在学习过程中，过了几天再启动虚拟机，启动hadoop后再启动别的框架会报错：
Exception in thread "main" java.lang.RuntimeException: org.apache.hadoop.ipc.RemoteException(org.apache.hadoop.hdfs.server.namenode.Saf eModeException): Cannot create directory /tmp/hive/root/52105826-b5a2-4aa7-965d-195920589030. Name node is in safe mode.The reported blocks 431 has reached the threshold 0.9990 of total blocks 431. The number of live datanodes 3 has reached the minimum nu mber 0. In safe mode extension. Safe mode will be turned off automatically in 13 seconds. at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.checkNameNodeSafeMode(FSNamesystem.java:1364) at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.mkdirsInt(FSNamesystem.java:4216) at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.mkdirs(FSNamesystem.java:4191) at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.mkdirs(NameNodeRpcServer.java:813) at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.mkdirs(ClientNamenodeProtocolServerSideTransl atorPB.java:600) at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenod eProtocolProtos.java)
hadoop 处于安全模式，所以需要退出安全模式，一般以如下方法可以解决：
hadoop dfsadmin -safemode leave
实在不行还可以用如下方式：
hdfs dfsadmin -safemode leave