Guava通過接口LoadingCache提供了一個非常強大的基於內存的LoadingCache<K，V>。在緩存中自動加載值，它提供了許多實用的方法，在有緩存需求時非常有用。

接口聲明

以下是forcom.google.common.cache.LoadingCache<K，V>接口的聲明：

@Beta
@GwtCompatible
public interface LoadingCache<K,V>
   extends Cache<K,V>, Function<K,V>

接口方法

S.N.	方法及說明
1	V apply(K key) 不推薦使用。提供滿足功能接口;使用get(K)或getUnchecked(K)代替。
2	ConcurrentMap<K,V> asMap() 返回存儲在該緩存作爲一個線程安全的映射條目的視圖。
3	V get(K key) 返回一個鍵在這個高速緩存中，首先裝載如果需要該值相關聯的值。
4	ImmutableMap<K,V> getAll(Iterable<? extends K> keys) 返回一個鍵相關聯的值的映射，創建或必要時檢索這些值。
5	V getUnchecked(K key) 返回一個鍵在這個高速緩存中，首先裝載如果需要該值相關聯的值。
6	void refresh(K key) 加載鍵key，可能是異步的一個新值。

LoadingCache 示例

使用所選擇的編輯器創建下面的java程序 C:/> Guava

GuavaTester.java

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;

import com.google.common.base.MoreObjects;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;

public class GuavaTester {
   public static void main(String args[]){
      //create a cache for employees based on their employee id
      LoadingCache
 
   employeeCache 
  =
   
  CacheBuilder
  .
  newBuilder
  ()
   
  .
  maximumSize
  (
  100
  )
   
  // maximum 100 records can be cached
   
  .
  expireAfterAccess
  (
  30
  ,
   
  TimeUnit
  .
  MINUTES
  )
   
  // cache will expire after 30 minutes of access
   
  .
  build
  (
  new
   
  CacheLoader
  
   (){
    
   // build the cacheloader
    
   @Override
    
   public
    
   Employee
    load
   (
   String
    empId
   )
    
   throws
    
   Exception
    
   {
    
   //make the expensive call
    
   return
    getFromDatabase
   (
   empId
   );
    
   }
    
   });
    
   try
    
   {
    
   //on first invocation, cache will be populated with corresponding
    
   //employee record
    
   System
   .
   out
   .
   println
   (
   "Invocation #1"
   );
    
   System
   .
   out
   .
   println
   (
   employeeCache
   .
   get
   (
   "100"
   ));
    
   System
   .
   out
   .
   println
   (
   employeeCache
   .
   get
   (
   "103"
   ));
    
   System
   .
   out
   .
   println
   (
   employeeCache
   .
   get
   (
   "110"
   ));
    
   //second invocation, data will be returned from cache
    
   System
   .
   out
   .
   println
   (
   "Invocation #2"
   );
    
   System
   .
   out
   .
   println
   (
   employeeCache
   .
   get
   (
   "100"
   ));
    
   System
   .
   out
   .
   println
   (
   employeeCache
   .
   get
   (
   "103"
   ));
    
   System
   .
   out
   .
   println
   (
   employeeCache
   .
   get
   (
   "110"
   ));
    
   }
    
   catch
    
   (
   ExecutionException
    e
   )
    
   {
    e
   .
   printStackTrace
   ();
    
   }
    
   }
    
   private
    
   static
    
   Employee
    getFromDatabase
   (
   String
    empId
   ){
    
   Employee
    e1 
   =
    
   new
    
   Employee
   (
   "Mahesh"
   ,
    
   "Finance"
   ,
    
   "100"
   );
    
   Employee
    e2 
   =
    
   new
    
   Employee
   (
   "Rohan"
   ,
    
   "IT"
   ,
    
   "103"
   );
    
   Employee
    e3 
   =
    
   new
    
   Employee
   (
   "Sohan"
   ,
    
   "Admin"
   ,
    
   "110"
   );
    
   Map
   
     database 
    =
     
    new
     
    HashMap
    
     ();
      database
     .
     put
     (
     "100"
     ,
      e1
     );
      database
     .
     put
     (
     "103"
     ,
      e2
     );
      database
     .
     put
     (
     "110"
     ,
      e3
     );
      
     System
     .
     out
     .
     println
     (
     "Database hit for"
      
     +
      empId
     );
      
     return
      database
     .
     get
     (
     empId
     );
      
     }
      
     }
      
     class
      
     Employee
      
     {
      
     String
      name
     ;
      
     String
      dept
     ;
      
     String
      emplD
     ;
      
     public
      
     Employee
     (
     String
      name
     ,
      
     String
      dept
     ,
      
     String
      empID
     ){
      
     this
     .
     name 
     =
      name
     ;
      
     this
     .
     dept 
     =
      dept
     ;
      
     this
     .
     emplD 
     =
      empID
     ;
      
     }
      
     public
      
     String
      getName
     ()
      
     {
      
     return
      name
     ;
      
     }
      
     public
      
     void
      setName
     (
     String
      name
     )
      
     {
      
     this
     .
     name 
     =
      name
     ;
      
     }
      
     public
      
     String
      getDept
     ()
      
     {
      
     return
      dept
     ;
      
     }
      
     public
      
     void
      setDept
     (
     String
      dept
     )
      
     {
      
     this
     .
     dept 
     =
      dept
     ;
      
     }
      
     public
      
     String
      getEmplD
     ()
      
     {
      
     return
      emplD
     ;
      
     }
      
     public
      
     void
      setEmplD
     (
     String
      emplD
     )
      
     {
      
     this
     .
     emplD 
     =
      emplD
     ;
      
     }
      
     @Override
      
     public
      
     String
      toString
     ()
      
     {
      
     return
      
     MoreObjects
     .
     toStringHelper
     (
     Employee
     .
     class
     )
      
     .
     add
     (
     "Name"
     ,
      name
     )
      
     .
     add
     (
     "Department"
     ,
      dept
     )
      
     .
     add
     (
     "Emp Id"
     ,
      emplD
     ).
     toString
     ();
      
     }
      
     }

驗證結果

使用javac編譯器如下編譯類

C:\Guava>javac GuavaTester.java

現在運行GuavaTester看到的結果

C:\Guava>java GuavaTester

看看結果：

Invocation #1
Database hit for100
Employee{Name=Mahesh, Department=Finance, Emp Id=100}
Database hit for103
Employee{Name=Rohan, Department=IT, Emp Id=103}
Database hit for110
Employee{Name=Sohan, Department=Admin, Emp Id=110}
Invocation #2
Employee{Name=Mahesh, Department=Finance, Emp Id=100}
Employee{Name=Rohan, Department=IT, Emp Id=103}
Employee{Name=Sohan, Department=Admin, Emp Id=110}

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