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。
