blocks|key|1495321|text|您可以使用LinkedHashMap+(Java+1.4%2B)：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1495322|//+Create+cache
final+int+MAX_ENTRIES+=+100;
Map+cache+=+new+LinkedHashMap(MAX_ENTRIES%2B1,+.75F,+true)+{
++++//+This+method+is+called+just+after+a+new+entry+has+been+added
++++public+boolean+removeEldestEntry(Map.Entry+eldest)+{
++++++++return+size()+>+MAX_ENTRIES;
++++}
};

//+Add+to+cache
Object+key+=+"key";
cache.put(key,+object);

//+Get+object
Object+o+=+cache.get(key);
if+(o+==+null+&&+!cache.containsKey(key))+{
++++//+Object+not+in+cache.+If+null+is+not+a+possible+value+in+the+cache,
++++//+the+call+to+cache.contains(key)+is+not+needed
}

//+If+the+cache+is+to+be+used+by+multiple+threads,
//+the+cache+must+be+wrapped+with+code+to+synchronize+the+methods
cache+=+(Map)Collections.synchronizedMap(cache);|code-block|syntax|javascript|1495323|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/6/docs/api/index.html?java/util/LinkedHashMap.html^0|5|D|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

You can use a <a href="http://docs.oracle.com/javase/6/docs/api/index.html?java/util/LinkedHashMap.html" rel="noreferrer">LinkedHashMap</a> (Java 1.4+) :

<pre><code>// Create cache
final int MAX_ENTRIES = 100;
Map cache = new LinkedHashMap(MAX_ENTRIES+1, .75F, true) {
 // This method is called just after a new entry has been added
 public boolean removeEldestEntry(Map.Entry eldest) {
 return size() &gt; MAX_ENTRIES;
 }
};

// Add to cache
Object key = "key";
cache.put(key, object);

// Get object
Object o = cache.get(key);
if (o == null &amp;&amp; !cache.containsKey(key)) {
 // Object not in cache. If null is not a possible value in the cache,
 // the call to cache.contains(key) is not needed
}

// If the cache is to be used by multiple threads,
// the cache must be wrapped with code to synchronize the methods
cache = (Map)Collections.synchronizedMap(cache);
</code></pre>

blocks|key|1710438|text|这是一个古老的问题，但我想为后人列出ConcurrentLinkedHashMap，它与LRUMap不同，它是线程安全的。使用起来非常简单：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1710439|ConcurrentMap<K,+V>+cache+=+new+ConcurrentLinkedHashMap.Builder<K,+V>()
++++.maximumWeightedCapacity(1000)
++++.build();|code-block|syntax|javascript|1710440|并且文档中有一些很好的examples，比如如何使LRU缓存基于大小而不是基于项数。|1710441|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/ben-manes/concurrentlinkedhashmap|1|https://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/map/LRUMap.html|2|https://github.com/ben-manes/concurrentlinkedhashmap/wiki/ExampleUsage^0|I|N|0|18|6|1|0|0|B|8|2|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]|$A|10|B|11|1|12]]|C|$]]|$1|D|3|E|5|F|7|13|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|14|8|@]|9|@$A|15|B|16|1|17]]|C|$]]|$1|K|3|-4|5|6|7|18|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]|S|$5|N|O|P|C|$Q|T]]|U|$5|N|O|P|C|$Q|V]]]]

This is an old question, but for posterity I wanted to list <a href="https://github.com/ben-manes/concurrentlinkedhashmap" rel="noreferrer">ConcurrentLinkedHashMap</a>, which is thread safe, unlike <a href="https://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/map/LRUMap.html" rel="noreferrer">LRUMap</a>. Usage is quite easy:

<pre><code>ConcurrentMap&lt;K, V&gt; cache = new ConcurrentLinkedHashMap.Builder&lt;K, V&gt;()
 .maximumWeightedCapacity(1000)
 .build();
</code></pre>

And the documentation has some good <a href="https://github.com/ben-manes/concurrentlinkedhashmap/wiki/ExampleUsage" rel="noreferrer">examples</a>, like how to make the LRU cache size-based instead of number-of-items based.

blocks|key|1495398|text|在Java语言中，Here是一种非常简单且易于使用的LRU缓存。虽然它是简短和简单的，但它是生产质量。代码经过了解释(请看README.md)，并且有一些单元测试。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1495399|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/barakb/Cache^0|9|4|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

<a href="https://github.com/barakb/Cache" rel="nofollow">Here</a> is a very simple and easy to use LRU cache in Java.
Although it is short and simple it is production quality.
The code is explained (look at the README.md) and has some unit tests.

blocks|key|1495451|text|这是我的实现，它让我在内存中保留了最优数量的元素。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1495452|重点是，我不需要跟踪当前正在使用的对象，因为我使用了MRU对象的LinkedHashMap和LRU对象的WeakHashMap的组合。所以缓存容量不小于MRU大小加上GC允许的值。当对象从MRU上掉下来时，只要GC拥有它们，它们就会一直保存在LRU中。|1495453|public+class+Cache<K,V>+{
final+Map<K,V>+MRUdata;
final+Map<K,V>+LRUdata;

public+Cache(final+int+capacity)
{
++++LRUdata+=+new+WeakHashMap<K,+V>();

++++MRUdata+=+new+LinkedHashMap<K,+V>(capacity%2B1,+1.0f,+true)+{
++++++++protected+boolean+removeEldestEntry(Map.Entry<K,V>+entry)
++++++++{
++++++++++++if+(this.size()+>+capacity)+{
++++++++++++++++LRUdata.put(entry.getKey(),+entry.getValue());
++++++++++++++++return+true;
++++++++++++}
++++++++++++return+false;
++++++++};
++++};
}

public+synchronized+V+tryGet(K+key)
{
++++V+value+=+MRUdata.get(key);
++++if+(value!=null)
++++++++return+value;
++++value+=+LRUdata.get(key);
++++if+(value!=null)+{
++++++++LRUdata.remove(key);
++++++++MRUdata.put(key,+value);
++++}
++++return+value;
}

public+synchronized+void+set(K+key,+V+value)
{
++++LRUdata.remove(key);
++++MRUdata.put(key,+value);
}
}|code-block|syntax|javascript|1495454|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Here is my implementation which lets me keep an optimal number of elements in memory.

The point is that I do not need to keep track of what objects are currently being used since I'm using a combination of a LinkedHashMap for the MRU objects and a WeakHashMap for the LRU objects.
So the cache capacity is no less than MRU size plus whatever the GC lets me keep. Whenever objects fall off the MRU they go to the LRU for as long as the GC will have them.

<pre><code>public class Cache&lt;K,V&gt; {
final Map&lt;K,V&gt; MRUdata;
final Map&lt;K,V&gt; LRUdata;

public Cache(final int capacity)
{
 LRUdata = new WeakHashMap&lt;K, V&gt;();

 MRUdata = new LinkedHashMap&lt;K, V&gt;(capacity+1, 1.0f, true) {
 protected boolean removeEldestEntry(Map.Entry&lt;K,V&gt; entry)
 {
 if (this.size() &gt; capacity) {
 LRUdata.put(entry.getKey(), entry.getValue());
 return true;
 }
 return false;
 };
 };
}

public synchronized V tryGet(K key)
{
 V value = MRUdata.get(key);
 if (value!=null)
 return value;
 value = LRUdata.get(key);
 if (value!=null) {
 LRUdata.remove(key);
 MRUdata.put(key, value);
 }
 return value;
}

public synchronized void set(K key, V value)
{
 LRUdata.remove(key);
 MRUdata.put(key, value);
}
}
</code></pre>

I know it's simple to implement, but I want to reuse something that already exist.

Problem I want to solve is that I load configuration (from XML so I want to cache them) for different pages, roles, ... so the combination of inputs can grow quite much (but in 99% will not). To handle this 1%, I want to have some max number of items in cache...

Till know I have found org.apache.commons.collections.map.LRUMap in apache commons and it looks fine but want to check also something else. Any recommendations?

Easy, simple to use LRU cache in java

单元测试

Java

我知道这很容易实现，但是我想重用已经存在的东西。我想解决的问题是，我加载配置(从XML，所以我想缓存它们)，为不同的页面，角色，...因此，输入的组合可以增长很多(但99%不会)。为了处理这1%，我希望在缓存中有一些最大数量的项目...直到我知道我已经在apache commons中发现了org.apache.comm...

问在java中简单易用的LRU缓存
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