jtulach@319: package org.apidesign.javamonitorflaws;
jtulach@319: 
jtulach@319: import java.util.HashMap;
jtulach@319: import java.util.Map;
jtulach@319: 
jtulach@319: /** Classical caching support class that makes sure there is
jtulach@319:  * always one "To" value for each "From" one returned from the {@link #get}
jtulach@319:  * method. However it does not prevent multiple threads to call
jtulach@319:  * {@link #createItem} multiple times for the same "From" value.
jtulach@319:  * <p>
jtulach@319:  * In contrast to {@link Cache}, this is correctly synchronized.
jtulach@319:  *
jtulach@319:  * @author Jaroslav Tulach <jtulach@netbeans.org>
jtulach@319:  */
jtulach@319: // BEGIN: monitor.pitfalls.CacheOK
jtulach@319: public abstract class CacheOK<From,To> {
jtulach@320:     private final Object LOCK = new Object();
jtulach@319: 
jtulach@319:     private Map<From,To> cache;
jtulach@319: 
jtulach@319:     protected abstract To createItem(From f);
jtulach@319: 
jtulach@319:     public final To get(From f) {
jtulach@319:         To t = inspectValue(f);
jtulach@319:         if (t != null) {
jtulach@319:             return t;
jtulach@319:         }
jtulach@319:         To newT = createItem(f);
jtulach@319:         return registerValue(f, newT);
jtulach@319:     }
jtulach@319: 
jtulach@319: 
jtulach@319:     private To inspectValue(From f) {
jtulach@319:         synchronized (LOCK) {
jtulach@319:             if (cache == null) {
jtulach@319:                 cache = new HashMap<From, To>();
jtulach@319:             }
jtulach@319:             return cache.get(f);
jtulach@319:         }
jtulach@319:     }
jtulach@319: 
jtulach@319:     private To registerValue(From f, To newT) {
jtulach@319:         synchronized (LOCK) {
jtulach@319:             To t = cache.get(f);
jtulach@319:             if (t == null) {
jtulach@319:                 cache.put(f, newT);
jtulach@319:                 return newT;
jtulach@319:             } else {
jtulach@319:                 return t;
jtulach@319:             }
jtulach@319:         }
jtulach@319:     }
jtulach@319: }
jtulach@319: // END: monitor.pitfalls.CacheOK