Re: My (lack of )wisdom about threads

Tom Anderson <>
Sun, 24 May 2009 21:54:50 +0100
On Sun, 24 May 2009, Arne Vajh?j wrote:

Tom Anderson wrote:

On Sat, 23 May 2009, Joshua Cranmer wrote:

Stefan Ram wrote:

  So, inspired by ?Swing application architecture question.?, here is
  my (lack of )wisdoms about threads:

    - To correctly use multi-threading, a special education is needed.
      I do not yet have taken the time to undergo this, so I need
      to refrain from using threads (that is, more than one thread).

I wouldn't say that special education is needed at Java's level of
abstraction. I've been able to do it to some degree, and I certainly never
had it yet. If you know what synchronized does and when you might want to
use volatile, you'll probably be fine. Java 5 and 6 (and possibly 7)
introduce new concurrency utilities that eases some use, especially if you
have a lot of threads interacting.

Some of them also make life more complicated, because you have more

I do like AtomicBoolean, though. It lets you implement something that's
like an optional synchronized block. Rather than:

private final Object lock = new Object();

synchronized (lock) {

You do:

private final AtomicBoolean lock = new AtomicBoolean();

if (lock.compareAndSet(false, true)) {
    try {
    finally {

This still guarantees mutual exclusion, but means that if a second thread
comes along while a first thread is in the critical section, rather than
blocking, it skips over the critical section. I used this construct to
guard some housekeeping work in a multithreaded program; something that
needs to be done quite often, but must only be done by one thread.

You could also use Semaphore.tryAcquire for a similar effect.

Aargh! Too many choices!

java.util.concurrent is a great package.

I assume that one could code the exact same thing oneself, but it is
always nice to get high quality code for free.

The atomic classes use special machine-code instructions (the sublime
ldarx/stwcx on PowerPC, the brutish lock cmpxchg on x86, etc), which you
wouldn't otherwise be able to write in java (without JNI, at least). You
could build classes with the same semantics using synchronized, but i
wouldn't say you could code the exact same thing oneself.

Apart from the atomics, though, i *think* all the stuff in
java.util.concurrent could be written in java by ordinary programmers, as
you say.


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