针对单一key加读写锁

针对单一key加读写锁

一、什么是读写锁
读写锁是JDK1.5提供的一个工具锁,适用于读多写少的场景,将读写分离,从而提高并发性。
二、读写锁的特点
读锁是共享锁,写锁是排他锁,读锁和写锁不能同时存在;
读锁不能升级为写锁;
写锁可以降级为读锁;
三、锁的本质
锁的本质就是锁住一块资源而不是一块代码. 在常见的一些代码实现都是加一把大锁,将这一块代码资源统一加锁,无法针对资源进行精确进行锁控制.
四、代码实现
`
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
@Slf4j
public class UserDefinedLock {

final ConcurrentHashMap<String, ReadWriteLock> map = new ConcurrentHashMap<>();

public UserDefinedLock() {

}

/**
 * 从map里获取锁 如果存在则返回 不存在则创建
 *
 * @param key key
 * @return lock
 */
public void createOrGetLock(String key) {
    synchronized (key.intern()) {
        map.compute(key, (k, lock) -> {
            if (lock == null) {
                lock = new ReentrantReadWriteLock();
            }
            return lock;
        });
    }
}

/**
 * 获取锁 会阻塞
 *
 * @param key key
 */
public void writeLock(String key) {
    map.get(key).writeLock().lock();
}

public void readLock(String key) {
    map.get(key).readLock().lock();
}

/**
 * 释放锁 必须由申请锁的线程进行调用
 *
 * @param key key
 */
public void unWriteLock(String key) {
    ReadWriteLock lock = map.get(key);
    if (null != lock) {
        try {
            lock.writeLock().unlock();
        } catch (Exception e) {
            log.error("释放写锁失败,key:{}, msg:{}", key, e.getMessage());
        }
    }
}

public void unReadLock(String key) {
    ReadWriteLock lock = map.get(key);
    if (null != lock) {
        try {
            lock.readLock().unlock();
        } catch (Exception e) {
            log.error("释放读锁失败,key:{}, msg:{}", key, e.getMessage());
        }
    }
}
  public static void main(String[] args) {
        UserDefinedLock userDefinedLock = new UserDefinedLock();
        ArrayList list = new ArrayList<String>();
    for (int i = 0; i < 5; i++) {
        String key = String.valueOf("测试" + i);
        new Thread(() -> {
            try {
                userDefinedLock.createOrGetLock(key);
                userDefinedLock.readLock(key);
                System.out.println("-----" + key + "----");
                list.add(key);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                userDefinedLock.unReadLock(key);
            }
        }, "read-" + key).start();
    }
}

}
`

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