Hadoop3.1.2 使用java连接 HDFS踩得坑

Hadoop3.1.2 使用java连接 HDFS踩得坑

hadoop3中DistributedFileSystem类找不到错误

按照hadoop2的示例，运行错误如下：

org.apache.hadoop.fs.UnsupportedFileSystemException: No FileSystem for scheme "hdfs"
	at org.apache.hadoop.fs.FileSystem.getFileSystemClass(FileSystem.java:3281)
	at org.apache.hadoop.fs.FileSystem.createFileSystem(FileSystem.java:3301)
	at org.apache.hadoop.fs.FileSystem.access$200(FileSystem.java:124)
	at org.apache.hadoop.fs.FileSystem$Cache.getInternal(FileSystem.java:3352)
	at org.apache.hadoop.fs.FileSystem$Cache.get(FileSystem.java:3320)
	at org.apache.hadoop.fs.FileSystem.get(FileSystem.java:479)
	at xin.tomdonkey.HadoopDemo.initHDFS(HadoopDemo.java:53)
	at xin.tomdonkey.HadoopDemo.main(HadoopDemo.java:63)
Exception in thread "main" java.lang.NullPointerException
	at xin.tomdonkey.HadoopDemo.main(HadoopDemo.java:63)

如果加入了如下代码：

	Configuration conf = new Configuration();
	conf.set("fs.hdfs.impl", "org.apache.hadoop.hdfs.DistributedFileSystem");

则错误变成：

Exception in thread "main" java.lang.RuntimeException: java.lang.ClassNotFoundException: Class org.apache.hadoop.hdfs.DistributedFileSystem not found
	at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:2595)
	at org.apache.hadoop.fs.FileSystem.getFileSystemClass(FileSystem.java:3269)
	at org.apache.hadoop.fs.FileSystem.createFileSystem(FileSystem.java:3301)
	at org.apache.hadoop.fs.FileSystem.access$200(FileSystem.java:124)
	at org.apache.hadoop.fs.FileSystem$Cache.getInternal(FileSystem.java:3352)
	at org.apache.hadoop.fs.FileSystem$Cache.get(FileSystem.java:3320)
	at org.apache.hadoop.fs.FileSystem.get(FileSystem.java:479)
	at xin.tomdonkey.HadoopDemo.initHDFS(HadoopDemo.java:53)
	at xin.tomdonkey.HadoopDemo.main(HadoopDemo.java:63)
Caused by: java.lang.ClassNotFoundException: Class org.apache.hadoop.hdfs.DistributedFileSystem not found
	at org.apache.hadoop.conf.Configuration.getClassByName(Configuration.java:2499)
	at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:2593)
	... 8 more

显然错误是由于缺少hdfs的scheme 引起的，也就是FileSystem 缺少相对应hdfs的实现。
这是因为org.apache.hadoop.hdfs.DistributedFileSystem类
在hadoop2中hadoop-hdfs-3.1.2.jar中
在hadoop3中被迁移到了：hadoop-hdfs-client-3.1.2中
按照原来的maven依赖进行依赖，是不行的，再加入就可以了

        <dependency>
            <groupId>org.apache.hadoopgroupId>
            <artifactId>hadoop-clientartifactId>
            <version>3.1.2version>
        dependency>