Lucene的spellchecker用于纠错的问题

import java.io.File;
import java.io.IOException;
import javax.servlet.http.HttpServletRequest;
import org.apache.lucene.search.spell.PlainTextDictionary;
import org.apache.lucene.search.spell.SpellChecker;
import org.apache.lucene.store.Directory;
import org.apache.struts2.ServletActionContext;
import com.opensymphony.xwork2.ActionSupport;

public class check extends ActionSupport{

private String searching;
private String[] suggestions;

public String getSearching() {
return searching;
}

         public void setSearching(String searching) {
this.searching = searching;
}

public String[] getSuggestions() {
return suggestions;
}

public void setSuggestions(String[] suggestions) {
this.suggestions = suggestions;
}
public String execute() throws IOException{


   Directory spellIndexDirectory = null;
   //实例化拼写检查器
   SpellChecker sp = new SpellChecker(spellIndexDirectory);


   //对词典进行索引
   sp.indexDictionary(new PlainTextDictionary(new File("dictionary.txt")));

   //“错误”的搜索
   String search = searching;

   //建议个数
   int suggestionNumber = 1;

   //获取建议的关键字
   suggestions = sp.suggestSimilar(search, suggestionNumber);

   System.out.println(suggestions[0]);

   return SUCCESS;
}
}
从前台得到searching的内容，dictionary.txt已经建好。运行时抛出异常如下
Struts Problem Report
Struts has detected an unhandled exception:

Messages:
File: org/apache/lucene/index/SegmentInfos.java
Line number: 151

--------------------------------------------------------------------------------

Stacktraces
java.lang.NullPointerException
    org.apache.lucene.index.SegmentInfos.getCurrentSegmentGeneration(SegmentInfos.java:151)
    org.apache.lucene.index.IndexReader.indexExists(IndexReader.java:626)
    org.apache.lucene.search.spell.SpellChecker.setSpellIndex(SpellChecker.java:141)
    org.apache.lucene.search.spell.SpellChecker.(SpellChecker.java:110)
    org.apache.lucene.search.spell.SpellChecker.(SpellChecker.java:124)
    check.execute(check.java:39)
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    java.lang.reflect.Method.invoke(Method.java:597)

...

我感觉就是 Directory spellIndexDirectory = null; 这里写错了，可是该怎么改呢，我看了API没看懂，问题如下
1.这个Directory是一个索引么
2.如果是一个索引，那他已经建好了还是没有被创建
3.如何创建他
4.如果不是我想的这样，那到底是什么地方错了，如何解决呢

先谢谢大家了 --------------------编程问答-------------------- 先获取在赋值给directory，看答案 --------------------编程问答-------------------- ls写的太简洁了，木有看懂啊，求助ing --------------------编程问答-------------------- 继续顶，顶啊顶 --------------------编程问答-------------------- 空指针啊也就是说某一个对象是null然后调用了他的方法之类的打个断电不可以吗? --------------------编程问答-------------------- sp.indexDictionary(new PlainTextDictionary(new File("dictionary.txt")));
很明显这是错误的给你举个例子吧
String s=new String(null);
s.indexOf();
你可以试一下，个人见解 --------------------编程问答-------------------- spellchecker.indexDictionary(new PlainTextDictionary(new File("myfile.txt")));
上面是api里的例子，我写的没错，不是这出的错误 --------------------编程问答-------------------- Directory spellIndexDirectory = null;
//实例化拼写检查器
SpellChecker sp = new SpellChecker(spellIndexDirectory);

spellIndexDirectory本来就是null呀，给个值就可以了
//创建词典

Directory directory = FSDirectory.open(new File("E:/index2"));

//实例化拼写检查器
final SpellChecker sp = new SpellChecker(directory);

补充：Java ,  Web 开发