黑马程序员-java5的线程锁技术Lock,Condition

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java.util.concurrent.locks.Lock

   

 

格式：Lock lock=new ReentrantLock();

lock.lock();
try {
} finally {
lock.unlock();
}

读写锁：分为读锁和写锁，多个读锁不互斥，读锁与写锁互斥，写锁与写锁互斥，这是由jvm自己控制的。

小结：读锁控制读线程与写线程的互斥，就是说多个线程可以同时读，但写线程就不能进来，如果不加读锁，写线程就会进来，这就是读锁的作用。

写锁：控制写线程与写线程的互斥。

格式：

ReadWriteLock rwl = new ReentrantReadWriteLock();
try {
rwl.readLock().lock();
} finally {
rwl.readLock().unlock();
}
try {
rwl.writeLock().lock();
} finally {
rwl.writeLock().unlock();
}

小结：读锁中的lock方法



 

写锁中的lock方法



 

DataDemo类

 

public class DataDemo {

private String data;

public void put(String data) {
System.out.println(Thread.currentThread().getName() + "be ready write");
try {
Thread.sleep(200);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "have write data"
+ data);
this.data = data;
}

public String get() {
System.out.println(Thread.currentThread().getName() + "be ready read");
try {
Thread.sleep(20);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "have read data"
+ data);
return data;
}
}

ReadWriteLockTest类

import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReadWriteLockTest {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
final ReadWriteLock rwl = new ReentrantReadWriteLock();
final DataDemo dataDemo = new DataDemo();
for (int i = 0; i < 3; i++) {
new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
while (true) {
// rwl.readLock().lock();
dataDemo.get();
// rwl.readLock().unlock();
try {
Thread.currentThread().sleep(20);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}).start();
}
for (int j = 0; j < 3; j++) {
new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
Random random = new Random();
while (true) {
rwl.writeLock().lock();
String str = random.nextInt() + "abc";
dataDemo.put(str);

rwl.writeLock().unlock();
try {
Thread.currentThread().sleep(20);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}).start();
}

}

}

输出结果：

注意我注释掉了读锁


 

所以出现这个结果很正常，因为读线程么有加读锁，如果加了的话，如果写线程没有释放锁，读线程rwl.readLock().lock();就会阻塞，如果不加读锁，读线程就不会阻塞。

自己写个缓存系统：

CacheDemo类

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class CacheDemo {

static Map<String, Object> map = new HashMap<String, Object>();
static Object obj;
static ReadWriteLock lock = new ReentrantReadWriteLock();

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub

}

public static Object get(String str) {
try {
lock.readLock().lock();
if (map.get(str) == null) {
try {
lock.readLock().unlock();
lock.writeLock().lock();
if (map.get(str) == null) {
obj = "";// 查询数据库
}
} finally {
lock.writeLock().unlock();
lock.readLock().lock();
}
}
} finally {
lock.readLock().unlock();
}
return obj;
}

}

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<a href="http://www.itheima.com"target="blank">ASP.Net+Unity开发</a>、<a href="http://www.itheima.com"target="blank">.Net培训</a>、期待与您交流！ ----------------------