Saturday, September 16, 2006

Don't swallow InterruptedException. Call Thread.currentThread().interrupt() instead.

Have you ever written the following code?
try {
} catch(InterruptedException swallowed) {
// just logging is also not a useful option here....

I have! I newer knew what the heck to do with those annoying InterruptedException when I simply wanted to call Thread.sleep(..). What is this InterruptedException? Why is it thrown? Why can't I ignore it?

Before I explain the whys, here is what you should do (if you don't re-throw):
try {
} catch(InterruptedException e) {
// Restore the interrupted status

What is the InterruptedException?

There is no way to simply stop a running thread in java (don't even consider using the deprecated method stop()). Stopping threads is cooperative in java. Calling Thread.interrupt() is a way to tell the thread to stop what it is doing. If the thread is in a blocking call, the blocking call will throw an InterruptedException, otherwise the interrupted flag of the tread will be set. A Thread or a Runnable that is interruptable should check from time to time Thread.currentThread().isInterrupted(). If it returns true, cleanup and return.

Why is it InterruptedException thrown?

The problem is that blocking calls like sleep() and wait(), can take very long till the check can be done. Therefore they throw an InterruptedException. However the isInterrupted is cleared when the InterruptedException is thrown! (I have some vague idea why this is the case, but for whatever reason this is done, that is how it is!)

Why can't InterruptedException be simply ignored?

It should be clear by now: because ignoring an InterruptedException means resetting the interrupted status of the thread. For example, worker threads take runnable from a queue and execute they may check the interrupted status periodically. If you swallow it the thread would not know that it was interrupted and would happily continue to run.

Some more thoughts
Unfortunately it is not specified that Thread.interrupt() can only be used for cancellation. It can be used for anything that requires to set a flag on a thread. So, ending your task or runnable early might be the wrong choice, if the interrupted status is used for something else. But common practice is to use it for cancellation. But even if it is used for something else, you code does not have the right to reset the interrupt flag (unless you are the owner of the thread).

To learn more read the nice article Dealing with InterruptedException by Brian Goetz.

Or read Java Concurrency in Practice By BriaBrian Goetz, Tim Peierls, Joshua Bloch, Joseph Bowbeer, David Holmes, and Doug Lea. It's a great book! If you program in java 5 this book is a must read!. I bought the book after I read Brians artikle. I have not yet read the entire book, but what I have read impresses me. It gives a lot of theorie and options. Very competent and complete, but I'm missing a set of simple patterns for concurrent programming. I'm looking forward to Doug Leas 3rd edition of Concurrent Programming in Java

Summary: If you don't know what to do with an InterruptedException call Thread.currentThread().interrupt()

Sunday, September 10, 2006

Pitfalls of Long.parseLong/Long.decode...

What is wrong with this code?
assert 0x0cc175b9c0f1b6a8L == Long.parseLong("0cc175b9c0f1b6a8",16);
assert 0xd41d8cd98f00b204L == Long.parseLong("d41d8cd98f00b204",16);
The second Long.parseLong will fail in the second line:
java.lang.NumberFormatException: For input string: "d41d8cd98f00b204"
The reason is that javas long is signed, but 0xd41d8cd98f00b204L is an unsigned long. Interesting, but annoying, if you want to parse an unsigned long in java. It makes Parsing and verifying hex, octal and decimal numbers more complicated, if you have a field that represents a C/C++ unsigned long (a 64 bit number)....

The solution is to use BigInteger:
assert 0x0cc175b9c0f1b6a8L == new BigInteger("0cc175b9c0f1b6a8",16).longValue();
assert 0xd41d8cd98f00b204L == new BigInteger("d41d8cd98f00b204",16).longValue();
However, you must ensure that the BigInteger does not exceed 64 bits. So here's my unsigned long parser
public static long parseUnsignedLong(String s, int radix) 
throws NumberFormatException {
BigInteger b= new BigInteger(s,radix);
throw new NumberFormatException(s+" is to big!");
return b.longValue();
Note: if you want to do any kind arithmetic with unsigned longs (like min/max checking) you have to use BigInteger objects.... Unless you are only interested in the bits of the long, the return value of my parseUnsignedLong is not really useful: it should better return the BigInteger!