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Java Forum / General / January 2008BufferedInputStream -- does not recommend "close()"?
The operation »close()« is mentioned in the class documentation http://java.sun.com/javase/6/docs/api/java/io/BufferedInputStream.html But nowhere does it recommend to use »close()«. I had expected the constructor documentation to say something like »Whenever an object of this class has been constructed successfully it needs to be closed at some instant in the future to avoid resource leaks, because only by closing all resource allocated by the construction will be released again.« But this is not so. The tutorial also does not recommend close: http://java.sun.com/docs/books/tutorial/essential/io/buffers.html I used to believe that closing was important. Why is it not recommended in the documentation nor the tutorial? (When insisting on the use of »close()«, I'd like to have some evidence for its necessity.) > The operation »close()« is mentioned in the class documentation > [quoted text clipped - 22 lines] > (When insisting on the use of »close()«, I'd > like to have some evidence for its necessity.) Huh. I am slightly mystified as well. But I believe a plausible explanation might be that BufferedInputStream itself doesn't hold any system resources; it simply forwards the "close" call onto the InputStream it wraps. It is those wrapped objects whose "close" methods must be called. So since for the BufferedInputStream itself, the "close" call isn't so critical, maybe (just maybe) Sun decided not to insert a stern warning into the documentation of that class. It is best practice to close all streams (and readers/writers) when you are done with them. Nasty unpredictible things can happen otherwise. -- Adam Maass > The operation »close()« is mentioned in the class documentation > [quoted text clipped - 22 lines] > (When insisting on the use of »close()«, I'd > like to have some evidence for its necessity.) Well, you could read what close() does and infer its necessity from that. <http://java.sun.com/javase/6/docs/api/java/io/BufferedInputStream.html#close()> > Closes this input stream and releases any system resources associated with the stream. Ask yourself in what universe it's a good idea to leave system resources unreleased after they're not needed.  SignatureLew > You could read what close() does and infer its necessity from that. > <http://java.sun.com/javase/6/docs/api/java/io/BufferedInputStream.html#close()> [quoted text clipped - 3 lines] > Ask yourself in what universe it's a good idea to leave system resources > unreleased after they're not needed. Um... one in which system resources are unlimited? ;-> (Sorry, I just couldn't resist.) -- Adam Maass >> You could read what close() does and infer its necessity from that. >> <http://java.sun.com/javase/6/docs/api/java/io/BufferedInputStream.html#close()> [quoted text clipped - 7 lines] > Um... one in which system resources are unlimited? ;-> (Sorry, I just > couldn't resist.) How would that make it a good idea not to release resources? All that does it make it not a terrible idea. Now, getting away from science fiction ... SignatureLew >>> Ask yourself in what universe it's a good idea to leave system >>> resources unreleased after they're not needed. [quoted text clipped - 3 lines] > make it not a terrible idea. > Now, getting away from science fiction ... As programmers, aren't we all a bit responsible to not let the vendors of memory-modules starve ... ?-) > Why is it not recommended in the documentation nor the tutorial? Too obvious? Java requires explicit close of all files. Various horrible things will happen if you don't, e.g. holds onto RAM, does not write out last bytes written, does not update directory with times, does not update directory with length, does not unlock file if there were some locking. Is a similar way, Frames require an explicit dispose. Oddly old languages like FORTRAN and Abundance closed files automatically that were left open on exit. I guess the idea is the designers of Java wanted to encourage you to close early. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >Oddly old languages like FORTRAN and Abundance closed files >automatically that were left open on exit. Early closing also in important in servers that are not expected to exit anytime soon, so that they will not exhaust resources. Roedy Green schrieb: >> Why is it not recommended in the documentation nor the tutorial? > [quoted text clipped - 9 lines] > automatically that were left open on exit. I guess the idea is the > designers of Java wanted to encourage you to close early. Java also closes all open streams on exit.. Well it might rather be the OS that cleans up any ressource of programs. > Roedy Green schrieb: > Java also closes all open streams on exit.. > Well it might rather be the OS that cleans up any ressource of > programs. That's easy to check; the OS can close files, but it doesn't know enough to flush data Java still has buffered. import java.io.*; class NoFlush { public static void main(String[] args) throws Exception { Writer w = new FileWriter("a.log"); w.write("abcd"); w.flush(); w.write("defg"); } } % ls -l a.log -rwx------+ 1 mschilling None 4 Jan 7 17:07 a.log Yes, the file is getting closed; no, Java is not flushing the stream. >> Roedy Green schrieb: > [quoted text clipped - 22 lines] > > Yes, the file is getting closed; no, Java is not flushing the stream. Which is exactly the same behavior as C with setbuf/setvbuf. SignatureJohn W. Kennedy "The grand art mastered the thudding hammer of Thor And the heart of our lord Taliessin determined the war." -- Charles Williams. "Mount Badon" >Java also closes all open streams on exit.. >Well it might rather be the OS that cleans up any ressource of programs. When did that start happening? IIRC some time ago, I forgot to close is some program and the file did not get written. Adding the close fixed it. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >> Java also closes all open streams on exit.. >> Well it might rather be the OS that cleans up any ressource of programs. > > When did that start happening? IIRC some time ago, I forgot to close > is some program and the file did not get written. Adding the close > fixed it. See upthread: Mike Schilling wrote: >> That's easy to check; the OS can close files, but it doesn't know >> enough to flush data Java still has buffered. The "file" in question may well be something only the Java program knows about - from the OS's point of view it might still be a non-existent or zero-length file at the time the JVM exits. It might also be that the JVM is better about telling the OS of its crashes than it used to be? Even if so, there are still no guarantees about the completeness or consistency or usability of an OS file after a Java program leaves without properly close()ing its connection to it. That much hasn't changed. SignatureLew Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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