B: 2
B is sleeping.
B is unlocking count.
java.util.concurrent.locks also defines the ReadWriteLock interface. This interface
specifies a reentrant lock that maintains separate locks for read and write access. This
enables multiple locks to be granted for readers of a resource as long as the resource is not
being written. ReentrantReadWriteLock provides an implementation of ReadWriteLock.
Atomic Operations
java.util.concurrent.atomic offers an alternative to the other synchronization features when
reading or writing the value of some types of variables. This package offers methods that
get, set, or compare the value of a variable in one uninterruptible (that is, atomic) operation.
This means that no lock or other synchronization mechanism is required.
Atomic operations are accomplished through the use of classes, such as AtomicInteger and
AtomicLong, and methods such as get( ), set( ), compareAndSet( ), decrementAndGet( ),
and getAndSet( ), which perform the action indicated by their names.
Here is an example that demonstrates how access to a shared integer can be synchronized
by the use of AtomicInteger:
// A simple example of Atomic.
import java.util.concurrent.atomic.*;
class AtomicDemo {
public static void main(String args[]) {
new AtomThread("A");
new AtomThread("B");
new AtomThread("C");
class Shared {
static AtomicInteger ai = new AtomicInteger(0);
// A thread of execution that increments count.
class AtomThread implements Runnable {
String name;
AtomThread(String n) {
name = n;
new Thread(this).start();
public void run() {
System.out.println("Starting " + name);
for(int i=1; i <= 3; i++)
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