自写filter步骤和注意事项

hbase本身提供了很多filter来实现服务器端过滤的功能，诸如

Filter, FilterBase, CompareFilter;

一些Comparators类;

Comparison Filters;

Dedicated Filters;

Decorating Filters;

FilterList;

Custom Filters。

但是这些filter往往不能满足我们的实际需要，因此我们可以针对自己需要来自写filter，然后加到hbase集群中。

主要步骤：

1.新建类，继承自FilterBase或WritableByteArrayComparable，需求不同，需要创建的类不同，不详述

2.将相关类打成jar包

3.将jar包copy到hbase集群中各机器的  $HBASE_HOME/lib  目录下，并重启集群

4.在客户端开发时，添加此jar包到路经

OK，可以使用了。

注意事项：

1.新建类中，需要添加常量 LOG. 否则出现

can't seal package org.apache.hadoop.hbase.filter: already loaded

异常提示

2.必须将jar包copy到hbase集群中各机器的  $HBASE_HOME/lib  目录下，并重启hbase集群

参考代码

如下（网友写的，帮忙解决了点问题）：

/**
 * Copyright 2008 The Apache Software Foundation
 *
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.hbase.filter;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.hbase.KeyValue;
import org.apache.hadoop.hbase.filter.FilterBase;

/**
 * A filter, based on the PageFilter, takes two arguments: limit and offset.
 * This filter can be used for specified row count, in order to efficient
 * lookups and paginated results for end users.
 * 
 */
public class RowPaginationFilter extends FilterBase {
    <span style="color: rgb(255, 0, 0);">static final Log LOG = LogFactory.getLog(RowPaginationFilter.class);
</span>    private int rowsAccepted = 0;
    private int limit = 0;
    private int offset = 0;

    /**
     * Default constructor, filters nothing. Required though for RPC
     * deserialization.
     */
    public RowPaginationFilter() {
    }

    /**
     * Constructor that takes a maximum page size.
     * 
     * get row from offset to offset+limit ( offset<= row<=offset+limit )
     * @param offset start position
     * @param limit count from offset position
     */
    public RowPaginationFilter(final int offset, final int limit) {
        this.offset = offset;
        this.limit = limit;
    }

    @Override
    public void reset() {
        // noop
    }
    @Override
    public boolean filterAllRemaining() {
        return this.rowsAccepted > this.limit+this.offset;
    }
    @Override
    public boolean filterRowKey(byte[] rowKey, int offset, int length) {
        return false;
    }
    @Override
    public void readFields(final DataInput in) throws IOException {
        this.offset = in.readInt();
        this.limit = in.readInt();
    }
    @Override
    public void write(final DataOutput out) throws IOException {
        out.writeInt(offset);
        out.writeInt(limit);
    }
    @Override
    public ReturnCode filterKeyValue(KeyValue v) {
        return ReturnCode.INCLUDE;
    }
    //true to exclude row, false to include row.
    @Override
    public boolean filterRow() {        
        boolean isExclude = this.rowsAccepted < this.offset || this.rowsAccepted>=this.limit+this.offset;
        rowsAccepted++;
        return isExclude;
    }
}