java - Servlet中注入(inject)的bean中的NullPointerException

我有两个servlet：ServletUtils和ServletAditional。
ServletAditional使用ServletUtils中的一种称为isAllowed的方法来检查用户是否可以访问该网页。问题是，每次ServletAditional尝试使用此方法时，都会因bean UserCalculation导致的NullPointer异常而结束，该异常为null。
快速浏览调试模式下的servlet之后，我发现ServletUtils中的init方法从未调用过。我试图在登录时设置bean，甚至在Login init中强制init，但是bean仍然为空。
我的问题是，有没有一种方法可以初始化ServletUtils并设置Bean，而不必重新编写每个Servlet？

ServletUtils：

public class ServletUtils extends HttpServlet{
private static final long serialVersionUID = 1L;
private HttpServletRequest request;
private HttpServletResponse response;
private UserCalculation userCalculation;
protected User user;

public void setUserCalculation(UserCalculation userCalculation) {
    this.userCalculation = userCalculation;
}

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {
    //does nothing, not necessary
}

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    //does nothing, not necessary
}

@Override
public void init(ServletConfig config) {
    ApplicationContext ctx = WebApplicationContextUtils.getRequiredWebApplicationContext(config.getServletContext());
    this.userCalculation = (UserCalculation) ctx.getBean("UserCalculation");
}

public boolean isAllowed(int idRole) throws ServletException, IOException{
    User user=(User) getSessionAttribute("user");
    return userCalculation.checkRole(user, idRole);
}
}

ServletAditional：

public class ServletAdicional extends ServletUtils {
private static final long serialVersionUID = 1L;
private int role=1;
private AditionalService service;

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    setRequestResponse(request, response);

    if (!isLogedIn()){
        redirectLogin();
    }
    else if (isAllowed(role)){
        setDefaultAttributes();
        redirectTo("CalculateAditionals");
    }
    else{
        redirectToErrorPage();
    }
}
@Override
public void init(ServletConfig config) {
    ApplicationContext ctx = WebApplicationContextUtils.getRequiredWebApplicationContext(config.getServletContext());
    this.service = (AditionalService) ctx.getBean("AditionalService");
}
}

错误消息说：

Servlet.service() for servlet [aditionalReport] in context with path [/ElectronicaProject] threw exception
java.lang.NullPointerException
    at electronicaProject.servlets.ServletUtils.isAllowed(ServletUtils.java:82)
    at electronicaProject.servlets.ServletAditional.doGet(ServletAditional.java:31)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:936)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1004)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:312)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:744)

最佳答案

非常简单由于在两个类中都覆盖了init()方法，因此永远不会调用init()的ServletUtils方法，也永远不会初始化userCalculation。

进一步了解


Oracle Java Tutorial - Polymorphism
Oracle Java Tutorial - Overriding and Hiding Methods




尝试

public class ServletAdicional extends ServletUtils {
    @Override
    public void init(ServletConfig config) {
        super.init(config);
        ...
    }
}

让我们看一个简单的程序来理解它。

interface Init {
    public void init();
}

class ServletUtils implements Init {
    @Override
    public void init() {
        System.out.println("inside ServletUtils's init method");
    }
}

class ServletAdicional extends ServletUtils {
    @Override
    public void init() {
        System.out.println("inside ServletAdicional's init method");
    }
}

public static void main(String[] args) throws Exception {

    Init init = new ServletAdicional();
    init.init();
}

输出：

inside ServletAdicional's init method

一些要点：


还要在super.init(config);类中调用ServletUtils。
如果不是单例，请尝试避免在Servlet中使用实例成员，例如user，因为它被视为对Servlet的多次请求的共享对象。
如果您不想提供任何实现，则它们不会覆盖这些方法。
如果需要，不要将HttpServletRequest和HttpServletResponse作为Servlet的实例成员，而只需将其作为方法参数传递即可。