读写锁

前言

调试JDBC反序列化的时候，里面有这种操作，有点好奇就来看看

public static ConnectionUrl getConnectionUrlInstance(String connString, Properties info) {
        if (connString == null) {
            throw ExceptionFactory.createException(WrongArgumentException.class, Messages.getString("ConnectionString.0"));
        }
        String connStringCacheKey = buildConnectionStringCacheKey(connString, info);
        ConnectionUrl connectionUrl;

        rwLock.readLock().lock();
        connectionUrl = connectionUrlCache.get(connStringCacheKey);
        if (connectionUrl == null) {
            rwLock.readLock().unlock();
            rwLock.writeLock().lock();
            try {
                // Check again, in the meantime it could have been cached by another thread.
                connectionUrl = connectionUrlCache.get(connStringCacheKey);
                if (connectionUrl == null) {
                    ConnectionUrlParser connStrParser = ConnectionUrlParser.parseConnectionString(connString);
                    connectionUrl = Type.getConnectionUrlInstance(connStrParser, info);
                    connectionUrlCache.put(connStringCacheKey, connectionUrl);
                }
                rwLock.readLock().lock();
            } finally {
                rwLock.writeLock().unlock();
            }
        }
        rwLock.readLock().unlock();
        return connectionUrl;
    }

demo

package org.example;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReadWriteLockTest {

    private double data = 0;

    ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
    private ReentrantLock lock = new ReentrantLock();

    public void get(){
        try {
            rwl.readLock().lock();
            System.out.println("----Thread:"+Thread.currentThread().getName()+"----read first value:"+data);
            Thread.sleep(1000);
            System.out.println("----Thread:"+Thread.currentThread().getName()+"----read second value:"+data);
            rwl.readLock().unlock();
        } catch (Exception e) {
            // TODO: handle exception
        }
    }


    public void put(){
        try {
            rwl.writeLock().lock();
            data = Math.random();
            System.out.println("----Thread:"+Thread.currentThread().getName()+"----write first value:"+data);
            Thread.sleep(100);
            rwl.writeLock().unlock();
        } catch (Exception e) {
            // TODO: handle exception
        }
    }

}

package org.example;

public class MainReadWritLockTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        final ReadWriteLockTest rwlt = new ReadWriteLockTest();
        for(int i=0; i<5;i++){
            new Thread(new Runnable() {
                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    rwlt.get();
                }
            }).start();
        }
        for(int i=0; i<2;i++){
            new Thread(new Runnable() {
                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    rwlt.put();
                }
            }).start();
        }
    }

}

输出结果

----Thread:Thread-0----read first value:0.0
----Thread:Thread-3----read first value:0.0
----Thread:Thread-4----read first value:0.0
----Thread:Thread-2----read first value:0.0
----Thread:Thread-1----read first value:0.0
----Thread:Thread-4----read second value:0.0
----Thread:Thread-1----read second value:0.0
----Thread:Thread-0----read second value:0.0
----Thread:Thread-2----read second value:0.0
----Thread:Thread-3----read second value:0.0
----Thread:Thread-5----write first value:0.38882443875894124
----Thread:Thread-6----write first value:0.537279172993168

可以看到，在read second value这一段结束了，才开始write的操作，也就是说在没有释放readlock的时候,writelock是锁着的，也就是 读读不互斥，写写不互斥，读写有互斥
如果把锁注释，也就是下面的代码

package org.example;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReadWriteLockTest {

    private double data = 0;

    ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
    private ReentrantLock lock = new ReentrantLock();

    public void get(){
        try {
            System.out.println("----Thread:"+Thread.currentThread().getName()+"----read first value:"+data);
            Thread.sleep(1000);
            System.out.println("----Thread:"+Thread.currentThread().getName()+"----read second value:"+data);
        } catch (Exception e) {
            // TODO: handle exception
        }
    }


    public void put(){
        try {
            data = Math.random();
            System.out.println("----Thread:"+Thread.currentThread().getName()+"----write first value:"+data);
            Thread.sleep(100);
        } catch (Exception e) {
            // TODO: handle exception
        }
    }

}

结果如下

----Thread:Thread-2----read first value:0.0
----Thread:Thread-3----read first value:0.0
----Thread:Thread-0----read first value:0.0
----Thread:Thread-1----read first value:0.0
----Thread:Thread-4----read first value:0.0
----Thread:Thread-5----write first value:0.7311360215541859
----Thread:Thread-6----write first value:0.32755079667745735
----Thread:Thread-0----read second value:0.32755079667745735
----Thread:Thread-1----read second value:0.32755079667745735
----Thread:Thread-3----read second value:0.32755079667745735
----Thread:Thread-2----read second value:0.32755079667745735
----Thread:Thread-4----read second value:0.32755079667745735

可以看到write写到中间了

getConnectionUrlInstance

回到开头的代码，可以发现这里是在读取缓存的时候加锁，读完了就释放，然后在加上写锁，往缓存列表放入URL