Guava Cache 数据变化实现回调的监听器RemovalListener-白红宇

Guava Cache 数据变化实现回调的监听器RemovalListener

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发布时间：2019-06-13

本文共 9514 字，大约阅读时间需要 31 分钟。

当我们需要在缓存被移除的时候，得到通知产生回调，并做一些额外处理工作。这个时候RemovalListener就派上用场了。

public class Main {     // 创建一个监听器    private static class MyRemovalListener implements RemovalListener
     
       {    @Override    public void onRemoval(RemovalNotification
      
        notification) {        String tips = String.format("key=%s,value=%s,reason=%s", notification.getKey(), notification.getValue(), notification.getCause());        System.out.println(tips);    }    }     public static void main(String[] args) {     // 创建一个带有RemovalListener监听的缓存    Cache
       
         cache = CacheBuilder.newBuilder().removalListener(new MyRemovalListener()).build();     cache.put(1, 1);     // 手动清除    cache.invalidate(1);     System.out.println(cache.getIfPresent(1)); // null    } }

使用invalidate()清除缓存数据之后，注册的回调被触发了

下面是只有主动删除数据使的回调

public class CacheConnection {    public static RemovalListener
     
       myRemovalListener = new RemovalListener
      
       (){    @Override      public void onRemoval(RemovalNotification
       
         notification) {          String tips = String.format("key=%s,value=%s,reason=%s in myRemovalListener", notification.getKey(), notification.getValue(), notification.getCause());          System.out.println(tips);        //when expireAfterAccess to do        if (notification.getCause().equals("EXPIRED") && notification.getValue() != null) {          try {            notification.getValue().close();          } catch (SQLException e) {            System.out.printf("Exception in myRemovalListener:\n");            e.printStackTrace();          }                    System.out.printf("Remove %s in cacheConnection", notification.getKey());        }              }  };    public static Cache
        
          cacheConnection = CacheBuilder.newBuilder()        //设置cache的初始大小为20000，要合理设置该值        .initialCapacity(20000)        //设置并发数为5，即同一时间最多只能有5个线程往cache执行写入操作        .concurrencyLevel(100)        //设置cache中的数据在600秒没有被读写将自动删除        .expireAfterAccess(600, TimeUnit.SECONDS)       //设置监听，当出现自动删除时的回调      .removalListener(myRemovalListener)      //构建cache实例        .build();      public static Connection getCache(String key)  {    try {      Connection var = cacheConnection.getIfPresent(key);      return var;    } catch (Exception e) {      // TODO: handle exception      System.out.println("the value of cacheConnection is null");      e.printStackTrace();      return null;    }  }        public static void putCache(String key, Connection value) {     cacheConnection.put(key, value);   }    }

RemovalNotification中包含了缓存的key、value以及被移除的原因RemovalCause。通过源码可以看出，移除原因与容量管理方式是相对应的。下面是具体的消息

public enum RemovalCause {  /**   * The entry was manually removed by the user. This can result from the user invoking   * {
    @link Cache#invalidate}, {
    @link Cache#invalidateAll(Iterable)}, {
    @link Cache#invalidateAll()},   * {
    @link Map#remove}, {
    @link ConcurrentMap#remove}, or {
    @link Iterator#remove}.   */  EXPLICIT {    @Override    boolean wasEvicted() {      return false;    }  },   /**   * The entry itself was not actually removed, but its value was replaced by the user. This can   * result from the user invoking {
    @link Cache#put}, {
    @link LoadingCache#refresh}, {
    @link Map#put},   * {
    @link Map#putAll}, {
    @link ConcurrentMap#replace(Object, Object)}, or   * {
    @link ConcurrentMap#replace(Object, Object, Object)}.   */  REPLACED {    @Override    boolean wasEvicted() {      return false;    }  },   /**   * The entry was removed automatically because its key or value was garbage-collected. This   * can occur when using {
    @link CacheBuilder#weakKeys}, {
    @link CacheBuilder#weakValues}, or   * {
    @link CacheBuilder#softValues}.   */  COLLECTED {    @Override    boolean wasEvicted() {      return true;    }  },   /**   * The entry's expiration timestamp has passed. This can occur when using   * {
    @link CacheBuilder#expireAfterWrite} or {
    @link CacheBuilder#expireAfterAccess}.   */  EXPIRED {    @Override    boolean wasEvicted() {      return true;    }  },   /**   * The entry was evicted due to size constraints. This can occur when using   * {
    @link CacheBuilder#maximumSize} or {
    @link CacheBuilder#maximumWeight}.   */  SIZE {    @Override    boolean wasEvicted() {      return true;    }  };   /**   * Returns {
    @code true} if there was an automatic removal due to eviction (the cause is neither   * {
    @link #EXPLICIT} nor {
    @link #REPLACED}).   */  abstract boolean wasEvicted();}

监听器使用很简单，有几个特点需要注意下：

1、默认情况下，监听器方法是被同步调用的（在移除缓存的那个线程中执行）。如果监听器方法比较耗时，会导致调用者线程阻塞时间变长。下面这段代码，由于监听器执行需要2s，所以main线程调用invalidate()要2s后才能返回。

public class Main {     // 创建一个监听器    private static class MyRemovalListener implements RemovalListener
     
       {        @Override        public void onRemoval(RemovalNotification
      
        notification) {            String tips = String.format("key=%s,value=%s,reason=%s", notification.getKey(), notification.getValue(), notification.getCause());            System.out.println(tips);             try {                // 模拟耗时                Thread.sleep(2000);            } catch (InterruptedException e) {                e.printStackTrace();            }         }    }     public static void main(String[] args) {         // 创建一个带有RemovalListener监听的缓存        final Cache
       
         cache = CacheBuilder.newBuilder().removalListener(new MyRemovalListener()).build();        cache.put(1, 1);        cache.put(2, 2);         System.out.println("main...begin.");        cache.invalidate(1);// 耗时2s        System.out.println("main...over.");    } }

解决这个问题的方法是：使用异步监听RemovalListeners.asynchronous(RemovalListener, Executor)。

public class Main {     // 创建一个监听器    private static class MyRemovalListener implements RemovalListener
      
        {        @Override        public void onRemoval(RemovalNotification
       
         notification) {            String tips = String.format("key=%s,value=%s,reason=%s", notification.getKey(), notification.getValue(), notification.getCause());            System.out.println(tips);             try {                // 模拟耗时                Thread.sleep(2000);            } catch (InterruptedException e) {                e.printStackTrace();            }         }    }     public static void main(String[] args) {         RemovalListener
        
          async = RemovalListeners.asynchronous(new MyRemovalListener(), Executors.newSingleThreadExecutor());        // 创建一个带有RemovalListener监听的缓存        final Cache
         
           cache = CacheBuilder.newBuilder().removalListener(async).build();        cache.put(1, 1);        cache.put(2, 2);         System.out.println("main...begin.");        cache.invalidate(1);// main线程立刻返回        System.out.println("main...over.");    } }

2、创建cache的时候只能添加1个监听器，这个监听器对象会被多个线程共享，所以如果监听器需要操作共享资源，那么一定要做好同步控制。下面这段代码可以看出：2个线程会交替执行监听器的发方法。

public class Main {     // 创建一个监听器    private static class MyRemovalListener implements RemovalListener
      
        {        @Override        public void onRemoval(RemovalNotification
       
         notification) {            String tips = String.format("key=%s,value=%s,reason=%s", notification.getKey(), notification.getValue(), notification.getCause());            System.out.println(tips);             try {                // 模拟耗时                Thread.sleep(2000);            } catch (InterruptedException e) {                e.printStackTrace();            }             System.out.println("process over.");        }    }     public static void main(String[] args) {         // 创建一个带有RemovalListener监听的缓存        final Cache
        
          cache = CacheBuilder.newBuilder().removalListener(new MyRemovalListener()).build();        cache.put(1, 1);        cache.put(2, 2);         new Thread(new Runnable() {            @Override            public void run() {                System.out.println("thread1...trigger RemovalListener begin.");                cache.invalidate(1);                System.out.println("thread1...trigger RemovalListener over.");            }        }).start();         new Thread(new Runnable() {            @Override            public void run() {                System.out.println("thread2...trigger RemovalListener begin.");                cache.invalidate(2);                System.out.println("thread2...trigger RemovalListener over.");            }        }).start();    } }

3、监听器中抛出的任何异常，在被记录到日志后，会被guava丢弃，不会导致监听器不可用。下面这段代码可以看到：监听器中抛出的异常只是被记录了(打印到了控制台)，并没有导致JVM退出，之后缓存被移除一样可以再次触发。

public class Main {     // 创建一个监听器    private static class MyRemovalListener implements RemovalListener
      
        {        @Override        public void onRemoval(RemovalNotification
       
         notification) {            String tips = String.format("key=%s,value=%s,reason=%s", notification.getKey(), notification.getValue(), notification.getCause());            System.out.println(tips);             throw new RuntimeException();        }    }     public static void main(String[] args) {         // 创建一个带有RemovalListener监听的缓存        final Cache
        
          cache = CacheBuilder.newBuilder().removalListener(new MyRemovalListener()).build();        cache.put(1, 1);        cache.put(2, 2);         cache.invalidate(1);        cache.invalidate(2);    } }

学习：https://blog.csdn.net/zhangjikuan/article/details/76408578

转载于:https://www.cnblogs.com/wynjauu/articles/9301821.html

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