使用 getServletConfig().getServletContext().getAttribute() (Java) 时获取 NPE

Question

I am using Servlets from not a long time. And i am getting this problem, which i can't terminate myself. I am using Servlets to create a small web project, and i tried to add my Service and DAO objects to ServletContext.

HTTP Status 500 - Error instantiating servlet class com.taxi.service.controller.LoginController

type Exception report

message Error instantiating servlet class com.taxi.service.controller.LoginController

description The server encountered an internal error that prevented it from fulfilling this request.

exception

javax.servlet.ServletException: Error instantiating servlet class com.taxi.service.controller.LoginController
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:957)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:423)
    org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1079)
    org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:620)
    org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:318)
    java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.lang.Thread.run(Thread.java:745)
root cause

java.lang.NullPointerException
    javax.servlet.GenericServlet.getServletContext(GenericServlet.java:123)
    com.taxi.service.controller.LoginController.<init>(LoginController.java:16)
    sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    java.lang.reflect.Constructor.newInstance(Constructor.java:526)
    java.lang.Class.newInstance(Class.java:379)
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:957)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:423)
    org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1079)
    org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:620)
    org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:318)
    java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.lang.Thread.run(Thread.java:745)

My web.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">
    <display-name>Jean taxi-service</display-name>

    <listener>
        <listener-class>com.taxi.service.utils.ApplicationContext</listener-class>
    </listener>

    <servlet>
        <servlet-name>InitController</servlet-name>
        <servlet-class>com.taxi.service.controller.InitController</servlet-class>
    </servlet>

    <servlet>
        <servlet-name>LoginController</servlet-name>
        <servlet-class>com.taxi.service.controller.LoginController</servlet-class>
    </servlet>

    <servlet>
        <servlet-name>RegistrationController</servlet-name>
        <servlet-class>com.taxi.service.controller.RegistrationController</servlet-class>
    </servlet>

    <servlet>
        <servlet-name>PrivateAreaController</servlet-name>
        <servlet-class>com.taxi.service.controller.PrivateAreaController</servlet-class>
    </servlet>

    <servlet>
        <servlet-name>OrderController</servlet-name>
        <servlet-class>com.taxi.service.controller.OrderController</servlet-class>
    </servlet>

    <servlet>
        <servlet-name>ReviewController</servlet-name>
        <servlet-class>com.taxi.service.controller.OrderController</servlet-class>
    </servlet>

    <servlet>
        <servlet-name>AdminController</servlet-name>
        <servlet-class>com.taxi.service.controller.AdminController</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>InitController</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>LoginController</servlet-name>
        <url-pattern>/login</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>RegistrationController</servlet-name>
        <url-pattern>/registration</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>OrderController</servlet-name>
        <url-pattern>/orderCreation</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>ReviewController</servlet-name>
        <url-pattern>/review</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>PrivateAreaController</servlet-name>
        <url-pattern>/privateArea</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>AdminController</servlet-name>
        <url-pattern>/adminPanel</url-pattern>
    </servlet-mapping>

    <!-- <error-page>
        <location>/WEB-INF/pages/error.jsp</location>
    </error-page>
    -->

    <session-config>
        <session-timeout>45</session-timeout>
    </session-config>

    <resource-ref>
        <description>DB Connection</description>
        <res-ref-name>jdbc/order_board</res-ref-name>
        <res-type>javax.sql.DataSource</res-type>
        <res-auth>Container</res-auth>
    </resource-ref>

    <welcome-file-list>
        <welcome-file>/index.jsp</welcome-file>
    </welcome-file-list>

    <login-config>
        <auth-method>BASIC</auth-method>
    </login-config>

</web-app>

ServletContext listener class:

public class ApplicationContext implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        servletContextEvent.getServletContext().setAttribute("clientDao", new ClientDaoImpl(DataBaseUtil.getConnectionPoolInstance()));
        servletContextEvent.getServletContext().setAttribute("orderDao", new ClientDaoImpl(DataBaseUtil.getConnectionPoolInstance()));
        servletContextEvent.getServletContext().setAttribute("clientService", new ClientServiceImpl());
        servletContextEvent.getServletContext().setAttribute("orderService", new OrderServiceImpl());
    }

    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {

    }
}

Login controller class:

public class LoginController extends HttpServlet {

    private ClientService clientService = (ClientServiceImpl) getServletConfig().getServletContext().getAttribute("clientService");

    @Override
    public void doGet(HttpServletRequest request, HttpServletResponse response){
       ...
    }

    @Override
    public void doPost(HttpServletRequest request, HttpServletResponse response{
        /**
         *...something doing with clientService
         */
    }
}

When i am trying to get my Service object from ServletContext, which a were added, in these line i am getting NPE:

private ClientService clientService = (ClientServiceImpl) getServletConfig().getServletContext().getAttribute("clientService");

What should i do, to make this npe exception gone and my clientService work. Thanks a lot. ps I will read all the proposes and asnwers, and hope you can help me in this problem.

Answer 1

Initialize your field in an overriden init(ServletConfig) method. The ServletConfig is not available at instance initialization time.

@Override
public void init(ServletConfig config) throws ServletException {
    super.init(config);
    this.clientService = config.getServletContext().getAttribute("clientService");
}

From the javadoc of Servlet#init(ServletConfig)

Called by the servlet container to indicate to a servlet that the servlet is being placed into service.

The servlet container calls the init method exactly once after instantiating the servlet. The init method must complete successfully before the servlet can receive any requests.

The javadoc of the GenericServlet implementation of init(ServletConfig) further specifies

This implementation stores the ServletConfig object it receives from the servlet container for later use. When overriding this form of the method, call super.init(config) .