NullPointerException in bean injected in Servlet

Question

I have two servlets: ServletUtils and ServletAditional.
ServletAditional uses a method from ServletUtils called isAllowed to check whether the user can access the webpage or not. The problem is that every time ServletAditional tries to use this method ends in a NullPointer exception caused by the bean UserCalculation, which is null.
After having a quick look a the servlet in debug mode, I found that the init method in ServletUtils is never called. I tried to set the bean at login and even forcing the init in the Login init, but the bean is still null.
My question is, is there a way to initialize ServletUtils and set the bean without having to re write every 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");
}
}

Error message says:

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)

Answer 1

It's very simple. As you have overridden init() method in both the classes hence init() method of ServletUtils is never called and userCalculation is never initialized.

Read more about

• Oracle Java Tutorial - Polymorphism

• Oracle Java Tutorial - Overriding and Hiding Methods

---

Try

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

---

Let's have a look at simple program to understand it.

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();
}

output:

inside ServletAdicional's init method

---

Some Points:

• Call super.init(config); in ServletUtils class as well.
• Try to avoid instance members in Servlet such as user if it's not a singleton because it's treated as shared object for multiple requests to the Servlet.
• If you don't want to provide any implementation they don't override these methods.
• Don't make HttpServletRequest and HttpServletResponse as instance members of the Servlet if needed then simply pass it as method arguments.