Guava库的Cache详解

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8. Preload the Cache

We can insert multiple records in our cache using putAll() method. In the following example, we add multiple records into our cache using a Map:

1. @Test
   
2. public void whenPreloadCache_thenUsePutAll() {
   
3.     CacheLoader<String, String> loader;
   
4.     loader = new CacheLoader<String, String>() {
   
5.         @Override
   
6.         public String load(String key) {
   
7.             return key.toUpperCase();
   
8.         }
   
9.     };
   
10. 
    
11.     LoadingCache<String, String> cache;
    
12.     cache = CacheBuilder.newBuilder().build(loader);
    
13. 
    
14.     Map<String, String> map = new HashMap<String, String>();
    
15.     map.put("first", "FIRST");
    
16.     map.put("second", "SECOND");
    
17.     cache.putAll(map);
    
18. 
    
19.     assertEquals(2, cache.size());
    
20. }
    

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9. RemovalNotification

Sometimes, you need to take some actions when a record is removed from the cache; so, let’s discuss RemovalNotification.

We can register a RemovalListener to get notifications of a record being removed. We also have access to the cause of the removal – via the getCause() method.

In the following sample, a RemovalNotification is recieved when the forth element in the cache because of its size:

1. @Test
   
2. public void whenEntryRemovedFromCache_thenNotify() {
   
3.     CacheLoader<String, String> loader;
   
4.     loader = new CacheLoader<String, String>() {
   
5.         @Override
   
6.         public String load(final String key) {
   
7.             return key.toUpperCase();
   
8.         }
   
9.     };
   
10. 
    
11.     RemovalListener<String, String> listener;
    
12.     listener = new RemovalListener<String, String>() {
    
13.         @Override
    
14.         public void onRemoval(RemovalNotification<String, String> n){
    
15.             if (n.wasEvicted()) {
    
16.                 String cause = n.getCause().name();
    
17.                 assertEquals(RemovalCause.SIZE.toString(),cause);
    
18.             }
    
19.         }
    
20.     };
    
21. 
    
22.     LoadingCache<String, String> cache;
    
23.     cache = CacheBuilder.newBuilder()
    
24.       .maximumSize(3)
    
25.       .removalListener(listener)
    
26.       .build(loader);
    
27. 
    
28.     cache.getUnchecked("first");
    
29.     cache.getUnchecked("second");
    
30.     cache.getUnchecked("third");
    
31.     cache.getUnchecked("last");
    
32.     assertEquals(3, cache.size());
    
33. }
    

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10. Notes

Finally, here are a few additional quick notes about the Guava cache implementation:

1) it is thread-safe

2) you can insert values manually into the cache using put(key,value)

3) you can measure your cache performance using CacheStats ( hitRate(), missRate(), ..)

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11. Conclusion

We went through a lot of usecases of the Guava Cache in this tutorial – from simple usage to eviction of elements, refresh and preload of the cache and removal notifications.

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over.