java 利用Map做缓存一个简单实例

时间：2013-11-27 编辑：简简单单来源：一聚教程网

有时候需要将一批数据缓存，并且按照最近最少使用的规则淘汰。这时候就应该想到LRUMap。
LRUMap是一种尺寸固定的Map，当它的容量达到最大时采用最近最少使用的规则淘汰相应的元素。

代码如下

复制代码

LRUMap cache = newLRUMap(5);

cache.put("01", "news 1");
cache.put("02", "news 2");
cache.put("03", "news 3");
cache.put("04", "news 4");
cache.put("05", "news 5");

String news1 = (String)cache.get("01");
String news2 = (String)cache.get("02");
String news3 = (String)cache.get("04");
String news4 = (String)cache.get("03");
String news5 = (String)cache.get("02");
String news6 = (String)cache.get("05");

cache.put("06", "news 6");

Iterator it = cache.entrySet().iterator();
while(it.hasNext()){
Map.Entry entry = (Map.Entry)it.next();
System.out.println(entry.getValue());
}

LazyMap:当你需要获取某个键值但发现该键值不存在时自动填充该键值，那就该考虑使用LazyMap，LazyMap采用了装饰模式装饰一个Map，并采用一个Transformer来决定当键值不存在时填充的规则（或者Factory）。

代码如下

复制代码

Transformer reverseString = newTransformer(){
       publicObject transform(Object object){
           String name = (String)object;
           String reverse = StringUtils.reverse(name);
           returnreverse;
       }
    };

    Map names = newHashMap();
    Map lazynames = LazyMap.decorate(names, reverseString);

    System.out.println(lazynames.get("hello"));
    System.out.println(lazynames.get("thanks"));
    System.out.println(lazynames.get("not at all"));

    Factory factory = newFactory(){
       publicObject create(){
           return"NB";
       }
    };

    Map lazyname = LazyMap.decorate(names, factory);

    System.out.println(lazyname.get("woshishei"));
    System.out.println(lazyname.get("nishishei"));
    System.out.println(lazyname.get("not at all"));

综合使用LRUMap和LazyMap：

代码如下

复制代码

Transformer reverseString = newTransformer(){
       publicObject transform(Object object){
           String name = (String)object;
           String reverse = StringUtils.reverse(name);
           returnreverse;
       }
    };

    LRUMap lruMap = newLRUMap(5);
    Map lazyMap = LazyMap.decorate(lruMap, reverseString);

    String name0 = (String)lazyMap.get("test0");
    String name1 = (String)lazyMap.get("test0");
    String name2 = (String)lazyMap.get("test0");
    String name3 = (String)lazyMap.get("test2");
    String name4 = (String)lazyMap.get("test4");
    String name5 = (String)lazyMap.get("test5");
    String name6 = (String)lazyMap.get("test0");
    String name7 = (String)lazyMap.get("test6");
    String name8 = (String)lazyMap.get("test8");
    String name9 = (String)lazyMap.get("test5");

    Iterator it = lazyMap.entrySet().iterator();
    while(it.hasNext()){
       Map.Entry entry = (Map.Entry)it.next();
       System.out.println(entry.getValue());
    }

当然可以在Transformer里可以做任何事，比如从网站爬取数据，并转换成相应的对象等。
如下代码从pchome的ip地址库中查询IP地址，首先定义一个ArticleTransformer对象，每次根据传入的参数从一个URL查询数据并返回。

代码如下

复制代码

public class ArticleTransformer implements Transformer {

    protected final static int BUF_SIZE = 300;

    protected String queryURL;
    public String getQueryURL(){return queryURL;}
    public void setQueryURL(String queryURL){this.queryURL = queryURL;}

    protected long timeOut;
    public long getTimeOut(){return timeOut;}
    /**
     * @param timeOut 单位:ms
     */
    public void setTimeOut(long timeOut){this.timeOut = timeOut;}

    public Object transform(Object obj){
        ArticleRetriever retriever = new ArticleRetriever((String)obj);
        try{
            Thread thread = new Thread(retriever);
            thread.start();
            thread.join(timeOut);
            //Thread.sleep(3*1000);
        }catch(InterruptedException e){
            System.out.println("url request time out.");
        }
        return retriever.getResponse();
    }

    public class ArticleRetriever implements Runnable{
        private String name;
        private String response;

        public ArticleRetriever(String name){
            this.name = name;
        }

        public String getResponse(){
            return this.response;
        }

        public void run(){

            HttpURL url = null;

            try{
                HttpClient client = new HttpClient();

                url = new HttpURL(queryURL+name);

                GetMethod getMethod = new GetMethod(url.toString());

                getMethod.getParams().setContentCharset("UTF-8");

                client.executeMethod(getMethod);

                response = getMethod.getResponseBodyAsString();
            }catch(Exception e){
                System.out.println("Error get Article " + url);
            }

        }
    }

}

然后如下代码调用

代码如下

复制代码

String queryURL = "http://whois.pconline.com.cn/ip.jsp?ip=";
    ArticleTransformer transformer = new ArticleTransformer();
    transformer.setQueryURL(queryURL);
    transformer.setTimeOut(500);

    Map lruMap = new LRUMap(3);
    articleMap = LazyMap.decorate(lruMap, transformer);

    String ip1 = (String)articleMap.get("127.0.0.1");
    String ip2 = (String)articleMap.get("121.0.29.241");
    String ip3 = (String)articleMap.get("10.25.145.123");
    String ip4 = (String)articleMap.get("10.121.20.35");
    String ip5 = (String)articleMap.get("121.230.145.60");
    String ip6 = (String)articleMap.get("27.186.214.221");
    String ip7 = (String)articleMap.get("127.0.0.1");

    try {
        Writer writer = new FileWriter("D://hello.txt");

        Iterator it = articleMap.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry entry = (Map.Entry) it.next();
            System.out.println(entry.getValue());
            if(entry.getValue() == null){
                writer.write("nulln");
            }else{
                writer.write((String)entry.getValue());
            }
        }
        if(writer!=null) writer.close();
    } catch (IOException e) {
        System.out.println("Error write file");
    }

最后输出如下的结果：

江苏省泰州市(泰兴)电信
河北省保定市电信
Internet保留地址

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