ReentrantLock
In this chapter you will learn:
Use ReentrantLock
The shared resource can be controlled by a lock. Before a thread can access a shared resource, it has to get the lock that protects that resource. When access to the resource is complete, the lock is released.
If a second thread acquires the lock when the lock is in use, the second thread is suspended until the lock is released.
To acquire a lock, call lock().
If the lock is unavailable, lock() will wait.
To release a lock, call unlock().
To see if a lock is available, and to acquire it if it is,
call tryLock().
It returns true if the lock is acquired and false otherwise.
The newCondition() method returns a Condition
object associated with the lock.
With Condition object, you can control the lock through methods such
as await() and signal().
await() and signal() are similar to
Object.wait() and Object.notify().
java.util.concurrent.locks supplies an implementation of
Lock called ReentrantLock.
ReentrantLock implements a reentrant lock, which is a lock
that can be repeatedly entered
by the thread that currently holds the lock. Otherwise,
a thread seeking to acquire the lock will suspend until
the lock is not in use.
The Lock Methods listed below:
void lock()Waits until the invoking lock can be acquired.void lockInterruptibly() throws InterruptedExceptionWaits until the invoking lock can be acquired, unless interrupted.Condition newCondition()Returns a Condition object that is associated with the invoking lock.boolean tryLock()Attempts to acquire the lock. This method will not wait if the lock is unavailable. Instead, it returns true if the lock has been acquired and false if the lock is currently in use by another thread.boolean tryLock(long wait, TimeUnit tu) throws InterruptedExceptionAttempts to acquire the lock. If the lock is unavailable, this method will wait no longer than the period specified by wait, which is in tu units. It returns true if the lock has been acquired and false if the lock cannot be acquired within the specified period.void unlock()Releases the lock.
/*/*from j av a 2 s . com*/
License for Java 1.5 'Tiger': A Developer's Notebook
(O'Reilly) example package
Java 1.5 'Tiger': A Developer's Notebook (O'Reilly)
by Brett McLaughlin and David Flanagan.
ISBN: 0-596-00738-8
You can use the examples and the source code any way you want, but
please include a reference to where it comes from if you use it in
your own products or services. Also note that this software is
provided by the author "as is", with no expressed or implied warranties.
In no event shall the author be liable for any direct or indirect
damages arising in any way out of the use of this software.
*/
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
class LinkList<E> {
// The value of this node
E value;
// The rest of the list
LinkList<E> rest;
// A lock for this node
Lock lock;
// Signals when the value of this node changes
Condition valueChanged;
// Signals when the node this is connected to changes
Condition linkChanged;
public LinkList(E value) {
this.value = value;
rest = null;
lock = new ReentrantLock();
valueChanged = lock.newCondition();
linkChanged = lock.newCondition();
}
public void setValue(E value) {
lock.lock();
try {
this.value = value;
// Let waiting threads that the value has changed
valueChanged.signalAll();
} finally {
lock.unlock();
}
}
public void executeOnValue(E desiredValue, Runnable task)
throws InterruptedException {
lock.lock();
try {
// Checks the value against the desired value
while (!value.equals(desiredValue)) {
// This will wait until the value changes
valueChanged.await();
}
// When we get here, the value is correct -- Run the task
task.run();
} finally {
lock.unlock();
}
}
public void append(E value) {
// Start the pointer at this node
LinkList<E> node = this;
node.lock.lock();
while (node.rest != null) {
LinkList<E> next = node.rest;
// Here's the hand-over-hand locking
try {
// Lock the next node
next.lock.lock();
} finally {
// unlock the current node
node.lock.unlock();
}
// Traverse
node = next;
}
// We're at the final node, so append and then unlock
try {
node.rest = new LinkList<E>(value);
// Let any waiting threads know that this node's link has changed
node.linkChanged.signalAll();
} finally {
node.lock.unlock();
}
}
public void printUntilInterrupted(String prefix) {
// Start the pointer at this node
LinkList<E> node = this;
node.lock.lock();
while (true) {
LinkList<E> next;
try {
System.out.println(prefix + ": " + node.value);
// Wait for the next node if not available
while (node.rest == null) {
node.linkChanged.await();
}
// Get the next node
next = node.rest;
// Lock it - more hand-to-hand locking
next.lock.lock();
} catch (InterruptedException e) {
// reset the interrupt status
Thread.currentThread().interrupt();
return;
} finally {
node.lock.unlock();
}
// Traverse
node = next;
}
}
}
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What you will learn in the next chapter: