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