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上一篇文章""提到：如果缓存过期，恰好有多个线程读取同一个key的值，那么guava只允许一个线程去加载数据，其余线程阻塞。这虽然可以防止大量请求穿透缓存，但是效率低下。使用refreshAfterWrite可以做到：只阻塞加载数据的线程，其余线程返回旧数据。

public class GuavaCache4TestRefresh {	private static CountDownLatch latch = new CountDownLatch(1);	// 1s后刷新缓存	private static LoadingCache
   
     cache = CacheBuilder.newBuilder().refreshAfterWrite(1, TimeUnit.SECONDS)			.build(new CacheLoader
    
     () {				@Override				public String load(String key) throws Exception {					return callable.call();				}			});		// 模拟一个需要耗时2s的数据库查询任务	private static Callable
     
       callable = new Callable
      
       () {		@Override		public String call() throws Exception {			System.out.println("begin to mock query db...");			Thread.sleep(2000);			System.out.println("success to mock query db...");			return UUID.randomUUID().toString();		}	};	public static void main(String[] args) throws Exception {		// 手动添加一条缓存数据,睡眠1.5s让其过期		cache.put("name", "aty");		Thread.sleep(1500);		for (int i = 0; i < 8; i++) {			startThread(i);		}		// 让线程运行		latch.countDown();	}	private static void startThread(int id) {		Thread t = new Thread(new Runnable() {			@Override			public void run() {				try {					System.out.println(Thread.currentThread().getName() + "...begin");					latch.await();					Stopwatch watch = Stopwatch.createStarted();					System.out.println(Thread.currentThread().getName() + "...value..." + cache.get("name"));					watch.stop();					System.out.println(Thread.currentThread().getName() + "...finish,cost time="							+ watch.elapsed(TimeUnit.SECONDS));				} catch (Exception e) {					e.printStackTrace();				}			}		});		t.setName("Thread-" + id);		t.start();	}}
      
     
    
   

Thread-0...beginThread-2...beginThread-1...beginThread-3...beginThread-4...beginThread-5...beginThread-6...beginThread-7...beginbegin to mock query db...Thread-1...value...atyThread-0...value...atyThread-1...finish,cost time=0Thread-3...value...atyThread-5...value...atyThread-7...value...atyThread-5...finish,cost time=0Thread-3...finish,cost time=0Thread-4...value...atyThread-0...finish,cost time=0Thread-6...value...atyThread-4...finish,cost time=0Thread-7...finish,cost time=0Thread-6...finish,cost time=0success to mock query db...Thread-2...value...bbae863c-bd28-4744-bd5e-227300d1662aThread-2...finish,cost time=2

通过输出结果可以看出：当缓存数据过期的时候，只有Thread-2真正去加载数据的线程会阻塞一段时间，其余线程立马返回过期的值，显然这种处理方式更符合实际的使用场景。

有一点需要注意：我们手动向缓存中添加了一条数据，并让其过期。如果没有这行代码，程序执行结果如下。

Thread-0...beginThread-2...beginThread-1...beginThread-3...beginThread-4...beginThread-5...beginThread-6...beginThread-7...beginbegin to mock query db...success to mock query db...Thread-0...value...cc22caf5-23bb-4365-b143-6d5d64447cc9Thread-5...value...cc22caf5-23bb-4365-b143-6d5d64447cc9Thread-1...value...cc22caf5-23bb-4365-b143-6d5d64447cc9Thread-5...finish,cost time=2Thread-7...value...cc22caf5-23bb-4365-b143-6d5d64447cc9Thread-2...value...cc22caf5-23bb-4365-b143-6d5d64447cc9Thread-0...finish,cost time=2Thread-4...value...cc22caf5-23bb-4365-b143-6d5d64447cc9Thread-3...value...cc22caf5-23bb-4365-b143-6d5d64447cc9Thread-4...finish,cost time=2Thread-2...finish,cost time=2Thread-7...finish,cost time=2Thread-1...finish,cost time=2Thread-3...finish,cost time=2Thread-6...value...cc22caf5-23bb-4365-b143-6d5d64447cc9Thread-6...finish,cost time=2

由于缓存没有数据，导致一个线程去加载数据的时候，别的线程都阻塞了(因为没有旧值可以返回)。所以一般系统启动的时候，我们需要将数据预先加载到缓存，不然就会出现这种情况。

还有一个问题不爽：真正加载数据的那个线程一定会阻塞，我们希望这个加载过程是异步的。这样就可以让所有线程立马返回旧值，在后台刷新缓存数据。refreshAfterWrite默认的刷新是同步的，会在调用者的线程中执行。我们可以改造成异步的，实现CacheLoader.reload()，对上面代码进行如下修改：

// guava线程池,用来产生ListenableFuture	private static ListeningExecutorService service = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(10));	// 1s后刷新缓存	private static LoadingCache
   
     cache = CacheBuilder.newBuilder().refreshAfterWrite(1, TimeUnit.SECONDS)			.build(new CacheLoader
    
     () {				@Override				public String load(String key) throws Exception {					return callable.call();				}								@Override				public ListenableFuture
     
       reload(String key, String oldValue) throws Exception {					System.out.println("......后台线程池异步刷新:" + key);					return service.submit(callable);				}			});
     
    
   

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