>
Fri, 03 Apr 2009 07:58:45 -0400
Richard Maher wrote:

If when an object's constructor throws an exception that object has not in
fact been intantiated, then what happens to the bits/objects/variables that
were initialized/instantiated before the exception was thrown?

Java by default zero-initializes every single field in your class before
it gets around to invoking the constructor. Indeed, the JVM sees the
constructor as merely "just another method," albeit one with a funky name.

For example, if I open a file, or connect a socket while constructing an
object and then throw an exception before the constructor/instantiation
completes, then what happens to the file or socket connections that were
successfully instantiated?

The same things that happen when an exception abruptly terminates code
using file or socket connections in regular methods. The most correct
thing to do would be the try/finally idiom for streams.

a) Garbage Collected in the fullness of time as they're no longer

All objects are GC'd once they lose all references (and the GC gets
around to collecting them). If they're local variables, they die when
the method loses scope; if they're instance variables, they die when the
instance loses references, which will likely be soon anyways.

b) Destroyed, and o/ride finalize() method that's called immediately?

finalize() is only called sometime after GC happens--an event that could
happen now or sometime next year. Or never, in some cases.

c) It's considered extremely bad form to do this in a constructor. Have an
init() method instead?

An init() method would be little different from the <init>() method that
the constructor is anyways.

d) Make sure you close the file/Socket before throwing the execption

Well, this is good practice no matter what the circumstances.

Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

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