1、什麼是線程同步
概念上的東西在這裏就不過多介紹了,線程同步,也叫做線程間通信,就是多個線程共同協作完成一件事情。
爲了完成一件事情,可能需要分很多的部分或步驟,每個部分由一個或一類線程來負責,而各個部分的處理又有先後順序,這就對相互協作的線程有了一定的約束,也就是進程同步。
2、Java中如何實現線程同步
在Java中,線程的同步涉及到synchronized和三個方法wait()、notify()和notifyAll()。
synchronized在上篇中已經講過了,這裏就不再重複了。
wait()方法與Thread類的sleep()和yield()方法類似,都是讓當線程睡眠,或者說是暫停執行;與之不同的是wait()方法會釋放掉當前對象的鎖,也因爲此wait()方法必須在synchronized塊裏才能被調用。
notify()和notifyAll()方法用於喚醒之前調用wait()方法睡眠的線程,與wait()方法一樣,notify()和notifyAll()也必須在synchronized塊裏才能被調用。
注意wait()、notify()和notifyAll()三個方法都是Object類的,而不是Thread類的。因爲這三個方法都涉及的鎖的操作,而鎖的操作適用於所有的對象。
3、線程同步事例
下面的這例子,假設有一個籃子,最多可以裝5個蘋果,producer線程不斷地往裏面放蘋果,consumer線程不斷的吃蘋果。
public class Basket
{
private int numOfPlace = 5;
private int numOfApple = 0;
public synchronized void addApple() throws InterruptedException
{
while (numOfPlace <= 0)
wait();
Thread.sleep(100);
numOfPlace--;
numOfApple++;
System.out.println("Producing... [" + (numOfApple) + "] apple left.");
notify();
}
public synchronized void eatApple() throws InterruptedException
{
while (numOfApple <= 0)
wait();
numOfPlace++;
numOfApple--;
Thread.sleep(100);
System.out.println("Eating... [" + (numOfApple) + "] apple left.");
notify();
}
public static void main(String[] args) throws InterruptedException
{
Basket box = new Basket();
ExecutorService exec = Executors.newCachedThreadPool();
exec.execute(new Producer(box));
exec.execute(new Consumer(box));
TimeUnit.SECONDS.sleep(3);
exec.shutdownNow();
}
}
class Producer implements Runnable
{
private Basket box;
public Producer(Basket box)
{
this.box = box;
}
@Override
public void run()
{
while (true)
{
try
{
box.addApple();
Thread.yield();
}
catch (InterruptedException e)
{
e.printStackTrace();
return;
}
}
}
}
class Consumer implements Runnable
{
private Basket box;
public Consumer(Basket box)
{
this.box = box;
}
@Override
public void run()
{
while (true)
{
try
{
box.eatApple();
Thread.yield();
}
catch (InterruptedException e)
{
e.printStackTrace();
return;
}
}
}
}
Basket.addApple()方法中,首先檢查籃子還有沒有空間可以裝入新蘋果,如果沒有,那麼producer線程就調用Basket對象的wait()方法,等侍並釋放Basket對象的鎖;接着consumer線程就有機會取得Basket對象的鎖並進入eatApple()方法,當consumer方法成功吃掉一個蘋果後,籃子就有空間裝入新的蘋果了,這時eatApple()方法中調用Basket對象的notifyAll()方法通知producer線程;這時prodocer線程又會重新獲得Basket對象的鎖,並再次檢查是否有空間裝入新的蘋果,成功裝入一個蘋果後,也會調用Basket對象的notifyAll()方法通知consumber線程。
上面這個例子中producer和consumer線程就是通過wait()和notify()方法來通信,實現共同協作的。
4、使用Lock和Condition實現線程同步
Java1.5提供了Lock和Condition接口,也可以實現線程同步,下面使用了Lock和Condition的方式重寫了上面的例子:
public class Basket
{
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
private int numOfPlace = 5;
private int numOfApple = 0;
public void addApple() throws InterruptedException
{
try
{
lock.lock();
while (numOfPlace <= 0)
condition.await();
Thread.sleep(100);
numOfPlace--;
numOfApple++;
System.out.println("Producing... [" + (numOfApple) + "] apple left.");
condition.signalAll();
}
finally
{
lock.unlock();
}
}
public void eatApple() throws InterruptedException
{
try
{
lock.lock();
while (numOfApple <= 0)
condition.await();
numOfPlace++;
numOfApple--;
Thread.sleep(100);
System.out.println("Eating... [" + (numOfApple) + "] apple left.");
condition.signalAll();
}
finally
{
lock.unlock();
}
}
public static void main(String[] args) throws InterruptedException
{
Basket box = new Basket();
ExecutorService exec = Executors.newCachedThreadPool();
exec.execute(new Producer(box));
exec.execute(new Consumer(box));
TimeUnit.SECONDS.sleep(3);
exec.shutdownNow();
}
}
class Producer implements Runnable
{
private Basket box;
public Producer(Basket box)
{
this.box = box;
}
@Override
public void run()
{
while (true)
{
try
{
box.addApple();
Thread.yield();
}
catch (InterruptedException e)
{
e.printStackTrace();
return;
}
}
}
}
class Consumer implements Runnable
{
private Basket box;
public Consumer(Basket box)
{
this.box = box;
}
@Override
public void run()
{
while (true)
{
try
{
box.eatApple();
Thread.yield();
}
catch (InterruptedException e)
{
e.printStackTrace();
return;
}
}
}
}
新的方法提供了更好的靈活性,但也增加了出錯的可能性,如果沒有特殊的需求,還是採用原來的方式來得實在。