自己写Lucene分词器原理篇——CJKAnalyzer简单讲解

其中CJK中日韩统一表意文字（CJK Unified Ideographs），目的是要把分别来自中文、日文、韩文、越文中，本质、意义相同、形状一样或稍异的表意文字（主要为汉字，但也有仿汉字如日本国字、韩国独有汉字、越南的喃字）于ISO 10646及Unicode标准内赋予相同编码。CJK 是中文（Chinese）、日文（Japanese）、韩文（Korean）三国文字的缩写。顾名思义，它能够支持这三种文字。

实际上，CJKAnalyzer支持中文、日文、韩文和朝鲜文。

CJKAnalyzer的所有类：

CJKAnalyzer是主类

CJKWidthFilter是负责格式化字符，主要是折叠变种的半宽片假名成等价的假名

CJKBigramFilter是负责把两个CJK字符切割成两个，只要是CJK字符就会两两切割，ABC->AB,BC

CJKTokenizer是兼容低版本的分析器

1.CJKAnalyzer的主要部分，可以看出，先判断版本号，低版本直接用CJKTokenizer，高版本的先用standardanalyzer把非英文字母数字一个个分开，再用CJKWidthFilter格式化CJK字符，再用LowerCaseFilter转换英文成小写，再用CJKBigramFilter把CJK字符切割成两两的。

protected TokenStreamComponents createComponents(String fieldName,
      Reader reader) {
    if (matchVersion.onOrAfter(Version.LUCENE_36)) {
      final Tokenizer source = new StandardTokenizer(matchVersion, reader);
      // run the widthfilter first before bigramming, it sometimes combines characters.
      TokenStream result = new CJKWidthFilter(source);
      result = new LowerCaseFilter(matchVersion, result);
      result = new CJKBigramFilter(result);
      return new TokenStreamComponents(source, new StopFilter(matchVersion, result, stopwords));
    } else {
      final Tokenizer source = new CJKTokenizer(reader);
      return new TokenStreamComponents(source, new StopFilter(matchVersion, source, stopwords));
    }
  }

2.CJKWidthFilter的部分主要是折叠变种的半宽片假名成等价的假名，有点类似全角符号转换成半角符号，含义是一样的，但是表示不一样，主要针对日文。

3.CJKBigramFilter，该fileter可以选择既输出单字切割，同时也输出双字切割；或者只输出双字切割

public boolean incrementToken() throws IOException {
    while (true) {
    //判断之前是否暂存了双CJK字符 
       if (hasBufferedBigram()) {
        
        // case 1: we have multiple remaining codepoints buffered,
        // so we can emit a bigram here.
        //如果选择了要输出单字切割
        if (outputUnigrams) {

          // when also outputting unigrams, we output the unigram first,
          // then rewind back to revisit the bigram.
          // so an input of ABC is A + (rewind)AB + B + (rewind)BC + C
          // the logic in hasBufferedUnigram ensures we output the C, 
          // even though it did actually have adjacent CJK characters.

          if (ngramState) {
            flushBigram();//写双字
          } else {
            flushUnigram();//写单字，然后后退一个字符(rewind)
            index--;
          }
         //这个个对应上面的判断，实现双字输出
          ngramState = !ngramState;
        } else {
          flushBigram();
        }
        return true;
      } else if (doNext()) {

        // case 2: look at the token type. should we form any n-grams?
        
        String type = typeAtt.type();
           //判断字符属性，是否是CJK字符
        if (type == doHan || type == doHiragana || type == doKatakana || type == doHangul) {
          
          // acceptable CJK type: we form n-grams from these.
          // as long as the offsets are aligned, we just add these to our current buffer.
          // otherwise, we clear the buffer and start over.
          
          if (offsetAtt.startOffset() != lastEndOffset) { // unaligned, clear queue
            if (hasBufferedUnigram()) {
              
              // we have a buffered unigram, and we peeked ahead to see if we could form
              // a bigram, but we can't, because the offsets are unaligned. capture the state 
              // of this peeked data to be revisited next time thru the loop, and dump our unigram.
              
              loneState = captureState();
              flushUnigram();
              return true;
            }
            index = 0;
            bufferLen = 0;
          }
          refill();
        } else {
          
          // not a CJK type: we just return these as-is.
          
          if (hasBufferedUnigram()) {
            
            // we have a buffered unigram, and we peeked ahead to see if we could form
            // a bigram, but we can't, because its not a CJK type. capture the state 
            // of this peeked data to be revisited next time thru the loop, and dump our unigram.
            
            loneState = captureState();
            flushUnigram();
            return true;
          }
          return true;
        }
      } else {
        
        // case 3: we have only zero or 1 codepoints buffered, 
        // so not enough to form a bigram. But, we also have no
        // more input. So if we have a buffered codepoint, emit
        // a unigram, otherwise, its end of stream.
        
        if (hasBufferedUnigram()) {
          flushUnigram(); // flush our remaining unigram
          return true;
        }
        return false;
      }
    }
  }