Java，多个线程对同一个数据源的访问

当多个线程对同一个数据源进行访问时，应对线程同步或加锁。
为何？举个简单的例子：有一个共享的数据源dataSource，其值为0。有两个线程，Thread1和Thread2。
Thread1的任务是将dataSource连续自增10次，
Thread2的任务是将dataSource连续自减10次，
当两个线程的任务都完成时，最终的dataSource的值应为0，事实上，可能不为0，请看示例1：

import static java.lang.System.out;
public class LockTest1
{
private static int dataSource;
private static class Task1 implements Runnable
{
public void run()
{
for (int i = 0; i < 10; ++i)
{
out.printf("Task1 --- dataSource: %d\n", dataSource);
int t = dataSource + 1;

try
{
Thread.sleep(1);
}
catch (InterruptedException ex)
{
ex.printStackTrace();
}
finally
{
dataSource = t;
}
}
}
}
private static class Task2 implements Runnable
{
public void run()
{
for (int i = 0; i < 10; ++i)
{
out.printf("Task2 --- dataSource: %d\n", dataSource);
int t = dataSource - 1;
try
{
Thread.sleep(10);
dataSource = t;
}
catch (InterruptedException ex)
{
ex.printStackTrace();
}
finally
{
dataSource = t;
}
}
}
}
public static void main(String[] args) throws InterruptedException
{
Thread thread1 = new Thread(new Task1());
Thread thread2 = new Thread(new Task2());
thread1.start();
thread2.start();
Thread.sleep(1000);
out.printf("The value of dataSource is %d.", dataSource);
}
}

最终的结果出人意料，dataSource的值不为0。

线程之间的切换执行，在访问同一数据源时会出差错。

如何避免这种错误？加锁或同步。对示例1进行修改，请看示例2：

import static java.lang.System.out;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class LockTest2
{
private static int dataSource;
private static ReentrantReadWriteLock.WriteLock lock = new ReentrantReadWriteLock().writeLock();
private static class Task1 implements Runnable
{
public void run()
{
lock.lock();
for (int i = 0; i < 10; ++i)
{
int t = dataSource + 1;
out.printf("Task1 --- 自增前 --- dataSource: %d\n", dataSource);
try
{
Thread.sleep(1);
}
catch (InterruptedException ex)
{
ex.printStackTrace();
}
finally
{
dataSource = t;
out.printf("Task1 --- 自增后 --- dataSource: %d\n", dataSource);
}
}
lock.unlock();
}
}
private static class Task2 implements Runnable
{
public void run()
{
lock.lock();
for (int i = 0; i < 10; ++i)
{
out.printf("Task2 --- 自增前 --- dataSource: %d\n", dataSource);
int t = dataSource - 1;
try
{
Thread.sleep(10);
dataSource = t;
}
catch (InterruptedException ex)
{
ex.printStackTrace();
}
finally
{
dataSource = t;
out.printf("Task2 --- 自增后 --- dataSource: %d\n", dataSource);
}
}
lock.unlock();
}
}
public static void main(String[] args) throws InterruptedException
{
Thread thread1 = new Thread(new Task1());
Thread thread2 = new Thread(new Task2());
thread1.start();
thread2.start();
Thread.sleep(300);
//睡眠一段时间，确保thread1和thread2执行完毕后再打印dataSource的值
out.printf("dataSource的值为%d.", dataSource);
}
}

此示例中，多个线程对同一数据源的修改可能会造成混乱，

所以我使用ReentrantReadWriteLock.WriteLock（写锁），

来保证每个线程对数据源修改时不被其他线程干扰。

 

对于另外一种使用synchronized关键字对线程进行同步的方法，

在此不做描述，其用法较简单，可自行百度^_^