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

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