做了个简单的springboot项目，一开始所有运行正常，之后写了一天的代码，再运行相同的代码，就一直报以下异常：

Whitelabel Error Page

This application has no explicit mapping for /error, so you are seeing this as a fallback.

Fri Apr 07 13:02:50 CST 2017
There was an unexpected error (type=Internal Server Error, status=500).
No message available

采用debug运行测试，控制台提示：

java.lang.NullPointerException: null
at org.springframework.boot.web.servlet.view.velocity.EmbeddedVelocityToolboxView.setContextToolbox(EmbeddedVelocityToolboxView.java:65) ~[spring-boot-1.3.8.RELEASE.jar:1.3.8.RELEASE]
at org.springframework.boot.web.servlet.view.velocity.EmbeddedVelocityToolboxView.createVelocityContext(EmbeddedVelocityToolboxView.java:54) ~[spring-boot-1.3.8.RELEASE.jar:1.3.8.RELEASE]
at org.springframework.web.servlet.view.velocity.VelocityView.renderMergedTemplateModel(VelocityView.java:288) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.servlet.view.AbstractTemplateView.renderMergedOutputModel(AbstractTemplateView.java:167) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:303) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1246) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1029) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:973) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:895) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:967) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:858) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:622) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:843) ~[spring-webmvc-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.0.37.jar:8.0.37]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:89) ~[spring-web-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77) ~[spring-web-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:121) ~[spring-web-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.2.8.RELEASE.jar:4.2.8.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212) ~[tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106) [tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502) [tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141) [tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) [tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88) [tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528) [tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1100) [tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:687) [tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520) [tomcat-embed-core-8.0.37.jar:8.0.37]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476) [tomcat-embed-core-8.0.37.jar:8.0.37]
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [na:1.8.0_51]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [na:1.8.0_51]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.0.37.jar:8.0.37]
at java.lang.Thread.run(Unknown Source) [na:1.8.0_51]

2017-04-07 13:02:50.306 ERROR 6276 --- [nio-8080-exec-1] org.apache.velocity : ResourceManager : unable to find resource 'error.html' in any resource loader.

既然是空指针异常，说明是我的代码哪里有问题，所以查看代码是否真的有空指针异常：

代码如下：

@Controller
public class HomeController {

    @RequestMapping(path = {"/index"}, method = {RequestMethod.GET, RequestMethod.POST})
    public String index(){
        return "index";
    }
}
测试运行的代码就这么一小部分，不应该有空指针异常的，但是，它又提示我，是空指针异常，所以，我很无奈的，一个个排查其他的代码是否有异常。认真看了其他代码，发现没有什么错误。
于是，开始百度，得知，可能是spring那些注解没有写上去，导致底层报错。又将所有的spring注解之类的代码全部检查了遍，确保都正确后，还是无效。
最后，采用最笨的方法，将代码一行行的重新敲，每写一个代码，就全部测试一遍，终于，发现了导致空指针异常的真正bug所在：
在配置文件application.properties:

spring.velocity.suffix=.html
spring.datasource.url=jdbc:mysql://localhost:3306/wenda?useUnicode=true&characterEncoding=utf8&useSSL=false
spring.datasource.username=root
spring.datasource.password=1234
mybatis.config-location=classpath:mybatis-config.xml
spring.velocity.toolbox-config-location=toolbox.xml

引入了最后一个文件，但是，在项目中忘记添加了，因为暂时还不需要这个功能，所以，由于项目中缺失这个配置文件，它找不到，因此，报空指针异常。
总结：
1.空指针异常，查看控制台的输出，控制台中没有一行提示我自己写的代码哪里出错误了，因此，一定不是我自己 写的代码的问题，而是，spring底层机制的问题，这个时候就需要去思考原理，
接下来，排除代码的可能性，那就剩下的是spring配置文件的问题了，仔细查看配置文件之间的联系，确保正确无误后，就应该没啥问题了。
这个bug虽然很简单，但是，自己也花费了差不多一天的时间才解决。主要是，空指针异常，让我思路跑偏了，一直在找自己代码哪里有错。根本原因，一方面自己编程不细心，另一方面，自己写代码经验
还不够丰富，控制台输出的异常信息还不是很会利用。
2.whitelabel error page异常一定是有原因的，比如，访问路径不对，解析不对，注解忘记引入等。另外，对于springboot项目，

WendaApplication这个类要放在groupId对应的包下，其他自己新建的包都应该是它的一个子包。注意，目录结构的关联。

Whitelabel Error Page异常

Whitelabel Error Page

Whitelabel Error Page异常的相关教程结束。

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