java链接mongodb 主从复制，操作数据库一直报异常

mongodb启动
             主节点:mongod --master --dbpath E:\tool\MongoDB\data\masterdb
             从节点:mongod --slave --source  127.0.0.1:27017 --port 27018 --dbpath E:\tool\MongoDB\data\slavedb
 启动正常
java代码链接：
new MongoClient(new MongoClientURI("mongodb://127.0.0.1:27017,127.0.0.1:27018/chat?slaveOk=true;authMechanism=MONGODB-CR&maxPoolSize=500")).getDatabase(dbName);
在查询的时候一直报错：
2017-12-21 10:30:13.676 [INFO ] [cluster]  No server chosen by ReadPreferenceServerSelector{readPreference=ReadPreference{name=secondaryPreferred}} from cluster description ClusterDescription{type=UNKNOWN, connectionMode=MULTIPLE, serverDescriptions=[]}. Waiting for 30000 ms before timing out
com.mongodb.MongoTimeoutException: Timed out after 30000 ms while waiting for a server that matches ReadPreferenceServerSelector{readPreference=ReadPreference{name=secondaryPreferred}}. Client view of cluster state is {type=UNKNOWN, servers=[]
at com.mongodb.connection.BaseCluster.createTimeoutException(BaseCluster.java:377)
at com.mongodb.connection.BaseCluster.selectServer(BaseCluster.java:104)
at com.mongodb.binding.ClusterBinding$ClusterBindingConnectionSource.<init>(ClusterBinding.java:75)
at com.mongodb.binding.ClusterBinding$ClusterBindingConnectionSource.<init>(ClusterBinding.java:71)
at com.mongodb.binding.ClusterBinding.getReadConnectionSource(ClusterBinding.java:63)
at com.mongodb.operation.OperationHelper.withConnection(OperationHelper.java:402)
at com.mongodb.operation.FindOperation.execute(FindOperation.java:510)
at com.mongodb.operation.FindOperation.execute(FindOperation.java:81)
at com.mongodb.Mongo.execute(Mongo.java:836)
at com.mongodb.Mongo$2.execute(Mongo.java:823)
at com.mongodb.FindIterableImpl$FindOperationIterable.first(FindIterableImpl.java:216)
at com.mongodb.FindIterableImpl.first(FindIterableImpl.java:156)
at com.eqianzhuang.commons.mongo.MongoDbUtils.findOne(MongoDbUtils.java:279)