Java多线程技术-Lock/Condition
2024-08-25 06:05:04
在java1.5中Lock对象来实现同步的效果,而且使用上更方便。
使用ReentrantLock实现同步
public class MyService { private Lock lock = new ReentrantLock(); public void methodA(){
try {
lock.lock();
System.out.println("methodA begin threadName=" + Thread.currentThread().getName()+" time="+System.currentTimeMillis());
Thread.sleep(5000);
System.out.println("methodA end threadName=" + Thread.currentThread().getName()+" time="+System.currentTimeMillis());
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
} public void methodB(){
try {
lock.lock();
System.out.println("methodB begin threadName=" + Thread.currentThread().getName()+" time="+System.currentTimeMillis());
Thread.sleep(5000);
System.out.println("methodB end threadName=" + Thread.currentThread().getName()+" time="+System.currentTimeMillis());
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
} public class Test {
public static void main(String[] args) throws InterruptedException {
MyService service = new MyService();
new Thread(()->service.methodA()).start();
new Thread(()->service.methodA()).start();
new Thread(()->service.methodB()).start();
new Thread(()->service.methodB()).start(); }
}
测试结果:调用lock.lock()的线程就持有了“对象监视器”,其他线程只有等待锁被释放时再次争抢,效果和synchronized一样。
methodA begin threadName=Thread-0 time=1516242293668
methodA end threadName=Thread-0 time=1516242298669
methodA begin threadName=Thread-1 time=1516242298669
methodA end threadName=Thread-1 time=1516242303671
methodB begin threadName=Thread-2 time=1516242303671
methodB end threadName=Thread-2 time=1516242308672
methodB begin threadName=Thread-3 time=1516242308672
methodB end threadName=Thread-3 time=1516242313674
使用Condition实现等待/通知
public class MyService {
private Lock lock = new ReentrantLock();
private Condition condition = lock.newCondition();
public void await(){
try {
lock.lock();
System.out.println("await时间为"+System.currentTimeMillis());
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
} public void signal(){
try {
lock.lock();
System.out.println("signal时间为"+System.currentTimeMillis());
condition.signal();
} finally {
lock.unlock();
}
}
} public class Run { public static void main(String[] args) throws InterruptedException {
MyService service = new MyService();
new Thread(()->service.await()).start();
Thread.sleep(3000);
service.signal();
}
}
相似处:
1. 在使用Condition的await方法和signal方法之前比较先调用lock.lock(),否则会抛出异常,跟wait和nofity一样,需在synchronized的代码里运行一样
2. Object.wait() --> Condition.await(). Object.notify()--> Condition.signal(), Object.notifyAll() -->Condition.signalAll()
优点:
在一个Lock对象里面可以创建多个Condition实例,线程对象可以注册在指定的Condition中,从而可以有选择性的进行线程通知,在调度上更加灵活。
使用多个Condition实现通知部分线程
public class MyService {
private Lock lock = new ReentrantLock();
private Condition conditionA = lock.newCondition();
private Condition conditionB = lock.newCondition();
public void awaitA(){
try {
lock.lock();
System.out.println("begin awaitA时间为"+System.currentTimeMillis() + "Thread name="+Thread.currentThread().getName());
conditionA.await();
System.out.println("end awaitA时间为"+System.currentTimeMillis() + "Thread name="+Thread.currentThread().getName());
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
} public void awaitB(){
try {
lock.lock();
System.out.println("begin awaitB时间为"+System.currentTimeMillis() + "Thread name="+Thread.currentThread().getName());
conditionB.await();
System.out.println("end awaitB时间为"+System.currentTimeMillis() + "Thread name="+Thread.currentThread().getName());
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
} public void signalAll_A(){
try {
lock.lock();
System.out.println("signalAll_B 时间为"+System.currentTimeMillis() + "Thread name="+Thread.currentThread().getName());
conditionA.signalAll();
} finally {
lock.unlock();
}
} public void signalAll_B(){
try {
lock.lock();
System.out.println("signalAll_B 时间为"+System.currentTimeMillis() + "Thread name="+Thread.currentThread().getName());
conditionB.signalAll();
} finally {
lock.unlock();
}
}
} public class Run { public static void main(String[] args) throws InterruptedException {
MyService service = new MyService();
new Thread(()->service.awaitA()).start();
new Thread(()->service.awaitB()).start();
Thread.sleep(3000);
service.signalAll_A();
}
}
测试结果:线程B没有被唤醒
begin awaitA时间为1516244219035Thread name=Thread-0
begin awaitB时间为1516244219036Thread name=Thread-1
signalAll_B 时间为1516244222035Thread name=main
end awaitA时间为1516244222035Thread name=Thread-0
生产者和消费者
public class MyList {
private Lock lock = new ReentrantLock();
private Condition condition = lock.newCondition();
private volatile List<String> list = new ArrayList<>(10); public void get() {
try {
System.out.println("get,listSize="+list.size());
lock.lock();
while (list.size() == 0) {
System.out.println("get 等待,ThreadName=" + Thread.currentThread().getName());
condition.await();
}
System.out.println("get -1");
list.remove(0);
condition.signalAll();
} catch (InterruptedException e) {
e.printStackTrace();
} finally{
lock.unlock();
}
} public void set() {
try {
System.out.println("set,listSize="+list.size());
lock.lock();
while (list.size() == 10) {
System.out.println("set 等待,ThreadName=" + Thread.currentThread().getName());
condition.await();
}
System.out.println("set+1");
list.add("A");
condition.signalAll();
} catch (InterruptedException e) {
e.printStackTrace();
} finally{
lock.unlock();
}
}
} public class Run { public static void main(String[] args) throws InterruptedException {
MyList consumer = new MyList();
for(int i =0;i<10;i++){
new Thread(()->{
while(true){ consumer.set();}
}).start();
new Thread(()->{
while(true){ consumer.get();}
}).start();
}
}
}
ReentrantReadWriteLock
ReentrantLock具有完全互斥排他的效果,即在同一时间内只有一个线程在执行ReentrantLock.lock()方法后面的任务。但是效率相对低下,而是用ReentrantReadWriteLock读写锁,可以相对提高效率。读写锁也就是有两个锁,一个是和读相关的锁,也称共享锁,一个是和写相关的锁,也叫排它锁。只有读读锁之间不互斥,其他都互斥。
读读锁不互斥的例子
public class Service { private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
public void read(){
try {
lock.readLock().lock();
System.out.println("获得读锁"+Thread.currentThread().getName()+" " + System.currentTimeMillis());
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.readLock().unlock();
}
}
} public class Run { public static void main(String[] args) throws InterruptedException {
Service service = new Service();
new Thread(()->service.read()).start();
new Thread(()->service.read()).start();
}
}
测试结果:几乎同时获得读锁,说明它们不互斥的
获得读锁Thread-0 1516246020468
获得读锁Thread-1 1516246020469
读写互斥的例子
public class Service { private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
public void read(){
try {
lock.readLock().lock();
System.out.println("获得读锁"+Thread.currentThread().getName()+" " + System.currentTimeMillis());
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.readLock().unlock();
}
} public void write(){
try {
lock.writeLock().lock();
System.out.println("获得写锁"+Thread.currentThread().getName()+" " + System.currentTimeMillis());
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.writeLock().unlock();
}
}
} public class Run { public static void main(String[] args) throws InterruptedException {
Service service = new Service();
new Thread(()->service.read()).start();
new Thread(()->service.write()).start();
}
}
测试结果:写操作需要等待读锁释放后才能获得
获得读锁Thread-0 1516246209130
获得写锁Thread-1 1516246219132
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