刚开始学习mongodb，编写了一个简单的spring boot程序连接mongo
，使用如下配置可以正确连接到mongo：

spring.data.mongodb.host=10.159.14.19
spring.data.mongodb.port=9001
spring.data.mongodb.authentication-database=admin
spring.data.mongodb.database=esign
spring.data.mongodb.username=esign_write
spring.data.mongodb.password=NTc2OTQ1YjcwN2Z

但是使用下面的配置就会报错：

spring.data.mongodb.uri=mongodb://esign_write:NTc2OTQ1YjcwN2Z@10.159.14.19:9001/esign
spring.data.mongodb.authentication-database=admin

错误信息如下：

com.mongodb.MongoSecurityException: Exception authenticating MongoCredential{mechanism=null, userName='esign_write', source='esign', password=<hidden>, mechanismProperties={}}
    at com.mongodb.connection.SaslAuthenticator.authenticate(SaslAuthenticator.java:61) ~[mongodb-driver-core-3.2.2.jar:na]
    at com.mongodb.connection.DefaultAuthenticator.authenticate(DefaultAuthenticator.java:32) ~[mongodb-driver-core-3.2.2.jar:na]
    at com.mongodb.connection.InternalStreamConnectionInitializer.authenticateAll(InternalStreamConnectionInitializer.java:99) ~[mongodb-driver-core-3.2.2.jar:na]
    at com.mongodb.connection.InternalStreamConnectionInitializer.initialize(InternalStreamConnectionInitializer.java:44) ~[mongodb-driver-core-3.2.2.jar:na]
    at com.mongodb.connection.InternalStreamConnection.open(InternalStreamConnection.java:115) ~[mongodb-driver-core-3.2.2.jar:na]
    at com.mongodb.connection.DefaultServerMonitor$ServerMonitorRunnable.run(DefaultServerMonitor.java:128) ~[mongodb-driver-core-3.2.2.jar:na]
    at java.lang.Thread.run(Thread.java:745) [na:1.8.0_121]
Caused by: com.mongodb.MongoCommandException: Command failed with error 18: 'Authentication failed.' on server 10.159.14.19:9001. The full response is { "ok" : 0.0, "errmsg" : "Authentication failed.", "code" : 18, "codeName" : "AuthenticationFailed" }
    at com.mongodb.connection.CommandHelper.createCommandFailureException(CommandHelper.java:170) ~[mongodb-driver-core-3.2.2.jar:na]
    at com.mongodb.connection.CommandHelper.receiveCommandResult(CommandHelper.java:123) ~[mongodb-driver-core-3.2.2.jar:na]
    at com.mongodb.connection.CommandHelper.executeCommand(CommandHelper.java:32) ~[mongodb-driver-core-3.2.2.jar:na]
    at com.mongodb.connection.SaslAuthenticator.sendSaslStart(SaslAuthenticator.java:95) ~[mongodb-driver-core-3.2.2.jar:na]
    at com.mongodb.connection.SaslAuthenticator.authenticate(SaslAuthenticator.java:45) ~[mongodb-driver-core-3.2.2.jar:na]
    ... 6 common frames omitted

这个是什么原因？