在Java如何保证方法是线程安全的

 

废话开篇

都说Java程序好些，但是我觉得Java编程这东西，没个十年八年的真不敢说自己精通Java编程，由于工作原因，开始转战Java多线程，以前没怎么接触过，所以想留点脚印在这两条路上。

切入正题

开门见山，今天看到别人写的一段关于方法是否线程安全的文章，比较简单，但是由于自己也是刚开始入门，所以就迈下了第一步。由于注释还算比较详细，所以就不废话了，直接上code。

此方法不是线程安全的

/**  
 * @Title: NotThreadSafeCounter.java
 * @Package never.uncategory
 * @Description: * The method is not thread-safe, because the 
 * counter++ operation is not atomic, which means it consists 
 * more than one atomic operations. In this case, one is 
 * accessing value and the other is increasing the value by one. 
 * @author "Never" xzllc2010#gmail.com  
 * @date Mar 14, 2014 7:44:24 PM
 */
/*

 */
package never.uncategory;

public class NotThreadSafeCounter extends Thread {

    private static int counter = 0;

    public void run() {
        System.out.println("counter:" + getCount());
    }

    public static int getCount() {

        try {
            Thread.sleep(1500);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return counter++;

    }

    public static void main(String[] args) {

        for (int i = 0; i < 5; i++) {
            new NotThreadSafeCounter().start();
        }
    }

}

此方法是线程安全的（synchronized）

/**  
 * @Title: ThreadSafeCounter.java
 * @Package never.uncategory
 * @Description: Adding synchronized to this method will makes 
 * it thread-safe,When synchronized is added to a static method, 
 * the Class object is the object which is locked.If the method 
 * is not static, then adding synchronized keyword willsynchronize 
 * the instance of the class, not the Class object.
 * @author "Never" xzllc2010#gmail.com  
 * @date Mar 14, 2014 8:09:45 PM
 */
package never.uncategory;

public class ThreadSafeCounter {

    private static int counter = 0;

    public void run() {
        System.out.println("counter:" + getCount());
    }

    public static synchronized int getCount() {
        return counter++;
    }

    public static void main(String[] args) {

        for (int i = 0; i < 5; i++) {
            new NotThreadSafeCounter().start();
        }
    }

}

此方法也是线程安全的（AtomicInteger）

/**  
* @Title: ThreadSafeCounter2.java
* @Package never.uncategory
* @Description: using AtomicInteger for safe counter
* @author "Never" xzllc2010#gmail.com  
* @date Mar 14, 2014 8:14:33 PM
*/ 
package never.uncategory;

import java.util.concurrent.atomic.AtomicInteger;

public class ThreadSafeCounter2 {

    private static AtomicInteger counter = new AtomicInteger(0);

    public void run() {
        System.out.println("counter:" + getCount());
    }

    public static int getCount() {
        return counter.getAndIncrement();
    }

    public static void main(String[] args) {

        for (int i = 0; i < 5; i++) {
            new NotThreadSafeCounter().start();
        }
    }
}

时光的尾巴

刚入门，如有疑问，多指教和交流。