title: Spring application start error: sources must not be empty
body: <p>I'm following a Spring tutorial, and when I tried to start the spring application I got the following error:</p>
  
  <blockquote>
    <p>2016-01-20 23:18:15.907  INFO 5271 --- [           main]
    o.s.boot.SpringApplication               : Starting SpringApplication
    v1.3.1.RELEASE on  ... </p>
    
    <p>2016-01-20 23:18:15.911  INFO 5271 --- [        main]
    o.s.boot.SpringApplication               : No active profile set,
    falling back to default profiles: default </p>
    
    <p>2016-01-20 23:18:15.918 ERROR 5271 --- [           main]
    o.s.boot.SpringApplication            : Application startup failed
    java.lang.IllegalArgumentException: Sources must not be empty     at
    org.springframework.util.Assert.notEmpty(Assert.java:276)
    ~[spring-core-4.2.4.RELEASE.jar:4.2.4.RELEASE]    at
    org.springframework.boot.SpringApplication.doRun(SpringApplication.java:352)
    [spring-boot-1.3.1.RELEASE.jar:1.3.1.RELEASE]     at
    org.springframework.boot.SpringApplication.run(SpringApplication.java:305)
    [spring-boot-1.3.1.RELEASE.jar:1.3.1.RELEASE]     at
    org.springframework.boot.SpringApplication.run(SpringApplication.java:1124)
    [spring-boot-1.3.1.RELEASE.jar:1.3.1.RELEASE]     at
    org.springframework.boot.SpringApplication.main(SpringApplication.java:1140)
    [spring-boot-1.3.1.RELEASE.jar:1.3.1.RELEASE]</p>
  </blockquote>
  
  <p>What is this "Sources must not be empty" error?</p>
  
  <p>I am using an Eclipse - Maven project for this tutorial project and I have updated the project. I have also cleaned and rebuilt but I still got this error.</p>