java ee: class not found despite valid import?!

问题

I use jersey to create some sort of RESTful API. To get information about the current request, I use the HttpServletRequest Object. I am able to compile the project without problems or errors, but when I run it, I get the following error:

[INFO] --- exec-maven-plugin:1.2.1:java (default-cli) @ backend ---
[WARNING] 
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:297)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NoClassDefFoundError: javax/servlet/http/HttpServletRequest
    at java.lang.Class.getDeclaredMethods0(Native Method)
    at java.lang.Class.privateGetDeclaredMethods(Class.java:2570)
    at java.lang.Class.getDeclaredMethods(Class.java:1855)
    at org.glassfish.jersey.server.model.IntrospectionModeller$2.run(IntrospectionModeller.java:236)
    at java.security.AccessController.doPrivileged(Native Method)
    at org.glassfish.jersey.server.model.IntrospectionModeller.getAllDeclaredMethods(IntrospectionModeller.java:230)
    at org.glassfish.jersey.server.model.IntrospectionModeller.checkForNonPublicMethodIssues(IntrospectionModeller.java:170)
    at org.glassfish.jersey.server.model.IntrospectionModeller.doCreateResourceBuilder(IntrospectionModeller.java:118)
    at org.glassfish.jersey.server.model.IntrospectionModeller.access$000(IntrospectionModeller.java:80)
    at org.glassfish.jersey.server.model.IntrospectionModeller$1.call(IntrospectionModeller.java:111)
    at org.glassfish.jersey.server.model.IntrospectionModeller$1.call(IntrospectionModeller.java:108)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:315)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:297)
    at org.glassfish.jersey.internal.Errors.processWithException(Errors.java:255)
    at org.glassfish.jersey.server.model.IntrospectionModeller.createResourceBuilder(IntrospectionModeller.java:108)
    at org.glassfish.jersey.server.model.Resource.from(Resource.java:744)
    at org.glassfish.jersey.server.ApplicationHandler.initialize(ApplicationHandler.java:400)
    at org.glassfish.jersey.server.ApplicationHandler.access$500(ApplicationHandler.java:163)
    at org.glassfish.jersey.server.ApplicationHandler$3.run(ApplicationHandler.java:323)
    at org.glassfish.jersey.internal.Errors$2.call(Errors.java:289)
    at org.glassfish.jersey.internal.Errors$2.call(Errors.java:286)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:315)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:297)
    at org.glassfish.jersey.internal.Errors.processWithException(Errors.java:286)
    at org.glassfish.jersey.server.ApplicationHandler.<init>(ApplicationHandler.java:320)
    at org.glassfish.jersey.server.ApplicationHandler.<init>(ApplicationHandler.java:285)
    at org.glassfish.jersey.grizzly2.httpserver.GrizzlyHttpContainer.<init>(GrizzlyHttpContainer.java:331)
    at org.glassfish.jersey.grizzly2.httpserver.GrizzlyHttpServerFactory.createHttpServer(GrizzlyHttpServerFactory.java:116)
    at com.getbro.api.Main.startServer(Main.java:32)
    at com.getbro.api.Main.main(Main.java:42)
    ... 6 more
Caused by: java.lang.ClassNotFoundException: javax.servlet.http.HttpServletRequest
    at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    ... 36 more

I think it's strange, because I import this class per maven:

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>servlet-api</artifactId>
    <version>2.5</version>
    <scope>provided</scope>
</dependency>

... so it should work like expected, but it does not!

回答1:

You import it in provided scope. That way you just promise don't worry, it'll be there, and nothing is added to war. Use scope compile to actually import dependancies.

回答2:

Your dependency has a provided scope, which means that the artifact will be available for compiling, but not during runtime. More info on this topic in the Maven documentation or this StackOverflow question.

That's the right thing to do, because the Servlet API is not something you want to have within your WAR, EAR, or whatever package you're building. Instead, this servlet-api JAR will be provided by your application server (Tomcat, Jetty...).

If you're running your web service project from Eclipse, you need to tell him which server runtime it has to use. (If you're using an IDE other than Eclipse, you'll have to do something similar):

• Window > Preferences > Server > Runtime environments.
• If there is already a runtime environment defined for your server (i.e. Tomcat 6.x if that's what you're using), cancel this window. Otherwise, create it by using the Add... button.

Once there is a runtime environment defined:

• Right click on your project > Build path > Configure build path....
• Libraries tab > Add library.
• Select the appropiate server runtime and finish.

来源：https://stackoverflow.com/questions/25338377/java-ee-class-not-found-despite-valid-import