HDFS Java API的使用举例

　　HDFS是Hadoop应用程序使用的主要分布式存储。HDFS集群主要由管理文件系统元数据的NameNode和存储实际数据的DataNodes组成，HDFS架构图描述了NameNode，DataNode和客户端之间的基本交互。客户端联系NameNode进行文件元数据或文件修改，并直接使用DataNodes执行实际的文件I / O。

　　Hadoop支持shell命令直接与HDFS进行交互，同时也支持JAVA API对HDFS的操作，例如，文件的创建、删除、上传、下载、重命名等。

　　HDFS中的文件操作主要涉及以下几个类：

　　　　Configuration：提供对配置参数的访问

　　　　FileSystem：文件系统对象

　　　　Path：在FileSystem中命名文件或目录。 路径字符串使用斜杠作为目录分隔符。 如果以斜线开始，路径字符串是绝对的

　　　　FSDataInputStream和FSDataOutputStream：这两个类分别是HDFS中的输入和输出流

下面是JAVA API对HDFS的操作过程：

1.项目结构

2.pom.xml配置

 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
     <modelVersion>4.0.0</modelVersion>

     <groupId>com.zjl</groupId>
     <artifactId>myhadoop</artifactId>
     <version>0.0.1-SNAPSHOT</version>
     <packaging>jar</packaging>

     <name>myhadoop</name>
     <url>http://maven.apache.org</url>

     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <hadoop.version>2.5.0</hadoop.version>
     </properties>

     <dependencies>
         <dependency>
             <groupId>org.apache.hadoop</groupId>
             <artifactId>hadoop-client</artifactId>
             <version>${hadoop.version}</version>
         </dependency>
         <dependency>
             <groupId>junit</groupId>
             <artifactId>junit</artifactId>
             <version>3.8.1</version>
         </dependency>
     </dependencies>
 </project>

3.拷贝hadoop安装目录下与HDFS相关的配置（core-site.xml，hdfs-site.xml，log4j.properties）到resource目录下

 [hadoop@hadoop01 ~]$ cd /opt/modules/hadoop-2.6./etc/hadoop/
 [hadoop@hadoop01 hadoop]$ cp core-site.xml hdfs-site.xml log4j.properties /opt/tools/workspace/myhadoop/src/main/resource/
 [hadoop@hadoop01 hadoop]$

（1）core-site.xml

 <?xml version="1.0" encoding="UTF-8"?>
 <?xml-stylesheet type="text/xsl" href="configuration.xsl"?>
 <!--
   Licensed 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. See accompanying LICENSE file.
 -->

 <!-- Put site-specific property overrides in this file. -->

 <configuration>
     <property>
         <name>fs.defaultFS</name>
         <!-- 如果没有配置，默认会从本地文件系统读取数据 -->
         <value>hdfs://hadoop01.zjl.com:9000</value>
     </property>
     <property>
         <name>hadoop.tmp.dir</name>
         <!-- hadoop文件系统依赖的基础配置，很多路径都依赖它。如果hdfs-site.xml中不配置namenode和datanode的存放位置，默认就放在这个路径中 -->
         <value>/opt/modules/hadoop-2.6.5/data/tmp</value>
     </property>
 </configuration>

（2）hdfs-site.xml

 <?xml version="1.0" encoding="UTF-8"?>
 <?xml-stylesheet type="text/xsl" href="configuration.xsl"?>
 <!--
   Licensed 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. See accompanying LICENSE file.
 -->

 <!-- Put site-specific property overrides in this file. -->

 <configuration>
     <property>
         <!-- default value 3 -->
         <name>dfs.replication</name>
         <value>1</value>
     </property>
 </configuration>

（3）采用默认即可，打印hadoop的日志信息所需的配置文件。如果不配置，运行程序时eclipse控制台会提示警告

4.启动hadoop的hdfs的守护进程，并在hdfs文件系统中创建文件（文件共步骤5中java程序读取）

 [hadoop@hadoop01 hadoop]$ cd /opt/modules/hadoop-2.6./
 [hadoop@hadoop01 hadoop-2.6.]$ sbin/start-dfs.sh
 // :: WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
 Starting namenodes on [hadoop01.zjl.com]
 hadoop01.zjl.com: starting namenode, logging to /opt/modules/hadoop-2.6./logs/hadoop-hadoop-namenode-hadoop01.zjl.com.out
 hadoop01.zjl.com: starting datanode, logging to /opt/modules/hadoop-2.6./logs/hadoop-hadoop-datanode-hadoop01.zjl.com.out
 Starting secondary namenodes [0.0.0.0]
 0.0.0.0: starting secondarynamenode, logging to /opt/modules/hadoop-2.6./logs/hadoop-hadoop-secondarynamenode-hadoop01.zjl.com.out
 // :: WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
 [hadoop@hadoop01 hadoop-2.6.]$ jps
  NameNode
  Jps
  SecondaryNameNode
  DataNode
  org.eclipse.equinox.launcher_1.3.201.v20161025-.jar
 [hadoop@hadoop01 hadoop-2.6.]$ bin/hdfs dfs -mkdir -p /user/hadoop/mapreduce/wordcount/input
 // :: WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
 [hadoop@hadoop01 hadoop-2.6.]$ cat wcinput/wc.input
 hadoop yarn
 hadoop mapreduce
 hadoop hdfs
 yarn nodemanager
 hadoop resourcemanager
 [hadoop@hadoop01 hadoop-2.6.]$ bin/hdfs dfs -put wcinput/wc.input /user/hadoop/mapreduce/wordcount/input/
 // :: WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
 [hadoop@hadoop01 hadoop-2.6.]$

5.java代码

 package com.zjl.myhadoop;

 import java.io.File;
 import java.io.FileInputStream;

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

 /**
  *
  * @author hadoop
  *
  */
 public class HdfsApp {

     /**
      * get file system
      * @return
      * @throws Exception
      */
     public static FileSystem getFileSystem() throws Exception{
         //read configuration
         //core-site.xml,core-default-site.xml,hdfs-site.xml,hdfs-default-site.xml
         Configuration conf = new Configuration();
         //create file system
         FileSystem fileSystem = FileSystem.get(conf);
         return fileSystem;
     }

     /**
      * read file from hdfs file system,output to the console
      * @param fileName
      * @throws Exception
      */
     public static void read(String fileName) throws Exception {
         //read path
         Path readPath = new Path(fileName);
         //get file system
         FileSystem fileSystem = getFileSystem();
         //open file
         FSDataInputStream inStream = fileSystem.open(readPath);
         try{
             //read file
             IOUtils.copyBytes(inStream, System.out, 4096, false);
         }catch (Exception e) {
             e.printStackTrace();
         }finally {
             //io close
             IOUtils.closeStream(inStream);
         }
     }

     public static void upload(String inFileName, String outFileName) throws Exception {

         //file input stream,local file
         FileInputStream inStream = new FileInputStream(new File(inFileName));

         //get file system
         FileSystem fileSystem = getFileSystem();
         //write path,hdfs file system
         Path writePath = new Path(outFileName);

         //output stream
         FSDataOutputStream outStream = fileSystem.create(writePath);
         try{
             //write file
             IOUtils.copyBytes(inStream, outStream, 4096, false);
         }catch (Exception e) {
             e.printStackTrace();
         }finally {
             //io close
             IOUtils.closeStream(inStream);
             IOUtils.closeStream(outStream);
         }
     }
     public static void main( String[] args ) throws Exception {
         //1.read file from hdfs to console
 //        String fileName = "/user/hadoop/mapreduce/wordcount/input/wc.input";
 //        read(fileName);

         //2.upload file from local file system to hdfs file system
         //file input stream,local file
         String inFileName = "/opt/modules/hadoop-2.6.5/wcinput/wc.input";
         String outFileName = "/user/hadoop/put-wc.input";
         upload(inFileName, outFileName);
     }
 }

6.调用方法 read(fileName)

7.进入hdfs文件系统查看/user/hadoop目录

8.调用upload(inFileName, outFileName)，然后刷新步骤7的页面，文件上传成功