what methods of HttpServlet are abstract?

[Sekhar Kadiyala]

Hi,

Confusion # 1

I understand that HttpServlet class is an abstract class. But when i was going thru the specifications, i couldn't identify which methods of it are abstract.

Also i came accross the following statement in the specs.

A subclass of HttpServlet must override at least one method, usually one of these:

doGet, if the servlet supports HTTP GET requests
doPost, for HTTP POST requests
doPut, for HTTP PUT requests
doDelete, for HTTP DELETE requests
init and destroy, to manage resources that are held for the life of the servlet
getServletInfo, which the servlet uses to provide information about itself

Why is such requirement? From my SCJP exp., if i have an abstract class and i am sub-classing it, then i must implement all the abstract methods otherwise delcare the sub-class also abstract.

For example sake, i can compile my class which extends HttpServlet, even if i don't override doGet method. It means doGet method is not a abstract method.

I am assuming same is the case with rest of the methods too. In such case, why do i need to implement at least one of these methods? what rule is governing this?

Confusion # 2

Assuming doGet method is not an abstract method, HttpServlet class should provide implementation for the same. Is this correct?

Thank you in advacne!

Sekhar -- SCWCD 1.3 scheduled for September 10th!!

[Bahadar Khan]

You are right, this is confusing. If you look at the javadocs for HttpServlet it is declared as abstract still its methods are protected and none of those is abstracted. The answer to this is the Web Container like Tomcat etc, they are supposed to provide the default implementation for this class so that we should be able to use it. Hence all the methods mentioned here are not our responsibility to override. Obviously there is no point in writing servlet if we don't intercept the requests from clients hence it is required to at least implement one of the methods.

Secondly, if you look at the sibling classes of HttpSerlet namely HttpServletRequest and HttpServletResponse etc, all of these are interfaces and we don't implement those, but still we make use of these. The answer lies in the earlier explanations as these too are implemented by Server provider.

[Bryan Basham]

Hello Sekhar,

The HttpServlet class is abstract, but all of its methods are implemented. There is nothing inherently wrong with this, but as you pointed out a little confusing. Java has no requirement that an abstract class must have one or more abstract methods. It just means that the class cannot be instantiated by the JVM (and enforced by the complier).

The reason why the doXyz methods are implemented is that the spec writers didn't want to force the servlet developer to have to implement all of these: doGet, doPost, doPut, doDelete, and so on. Most of these HTTP methods are of no interest to the casual servlet developer. So... the HttpServlet class implements all of these methods and the typical behavior is to send an 501 (Not Implemented) HTTP status code. Therefore, (as the javadocs state) you must override one or more of these methods to get the behaviour you are trying to achieve. So... the HttpServlet implementations are merely a convience for the servlet developer. Make sense?


-Bryan

[Sekhar Kadiyala]

Thank you, Brayn. Things are little more clear now. But as a continuation of this discussion, if the methods are implemented (for example doGet) then, if my servlet doesn't have doGet method and i am trying to access the servlet thru address bar (which will invoke the HTTP method GET), then technically i shouldn't be getting the error as my servlet extends HttpServlet. But it does throw the error saying

doGet method not found

.

This is where i think i am still confused. If doGet method is already implemenetd, then i am assuming the default implementation wouldn't do anything, so even if i call my servlet without defining doGet, i should get no output, but not the error.

[Bahadar Khan]

Thanks Bryan.

You have corrected me too. I was under the wrong assumption. And sorry Sekhar, I didn't mean to provide you with the wrong information. It was what I knew about this.

Thanks again Bryan.

[Yi Meng]

in weblogic 8.1, the default implementation is as follows:

 // Decompiled by DJ v3.7.7.81 Copyright 2004 Atanas Neshkov  Date: 09/09/2004 11:41:52 AM
// Home Page : <a href="http://members.fortunecity.com/neshkov/dj.html" target="_blank" rel="nofollow">http://members.fortunecity.com/neshkov/dj.html</a>  - Check often for new version!
// Decompiler options: packimports(3) 
// Source File Name:   HttpServlet.java
 
package javax.servlet.http;
 
import java.io.*;
import java.lang.reflect.Method;
import java.text.MessageFormat;
import java.util.Enumeration;
import java.util.ResourceBundle;
import javax.servlet.*;
 
// Referenced classes of package javax.servlet.http:
//            NoBodyResponse, HttpServletRequest, HttpServletResponse
 
public abstract class HttpServlet extends GenericServlet
    implements Serializable
{
 
    public HttpServlet()
    {
    }
 
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_get_not_supported");
        if(protocol.endsWith("1.1"))
            resp.sendError(405, msg);
        else
            resp.sendError(400, msg);
    }
 
    protected long getLastModified(HttpServletRequest req)
    {
        return -1L;
    }
 
    protected void doHead(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        NoBodyResponse response = new NoBodyResponse(resp);
        doGet(req, response);
        response.setContentLength();
    }
 
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_post_not_supported");
        if(protocol.endsWith("1.1"))
            resp.sendError(405, msg);
        else
            resp.sendError(400, msg);
    }
 
    protected void doPut(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_put_not_supported");
        if(protocol.endsWith("1.1"))
            resp.sendError(405, msg);
        else
            resp.sendError(400, msg);
    }
 
    protected void doDelete(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_delete_not_supported");
        if(protocol.endsWith("1.1"))
            resp.sendError(405, msg);
        else
            resp.sendError(400, msg);
    }
 
    private Method[] getAllDeclaredMethods(Class c)
    {
        if(c.getName().equals("javax.servlet.http.HttpServlet"))
            return null;
        int j = 0;
        Method parentMethods[] = getAllDeclaredMethods(c.getSuperclass());
        Method thisMethods[] = c.getDeclaredMethods();
        if(parentMethods != null)
        {
            Method allMethods[] = new Method[parentMethods.length + thisMethods.length];
            for(int i = 0; i < parentMethods.length; i++)
            {
                allMethods[i] = parentMethods[i];
                j = i;
            }
 
            for(int i = ++j; i < thisMethods.length + j; i++)
                allMethods[i] = thisMethods[i - j];
 
            return allMethods;
        } else
        {
            return thisMethods;
        }
    }
 
    protected void doOptions(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        Method methods[] = getAllDeclaredMethods(getClass());
        boolean ALLOW_GET = false;
        boolean ALLOW_HEAD = false;
        boolean ALLOW_POST = false;
        boolean ALLOW_PUT = false;
        boolean ALLOW_DELETE = false;
        boolean ALLOW_TRACE = true;
        boolean ALLOW_OPTIONS = true;
        for(int i = 0; i < methods.length; i++)
        {
            Method m = methods[i];
            if(m.getName().equals("doGet"))
            {
                ALLOW_GET = true;
                ALLOW_HEAD = true;
            }
            if(m.getName().equals("doPost"))
                ALLOW_POST = true;
            if(m.getName().equals("doPut"))
                ALLOW_PUT = true;
            if(m.getName().equals("doDelete"))
                ALLOW_DELETE = true;
        }
 
        String allow = null;
        if(ALLOW_GET && allow == null)
            allow = "GET";
        if(ALLOW_HEAD)
            if(allow == null)
                allow = "HEAD";
            else
                allow = allow + ", HEAD";
        if(ALLOW_POST)
            if(allow == null)
                allow = "POST";
            else
                allow = allow + ", POST";
        if(ALLOW_PUT)
            if(allow == null)
                allow = "PUT";
            else
                allow = allow + ", PUT";
        if(ALLOW_DELETE)
            if(allow == null)
                allow = "DELETE";
            else
                allow = allow + ", DELETE";
        if(ALLOW_TRACE)
            if(allow == null)
                allow = "TRACE";
            else
                allow = allow + ", TRACE";
        if(ALLOW_OPTIONS)
            if(allow == null)
                allow = "OPTIONS";
            else
                allow = allow + ", OPTIONS";
        resp.setHeader("Allow", allow);
    }
 
    protected void doTrace(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String CRLF = "\r\n";
        String responseString = "TRACE " + req.getRequestURI() + " " + req.getProtocol();
        for(Enumeration reqHeaderEnum = req.getHeaderNames(); reqHeaderEnum.hasMoreElements() :wink: 
        {
            String headerName = (String)reqHeaderEnum.nextElement();
            responseString = responseString + CRLF + headerName + ": " + req.getHeader(headerName);
        }
 
        responseString = responseString + CRLF;
        int responseLength = responseString.length();
        resp.setContentType("message/http");
        resp.setContentLength(responseLength);
        ServletOutputStream out = resp.getOutputStream();
        out.print(responseString);
        out.close();
    }
 
    protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String method = req.getMethod();
        if(method.equals("GET"))
        {
            long lastModified = getLastModified(req);
            if(lastModified == -1L)
            {
                doGet(req, resp);
            } else
            {
                long ifModifiedSince = req.getDateHeader("If-Modified-Since");
                if(ifModifiedSince < (lastModified / 1000L) * 1000L)
                {
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else
                {
                    resp.setStatus(304);
                }
            }
        } else
        if(method.equals("HEAD"))
        {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);
        } else
        if(method.equals("POST"))
            doPost(req, resp);
        else
        if(method.equals("PUT"))
            doPut(req, resp);
        else
        if(method.equals("DELETE"))
            doDelete(req, resp);
        else
        if(method.equals("OPTIONS"))
            doOptions(req, resp);
        else
        if(method.equals("TRACE"))
        {
            doTrace(req, resp);
        } else
        {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object errArgs[] = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(501, errMsg);
        }
    }
 
    private void maybeSetLastModified(HttpServletResponse resp, long lastModified)
    {
        if(resp.containsHeader("Last-Modified"))
            return;
        if(lastModified >= 0L)
            resp.setDateHeader("Last-Modified", lastModified);
    }
 
    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException
    {
        HttpServletRequest request;
        HttpServletResponse response;
        try
        {
            request = (HttpServletRequest)req;
            response = (HttpServletResponse)res;
        }
        catch(ClassCastException classcastexception)
        {
            throw new ServletException("non-HTTP request or response");
        }
        service(request, response);
    }
 
    private static final String METHOD_DELETE = "DELETE";
    private static final String METHOD_HEAD = "HEAD";
    private static final String METHOD_GET = "GET";
    private static final String METHOD_OPTIONS = "OPTIONS";
    private static final String METHOD_POST = "POST";
    private static final String METHOD_PUT = "PUT";
    private static final String METHOD_TRACE = "TRACE";
    private static final String HEADER_IFMODSINCE = "If-Modified-Since";
    private static final String HEADER_LASTMOD = "Last-Modified";
    private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings";
    private static ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.http.LocalStrings");
 
} 

[Bryan Basham]

Originally posted by Sekhar Kadiyala:
But as a continuation of this discussion, if the methods are implemented (for example doGet) then, if my servlet doesn't have doGet method and i am trying to access the servlet thru address bar (which will invoke the HTTP method GET), then technically i shouldn't be getting the error as my servlet extends HttpServlet. But it does throw the error saying doGet method not found

OK, there are "compiler errors" and "HTTP errors". In your scenario, you will not get a compiler error (because HttpServlet implements doGet), but you will get an HTTP error when you invoke the servlet from the browser. Is this the behavior that you are seeing? If so, then have I sufficiently explained why this is happening? If not, then please let me know what you see in the browser.

HTH,
Bryan

[Sekhar Kadiyala]

OK, there are "compiler errors" and "HTTP errors".

This point has made it all clear. Thank you very much!