Read-Write Lock模式 --大家一起讀可以，讀的時候不要寫

定義：

線程執行讀取操作時，實例的狀態不會發生改變，所以多個線程可以同時讀取。線程執行寫取操作時，實例的狀態可能會發生改變，所以多個線程不可以同時寫入。

引例

類的一覽表

名字	說明
main	測試行爲的類
data	可以讀寫的類
writerthread	寫入線程
readthread	讀取線程
readwritelock	提供讀寫鎖

 

public class Main {
	public static void main(String[] args) {
		Data data = new Data(10);
		new ReaderThread(data).start();
		new ReaderThread(data).start();
		new ReaderThread(data).start();
		new ReaderThread(data).start();
		new ReaderThread(data).start();
		new ReaderThread(data).start();
		new WriterThread(data, "ABCDEFGHIJKLMNOPQRSTUVWXYZ").start();
		new WriterThread(data, "abcdefghijklmnopqrstuvwxyz").start();
	}
}

public class Data {
    private final char[] buffer;
    private final ReadWriteLock lock = new ReadWriteLock();
    public Data(int size) {
        this.buffer = new char[size];
        for (int i = 0; i < buffer.length; i++) {
            buffer[i] = '*';
        }
    }
    public char[] read() throws InterruptedException {
        lock.readLock();
        try {
            return doRead();
        } finally {
            lock.readUnlock();
        }
    }
    public void write(char c) throws InterruptedException {
        lock.writeLock();
        try {
            doWrite(c);
        } finally {
            lock.writeUnlock();
        }
    }
    private char[] doRead() {
        char[] newbuf = new char[buffer.length];
        for (int i = 0; i < buffer.length; i++) {
            newbuf[i] = buffer[i];
        }
        slowly();
        return newbuf;
    }
    private void doWrite(char c) {
        for (int i = 0; i < buffer.length; i++) {
            buffer[i] = c;
            slowly();
        }
    }
    private void slowly() {
        try {
            Thread.sleep(50);
        } catch (InterruptedException e) {
        }
    }
}

 

public class WriterThread extends Thread {
    private static final Random random = new Random();
    private final Data data;
    private final String filler;
    private int index = 0;
    public WriterThread(Data data, String filler) {
        this.data = data;
        this.filler = filler;
    }
    public void run() {
        try {
            while (true) {
                char c = nextchar();
                data.write(c);
                Thread.sleep(random.nextInt(3000));
            }
        } catch (InterruptedException e) {
        }
    }
    private char nextchar() {
        char c = filler.charAt(index);
        index++;
        if (index >= filler.length()) {
            index = 0;
        }
        return c;
    }
}

public class ReaderThread extends Thread {
    private final Data data;
    public ReaderThread(Data data) {
        this.data = data;
    }
    public void run() {
        try {
            while (true) {
                char[] readbuf = data.read();
                System.out.println(Thread.currentThread().getName() + " reads " + String.valueOf(readbuf));
            }
        } catch (InterruptedException e) {
        }
    }
}

 

public final class ReadWriteLock {
    private int readingReaders = 0; // (A)...實際正在讀取的執行緒數量
    private int waitingWriters = 0; // (B)...正在等待寫入的執行緒數量
    private int writingWriters = 0; // (C)...實際正在寫入的執行緒數量
    private boolean preferWriter = true; // 寫入優先的話，值爲true

    public synchronized void readLock() throws InterruptedException {
        while (writingWriters > 0 || (preferWriter && waitingWriters > 0)) {
            wait();
        }
        readingReaders++;                       //  (A)實際正在讀取的線程數量加1
    }

    public synchronized void readUnlock() {
        readingReaders--;                       //  (A)實際正在讀取的線程數量減1
        preferWriter = true;
        notifyAll();
    }

    public synchronized void writeLock() throws InterruptedException {
        waitingWriters++;                       // (B)正在等待寫入的線程數量加1
        try {
            while (readingReaders > 0 || writingWriters > 0) {
                wait();
            }
        } finally {
          waitingWriters--;                   // (B)正在等待寫入的線程數量減1
        }
        writingWriters++;                       //  (C)實際正在寫入的線程數量加1
    }

    public synchronized void writeUnlock() {
        writingWriters--;                       // (C)實際正在寫入的線程數量減
        preferWriter = false;
        notifyAll();
    }
}

Read-Write Lock模式角色

reader：讀者

writer：寫者

sharedresource：共享資源

readwritelock：讀寫鎖

何時使用Read-Write Lock模式

1.適合讀取操作繁重時

2.讀取頻率比寫入頻率高時