使用HBase的一个典型例子，涉及了HBase中很多概念

一个使用HBase的例子，如下。

import java.io.IOException;
import org.apache.hadoop.hbase.client.HTable;
import org.apache.hadoop.hbase.client.Scanner;
import org.apache.hadoop.hbase.io.BatchUpdate;
import org.apache.hadoop.hbase.io.Cell;
import org.apache.hadoop.hbase.io.RowResult;

public class MyClient {

public static void main(String args[]) throws IOException {
    // You need a configuration object to tell the client where to connect.
    // But don't worry, the defaults are pulled from the local config file.
    HBaseConfiguration config = new HBaseConfiguration();

    // This instantiates an HTable object that connects you to the "myTable"
    // table. 
    HTable table = new HTable(config, "myTable");

    // To do any sort of update on a row, you use an instance of the BatchUpdate
    // class. A BatchUpdate takes a row and optionally a timestamp which your
    // updates will affect. 
    BatchUpdate batchUpdate = new BatchUpdate("myRow");

    // The BatchUpdate#put method takes a Text that describes what cell you want
    // to put a value into, and a byte array that is the value you want to 
    // store. Note that if you want to store strings, you have to getBytes() 
    // from the string for HBase to understand how to store it. (The same goes
    // for primitives like ints and longs and user-defined classes - you must 
    // find a way to reduce it to bytes.)
    batchUpdate.put("myColumnFamily:columnQualifier1", 
      "columnQualifier1 value!".getBytes());

    // Deletes are batch operations in HBase as well. 
    batchUpdate.delete("myColumnFamily:cellIWantDeleted");

    // Once you've done all the puts you want, you need to commit the results.
    // The HTable#commit method takes the BatchUpdate instance you've been 
    // building and pushes the batch of changes you made into HBase.
    table.commit(batchUpdate);

    // Now, to retrieve the data we just wrote. The values that come back are
    // Cell instances. A Cell is a combination of the value as a byte array and
    // the timestamp the value was stored with. If you happen to know that the 
    // value contained is a string and want an actual string, then you must 
    // convert it yourself.
    Cell cell = table.get("myRow", "myColumnFamily:columnQualifier1");
    String valueStr = new String(cell.getValue());
    
    // Sometimes, you won't know the row you're looking for. In this case, you
    // use a Scanner. This will give you cursor-like interface to the contents
    // of the table.
    Scanner scanner = 
      // we want to get back only "myColumnFamily:columnQualifier1" when we iterate
      table.getScanner(new String[]{"myColumnFamily:columnQualifier1"});
    
    
    // Scanners in HBase 0.2 return RowResult instances. A RowResult is like the
    // row key and the columns all wrapped up in a single interface. 
    // RowResult#getRow gives you the row key. RowResult also implements 
    // Map, so you can get to your column results easily. 
    
    // Now, for the actual iteration. One way is to use a while loop like so:
    RowResult rowResult = scanner.next();
    
    while(rowResult != null) {
      // print out the row we found and the columns we were looking for
      System.out.println("Found row: " + new String(rowResult.getRow()) + " with value: " +
       rowResult.get("myColumnFamily:columnQualifier1".getBytes()));
      
      rowResult = scanner.next();
    }
    
    // The other approach is to use a foreach loop. Scanners are iterable!
    for (RowResult result : scanner) {
      // print out the row we found and the columns we were looking for
      System.out.println("Found row: " + new String(result.getRow()) + " with value: " +
       result.get("myColumnFamily:columnQualifier1".getBytes()));
    }
    
    // Make sure you close your scanners when you are done!
    scanner.close();
}
}

在这个例子中，使用了HBase中的很多概念，包括:
HBaseConfiguration: 用于告诉client如何连接，连接到哪个HBase的服务器上。
HTable：代表一个HBase表格。
BatchUpdate：用于表格中一行的更新。包括添加某个列，修改某列的值，删除某列等。
commit：table的一个方法。代表某个BatchUpdate操作可以生效了。类似于数据库中的commit操作。

Cell：table中对应某个（行key, 列值，时间戳）下的单元格值。
获取Cell的方法。For example:
table.get("myRow", "myColumnFamily:columnQualifier1");

scanner：用于遍历表格。
rowResult：遍历过程当中保存某行信息。

--

从上面可以看到，HBase中的数据都是Bytes。HBase并不care里面实际存的数据到底是什么数据，只要
该数据可以转化成byte[]即可。

评论

1 楼 canedy 2011-12-19  

import org.apache.hadoop.hbase.io.BatchUpdate;
这个包还能用吗？