编写线程安全的Java缓存读写机制 （原创）

一种习以为常的缓存写法：

IF value in cached THEN  return value from cacheELSE  compute value  save value in cache  return valueEND IF 

看上去逻辑无比正确，但实际上会造成2种问题：

1、这种方法是不线程安全的。

2、产生数值写入重复，造成错误的数据。

如下图，在线程1执行计算数值的过程中，线程2也进入数据检查，将多次写入数据，程序非常危险。

 

 

演示错误代码：

    //最容易产生的错误写法，先读取缓存，读不到就写缓存    public Long getNumber(final long index) {        if (cache.containsKey(index)) {            return cache.get(index);        }                final long value = getNumber(index - 1) + getNumber(index - 2);        cache.put(index, value);        return value;    }

 

1、传统的解决办法，使用重入锁 （getNumberByLock 方法）或者同步锁（getNumberBySynchroniz 方法）。

代码

import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future;import java.util.concurrent.TimeUnit;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;public class NaiveCacheExample {    private final Map<Long, Long> cache = new HashMap<>();    private Object o=new Object();    Lock lock =new ReentrantLock();        public NaiveCacheExample() {        cache.put(0L, 1L);        cache.put(1L, 1L);    }        //最容易产生的错误写法，先读取缓存，读不到就写缓存    public Long getNumber(final long index) {        if (cache.containsKey(index)) {            return cache.get(index);        }                final long value = getNumber(index - 1) + getNumber(index - 2);        cache.put(index, value);        return value;    }    //使用折返锁，使读写同步    public Long getNumberByLock(final long index) {               long value =0;        try {            lock.lock();                       if (cache.containsKey(index)) {                return cache.get(index);            }            value = getNumberByLock(index - 1) + getNumberByLock(index - 2);            cache.put(index, value);            return value;        }        catch (Exception e)        {}        finally        {            lock.unlock();        }                 return 0l;    }        //使用同步，使读写同步    public Long getNumberBySynchroniz(final long index) {        synchronized (o)        {        long value =0;        try {            if (cache.containsKey(index)) {                return cache.get(index);            }            value = getNumberBySynchroniz(index - 1) + getNumberBySynchroniz(index - 2);            cache.put(index, value);            return value;        }        catch (Exception e)        {}        finally        {                }        }        return 0l;    }    public static void main(final String[] args) {               NaiveCacheExample naiveCacheExample =new NaiveCacheExample();                Thread threadA =new Thread(new Runnable()        {            @Override            public void run() {                System.out.println(naiveCacheExample.getNumberBySynchroniz(1000));                            }                    }        ,"Thread-A");        threadA.start();                final Thread threadB = new Thread(new Runnable() {          public void run() {              System.out.println(naiveCacheExample.getNumberBySynchroniz(1000));          }        }, "Thread-B");        threadB.start();            }}

 

 

2、一个更好的缓存算法可以用 Callable 和 Future 。 缓存的值将存储在一个实例 ConcurrentMap 中 ，ConcurrentMap 是线程安全的。

代码：

import java.util.concurrent.Callable;import java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.ConcurrentMap;import java.util.concurrent.ExecutionException;import java.util.concurrent.Future;import java.util.concurrent.FutureTask;public class GenericCacheExample<K, V> {    private final ConcurrentMap<K, Future<V>> cache = new ConcurrentHashMap<>();    private Future<V> createFutureIfAbsent(final K key, final Callable<V> callable) {        Future<V> future = cache.get(key);        if (future == null) {            final FutureTask<V> futureTask = new FutureTask<V>(callable);            future = cache.putIfAbsent(key, futureTask);            if (future == null) {                future = futureTask;                futureTask.run();            }        }        return future;    }    public V getValue(final K key, final Callable<V> callable) throws InterruptedException, ExecutionException {        try {            final Future<V> future = createFutureIfAbsent(key, callable);            return future.get();        } catch (final InterruptedException e) {            cache.remove(key);            throw e;        } catch (final ExecutionException e) {            cache.remove(key);            throw e;        } catch (final RuntimeException e) {            cache.remove(key);            throw e;        }    }    public void setValueIfAbsent(final K key, final V value) {        createFutureIfAbsent(key, new Callable<V>() {            @Override            public V call() throws Exception {                return value;            }        });    }}

 

参考博客：

http://www.javacreed.com/how-to-cache-results-to-boost-performance/