用java運行Hadoop例程報錯:org.apache.hadoop.fs.LocalFileSystem cannot be cast to org.apache.所寫代碼以下:java
package com.pcitc.hadoop; import java.io.IOException; import org.apache.hadoop.conf.Configuration; import org.apache.hadoop.fs.FileSystem; import org.apache.hadoop.hdfs.DistributedFileSystem; import org.apache.hadoop.hdfs.protocol.DatanodeInfo; /** * 獲取HDFS集羣上全部節點名稱 * @author lenovo * */ public class GetList { public static void main(String[] args) throws IOException { Configuration conf = new Configuration(); conf.set("dfs.default.name", "hdfs://hadoopmaster:9000"); FileSystem fs = FileSystem.get(conf); DistributedFileSystem hdfs = (DistributedFileSystem) fs; DatanodeInfo[] dataNodeStats = hdfs.getDataNodeStats(); String[] names = new String[dataNodeStats.length]; for (int i = 0; i < dataNodeStats.length; i++) { names[i] = dataNodeStats[i].getHostName(); System.out.println("node" + i + "name" + names[i]); } } }
執行以後報以下錯誤:node
Exception in thread "main" java.lang.ClassCastException: org.apache.hadoop.fs.LocalFileSystem cannot be cast to org.apache.hadoop.hdfs.DistributedFileSystem
at org.apache.hadoop.examples.FindFileOnHDFS.getHDFSNodes(FindFileOnHDFS.java:43)
at org.apache.hadoop.examples.FindFileOnHDFS.main(FindFileOnHDFS.java:16)
緣由是DistributedFileSystem和LocalFileSystem都是FileSystem的子類,FileSystem.get(conf)獲得的是LocalFileSystem的instance, 這個類型應該是默認的,要得到DistributedFileSystem,須要配置conf對象,按照個人寫法我以爲應該是配了conf對象了,可是仍是保存,最後按照網上的說法進行相應修改就能夠了。直接上修改後的代碼以下(注意紅色部分):apache
package com.pcitc.hadoop; import java.io.IOException; import java.net.URI; import org.apache.hadoop.conf.Configuration; import org.apache.hadoop.fs.FileSystem; import org.apache.hadoop.hdfs.DistributedFileSystem; import org.apache.hadoop.hdfs.protocol.DatanodeInfo; /** * 獲取HDFS集羣上全部節點名稱 * * @author lenovo * */ public class GetList { public static void main(String[] args) throws IOException { Configuration conf = new Configuration(); // conf.set("dfs.default.name", "hdfs://hadoopmaster:9000"); String uri = "hdfs://hadoopmaster:9000"; FileSystem fs = FileSystem.get(URI.create(uri), conf); DistributedFileSystem hdfs = (DistributedFileSystem) fs; DatanodeInfo[] dataNodeStats = hdfs.getDataNodeStats(); String[] names = new String[dataNodeStats.length]; for (int i = 0; i < dataNodeStats.length; i++) { names[i] = dataNodeStats[i].getHostName(); System.out.println("node:" + i + ",name:" + names[i]); } } }