public class ScoreSortTest {public final static String INDEX_STORE_PATH = "index";public static void main(String[] args) throws Exception {IndexWriter writer = new IndexWriter(INDEX_STORE_PATH, new StandardAnalyzer(), true);writer.setUseCompoundFile(false);Document doc1 = new Document();Document doc2 = new Document();Document doc3 = new Document();Field f1 = new Field("bookname","bc bc", Field.Store.YES, Field.Index.TOKENIZED);Field f2 = new Field("bookname","ab bc", Field.Store.YES, Field.Index.TOKENIZED);Field f3 = new Field("bookname","ab bc cd", Field.Store.YES, Field.Index.TOKENIZED);doc1.add(f1);doc2.add(f2);doc3.add(f3);writer.addDocument(doc1);writer.addDocument(doc2);writer.addDocument(doc3);writer.close();IndexSearcher searcher = new IndexSearcher(INDEX_STORE_PATH);TermQuery q = new TermQuery(new Term("bookname", "bc"));q.setBoost(2f);Hits hits = searcher.search(q);for(int i=0; i<hits.length();i++){Document doc = hits.doc(i);System.out.print(doc.get("bookname") + "\t\t");System.out.println(hits.score(i));System.out.println(searcher.explain(q, hits.id(i)));//
        }}
}

bc bc    0.629606
0.629606 = (MATCH) fieldWeight(bookname:bc in 0), product of: 1.4142135 = tf(termFreq(bookname:bc)=2) 0.71231794 = idf(docFreq=3, numDocs=3) 0.625 = fieldNorm(field=bookname, doc=0) ab bc    0.4451987
0.4451987 = (MATCH) fieldWeight(bookname:bc in 1), product of: 1.0 = tf(termFreq(bookname:bc)=1) 0.71231794 = idf(docFreq=3, numDocs=3) 0.625 = fieldNorm(field=bookname, doc=1) ab bc cd    0.35615897
0.35615897 = (MATCH) fieldWeight(bookname:bc in 2), product of: 1.0 = tf(termFreq(bookname:bc)=1) 0.71231794 = idf(docFreq=3, numDocs=3) 0.5 = fieldNorm(field=bookname, doc=2) 

涉及到的源码：

idf的计算

idf是项在倒排文档中出现的频率，计算方式为

1. /** Implemented as <code>log(numDocs/(docFreq+1)) + 1</code>. */
2. @Override
3. public float idf(long docFreq, long numDocs) {
4. return (float)(Math.log(numDocs/(double)(docFreq+1)) + 1.0);
5. }

docFreq是根据指定关键字进行检索，检索到的Document的数量，我们测试的docFreq=14；numDocs是指索引文件中总共的Document的数量，我们测试的numDocs=1453。用计算器验证一下，没有错误，这里就不啰嗦了。

queryNorm的计算

queryNorm的计算在DefaultSimilarity类中实现，如下所示：

1. /** Implemented as <code>1/sqrt(sumOfSquaredWeights)</code>. */
2. public float queryNorm(float sumOfSquaredWeights) {
3. return (float)(1.0 / Math.sqrt(sumOfSquaredWeights));
4. }

这里，sumOfSquaredWeights的计算是在org.apache.lucene.search.TermQuery.TermWeight类中的sumOfSquaredWeights方法实现：

1. public float sumOfSquaredWeights() {
2. queryWeight = idf * getBoost();             // compute query weight
3. return queryWeight * queryWeight;          // square it
4. }

其实默认情况下，sumOfSquaredWeights = idf * idf，因为Lucune中默认的boost = 1.0。