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| 2005/5/2-3 [Computer/SW/Languages/Java] UID:37446 Activity:nil |
5/2 Could someone offer a tip on how to write thread-safe (Java) code?
I have a class which performs some work, and I want to allow only one
thread to be in the do_work function, but allow many threads to enqueue
work. How would I do this?
class Worker {
private void dowork(Work w) {...}
public void enqueuework(Work w) { ...}
private Queue workqueue; // A FIFO queue of Work objects.
...
}
\_ Look into the synchronized keyword. If nowhere else, the Java
language spec has an acceptable description.
\_ The above answer isn't quite enough. Here's how you do it
in general, gross terms, you'll have to do some tweaking and
look up the Java-specifics yourself:
\_ The above answer isn't quite enough. This isn't 100% correct,
but it should point you in the right direction:
-- The do work thread should start running in a loop
which locks a synch variable, process anything
in the queue, and then waits on condition variable
-- The enqueue method should lock the synch variable
put stuff into the queue, and signal the condition
variable
See also: http://csua.org/u/bxq (java.sun.com)
- ciyer
\_ Shouldn't it be sufficient to use the synchronized keyword on
the doWork method? The enqueuework method doesn't need to
be synchronized, right?
\_ It does if you're writing unsynchronized data structures
in the enqueueing operation. |
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| csua.org/u/bxq -> java.sun.com/docs/books/tutorial/essential/threads/waitAndNotify.html Feedback Form Trail: Essential Java Classes Lesson: Threads: Doing Two or More Tasks At Once Using the notifyAll and wait Methods Let's investigate how the code in CubbyHole's put and get methods helps the Producer and the Consumer coordinate their activities. The CubbyHol e stores its value in a private member variable called contents. CubbyH ole has another private member variable, available, that is a boolean. The available variable is true when the value has been put but not yet gotten and is false when the value has been gotten but not yet put. Her e's one possible implementation for the put and get methods: public synchronized int get() { //won't work! Wh at happens if the Producer hasn't put anything in the CubbyHole and ava ilable isn't true? Similarly, if the Produ cer calls put before the Consumer got the value, put doesn't do anythin g You really want the Consumer to wait until the Producer puts something i n the CubbyHole and the Producer to notify the Consumer when it's done so. Similarly, the Producer should wait until the Consumer takes a valu e (and notifies the Producer of its activities) before replacing it wit h a new value. The two threads must coordinate more fully and can use O bject's wait and notifyAll methods to do so. Here are the new get and put implementations that wait on and notify eac h other of their activities: public synchronized int get() { while (available == false) { try { //wait for Producer to put value wait(); The wait m ethod relinquishes the lock held by the Consumer on the CubbyHole (ther eby allowing the Producer to get the lock and update the CubbyHole) and then waits for notification from the Producer. When Producer puts some thing in the CubbyHole, it notifies Consumer by calling notifyAll. The Consumer then comes out of the wait state and the get method returns th e value in the CubbyHole. It waits for the Consumer thr ead to consume the current value before allowing the Producer to produc e a new one. The notifyAll method wakes up all threads waiting on the object in quest ion (in this case, the CubbyHole). The Objec t class also defines the notify method, which arbitrarily wakes up one of the threads waiting on this object. There are the three versions of the wait method contained in the Object class: wait() Waits indefinitely for notification. wait(long timeout, int nanos) Waits for notification or until timeout milliseconds plus nanos nanoseconds have elapsed. Note: Besides using these timed wait methods to synchronize threads, yo u also can use them in place of sleep. Both wait and sleep delay for th e requested amount of time. You can easily wake up wait with a notify b ut a sleeping thread cannot be awakened prematurely. This doesn't matte r too much for threads that don't sleep for long, but it could be impor tant for threads that sleep for minutes at a time. |
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