Java并发编程之线程管理（基本线程同步4）

1.3   读写Lock

Lock机制最具重大意义的改进之一就是读写锁（ReadWriteLock）接口和ReentrantReadWriteLock类，这个类是唯一一个实现这个读写锁的类。在这个类中，有两个锁，一个事读操作，一个事写操作。有超过一个线程并行的进行读操作，但是写操作只能是某一个线程。当一个线程做写操作时，其它的线程不能能够进行读操作。

下面使用一个实例来说明这一点。定义一个读者和写者，分别读和写价格。

先定义价格打印类PrincesInfo:

 importjava.util.concurrent.locks.ReadWriteLock;
importjava.util.concurrent.locks.ReentrantReadWriteLock;

/**
* This class simulates the store oftwo prices. We will
* have a writer that stores theprices and readers that
* consult this prices
*
*/
public class PricesInfo {

/**
* The two prices
*/
private doubleprice1;
private doubleprice2;

/**
* Lock to control the access to the prices
*/
private ReadWriteLock lock;

/**
* Constructor of the class. Initializes theprices and the Lock
*/
public PricesInfo(){
price1=1.0;
price2=2.0;
lock=new ReentrantReadWriteLock();
}

/**
* Returns the first price
* @return the first price
*/
public doublegetPrice1() {
lock.readLock().lock();
double value=price1;
lock.readLock().unlock();
return value;
}

/**
* Returns the second price
* @return the second price
*/
public doublegetPrice2() {
lock.readLock().lock();
double value=price2;
lock.readLock().unlock();
return value;
}

/**
* Establish the prices
* @param price1 The price of the first product
* @param price2 The price of the second product
*/
public voidsetPrices(doubleprice1, doubleprice2) {
lock.writeLock().lock();
this.price1=price1;
this.price2=price2;
lock.writeLock().unlock();
}
}

 
定义读者类Reader：

 /**
* This class implements a readerthat consults the prices
*
*/
public class Reader implements Runnable {

/**
* Class that stores the prices
*/
private PricesInfo pricesInfo;

/**
* Constructor of the class
* @param pricesInfo object that stores the prices
*/
public Reader (PricesInfo pricesInfo){
this.pricesInfo=pricesInfo;
}

/**
* Core method of the reader. Consults the twoprices and prints them
* to the console
*/
@Override
public voidrun() {
for (int i=0; i<10; i++){
System.out.printf("%s: Price 1: %f\n",Thread.currentThread().getName(),pricesInfo.getPrice1());
System.out.printf("%s: Price 2: %f\n",Thread.currentThread().getName(),pricesInfo.getPrice2());
}
}

}

定义写者类Writer：
/**
* This class implements a writerthat establish the prices
*
*/
public class Writer implements Runnable {

/**
* Class that stores the prices
*/
private PricesInfo pricesInfo;

/**
* Constructor of the class
* @param pricesInfo object that stores the prices
*/
public Writer(PricesInfo pricesInfo){
this.pricesInfo=pricesInfo;
}

/**
* Core method of the writer. Establish theprices
*/
@Override
public voidrun() {
for (int i=0; i<3; i++) {
System.out.printf("Writer: Attempt to modify the prices.\n");
pricesInfo.setPrices(Math.random()*10,Math.random()*8);
System.out.printf("Writer: Prices have been modified.\n");
try {
Thread.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

}

定义测试类ReadWriteLock：

public class ReadWriteLockTest {

/**
* Main class of the example
* @param args
*/
public staticvoidmain(String[] args) {

// Creates an object to store the prices
PricesInfo pricesInfo=new PricesInfo();

Reader readers[]=new Reader[5];
Thread threadsReader[]=new Thread[5];

// Creates five readers and threads to run them
for (int i=0; i<5; i++){
readers[i]=new Reader(pricesInfo);
threadsReader[i]=new Thread(readers[i]);
}

// Creates a writer and a thread to run it
Writer writer=new Writer(pricesInfo);
Thread threadWriter=new Thread(writer);

// Starts the threads
for (int i=0; i<5; i++){
threadsReader[i].start();
}
threadWriter.start();

}

}

运行结果：

Writer:Attempt to modify the prices.
Writer:Prices have been modified.
Thread-0:Price 1: 1.000000
Thread-3:Price 1: 1.000000
Thread-1:Price 1: 1.000000
Thread-1:Price 2: 1.844894
Thread-1:Price 1: 8.698622
Thread-1:Price 2: 1.844894
Thread-1:Price 1: 8.698622
Thread-1:Price 2: 1.844894
Thread-1:Price 1: 8.698622
Thread-1:Price 2: 1.844894
Thread-1:Price 1: 8.698622
Thread-1:Price 2: 1.844894
Thread-1:Price 1: 8.698622
Thread-1:Price 2: 1.844894
Thread-1:Price 1: 8.698622
Thread-1:Price 2: 1.844894
Thread-1:Price 1: 8.698622
Thread-1:Price 2: 1.844894
Thread-1:Price 1: 8.698622
Thread-1:Price 2: 1.844894
Thread-1:Price 1: 8.698622
Thread-1:Price 2: 1.844894
Thread-4:Price 1: 1.000000
Thread-4:Price 2: 1.844894
Thread-4:Price 1: 8.698622
Thread-4:Price 2: 1.844894
Thread-4:Price 1: 8.698622
Thread-4:Price 2: 1.844894
Thread-4:Price 1: 8.698622
Thread-4:Price 2: 1.844894
Thread-4:Price 1: 8.698622
Thread-4:Price 2: 1.844894
Thread-4:Price 1: 8.698622
Thread-4:Price 2: 1.844894
Thread-4:Price 1: 8.698622
Thread-4:Price 2: 1.844894
Thread-4:Price 1: 8.698622
Thread-4:Price 2: 1.844894
Thread-4:Price 1: 8.698622
Thread-4:Price 2: 1.844894
Thread-4:Price 1: 8.698622
Thread-4:Price 2: 1.844894
Thread-2:Price 1: 1.000000
Thread-3:Price 2: 1.844894
Thread-0:Price 2: 1.844894
Thread-0:Price 1: 8.698622
Thread-0:Price 2: 1.844894
Thread-0:Price 1: 8.698622
Thread-0:Price 2: 1.844894
Writer:Attempt to modify the prices.
Thread-0:Price 1: 8.698622
Thread-3:Price 1: 8.698622
Thread-2:Price 2: 1.844894
Thread-3:Price 2: 6.717506
Thread-0:Price 2: 6.717506
Thread-0:Price 1: 6.478280
Thread-0:Price 2: 6.717506
Writer:Prices have been modified.
Thread-0:Price 1: 6.478280
Thread-0:Price 2: 6.717506
Thread-0:Price 1: 6.478280
Thread-0:Price 2: 6.717506
Thread-0:Price 1: 6.478280
Thread-0:Price 2: 6.717506
Thread-0:Price 1: 6.478280
Thread-0:Price 2: 6.717506
Thread-0:Price 1: 6.478280
Thread-0:Price 2: 6.717506
Thread-3:Price 1: 6.478280
Thread-3:Price 2: 6.717506
Thread-3:Price 1: 6.478280
Thread-3:Price 2: 6.717506
Thread-3:Price 1: 6.478280
Thread-3:Price 2: 6.717506
Thread-3:Price 1: 6.478280
Thread-3:Price 2: 6.717506
Thread-3:Price 1: 6.478280
Thread-3:Price 2: 6.717506
Thread-3:Price 1: 6.478280
Thread-3:Price 2: 6.717506
Thread-3:Price 1: 6.478280
Thread-3:Price 2: 6.717506
Thread-3:Price 1: 6.478280
Thread-3:Price 2: 6.717506
Thread-2:Price 1: 6.478280
Thread-2:Price 2: 6.717506
Thread-2:Price 1: 6.478280
Thread-2:Price 2: 6.717506
Writer:Attempt to modify the prices.
Writer:Prices have been modified.
Thread-2:Price 1: 6.478280
Thread-2:Price 2: 7.824052
Thread-2:Price 1: 7.911020
Thread-2:Price 2: 7.824052
Thread-2:Price 1: 7.911020
Thread-2:Price 2: 7.824052
Thread-2:Price 1: 7.911020
Thread-2:Price 2: 7.824052
Thread-2:Price 1: 7.911020
Thread-2:Price 2: 7.824052
Thread-2:Price 1: 7.911020
Thread-2:Price 2: 7.824052
Thread-2:Price 1: 7.911020
Thread-2:Price 2: 7.824052