browse（）函数中有一行代码：

     filter = handler.buildFilter(sel);

用于构建filter，下面是FacetHandler的函数buildFilter（）的实现，其中buildRandomAccessFilter （）是主要实现函数：

	public final RandomAccessFilter buildFilter(BrowseSelection sel) throws IOException
	{
      String[] selections = sel.getValues();//相当于BooleanQuery的MUST，就是必须要有的值
      String[] notSelections = sel.getNotValues();//相当于NOT，不能有的属性值
      Properties prop=sel.getSelectionProperties();
      
      RandomAccessFilter filter = null;
      if (selections!=null && selections.length > 0)
      {
        if (sel.getSelectionOperation() == ValueOperation.ValueOperationAnd)
        {
          filter = buildRandomAccessAndFilter(selections,prop);
          if (filter == null)
          {
            filter = EmptyFilter.getInstance();
          }
        }
        else
        {
          filter = buildRandomAccessOrFilter(selections, prop,false);
          if (filter == null)
          {
            return EmptyFilter.getInstance();
          }
        }
      }
      
      if (notSelections!=null && notSelections.length>0)
      {
        RandomAccessFilter notFilter = buildRandomAccessOrFilter(notSelections, prop, true);
        if (filter==null)
        {
          filter = notFilter;
        }
        else
        {
          RandomAccessFilter andFilter = new RandomAccessAndFilter(Arrays.asList(new RandomAccessFilter[]{filter,notFilter}));
          filter = andFilter;
        }
      }
      
      return filter;
	}

 在基类FacetHandler中有函数：

	abstract public RandomAccessFilter buildRandomAccessFilter(String value,Properties selectionProperty) throws IOException;

那么在所有的具体实现类中都有自己的实现方法，以SimpleFacetHandler为例：

 @Override
  public RandomAccessFilter buildRandomAccessFilter(String value, Properties prop) throws IOException
  {
    //各个FacetHanlder实例基本都是在这个地方不相同而已，比如MultiFacetHandler在这一行是
    //  new MultiFacetFilter（this, value);
    FacetFilter f = new FacetFilter(this, value);
                                                                        
 AdaptiveFacetFilter af = new AdaptiveFacetFilter(new FacetDataCacheBuilder(){

		@Override
		public FacetDataCache build(BoboIndexReader reader) {
			return  getFacetData(reader);
		}

		@Override
		public String getName() {
			return _name;
		}
    	
    }, f, new String[]{value});
    return af;
  }

每个FacetHandler基本都有自己专有的Filter，都是继承于RandomAccessFilter，下面是所有的继承关系图：

public abstract class RandomAccessFilter extends Filter
{
  private static final long serialVersionUID = 1L;

  @Override 
  public DocIdSet getDocIdSet(IndexReader reader) throws IOException
  {
	if (reader instanceof BoboIndexReader){
      return getRandomAccessDocIdSet((BoboIndexReader)reader);
	}
	else{
	  throw new IllegalStateException("reader not instance of "+BoboIndexReader.class);
	}
  }
  
  public abstract RandomAccessDocIdSet getRandomAccessDocIdSet(BoboIndexReader reader) throws IOException;
  //返回该属性值在所有属性中的百分比
  public double getFacetSelectivity(BoboIndexReader reader) { return 0.50; }
  
}

 可以看到RandomAccessFilter的主要的功能函数是返回一个RandomAccessDocIdSet ，而RandomAccessDocIdSet接口的主要功能函数是get （int docid）和iterator ()。get（）判断某个doc是否满足条件：

public abstract class RandomAccessDocIdSet extends DocIdSet
{
  public abstract boolean get(int docId);
}

  以及从Filter继承来的iterator()函数，用于返回一个满足条件的doc的迭代器：

public abstract DocIdSetIterator iterator() throws IOException;