设计模式 Concurrency 之 ReadWriteLock 读写锁

1. 定义

• 这种模式允许同步的读操作,但是写操作会加排他锁.
• 意味着多线程可以进行读操作,但是修改数据的时候需要加排他锁
• 当一个线程在写数据时,其他读或写线程会一直阻塞到当前线程完成写操作

2. 例子

ReadWriteLock

package com.hqq.concurrency.read_write_lock;

import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;

/**
 * ReaderWriterLock
 * 读写锁
 * 控制读写权限
 * 允许多个读操作持有锁 但是如果加了写锁 读操作便会阻塞
 * 如果加了读锁 写操作就会阻塞
 * 是非公平锁
 * Created by heqianqian on 2017/7/30.
 */
public class ReaderWriterLock implements ReadWriteLock {


    private Object readerMutex = new Object();

    private int currentReaderCount;

    private ReadLock readLock = new ReadLock();

    private WriteLock writeLock = new WriteLock();

    /**
     * 全局互斥变量 用来表示写操作和读操作是否同时获得锁
     * <p>
     * 1.如果包含{@link readLock},表示当前线程持有读锁,其他线程仍然可以进行读操作
     * 2.如果包含{@link writeLock},表示当前线程持有写锁,其他线程既无法执行写操作也无法执行读操作
     */
    private Set<Object> globalMutex = new HashSet<>();

    @Override
    public Lock readLock() {
        return this.readLock;
    }

    @Override
    public Lock writeLock() {
        return this.writeLock;
    }

    private boolean doesWriterHoldLock() {
        return globalMutex.contains(writeLock);
    }

    private boolean doesReaderHoldLock() {
        return globalMutex.contains(readLock);
    }


    private boolean isLockFree() {
        return globalMutex.isEmpty();
    }

    private static void waitUnInterruptly(Object o) {
        try {
            o.wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    /**
     * 读锁
     */
    private class ReadLock implements Lock {

        @Override
        public void lock() {
            synchronized (readerMutex) {
                currentReaderCount++;//当前读操作数加1
                if (currentReaderCount == 1) {
                    //尝试为第一个读线程获得互斥锁
                    synchronized (globalMutex) {
                        while (true) {
                            //如果没有线程持有锁或者当前持有的是读锁
                            if (isLockFree() || doesReaderHoldLock()) {
                                globalMutex.add(this);
                                break;
                            } else {//否则阻塞等待
                                waitUnInterruptly(globalMutex);
                            }
                        }
                    }
                }
            }

        }

        @Override
        public void lockInterruptibly() throws InterruptedException {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean tryLock() {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
            throw new UnsupportedOperationException();
        }

        @Override
        public void unlock() {
            synchronized (readerMutex) {
                //当前读线程数减1
                currentReaderCount--;
                //当读线程数为0[即完成所有读操作时 移除当前线程 唤醒其他等待线程]
                if (currentReaderCount == 0) {
                    synchronized (globalMutex) {
                        globalMutex.remove(this);
                        //唤醒等待者 一般是读锁
                        globalMutex.notifyAll();
                    }
                }
            }
        }

        @Override
        public Condition newCondition() {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * 写锁
     */
    private class WriteLock implements Lock {

        @Override
        public void lock() {
            synchronized (globalMutex) {
                while (true) {
                    if (isLockFree()) {
                        globalMutex.add(this);
                        break;
                    } else if (doesWriterHoldLock()) {
                        waitUnInterruptly(globalMutex);
                    } else if (doesReaderHoldLock()) {
                        waitUnInterruptly(globalMutex);
                    } else {
                        throw new AssertionError("it should never reach here");
                    }
                }
            }
        }

        @Override
        public void lockInterruptibly() throws InterruptedException {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean tryLock() {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
            throw new UnsupportedOperationException();
        }

        @Override
        public void unlock() {
            synchronized (globalMutex) {
                globalMutex.remove(this);
                globalMutex.notifyAll();
            }
        }

        @Override
        public Condition newCondition() {
            throw new UnsupportedOperationException();
        }
    }
}

Reader

package com.hqq.concurrency.read_write_lock;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.locks.Lock;

/**
 * Reader
 * Created by heqianqian on 2017/7/30.
 */
public class Reader extends Thread {

    private static final Logger LOGGER = LoggerFactory.getLogger(Reader.class);

    private String name;

    private Lock readLock;

    public Reader(String name, Lock readLock) {
        this.name = name;
        this.readLock = readLock;
    }

    @Override
    public void run() {
        readLock.lock();
        try {
            read();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            readLock.unlock();
        }
    }

    private void read() throws InterruptedException {
        LOGGER.info("{} read begin", name);
        Thread.sleep(500);
        LOGGER.info("{} read end", name);
    }
}

Writer

package com.hqq.concurrency.read_write_lock;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.locks.Lock;

/**
 * Writer
 * Created by heqianqian on 2017/7/30.
 */
public class Writer extends Thread {

    private static final Logger LOGGER = LoggerFactory.getLogger(Writer.class);

    private String name;

    private Lock writeLock;

    public Writer(String name, Lock writeLock) {
        this.name = name;
        this.writeLock = writeLock;
    }

    @Override
    public void run() {
        writeLock.lock();
        try {
            write();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            writeLock.unlock();
        }
    }

    public void write() throws InterruptedException {
        LOGGER.info("{} write begin", name);
        Thread.sleep(500);
        LOGGER.info("{} write end", name);
    }
}

APP

package com.hqq.concurrency.read_write_lock;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.stream.IntStream;

/**
 * ReadWriteLock读写锁
 * 在一个多线程的环境下,不管是读还是写操作会尝试去同步共享资源.
 * 这会导致低性能,特别在读多于写的情况下,因为读操作对另一个线程来说的线程安全的
 * <p>
 * 这种模式允许同步的读操作,但是写操作会加排他锁.
 * 意味着多线程可以进行读操作,但是修改数据的时候需要加排他锁
 * 当一个线程在写数据时,其他读或写线程会一直阻塞到当前线程完成写操作
 * Created by heqianqian on 2017/7/30.
 */
public class App {

    private static final Logger LOGGER = LoggerFactory.getLogger(App.class);

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        ReaderWriterLock readerWriterLock = new ReaderWriterLock();

        //开启5个读线程
        IntStream.range(0, 5)
                .forEach(i -> executorService.submit(new Reader("Reader" + i, readerWriterLock.readLock())));

        //开启5个写线程
        IntStream.range(0, 5)
                .forEach(i -> executorService.submit(new Writer("Writer" + i, readerWriterLock.writeLock())));

        executorService.shutdown();

        try {
            executorService.awaitTermination(5, TimeUnit.SECONDS);
        } catch (InterruptedException e) {
            LOGGER.error("Error waiting for ExecutorService shutdown");
        }
    }

}

测试结果:

INFO  [2017-08-09 02:24:34,339] com.hqq.concurrency.read_write_lock.Reader: Reader2 read begin
INFO  [2017-08-09 02:24:34,336] com.hqq.concurrency.read_write_lock.Reader: Reader0 read begin
INFO  [2017-08-09 02:24:34,337] com.hqq.concurrency.read_write_lock.Reader: Reader1 read begin
INFO  [2017-08-09 02:24:34,352] com.hqq.concurrency.read_write_lock.Reader: Reader3 read begin
INFO  [2017-08-09 02:24:34,361] com.hqq.concurrency.read_write_lock.Reader: Reader4 read begin
INFO  [2017-08-09 02:24:34,849] com.hqq.concurrency.read_write_lock.Reader: Reader0 read end
INFO  [2017-08-09 02:24:34,849] com.hqq.concurrency.read_write_lock.Reader: Reader1 read end
INFO  [2017-08-09 02:24:34,849] com.hqq.concurrency.read_write_lock.Reader: Reader2 read end
INFO  [2017-08-09 02:24:34,865] com.hqq.concurrency.read_write_lock.Reader: Reader3 read end
INFO  [2017-08-09 02:24:34,866] com.hqq.concurrency.read_write_lock.Reader: Reader4 read end
INFO  [2017-08-09 02:24:34,866] com.hqq.concurrency.read_write_lock.Writer: Writer4 write begin
INFO  [2017-08-09 02:24:35,402] com.hqq.concurrency.read_write_lock.Writer: Writer4 write end
INFO  [2017-08-09 02:24:35,402] com.hqq.concurrency.read_write_lock.Writer: Writer0 write begin
INFO  [2017-08-09 02:24:35,904] com.hqq.concurrency.read_write_lock.Writer: Writer0 write end
INFO  [2017-08-09 02:24:35,904] com.hqq.concurrency.read_write_lock.Writer: Writer3 write begin
INFO  [2017-08-09 02:24:36,410] com.hqq.concurrency.read_write_lock.Writer: Writer3 write end
INFO  [2017-08-09 02:24:36,411] com.hqq.concurrency.read_write_lock.Writer: Writer1 write begin
INFO  [2017-08-09 02:24:36,911] com.hqq.concurrency.read_write_lock.Writer: Writer1 write end
INFO  [2017-08-09 02:24:36,912] com.hqq.concurrency.read_write_lock.Writer: Writer2 write begin
INFO  [2017-08-09 02:24:37,415] com.hqq.concurrency.read_write_lock.Writer: Writer2 write end

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