为什么从ServletContext上传数据会阻止加载servlet？

-1

我注入Servlet上下文对象来处理数据库。我使用我的执行ServletContextListener。但是，当我尝试使用我的servlet时，然后得到HTTP Status [500] – [Internal Server Error]。为什么从ServletContext上传数据会阻止加载servlet？

我ServletContextListener：

@WebListener 
public class ContextListener implements ServletContextListener { 

    @Override 
    public void contextInitialized(ServletContextEvent servletContextEvent) { 

     final ServletContext servletContext = servletContextEvent.getServletContext(); 

     servletContext.setAttribute("db", new DBJointHandler("database_scripts", "authentication_database")); 

    } 
    ... 
}

而且我的servlet：

public class AddUserServlet extends HttpServlet { 

    private DBJoint db = (DBJoint) getServletContext().getAttribute("db"); 

    @Override 
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 

     db.somethingToDo(); 
     ... 
    } 
}

，并在通话AddUserServlet时间：

HTTP Status [500] – [Internal Server Error] 

Type Exception Report 

Message Error instantiating servlet class [ru.pravvich.servlets.AddUserServlet] 

Description The server encountered an unexpected condition that prevented it from fulfilling the request. 

Exception 

javax.servlet.ServletException: Error instantiating servlet class [ru.pravvich.servlets.AddUserServlet] 
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) 
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) 
    org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624) 
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) 
    org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799) 
    org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) 
    org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:861) 
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455) 
    org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) 
    java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) 
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) 
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) 
    java.lang.Thread.run(Thread.java:745) 
Root Cause 

java.lang.NullPointerException 
    javax.servlet.GenericServlet.getServletContext(GenericServlet.java:123) 
    ru.pravvich.servlets.AddUserServlet.<init>(AddUserServlet.java:18) 
    sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) 
    sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) 
    sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) 
    java.lang.reflect.Constructor.newInstance(Constructor.java:423) 
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) 
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) 
    org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624) 
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) 
    org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799) 
    org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) 
    org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:861) 
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455) 
    org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) 
    java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) 
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) 
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) 
    java.lang.Thread.run(Thread.java:745)

这在调用的时候只发生通过浏览器形式的Servlet。如果我清理(DBJoint) getServletContext().getAttribute("DB") servlet的工作没有问题。但我需要这个对象与DB一起工作...

为什么？什么不对？如何解决这个问题？谢谢。

来源

2017-06-02 Pavel

ServletContext在构造servlet期间未定义。

你需要推迟初始化的db场，直到该servlet正在初始化：

public class AddUserServlet extends HttpServlet { 

    private DBJoint db; 

    @Override 
    void init(ServletConfig config) { 
     ServletContext servletContext = config.getServletContext(); 
     db = servletContext.getAttribute("db"); 
    } 

    ... 

}

来源

2017-06-02 14:09:37

为什么从ServletContext上传数据会阻止加载servlet？

回答

相关问题