1、重读配置文件core-site.xml

要利用Java客户端来存取HDFS上的文件，不得不说的是配置文件hadoop-0.20.2/conf/core-site.xml了，最初我就是在这里吃了大亏，所以我死活连不上HDFS，文件无法创建、读取。

<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="http://www.mamicode.com/configuration.xsl"?>

<configuration>
<!--- global properties -->
<property>
<name>hadoop.tmp.dir</name>
<value>/home/zhangzk/hadoop</value>
<description>A base for other temporary directories.</description>
</property>
<!-- file system properties -->
<property>
<name>fs.default.name</name>
<value>hdfs://linux-zzk-113:9000</value>
</property>
</configuration>

配置项：hadoop.tmp.dir表示命名节点上存放元数据的目录位置，对于数据节点则为该节点上存放文件数据的目录。 

配置项：fs.default.name表示命名的IP地址和端口号,缺省值是file:///，对于JavaAPI来讲，连接HDFS必须使用这里的配置的URL地址，对于数据节点来讲，数据节点通过该URL来访问命名节点。

2、利用JavaAPI来访问HDFS的文件与目录

package com.demo.hdfs;

import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URI;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.FSDataOutputStream;
import org.apache.hadoop.fs.FileStatus;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.util.Progressable;

/**
 * @author zhangzk
 *
 */
public class FileCopyToHdfs {

 public static void main(String[] args) throws Exception {
  try {
   //uploadToHdfs();   
   //deleteFromHdfs();
   //getDirectoryFromHdfs();
   appendToHdfs();
   readFromHdfs();
  } catch (Exception e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
  finally
  {
   System.out.println("SUCCESS");
  }
 }

 /**上传文件到HDFS上去*/

 private static void uploadToHdfs() throws FileNotFoundException,IOException {
  String localSrc = "d://qq.txt";
  String dst = "hdfs://192.168.0.113:9000/user/zhangzk/qq.txt";
  InputStream in = new BufferedInputStream(new FileInputStream(localSrc));
  Configuration conf = new Configuration();
  
  FileSystem fs = FileSystem.get(URI.create(dst), conf);
  OutputStream out = fs.create(new Path(dst), new Progressable() {
   public void progress() {
    System.out.print(".");
   }
  });
  IOUtils.copyBytes(in, out, 4096, true);
 }
 

 /**从HDFS上读取文件*/
 private static void readFromHdfs() throws FileNotFoundException,IOException {
  String dst = "hdfs://192.168.0.113:9000/user/zhangzk/qq.txt";  
  Configuration conf = new Configuration();  
  FileSystem fs = FileSystem.get(URI.create(dst), conf);
  FSDataInputStream hdfsInStream = fs.open(new Path(dst));
  
  OutputStream out = new FileOutputStream("d:/qq-hdfs.txt");
  byte[] ioBuffer = new byte[1024];
  int readLen = hdfsInStream.read(ioBuffer);

  while(-1 != readLen){
  out.write(ioBuffer, 0, readLen);  
  readLen = hdfsInStream.read(ioBuffer);
  }
  out.close();
  hdfsInStream.close();
  fs.close();
 }
 

 /**以append方式将内容添加到HDFS上文件的末尾;注意：文件更新，需要在hdfs-site.xml中添<property><name>dfs.append.support</name><value>true</value></property>*/
 private static void appendToHdfs() throws FileNotFoundException,IOException {
  String dst = "hdfs://192.168.0.113:9000/user/zhangzk/qq.txt";  
  Configuration conf = new Configuration();  
  FileSystem fs = FileSystem.get(URI.create(dst), conf);  
  FSDataOutputStream out = fs.append(new Path(dst));

  int readLen = "zhangzk add by hdfs java api".getBytes().length;

  while(-1 != readLen){
  out.write("zhangzk add by hdfs java api".getBytes(), 0, readLen);
  }
  out.close();
  fs.close();
 }
 

 /**从HDFS上删除文件*/
 private static void deleteFromHdfs() throws FileNotFoundException,IOException {
  String dst = "hdfs://192.168.0.113:9000/user/zhangzk/qq-bak.txt";  
  Configuration conf = new Configuration();  
  FileSystem fs = FileSystem.get(URI.create(dst), conf);
  fs.deleteOnExit(new Path(dst));
  fs.close();
 }
 

 /**遍历HDFS上的文件和目录*/
 private static void getDirectoryFromHdfs() throws FileNotFoundException,IOException {
  String dst = "hdfs://192.168.0.113:9000/user/zhangzk";  
  Configuration conf = new Configuration();  
  FileSystem fs = FileSystem.get(URI.create(dst), conf);
  FileStatus fileList[] = fs.listStatus(new Path(dst));
  int size = fileList.length;
  for(int i = 0; i < size; i++){
  System.out.println("name:" + fileList[i].getPath().getName() + "/t/tsize:" + fileList[i].getLen());
  }
  fs.close();
 } 

}

注意：对于append操作，从hadoop-0.21版本开始就不支持了，关于Append的操作可以参考Javaeye上的一篇文档。

http://dongyajun.javaeye.com/blog/643391