无法在首次请求时加载资源

Question

So this is really weird to me. I have this spring application I am writing and when I start the server and navigate to http://localhost/ims/resources/index.html for the first time it gives me the following error:

SEVERE: Servlet.service() for servlet [appServlet] in context with path [/ims] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
java.lang.NullPointerException
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:848)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:936)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:827)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:812)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:936)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1004)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:722)

However if I refresh the page I don't have any problems. It only happens on the very first request. This is what my resource config looks like:

@Configuration
@EnableWebMvc
@ComponentScan(basePackages = "com.mypackage.ims.web")
public class IMSAppConfig extends WebMvcConfigurerAdapter
{
    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry)
    {
        registry.addResourceHandler("/resources/**").addResourceLocations("ims/resources/");
    }

    ....
}

Any ideas why this might be happening?

Answer 1

You need to give the base package scanning details and other details in context.xml file. So that the configuration will be loaded once the project is deployed.

But in your case the package to search for the mapping urls is known to the spring only at the exectution time, So only after the program executes for the first time the spring maps down the urls and when you refresh (hit the server for the 2nd time) the url is found.