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

Question

我使用 Servlet 的时间不长。 我遇到了这个问题，我无法终止自己。 我正在使用 Servlet 创建一个小型 Web 项目，并尝试将我的 Service 和 DAO 对象添加到 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)

我的 web.xml 文件：

<?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 侦听器类：

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) {

    }
}

登录控制器类：

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
         */
    }
}

当我试图从 ServletContext 中获取我的服务对象时，在这些行中我得到了 NPE：

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

我该怎么做，才能使这个 npe 异常消失并且我的 clientService 工作。 非常感谢。 ps我会阅读所有的建议和答案，希望你能帮助我解决这个问题。

Answer 1

在覆盖的init(ServletConfig)方法中初始化您的字段。 ServletConfig在实例初始化时不可用。

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

来自Servlet#init(ServletConfig)的javadoc

由 servlet 容器调用以向 servlet 指示该 servlet 正在投入使用。

servlet 容器在实例化 servlet 后只调用一次init方法。 在 servlet 可以接收任何请求之前， init方法必须成功完成。

init(ServletConfig)的GenericServlet实现的javadoc进一步指定

此实现存储它从 servlet 容器接收到的ServletConfig对象以供以后使用。 当覆盖这种形式的方法时，调用super.init(config) 。