Java併發編程初級篇(十四)：使用讀寫所實現同步機制

Java API除了提供Lock()接口以外，還爲咱們提供了一個讀寫鎖接口ReadWriteLock，使用這個鎖的實現類ReentrantReadWriteLock能夠讓咱們把讀鎖和寫鎖進行分離，對同步數據進行修改的時候使用寫鎖，這時候其餘須要獲取寫鎖的線程會被掛起，同時使用讀鎖的線程也會被掛起。而讀取數據的時候使用讀鎖，這時候使用讀鎖的線程能夠併發訪問，以提升效率。

public interface ReadWriteLock {
    /**
     * Returns the lock used for reading.
     *
     * @return the lock used for reading.
     */
    Lock readLock();

    /**
     * Returns the lock used for writing.
     *
     * @return the lock used for writing.
     */
    Lock writeLock();
}

咱們模擬一個修改價格的例子，來看一下讀鎖與寫鎖是如何使用的。

建立一個價格類，裏面兩個商品的價格以及一個讀寫鎖。在修改價格的方法裏咱們使用寫鎖，在讀取商品價格的方法裏咱們使用讀鎖。修改商品價方法休眠兩秒來模擬修改價格的過程，並打印修改價格信息。

public class PriceInfo {
    private double price1;
    private double price2;
    private ReadWriteLock lock;

    public PriceInfo() {
        price1 = 1.0;
        price2 = 2.0;
        lock = new ReentrantReadWriteLock();
    }

    public double getPrice1() {
        lock.readLock().lock();
        double value = price1;
        lock.readLock().unlock();
        return price1;
    }

    public double getPrice2() {
        lock.readLock().lock();
        double value = price2;
        lock.readLock().unlock();
        return price2;
    }

    public void setPrice(double price1, double price2) {
        lock.writeLock().lock();
        System.out.printf("Writer: Attemp to modify the price! Price1:%f Price2:%f\n", price1, price2);
        this.price1 = price1;
        this.price2 = price2;
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.printf("Writer: Price has been modified: Price1:%f Price2:%f\n", price1, price2);
        lock.writeLock().unlock();
    }
}

建立一個價格修改線程，隨機休眠必定時間後修改價格。

public class Writer implements Runnable{
    private PriceInfo priceInfo;

    public Writer(PriceInfo priceInfo) {
        this.priceInfo = priceInfo;
    }

    @Override
    public void run() {
        double price1 = Math.random() * 10;
        double price2 = Math.random() * 8;

        try {
            Thread.sleep((long) (Math.random() * 10000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        priceInfo.setPrice(price1, price2);
    }
}

建立一個價格讀取線程，每次讀取兩個商品的價格並打印。

public class Reader implements Runnable {
    private PriceInfo priceInfo;

    public Reader(PriceInfo priceInfo) {
        this.priceInfo = priceInfo;
    }

    @Override
    public void run() {
        Date date = new Date();
        System.out.printf("%s: Price1 %f .%s\n",
                Thread.currentThread().getName(), priceInfo.getPrice1(), date);
        System.out.printf("%s: Price2 %f .%s\n",
                Thread.currentThread().getName(), priceInfo.getPrice2(), date);
    }
}

建立主方法類，啓動三個價格修改線程。而後每一個兩秒啓動一個線程讀取商品價格。

public class Main {
    public static void main(String[] args) {
        PriceInfo priceInfo = new PriceInfo();

        for (int i = 0; i < 3; i++) {
            Thread threadWriter = new Thread(new Writer(priceInfo));
            threadWriter.start();
        }

        while (true) {
            new Thread(new Reader(priceInfo)).start();
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

查看控制檯日誌，每次讀取價格都是修改價格以後的新價格，而且在修改價格的2秒內，讀取價格線程處於掛起狀態。

Thread-3: Price1 1.000000 .Fri Nov 25 11:16:35 CST 2016
Thread-3: Price2 2.000000 .Fri Nov 25 11:16:35 CST 2016
Writer: Attemp to modify the price! Price1:9.655775 Price2:4.031177
Writer: Price has been modified: Price1:9.655775 Price2:4.031177
Thread-4: Price1 9.655775 .Fri Nov 25 11:16:37 CST 2016
Thread-4: Price2 4.031177 .Fri Nov 25 11:16:37 CST 2016
Thread-5: Price1 9.655775 .Fri Nov 25 11:16:39 CST 2016
Thread-5: Price2 4.031177 .Fri Nov 25 11:16:39 CST 2016
Writer: Attemp to modify the price! Price1:2.347803 Price2:2.088409
Writer: Price has been modified: Price1:2.347803 Price2:2.088409
Thread-6: Price1 2.347803 .Fri Nov 25 11:16:41 CST 2016
Thread-6: Price2 2.088409 .Fri Nov 25 11:16:41 CST 2016
Thread-7: Price1 2.347803 .Fri Nov 25 11:16:43 CST 2016
Thread-7: Price2 2.088409 .Fri Nov 25 11:16:43 CST 2016
Writer: Attemp to modify the price! Price1:0.981288 Price2:6.281034
Writer: Price has been modified: Price1:0.981288 Price2:6.281034
Thread-8: Price1 0.981288 .Fri Nov 25 11:16:45 CST 2016
Thread-8: Price2 6.281034 .Fri Nov 25 11:16:45 CST 2016
Thread-9: Price1 0.981288 .Fri Nov 25 11:16:47 CST 2016
Thread-9: Price2 6.281034 .Fri Nov 25 11:16:47 CST 2016
Thread-10: Price1 0.981288 .Fri Nov 25 11:16:49 CST 2016
Thread-10: Price2 6.281034 .Fri Nov 25 11:16:49 CST 2016
Thread-11: Price1 0.981288 .Fri Nov 25 11:16:51 CST 2016
Thread-11: Price2 6.281034 .Fri Nov 25 11:16:51 CST 2016