Java线程间怎么实现同步
1、Object#wait(), Object#notify()让两个线程依次执行
/**
* 类AlternatePrintDemo.java的实现描述：交替打印
*/
class NumberPrint implements Runnable {
private int number;
public byte res[];
public static int count = 5;
public NumberPrint(int number, byte a[]) {
this.number = number;
res = a;
}
public void run() {
synchronized (res) {
while (count-- > 0) {
try {
res.notify();//唤醒等待res资源的线程，把锁交给线程（该同步锁执行完毕自动释放锁）
System.out.println(" " + number);
res.wait();//释放CPU控制权，释放res的锁，本线程阻塞，等待被唤醒。
System.out.println("------线程" + Thread.currentThread().getName() + "获得锁，wait()后的代码继续运行：" + number);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
public class AlternatePrintDemo {
public static void main(String args[]) {
final byte a[] = { 0 };//以该对象为共享资源
new Thread(new NumberPrint(1, a), "1").start();
new Thread(new NumberPrint(2, a), "2").start();
}
}
2、Condition#signal(), Condition#wait()让两个线程依次执行
/**
*
* 类ConditionDemo.java的实现描述：Condition 将 Object 监视器方法（wait、notify 和 notifyAll）分解成截然不同的对象，以便通过将这些对象与任意 Lock 实现组合使用，
* 为每个对象提供多个等待 set （wait-set）。其中，Lock 替代了 synchronized 方法和语句的使用，Condition 替代了 Object 监视器方法的使用。
*/
public class ConditionDemo {
public static void main(String[] args) {
final Business business = new Business();
new Thread(new Runnable() {
@Override
public void run() {
threadExecute(business, "sub");
}
}).start();
threadExecute(business, "main");
}
public static void threadExecute(Business business, String threadType) {
for (int i = 0; i < 10; i++) {
try {
if ("main".equals(threadType)) {
business.main(i);
} else {
business.sub(i);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
class Business {
private boolean bool = true;
private Lock lock = new ReentrantLock();
private Condition condition = lock.newCondition();
public /* synchronized */ void main(int loop) throws InterruptedException {
lock.lock();
try {
while (bool) {
condition.await();//this.wait();
}
System.out.println("main thread seq loop of " + loop);
bool = true;
condition.signal();//this.notify();
} finally {
lock.unlock();
}
}
public /* synchronized */ void sub(int loop) throws InterruptedException {
lock.lock();
try {
while (!bool) {
condition.await();//this.wait();
}
System.out.println("sub thread seq loop of " + loop);
bool = false;
condition.signal();//this.notify();
} finally {
lock.unlock();
}
}
}
Lock.Condition同理
import java.util.concurrent.locks.*;
class BoundedBuffer {
final Lock lock = new ReentrantLock(); //锁对象
final Condition notFull = lock.newCondition(); //写线程条件
final Condition notEmpty = lock.newCondition(); //读线程条件
final Object[] items = new Object[100]; //缓存队列
int putptr/* 写索引 */, takeptr/* 读索引 */, count/* 队列中存在的数据个数 */;
public void put(Object x) throws InterruptedException {
lock.lock();
try {
while (count == items.length)//如果队列满了
notFull.await();//阻塞写线程
items[putptr] = x;//赋值
if (++putptr == items.length)
putptr = 0;//如果写索引写到队列的最后一个位置了，那么置为0
++count;//个数++
notEmpty.signal();//唤醒读线程
} finally {
lock.unlock();
}
}
public Object take() throws InterruptedException {
lock.lock();
try {
while (count == 0)//如果队列为空
notEmpty.await();//阻塞读线程
Object x = items[takeptr];//取值
if (++takeptr == items.length)
takeptr = 0;//如果读索引读到队列的最后一个位置了，那么置为0
--count;//个数--
notFull.signal();//唤醒写线程
return x;
} finally {
lock.unlock();
}
}
}
3、两个线程使用Object#wait(), Object#notify()实现生产消费者模式。
/**
*
* 类ProducerConsumerDemo.java的实现描述：生产消费者模式
*/
public class ProducerConsumerDemo {
public static void main(String args[]) {
final Queue<Integer> sharedQ = new LinkedList<>();
Thread producer = new Producer(sharedQ);
Thread consumer = new Consumer(sharedQ);
producer.start();
consumer.start();
}
}
class Producer extends Thread {
private static final int MAX_COUNT = 10;
private Queue<Integer> sharedQ;
public Producer(Queue<Integer> sharedQ) {
super("Producer");
this.sharedQ = sharedQ;
}
@Override
public void run() {
for (int i = 0; i < MAX_COUNT; i++) {
synchronized (sharedQ) {
//waiting condition - wait until Queue is not empty
while (sharedQ.size() >= 1) {
try {
System.out.println("Queue is full, waiting");
sharedQ.wait();
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
System.out.println("producing : " + i);
sharedQ.add(i);
sharedQ.notify();
}
}
}
}
class Consumer extends Thread {
private Queue<Integer> sharedQ;
public Consumer(Queue<Integer> sharedQ) {
super("Consumer");
this.sharedQ = sharedQ;
}
@Override
public void run() {
while (true) {
synchronized (sharedQ) {
//waiting condition - wait until Queue is not empty
while (sharedQ.size() == 0) {
try {
System.out.println("Queue is empty, waiting");
sharedQ.wait();
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
int number = (int) sharedQ.poll();
System.out.println("consuming : " + number);
sharedQ.notify();
//termination condition
if (number == 3) {
break;
}
}
}
}
}
4、CountDownLatch实现类似计数器的功能。
/**
*
* 类CountDownLatchDemo.java的实现描述：CountDownLatch类位于java.util.concurrent包下，利用它可以实现类似计数器的功能.
* 调用await()方法的线程会被挂起，它会等待直到count值为0才继续执行
*/
public class CountDownLatchDemo {
public static void main(String[] args) {
final CountDownLatch latch = new CountDownLatch(2);
new Thread() {
public void run() {
try {
System.out.println("子线程" + Thread.currentThread().getName() + "正在执行");
Thread.sleep(3000);
System.out.println("子线程" + Thread.currentThread().getName() + "执行完毕");
latch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
};
}.start();
new Thread() {
public void run() {
try {
System.out.println("子线程" + Thread.currentThread().getName() + "正在执行");
Thread.sleep(3000);
System.out.println("子线程" + Thread.currentThread().getName() + "执行完毕");
latch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
};
}.start();
try {
System.out.println("等待2个子线程执行完毕...");
latch.await();
System.out.println("2个子线程已经执行完毕");
System.out.println("继续执行主线程");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
5、 CyclicBarrier(回环栅栏)可以实现让一组线程等待至某个状态之后再全部同时执行。
/**
* 类CyclicBarrierDemo.java的实现描述：字面意思回环栅栏，通过它可以实现让一组线程等待至某个状态之后再全部同时执行。
* 叫做回环是因为当所有等待线程都被释放以后，CyclicBarrier可以被重用。我们暂且把这个状态就叫做barrier，当调用await()方法之后，
* 线程就处于barrier了。
*/
public class CyclicBarrierDemo {
public static void main(String[] args) {
int N = 4;
//所有线程写入操作完之后，进行额外的其他操作可以为CyclicBarrier提供Runnable参数
CyclicBarrier barrier = new CyclicBarrier(N, new Runnable() {
@Override
public void run() {
System.out.println("当前线程" + Thread.currentThread().getName());
}
});
for (int i = 0; i < N; i++) {
if (i < N - 1) {
new Writer(barrier).start();
} else {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
new Writer(barrier).start();
}
}
System.out.println("CyclicBarrier重用");
for (int i = 0; i < N; i++) {
new Writer(barrier).start();
}
}
static class Writer extends Thread {
private CyclicBarrier cyclicBarrier;
public Writer(CyclicBarrier cyclicBarrier) {
this.cyclicBarrier = cyclicBarrier;
}
@Override
public void run() {
System.out.println("线程" + Thread.currentThread().getName() + "正在写入数据...");
try {
Thread.sleep(5000); //以睡眠来模拟写入数据操作
System.out.println("线程" + Thread.currentThread().getName() + "写入数据完毕，等待其他线程写入完毕");
try {
cyclicBarrier.await(2000, TimeUnit.MILLISECONDS);
} catch (TimeoutException e) {
e.printStackTrace();
}
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
System.out.println("所有线程写入完毕，继续处理其他任务...");
}
}
}
6、Semaphore用来控制同时访问某一资源的操作数量，或控制同时执行某个指定操作的数量。
/**
* 类SemaphoreDemo.java的实现描述：Semaphore用来控制同时访问某一资源的操作数量，或控制同时执行某个指定操作的数量。
* 主要通过控制一组虚拟的“许可”，当需要执行操作时首先申请获取许可，如果还有剩余的许可 并且获取成功，就执行操作；如果剩余许可为0，就阻塞当前线程；
* 操作执行完成后释放许可，排队的阻塞线程可以被唤醒重新获取许可继续执行。这里提到排队，其实就是利用AQS的队列进行排队。
*/
public class SemaphoreDemo {
public static void main(String[] args) {
// 线程池
ExecutorService exec = Executors.newCachedThreadPool();
// 只能5个线程同时访问
final Semaphore semp = new Semaphore(5);
// 模拟20个客户端访问
for (int index = 0; index < 20; index++) {
final int NO = index;
Runnable run = new Runnable() {
public void run() {
try {
// 获取许可
semp.acquire();
System.out.println("Accessing: " + NO);
Thread.sleep((long) (Math.random() * 10000));
// 访问完后，释放
semp.release();
} catch (InterruptedException e) {
}
}
};
exec.execute(run);
}
// 退出线程池
exec.shutdown();
}
}
