利用Map做缓存

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。

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）。  

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：

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。

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

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("null\\n");
        }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