运行Map Reduce报错Got exception: java.net.ConnectException: Call From master.localdomain/127.0.0.1 to

部署好Hadoop完全分布式集群上传hdfs测试，运行MapReduce出现报错：
报错信息：Got exception: java.net.ConnectException: Call From master.localdomain/127.0.0.1 to localhost:40721 failed on connection exception: java.net.ConnectException: 拒绝连接;

Got exception: java.net.ConnectException:
Call From master.localdomain/127.0.0.1 to localhost:40721 
failed on connection exception: java.net.ConnectException: 拒绝连接;

查看自己的hostname文件：

vi /etc/hostname

例如：默认是localhost.localdomain，但在修改主机名称时不小心修改了此文件，改回原来的内容就行了。

然后运行