错误案例1：

package com.net.thread.lock;import java.util.HashMap;import java.util.Map;import java.util.concurrent.locks.ReadWriteLock;import java.util.concurrent.locks.ReentrantReadWriteLock;/** * @author  * @Time：2017年8月23日 下午6:09:20 * @version 1.0 * @description 读写锁模拟缓存技术 ，错误写法1 */public class Example{    public static void main(String[] args)    {        Demo demo = new Demo();        new Thread(new Runnable()        {            @Override            public void run()            {                System.out.println(demo.get("java"));            }        }).start();        new Thread(new Runnable()        {            @Override            public void run()            {                System.out.println(demo.get("java"));            }        }).start();    }    static class Demo    {        ReadWriteLock rwl = new ReentrantReadWriteLock();        Map
     
       map = new HashMap
      
       ();        boolean flag;                public Object get(Object key)        {            rwl.readLock().lock();            Object value = null;            value = map.get(key);                        //if不安全，如果在这里边的value依旧是null，则根本没有安全保障                            if (value == null) {                flag = false;                rwl.readLock().unlock();                rwl.writeLock().lock();                System.out.println(Thread.currentThread().getName() + " 的flag：" + flag);                if (!flag) {                    value =  null;                    map.put(key, value);                    flag = true;                    try {                        Thread.sleep(2000);                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                }                rwl.writeLock().unlock();                rwl.readLock().lock();            }            rwl.readLock().unlock();            return value;        }    }}
      
     

 

错误案例2： 在前一个案例中进行改造，但不完全；此例子中的flag是局部变量，而局部变量实际上是线程安全，就相当于每个线程开始都有一个flag=false的结果，因此，多次调用，多次初始化，并没有起到缓存的作用

package com.net.thread.lock;import java.util.HashMap;import java.util.Map;import java.util.concurrent.locks.ReadWriteLock;import java.util.concurrent.locks.ReentrantReadWriteLock;/** * @author  * @Time：2017年8月23日 下午6:09:20 * @version 1.0 * @description 读写锁模拟缓存技术，错误写法2 */public class Example2{    public static void main(String[] args)    {        Demo demo = new Demo();        new Thread(new Runnable()        {            @Override            public void run()            {                System.out.println(demo.get("java"));            }        }).start();        new Thread(new Runnable()        {            @Override            public void run()            {                System.out.println(demo.get("java"));            }        }).start();    }    static class Demo    {        ReadWriteLock rwl = new ReentrantReadWriteLock();        Map
     
       map = new HashMap
      
       ();        int i = 1;                public Object get(Object key)        {            boolean flag = false;            rwl.readLock().lock();            Object value = map.get(key);            while (value == null)             {                System.out.println(Thread.currentThread().getName() + " 的flag：" + flag);                try {                    Thread.sleep(1000);                } catch (InterruptedException e) {                    e.printStackTrace();                }                rwl.readLock().unlock();                rwl.writeLock().lock();                if (!flag)                 {                    map.put(key, "初始化");                    flag = true;                    System.out.println(Thread.currentThread().getName() + "写完后的flag :" + flag + " 次数：" + i);                    i++;                }                rwl.writeLock().unlock();                rwl.readLock().lock();                value = map.get(key);            }                        rwl.readLock().unlock();            return value;        }    }}
      
     

 

正确写法：

package com.net.thread.lock;import java.util.HashMap;import java.util.Map;import java.util.concurrent.locks.ReadWriteLock;import java.util.concurrent.locks.ReentrantReadWriteLock;/** * @author  * @Time：2017年8月23日 下午6:09:20 * @version 1.0 * @description 读写锁模拟缓存技术,正确写法 */public class Example3{    public static void main(String[] args)    {        Demo demo = new Demo();        new Thread(new Runnable()        {            @Override            public void run()            {                System.out.println(demo.get("java"));            }        }).start();        new Thread(new Runnable()        {            @Override            public void run()            {                System.out.println(demo.get("java"));            }        }).start();    }    static class Demo    {        ReadWriteLock rwl = new ReentrantReadWriteLock();        Map
     
       map = new HashMap
      
       ();        int i = 1;        boolean flag = false;                public Object get(Object key)        {            rwl.readLock().lock();            Object value = map.get(key);            while (value == null)             {                System.out.println(Thread.currentThread().getName() + " 的flag：" + flag);                try {                    Thread.sleep(1000);                } catch (InterruptedException e) {                    e.printStackTrace();                }                rwl.readLock().unlock();                rwl.writeLock().lock();                if (!flag)                 {                    map.put(key, "初始化");                    flag = true;                    System.out.println(Thread.currentThread().getName() + "写完后的flag :" + flag + " 次数：" + i);                    i++;                }                rwl.writeLock().unlock();                rwl.readLock().lock();                value = map.get(key);            }                        rwl.readLock().unlock();            return value;        }    }}