运行 java 类时，Tomcat 服务器不会在 Eclipse 中启动。 引起：java.lang.NoClassDefFoundError：org/json/JSONException

Question

When trying to run my java class in Eclipse, the Tomcat server won't start. There are several errors about java.lang.NoClassDefFoundError: org/json/JSONException and java.lang.ClassNotFoundException: org.json.JSONException. I have imported org.json.JSONException in my project, and have added org.json.jar in my project build path, as an external jar. Org.json.jar is also included in my CLASSPATH environment variable. Any advice is greatly appreciated.

The console error in Eclipse when attempting to start the server states:

SEVERE: A child container failed during start
java.util.concurrent.ExecutionException: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/URL6]]
    at java.util.concurrent.FutureTask.report(Unknown Source)
    at java.util.concurrent.FutureTask.get(Unknown Source)
    at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:931)
    at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:841)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
    at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1412)
    at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1402)
    at java.util.concurrent.FutureTask.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/URL6]]
    at org.apache.catalina.util.LifecycleBase.handleSubClassException(LifecycleBase.java:440)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:198)
    ... 6 more
Caused by: java.lang.NoClassDefFoundError: org/json/JSONException
    at java.lang.Class.getDeclaredFields0(Native Method)
    at java.lang.Class.privateGetDeclaredFields(Unknown Source)
    at java.lang.Class.getDeclaredFields(Unknown Source)
    at org.apache.catalina.util.Introspection.getDeclaredFields(Introspection.java:110)
    at org.apache.catalina.startup.WebAnnotationSet.loadFieldsAnnotation(WebAnnotationSet.java:269)
    at org.apache.catalina.startup.WebAnnotationSet.loadApplicationServletAnnotations(WebAnnotationSet.java:137)
    at org.apache.catalina.startup.WebAnnotationSet.loadApplicationAnnotations(WebAnnotationSet.java:69)
    at org.apache.catalina.startup.ContextConfig.applicationAnnotationsConfig(ContextConfig.java:328)
    at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:779)
    at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:299)
    at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:123)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5063)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183)
    ... 6 more
Caused by: java.lang.ClassNotFoundException: org.json.JSONException
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1358)
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1180)
    ... 19 more

Project Build Configuration

CLASSPATH

Answer 1

Open the project's Deployment Assembly property page and make sure the jar is set to be deployed as part of the app. The CLASSPATH environment variable doesn't affect Tomcat, and you should stop relying on it in general.

Answer 2

for me, what worked was this...

in the servlet program, I omitted the forward slash "/" in the annotation declaration

---

@WebServlet("/MyServlet") //correct✔️

public class MyServlet extends HttpServlet{ . . .}

---

@WebServlet("MyServlet") //wrong❌

public class MyServlet extends HttpServlet{ . . .}