Java-Class Library-Guava-cache

Caching is an essential method to solve performance problems in our daily development. Simply put, cache is a memory space created to improve system performance.

Caching is widely used in many systems and architectures, such as:

1.CPU cache

2. Operating system cache

3. Local cache

4. Distributed cache

5. HTTP cache

6. Database cache

Wait, it can be said that in the field of computers and networks, Caching is everywhere. It can be said that as long as there is unequal hardware performance, cache will be present wherever network transmission is involved.

Guava Cache is a full-memory local cache implementation that provides a thread-safe implementation mechanism. Overall, Guava cache is the best choice for local caching. It is easy to use and has good performance.

There are two ways to create Guava Cache:

1. cacheLoader

2. callable callback

The cache created by these two methods is the same as the usual one The difference between using map to cache is that both methods implement a logic - get the value of key X from the cache. If the value has been cached, return the value in the cache. If it is not cached However, there is a method to obtain this value. But the difference is that the definition of cacheloader is relatively broad and is defined for the entire cache. It can be considered a unified method of loading value based on key value. The callable method is more flexible, allowing you to specify the

cacheLoader method implementation example when getting:

[code] @Test
    public void TestLoadingCache() throws Exception{
        LoadingCache<String,String> cahceBuilder=CacheBuilder
        .newBuilder()
        .build(new CacheLoader<String, String>(){
            @Override
            public String load(String key) throws Exception {        
                String strProValue="hello "+key+"!";                
                return strProValue;
            }

        });        

        System.out.println("jerry value:"+cahceBuilder.apply("jerry"));
        System.out.println("jerry value:"+cahceBuilder.get("jerry"));
        System.out.println("peida value:"+cahceBuilder.get("peida"));
        System.out.println("peida value:"+cahceBuilder.apply("peida"));
        System.out.println("lisa value:"+cahceBuilder.apply("lisa"));
        cahceBuilder.put("harry", "ssdded");
        System.out.println("harry value:"+cahceBuilder.get("harry"));
    }

callable callback implementation:

[code] @Test
    public void testcallableCache()throws Exception{
        Cache<String, String> cache = CacheBuilder.newBuilder().maximumSize(1000).build();  
        String resultVal = cache.get("jerry", new Callable<String>() {  
            public String call() {  
                String strProValue="hello "+"jerry"+"!";                
                return strProValue;
            }  
        });  
        System.out.println("jerry value : " + resultVal);

        resultVal = cache.get("peida", new Callable<String>() {  
            public String call() {  
                String strProValue="hello "+"peida"+"!";                
                return strProValue;
            }  
        });  
        System.out.println("peida value : " + resultVal);  
    }

　　输出：
　　jerry value : hello jerry!
　　peida value : hello peida!

cache parameter description:

Recycling parameters:

1. Size setting: CacheBuilder.maximumSize(long) CacheBuilder.weigher(Weigher) CacheBuilder.maxumumWeigher(long)

2. Time: expireAfterAccess (long, TimeUnit) expireAfterWrite(long, TimeUnit)

3. Reference: CacheBuilder.weakKeys() CacheBuilder.weakValues() CacheBuilder.softValues()

4. Explicit deletion: invalidate( key) invalidateAll(keys) invalidateAll()

　5. Delete listener: CacheBuilder.removalListener(RemovalListener)

　

　Refresh mechanism:

　1. LoadingCache.refresh(K) When generating a new value, the old value will still be used.

　2. CacheLoader.reload(K, V) allows the use of old values ​​during the generation of new values ​​

　3. CacheBuilder.refreshAfterWrite(long, TimeUnit) automatically refreshes the cache

[code] /**
     * 不需要延迟处理(泛型的方式封装)
     * @return
     */
    public  <K , V> LoadingCache<K , V> cached(CacheLoader<K , V> cacheLoader) {
          LoadingCache<K , V> cache = CacheBuilder
          .newBuilder()
          .maximumSize(2)
          .weakKeys()
          .softValues()
          .refreshAfterWrite(120, TimeUnit.SECONDS)
          .expireAfterWrite(10, TimeUnit.MINUTES)        
          .removalListener(new RemovalListener<K, V>(){
            @Override
            public void onRemoval(RemovalNotification<K, V> rn) {
                System.out.println(rn.getKey()+"被移除");  

            }})
          .build(cacheLoader);
          return cache;
    }

    /**
     * 通过key获取value
     * 调用方式 commonCache.get(key) ; return String
     * @param key
     * @return
     * @throws Exception
     */

    public  LoadingCache<String , String> commonCache(final String key) throws Exception{
        LoadingCache<String , String> commonCache= cached(new CacheLoader<String , String>(){
                @Override
                public String load(String key) throws Exception {
                    return "hello "+key+"!";    
                }
          });
        return commonCache;
    }

    @Test
    public void testCache() throws Exception{
        LoadingCache<String , String> commonCache=commonCache("peida");
        System.out.println("peida:"+commonCache.get("peida"));
        commonCache.apply("harry");
        System.out.println("harry:"+commonCache.get("harry"));
        commonCache.apply("lisa");
        System.out.println("lisa:"+commonCache.get("lisa"));
    }
peida:hello peida!
harry:hello harry!
peida被移除
lisa:hello lisa!

Callable Cache implementation based on generics:

[code]private static Cache<String, String> cacheFormCallable = null; 

    /**
     * 对需要延迟处理的可以采用这个机制；(泛型的方式封装)
     * @param <K>
     * @param <V>
     * @param key
     * @param callable
     * @return V
     * @throws Exception
     */
    public static <K,V> Cache<K , V> callableCached() throws Exception {
          Cache<K, V> cache = CacheBuilder
          .newBuilder()
          .maximumSize(10000)
          .expireAfterWrite(10, TimeUnit.MINUTES)
          .build();
          return cache;
    }

    private String getCallableCache(final String userName) {
           try {
             //Callable只有在缓存值不存在时，才会调用
             return cacheFormCallable.get(userName, new Callable<String>() {
                @Override
                public String call() throws Exception {
                    System.out.println(userName+" from db");
                    return "hello "+userName+"!";
               }
              });
           } catch (ExecutionException e) {
              e.printStackTrace();
              return null;
            } 
    }

    @Test
    public void testCallableCache() throws Exception{
         final String u1name = "peida";
         final String u2name = "jerry"; 
         final String u3name = "lisa"; 
         cacheFormCallable=callableCached();
         System.out.println("peida:"+getCallableCache(u1name));
         System.out.println("jerry:"+getCallableCache(u2name));
         System.out.println("lisa:"+getCallableCache(u3name));
         System.out.println("peida:"+getCallableCache(u1name));

    }

　　输出：

peida from db
peida:hello peida!
jerry from db
jerry:hello jerry!
lisa from db
lisa:hello lisa!
peida:hello peida!

　　说明：Callable只有在缓存值不存在时，才会调用，比如第二次调用getCallableCache(u1name)直接返回缓存中的值

　　guava Cache数据移除：

　　guava做cache时候数据的移除方式，在guava中数据的移除分为被动移除和主动移除两种。 

　　被动移除数据的方式，guava默认提供了三种方式： 

　　1.基于大小的移除:看字面意思就知道就是按照缓存的大小来移除，如果即将到达指定的大小，那就会把不常用的键值对从cache中移除。 

　　定义的方式一般为 CacheBuilder.maximumSize(long)，还有一种一种可以算权重的方法，个人认为实际使用中不太用到。就这个常用的来看有几个注意点， 

　　　　其一，这个size指的是cache中的条目数，不是内存大小或是其他； 

　　　　其二，并不是完全到了指定的size系统才开始移除不常用的数据的，而是接近这个size的时候系统就会开始做移除的动作； 

　　　　其三，如果一个键值对已经从缓存中被移除了，你再次请求访问的时候，如果cachebuild是使用cacheloader方式的，那依然还是会从cacheloader中再取一次值，如果这样还没有，就会抛出异常 

　　2.基于时间的移除：guava提供了两个基于时间移除的方法 

　　　　expireAfterAccess(long, TimeUnit) 这个方法是根据某个键值对最后一次访问之后多少时间后移除 

　　　　expireAfterWrite(long, TimeUnit) 这个方法是根据某个键值对被创建或值被替换后多少时间移除 

　　3.基于引用的移除： 

　　这种移除方式主要是基于java的垃圾回收机制，根据键或者值的引用关系决定移除 

　　主动移除数据方式，主动移除有三种方法： 

　　1.单独移除用 Cache.invalidate(key) 

　　2.批量移除用 Cache.invalidateAll(keys) 

　　3.移除所有用 Cache.invalidateAll() 

　　如果需要在移除数据的时候有所动作还可以定义Removal Listener，但是有点需要注意的是默认Removal Listener中的行为是和移除动作同步执行的，如果需要改成异步形式，可以考虑使用RemovalListeners.asynchronous(RemovalListener, Executor)

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