前一篇博客说了一下 HBase 的一些过滤器，今天看看 HBase 的分页过滤器。

在 HBase 中分页过滤是通过 PageFilter 来实现的，在创建这个参数的时候需要设置一个pageSize参数，通过这个参数来控制每页返回的行数，并且在每次查询时需要指定本次查询的起始行。

这里有一点需要注意，HBase中行键的排序是按字典顺序排列的，因此返回的结果也是按此顺序排列。

下面看一下分页过滤的代码片段

Filter filter = new PageFilter(10);
Table table = connection.getTable(TableName.valueOf(TABLE_NAME));

byte[] lastRow = null;
while(true) {
    Scan scan = new Scan();
    scan.setFilter(filter);
    if (lastRow != null) {
        scan.withStartRow(lastRow, false);
    }
    ResultScanner resultScanner = table.getScanner(scan);
    Iterator<Result> it = resultScanner.iterator();
    int count = 0;
    while (it.hasNext()) {
        Result result = it.next();
        printRow(result);
        lastRow = result.getRow();
        count ++;
    }
    resultScanner.close();
    if (count == 0) {
        break;
    }
}
table.close();

• 首先需要创建一个PageFilter对象，并设置每页10行。
• 然后通过Scan.withStartRow()来设置起始行，对于第一次查询，可以不用设置。其中第二个参数是用来标识是否需要包括指定起始行。
• 执行查询，对于每次查询设置了一个计数，当计数为 0 时，表示本次查询没有返回结果，说明查询遍历完成，此时跳出循环。

下面是可运行完整代码

package my.hbasestudy;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hbase.CompareOperator;
import org.apache.hadoop.hbase.HBaseConfiguration;
import org.apache.hadoop.hbase.TableName;
import org.apache.hadoop.hbase.client.*;
import org.apache.hadoop.hbase.exceptions.DeserializationException;
import org.apache.hadoop.hbase.filter.*;
import org.apache.hadoop.hbase.util.Bytes;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class TestPageFilter {
    private static final String TABLE_NAME = "user";

    private static final String COLUMN_FAMILY_BASE = "base";
    private static final String COLUMN_FAMILY_ADDRESS = "address";

    private static final String COLUMN_USERNAME = "username";
    private static final String COLUMN_PASSWORD = "password";
    private static final String COLUMN_HOME = "home";
    private static final String COLUMN_OFFICE = "office";

    private Connection connection;

    public static void main(String[] args) throws Exception {
        Configuration config = HBaseConfiguration.create();
        Connection connection = ConnectionFactory.createConnection(config);

        TestPageFilter t = new TestPageFilter(connection);
        t.test();

        connection.close();
    }

    public TestPageFilter(Connection connection) {
        this.connection = connection;
    }

    private void test() throws IOException, DeserializationException {
        createTable();
        prepare();

        Filter filter = new PageFilter(10);
        Table table = connection.getTable(TableName.valueOf(TABLE_NAME));

        byte[] lastRow = null;
        while(true) {
            Scan scan = new Scan();
            scan.setFilter(filter);
            if (lastRow != null) {
                scan.withStartRow(lastRow, false);
            }
            ResultScanner resultScanner = table.getScanner(scan);
            Iterator<Result> it = resultScanner.iterator();
            int count = 0;
            while (it.hasNext()) {
                Result result = it.next();
                printRow(result);
                lastRow = result.getRow();
                count ++;
            }
            resultScanner.close();
            if (count == 0) {
                break;
            }
        }
        table.close();

        deleteTable();
    }

    private void createTable() throws IOException {
        Admin admin = connection.getAdmin();

        try {
            TableDescriptor tableDesc = TableDescriptorBuilder.newBuilder(TableName.valueOf(TABLE_NAME))
                    .addColumnFamily(ColumnFamilyDescriptorBuilder.newBuilder(Bytes.toBytes(COLUMN_FAMILY_BASE)).build())
                    .addColumnFamily(ColumnFamilyDescriptorBuilder.newBuilder(Bytes.toBytes(COLUMN_FAMILY_ADDRESS)).build())
                    .build();
            admin.createTable(tableDesc);
        } finally {
            admin.close();
        }
    }

    private void deleteTable() throws IOException {
        Admin admin = connection.getAdmin();

        try {
            admin.disableTable(TableName.valueOf(TABLE_NAME));
            admin.deleteTable(TableName.valueOf(TABLE_NAME));
        } finally {
            admin.close();
        }
    }

    private void prepare() throws IOException {
        Table table = connection.getTable(TableName.valueOf(TABLE_NAME));

        List<Row> actions = new ArrayList<Row>();
        for (int i = 0; i < 100; i++) {
            Put put = new Put(Bytes.toBytes("row_" + i));
            put.addColumn(Bytes.toBytes(COLUMN_FAMILY_BASE), Bytes.toBytes(COLUMN_USERNAME), Bytes.toBytes("user_" + i));
            put.addColumn(Bytes.toBytes(COLUMN_FAMILY_BASE), Bytes.toBytes(COLUMN_PASSWORD), Bytes.toBytes("password_" + i));
            put.addColumn(Bytes.toBytes(COLUMN_FAMILY_ADDRESS), Bytes.toBytes(COLUMN_HOME), Bytes.toBytes("home_" + i));
            put.addColumn(Bytes.toBytes(COLUMN_FAMILY_ADDRESS), Bytes.toBytes(COLUMN_OFFICE), Bytes.toBytes("office_" + i));
            actions.add(put);
        }
        Object[] results = new Object[actions.size()];

        try {
            table.batch(actions, results);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        table.close();
    }

    private void filter(Filter filter) throws IOException {
        Table table = connection.getTable(TableName.valueOf(TABLE_NAME));
        Scan scan = new Scan();
        scan.setFilter(filter);
        ResultScanner resultScanner = table.getScanner(scan);
        Iterator<Result> it = resultScanner.iterator();
        while (it.hasNext()) {
            Result result = it.next();
            printRow(result);
        }
        resultScanner.close();
        table.close();
    }

    private void printRow(Result result) {
        if (Bytes.toString(result.getRow()) != null) {
            StringBuilder sb = new StringBuilder();
            sb.append(Bytes.toString(result.getRow()));
            sb.append("[");
            sb.append("base:username=" + Bytes.toString(result.getValue(Bytes.toBytes("base"), Bytes.toBytes("username"))));
            sb.append(", base:password=" + Bytes.toString(result.getValue(Bytes.toBytes("base"), Bytes.toBytes("password"))));
            sb.append(", address:home=" + Bytes.toString(result.getValue(Bytes.toBytes("address"), Bytes.toBytes("home"))));
            sb.append(", address:office=" + Bytes.toString(result.getValue(Bytes.toBytes("address"), Bytes.toBytes("office"))));
            sb.append("]");
            System.out.println(sb.toString());
        }
    }
}