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。