使用 HashMap 实现简单的线程安全缓存而不使用同步集合

已发表: 2013-11-29

使用 Hashmap 的 Java 内存缓存

缓存是本地内存的一个区域，用于保存经常访问的数据的副本，否则这些数据的获取或计算成本很高。 此类数据的示例包括对数据库、磁盘文件或报告的查询结果。

这是一个简单的 Java 示例，它是使用 HashMap 而不使用同步集合的线程安全。

CrunchifyCacheExample.java
Java
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package com . crunchify . tutorials ;
 
import java . util . ArrayList ;
import java . util . HashMap ;
import java . util . Iterator ;
import java . util . Map . Entry ;
 
/**
* @author Crunchify.com
*
*/
 
// Create Simple Cache object with the help of HashMap...
public class CrunchifyCacheExample < K , T > {
    
     private long timeToLive ;
     private HashMap < K , T > cacheMap ;
    
     protected class CrunchifyCacheObject {
         public long lastAccessed = System . currentTimeMillis ( ) ;
         public String value ;
        
         protected CrunchifyCacheObject ( String value ) {
             this . value = value ;
         }
     }
    
     public CrunchifyCacheExample ( long timeToLive , final long timeInterval , int max ) {
         this . timeToLive = timeToLive * 2000 ;
        
         cacheMap = new HashMap < K , T > ( max ) ;
        
         if ( timeToLive > 0 && timeInterval > 0) {
            
            Thread t = new Thread(new Runnable() {
                public void run() {
                    while (true) {
                        try {
                            Thread.sleep(timeInterval * 1000);
                         } catch ( InterruptedException ex ) {
                         }
                        
                     }
                 }
             } ) ;
            
             t . setDaemon ( true ) ;
             t . start ( ) ;
         }
     }
    
     // PUT method
     public void put ( K key , T value ) {
         synchronized ( cacheMap ) {
             cacheMap . put ( key , value ) ;
         }
     }
    
     // GET method
     @SuppressWarnings ( "unchecked" )
     public T get ( K key ) {
         synchronized ( cacheMap ) {
             CrunchifyCacheObject c = ( CrunchifyCacheObject ) cacheMap . get ( key ) ;
            
             if ( c == null )
                 return null ;
             else {
                 c . lastAccessed = System . currentTimeMillis ( ) ;
                 return ( T ) c . value ;
             }
         }
     }
    
     // REMOVE method
     public void remove ( String key ) {
         synchronized ( cacheMap ) {
             cacheMap . remove ( key ) ;
         }
     }
    
     // Get Cache Objects Size()
     public int size ( ) {
         synchronized ( cacheMap ) {
             return cacheMap . size ( ) ;
         }
     }
    
     // CLEANUP method
     public void cleanup ( ) {
        
         long now = System . currentTimeMillis ( ) ;
         ArrayList <String> deleteKey = null ;
        
         synchronized ( cacheMap ) {
             Iterator <? > itr = cacheMap . entrySet ( ) . iterator ( ) ;
            
             deleteKey = new ArrayList < String > ( ( cacheMap . size ( ) / 2 ) + 1 ) ;
             CrunchifyCacheObject c = null ;
            
             while ( itr . hasNext ( ) ) {
                 String key = ( String ) itr . next ( ) ;
                 c = ( CrunchifyCacheObject ) ( ( Entry <? , ?> ) itr ) . getValue ( ) ;
                 if ( c ! = null && (now > (timeToLive + c.lastAccessed))) {
                    deleteKey.add(key);
                 }
             }
         }
        
         for ( String key : deleteKey ) {
             synchronized ( cacheMap ) {
                 cacheMap . remove ( key ) ;
             }
            
             Thread . yield ( ) ;
         }
     }
}

您可能想查看更多 Java 示例。

奖励：Java 内存模型详细信息：

Java 内存模型