Why do I need to use finally to close resources?

Question

Most of the time, the only thing I see a finally block used for is something like

FileInputStream f;
try{
    f= new FileInputStream("sample.txt");
    //something that uses f and sometimes throws an exception
}
catch(IOException ex){
    /* Handle it somehow */
}
finally{
    f.close();
}

My question is, if f's scope ends with the enclosing block, why do we need to close it in the finally?

Creating a File reference doesn't throw exceptions (or create a file handle), for future reference. — Stefan Kendall, Oct 04 '11 at 18:00
FWIW, in Java 7, you can use the try-with-resources construct: http://download.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html — jtoberon, Oct 04 '11 at 18:04

score 20 · Accepted Answer · answered Oct 04 '11 at 17:59

20

Because garbage collection is not the same thing as resource cleanup.

For example, if you have a JDBC Connection object that goes out of scope, there's no signal sent to the database server to indicate that open cursors and connections are no longer needed. Without those messages, you'll eventually exhaust the number of cursors and connections available to you.

Same with file handles and any other resource. Clean up after thyself.

answered Oct 04 '11 at 17:59

duffymo

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Because the code in the finally block is guaranteed to be executed, even if an exception is thrown. – duffymo Oct 04 '11 at 18:01
1

@Joe good example. But never return something in a finally block if you have a catch block which throw an exception. in that case, your exception will never be thrown... i learn this one the hard way ;-) – Cygnusx1 Oct 04 '11 at 18:05

score 6 · Answer 2 · answered Oct 04 '11 at 17:59

Well you've given a bad example - I suspect you meant something like FileInputStream - but the basic reason is that Java doesn't have deterministic finalization.

The scope of the variable f ends with the block it's declared in (not the try block), but that doesn't mean there are necessarily no "live" references to the object any more - and the garbage collector will neither finalize the object nor garbage collect it in any deterministic manner.

Unless you want to leave resources hanging around for an arbitrary length of time (and delay garbage collection, as finalizers require an extra round of collection before the memory is finally released), you should explicitly close resources.

Basically Java does not support RAII in the same way that C++ does; you shouldn't try to use it as if it were C++.

Edited to reflect your suggestion: I was just coming up with a Closeable from memory, so thanks for giving me one that actually applies :P — Matt G, Oct 05 '11 at 06:00

score 2 · Answer 3 · answered Oct 04 '11 at 18:01

2

because finally is called everytime, even if you get an exception raised. the finally block insure you that the file/connection will be closed.

answered Oct 04 '11 at 18:01

Cygnusx1

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score 0 · Answer 4 · answered Oct 04 '11 at 18:00

The reason is that Java doesn't guarantee that an object will be garbage-collected as soon as a particular reference to it falls out of scope. So for objects that reference limited system resources, such as a file descriptor, it's not enough to wait for garbage collection.

Note though, that java.io.File is not actually such an object.

score 0 · Answer 5 · answered Oct 04 '11 at 18:04

We handled exception by try catch finally ,finally block every time execute but there is no guarantee of catch because catch block execute only if exception passed in parameter matched. For example if we have open any database connection so it is must we closed it before leave,tht must be implemented into finally.

Why do I need to use finally to close resources?

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