javax.servlet.http: abstract public class: HttpServlet

javax.servlet.http
abstract public class: HttpServlet [javadoc | source]

java.lang.Object
   javax.servlet.GenericServlet
      javax.servlet.http.HttpServlet

All Implemented Interfaces:
Serializable, ServletConfig, Servlet

Provides an abstract class to be subclassed to create an HTTP servlet suitable for a Web site. 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

There's almost no reason to override the service method. service handles standard HTTP requests by dispatching them to the handler methods for each HTTP request type (the doXXX methods listed above).

Likewise, there's almost no reason to override the doOptions and doTrace methods.

Servlets typically run on multithreaded servers, so be aware that a servlet must handle concurrent requests and be careful to synchronize access to shared resources. Shared resources include in-memory data such as instance or class variables and external objects such as files, database connections, and network connections. See the Java Tutorial on Multithreaded Programming for more information on handling multiple threads in a Java program.

author: Various -

version: $ - Version$

Constructor:
public HttpServlet() { } Does nothing, because this is an abstract class.

Constructor:

 public HttpServlet() {

}

Does nothing, because this is an abstract class.

Method from javax.servlet.http.HttpServlet Summary:
doDelete, doGet, doHead, doOptions, doPost, doPut, doTrace, getLastModified, service, service

Methods from javax.servlet.GenericServlet:
destroy, getInitParameter, getInitParameterNames, getServletConfig, getServletContext, getServletInfo, getServletName, init, init, log, log, service

Methods from java.lang.Object:
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method from javax.servlet.http.HttpServlet Detail:
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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } Called by the server (via the `service` method) to allow a servlet to handle a DELETE request. The DELETE operation allows a client to remove a document or Web page from the server. This method does not need to be either safe or idempotent. Operations requested through DELETE can have side effects for which users can be held accountable. When using this method, it may be useful to save a copy of the affected URL in temporary storage. If the HTTP DELETE request is incorrectly formatted, `doDelete` returns an HTTP "Bad Request" message.
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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } Called by the server (via the `service` method) to allow a servlet to handle a GET request. Overriding this method to support a GET request also automatically supports an HTTP HEAD request. A HEAD request is a GET request that returns no body in the response, only the request header fields. When overriding this method, read the request data, write the response headers, get the response's writer or output stream object, and finally, write the response data. It's best to include content type and encoding. When using a `PrintWriter` object to return the response, set the content type before accessing the `PrintWriter` object. The servlet container must write the headers before committing the response, because in HTTP the headers must be sent before the response body. Where possible, set the Content-Length header (with the javax.servlet.ServletResponse#setContentLength method), to allow the servlet container to use a persistent connection to return its response to the client, improving performance. The content length is automatically set if the entire response fits inside the response buffer. When using HTTP 1.1 chunked encoding (which means that the response has a Transfer-Encoding header), do not set the Content-Length header. The GET method should be safe, that is, without any side effects for which users are held responsible. For example, most form queries have no side effects. If a client request is intended to change stored data, the request should use some other HTTP method. The GET method should also be idempotent, meaning that it can be safely repeated. Sometimes making a method safe also makes it idempotent. For example, repeating queries is both safe and idempotent, but buying a product online or modifying data is neither safe nor idempotent. If the request is incorrectly formatted, `doGet` returns an HTTP "Bad Request" message.
protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { NoBodyResponse response = new NoBodyResponse(resp); doGet(req, response); response.setContentLength(); } Receives an HTTP HEAD request from the protected `service` method and handles the request. The client sends a HEAD request when it wants to see only the headers of a response, such as Content-Type or Content-Length. The HTTP HEAD method counts the output bytes in the response to set the Content-Length header accurately. If you override this method, you can avoid computing the response body and just set the response headers directly to improve performance. Make sure that the `doHead` method you write is both safe and idempotent (that is, protects itself from being called multiple times for one HTTP HEAD request). If the HTTP HEAD request is incorrectly formatted, `doHead` returns an HTTP "Bad Request" message.
protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { Method[] methods = getAllDeclaredMethods(this.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) if (allow==null) allow=METHOD_GET; if (ALLOW_HEAD) if (allow==null) allow=METHOD_HEAD; else allow += ", " + METHOD_HEAD; if (ALLOW_POST) if (allow==null) allow=METHOD_POST; else allow += ", " + METHOD_POST; if (ALLOW_PUT) if (allow==null) allow=METHOD_PUT; else allow += ", " + METHOD_PUT; if (ALLOW_DELETE) if (allow==null) allow=METHOD_DELETE; else allow += ", " + METHOD_DELETE; if (ALLOW_TRACE) if (allow==null) allow=METHOD_TRACE; else allow += ", " + METHOD_TRACE; if (ALLOW_OPTIONS) if (allow==null) allow=METHOD_OPTIONS; else allow += ", " + METHOD_OPTIONS; resp.setHeader("Allow", allow); } Called by the server (via the `service` method) to allow a servlet to handle a OPTIONS request. The OPTIONS request determines which HTTP methods the server supports and returns an appropriate header. For example, if a servlet overrides `doGet`, this method returns the following header: `Allow: GET, HEAD, TRACE, OPTIONS` There's no need to override this method unless the servlet implements new HTTP methods, beyond those implemented by HTTP 1.1.
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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } Called by the server (via the `service` method) to allow a servlet to handle a POST request. The HTTP POST method allows the client to send data of unlimited length to the Web server a single time and is useful when posting information such as credit card numbers. When overriding this method, read the request data, write the response headers, get the response's writer or output stream object, and finally, write the response data. It's best to include content type and encoding. When using a `PrintWriter` object to return the response, set the content type before accessing the `PrintWriter` object. The servlet container must write the headers before committing the response, because in HTTP the headers must be sent before the response body. Where possible, set the Content-Length header (with the javax.servlet.ServletResponse#setContentLength method), to allow the servlet container to use a persistent connection to return its response to the client, improving performance. The content length is automatically set if the entire response fits inside the response buffer. When using HTTP 1.1 chunked encoding (which means that the response has a Transfer-Encoding header), do not set the Content-Length header. This method does not need to be either safe or idempotent. Operations requested through POST can have side effects for which the user can be held accountable, for example, updating stored data or buying items online. If the HTTP POST request is incorrectly formatted, `doPost` returns an HTTP "Bad Request" message.
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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } Called by the server (via the `service` method) to allow a servlet to handle a PUT request. The PUT operation allows a client to place a file on the server and is similar to sending a file by FTP. When overriding this method, leave intact any content headers sent with the request (including Content-Length, Content-Type, Content-Transfer-Encoding, Content-Encoding, Content-Base, Content-Language, Content-Location, Content-MD5, and Content-Range). If your method cannot handle a content header, it must issue an error message (HTTP 501 - Not Implemented) and discard the request. For more information on HTTP 1.1, see RFC 2616 . This method does not need to be either safe or idempotent. Operations that `doPut` performs can have side effects for which the user can be held accountable. When using this method, it may be useful to save a copy of the affected URL in temporary storage. If the HTTP PUT request is incorrectly formatted, `doPut` returns an HTTP "Bad Request" message.
protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { int responseLength; String CRLF = "\r\n"; String responseString = "TRACE "+ req.getRequestURI()+ " " + req.getProtocol(); Enumeration reqHeaderEnum = req.getHeaderNames(); while( reqHeaderEnum.hasMoreElements() ) { String headerName = (String)reqHeaderEnum.nextElement(); responseString += CRLF + headerName + ": " + req.getHeader(headerName); } responseString += CRLF; responseLength = responseString.length(); resp.setContentType("message/http"); resp.setContentLength(responseLength); ServletOutputStream out = resp.getOutputStream(); out.print(responseString); out.close(); return; } Called by the server (via the `service` method) to allow a servlet to handle a TRACE request. A TRACE returns the headers sent with the TRACE request to the client, so that they can be used in debugging. There's no need to override this method.
protected long getLastModified(HttpServletRequest req) { return -1; } Returns the time the `HttpServletRequest` object was last modified, in milliseconds since midnight January 1, 1970 GMT. If the time is unknown, this method returns a negative number (the default). Servlets that support HTTP GET requests and can quickly determine their last modification time should override this method. This makes browser and proxy caches work more effectively, reducing the load on server and network resources.
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); if (method.equals(METHOD_GET)) { long lastModified = getLastModified(req); if (lastModified == -1) { // servlet doesn't support if-modified-since, no reason // to go through further expensive logic doGet(req, resp); } else { long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE); if (ifModifiedSince < (lastModified / 1000 * 1000)) { // If the servlet mod time is later, call doGet() // Round down to the nearest second for a proper compare // A ifModifiedSince of -1 will always be less maybeSetLastModified(resp, lastModified); doGet(req, resp); } else { resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED); } } } else if (method.equals(METHOD_HEAD)) { long lastModified = getLastModified(req); maybeSetLastModified(resp, lastModified); doHead(req, resp); } else if (method.equals(METHOD_POST)) { doPost(req, resp); } else if (method.equals(METHOD_PUT)) { doPut(req, resp); } else if (method.equals(METHOD_DELETE)) { doDelete(req, resp); } else if (method.equals(METHOD_OPTIONS)) { doOptions(req,resp); } else if (method.equals(METHOD_TRACE)) { doTrace(req,resp); } else { // // Note that this means NO servlet supports whatever // method was requested, anywhere on this server. // String errMsg = lStrings.getString("http.method_not_implemented"); Object[] errArgs = new Object[1]; errArgs[0] = method; errMsg = MessageFormat.format(errMsg, errArgs); resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg); } } Receives standard HTTP requests from the public `service` method and dispatches them to the `do`XXX methods defined in this class. This method is an HTTP-specific version of the javax.servlet.Servlet#service method. There's no need to override this method.
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { HttpServletRequest request; HttpServletResponse response; try { request = (HttpServletRequest) req; response = (HttpServletResponse) res; } catch (ClassCastException e) { throw new ServletException("non-HTTP request or response"); } service(request, response); } Dispatches client requests to the protected `service` method. There's no need to override this method.

Method from javax.servlet.http.HttpServlet Detail:

 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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
}

service

This method does not need to be either safe or idempotent. Operations requested through DELETE can have side effects for which users can be held accountable. When using this method, it may be useful to save a copy of the affected URL in temporary storage.

If the HTTP DELETE request is incorrectly formatted, doDelete returns an HTTP "Bad Request" message.

 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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
}

service

Overriding this method to support a GET request also automatically supports an HTTP HEAD request. A HEAD request is a GET request that returns no body in the response, only the request header fields.

When overriding this method, read the request data, write the response headers, get the response's writer or output stream object, and finally, write the response data. It's best to include content type and encoding. When using a PrintWriter object to return the response, set the content type before accessing the PrintWriter object.

The servlet container must write the headers before committing the response, because in HTTP the headers must be sent before the response body.

Where possible, set the Content-Length header (with the javax.servlet.ServletResponse#setContentLength method), to allow the servlet container to use a persistent connection to return its response to the client, improving performance. The content length is automatically set if the entire response fits inside the response buffer.

When using HTTP 1.1 chunked encoding (which means that the response has a Transfer-Encoding header), do not set the Content-Length header.

The GET method should be safe, that is, without any side effects for which users are held responsible. For example, most form queries have no side effects. If a client request is intended to change stored data, the request should use some other HTTP method.

The GET method should also be idempotent, meaning that it can be safely repeated. Sometimes making a method safe also makes it idempotent. For example, repeating queries is both safe and idempotent, but buying a product online or modifying data is neither safe nor idempotent.

If the request is incorrectly formatted, doGet returns an HTTP "Bad Request" message.

 protected  void doHead(HttpServletRequest req,
    HttpServletResponse resp) throws ServletException, IOException {
        NoBodyResponse response = new NoBodyResponse(resp);
        doGet(req, response);
        response.setContentLength();
}

Receives an HTTP HEAD request from the protected service method and handles the request. The client sends a HEAD request when it wants to see only the headers of a response, such as Content-Type or Content-Length. The HTTP HEAD method counts the output bytes in the response to set the Content-Length header accurately.

If you override this method, you can avoid computing the response body and just set the response headers directly to improve performance. Make sure that the doHead method you write is both safe and idempotent (that is, protects itself from being called multiple times for one HTTP HEAD request).

If the HTTP HEAD request is incorrectly formatted, doHead returns an HTTP "Bad Request" message.

 protected  void doOptions(HttpServletRequest req,
    HttpServletResponse resp) throws ServletException, IOException {
        Method[] methods = getAllDeclaredMethods(this.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)
            if (allow==null) allow=METHOD_GET;
        if (ALLOW_HEAD)
            if (allow==null) allow=METHOD_HEAD;
            else allow += ", " + METHOD_HEAD;
        if (ALLOW_POST)
            if (allow==null) allow=METHOD_POST;
            else allow += ", " + METHOD_POST;
        if (ALLOW_PUT)
            if (allow==null) allow=METHOD_PUT;
            else allow += ", " + METHOD_PUT;
        if (ALLOW_DELETE)
            if (allow==null) allow=METHOD_DELETE;
            else allow += ", " + METHOD_DELETE;
        if (ALLOW_TRACE)
            if (allow==null) allow=METHOD_TRACE;
            else allow += ", " + METHOD_TRACE;
        if (ALLOW_OPTIONS)
            if (allow==null) allow=METHOD_OPTIONS;
            else allow += ", " + METHOD_OPTIONS;
        resp.setHeader("Allow", allow);
}

service

doGet

Allow: GET, HEAD, TRACE, OPTIONS

There's no need to override this method unless the servlet implements new HTTP methods, beyond those implemented by HTTP 1.1.

 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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
}

service

The servlet container must write the headers before committing the response, because in HTTP the headers must be sent before the response body.

When using HTTP 1.1 chunked encoding (which means that the response has a Transfer-Encoding header), do not set the Content-Length header.

This method does not need to be either safe or idempotent. Operations requested through POST can have side effects for which the user can be held accountable, for example, updating stored data or buying items online.

If the HTTP POST request is incorrectly formatted, doPost returns an HTTP "Bad Request" message.

 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(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
}

service

When overriding this method, leave intact any content headers sent with the request (including Content-Length, Content-Type, Content-Transfer-Encoding, Content-Encoding, Content-Base, Content-Language, Content-Location, Content-MD5, and Content-Range). If your method cannot handle a content header, it must issue an error message (HTTP 501 - Not Implemented) and discard the request. For more information on HTTP 1.1, see RFC 2616 .

This method does not need to be either safe or idempotent. Operations that doPut performs can have side effects for which the user can be held accountable. When using this method, it may be useful to save a copy of the affected URL in temporary storage.

If the HTTP PUT request is incorrectly formatted, doPut returns an HTTP "Bad Request" message.

 protected  void doTrace(HttpServletRequest req,
    HttpServletResponse resp) throws ServletException, IOException {
        
        int responseLength;
        String CRLF = "\r\n";
        String responseString = "TRACE "+ req.getRequestURI()+
            " " + req.getProtocol();
        Enumeration reqHeaderEnum = req.getHeaderNames();
        while( reqHeaderEnum.hasMoreElements() ) {
            String headerName = (String)reqHeaderEnum.nextElement();
            responseString += CRLF + headerName + ": " +
                req.getHeader(headerName); 
        }
        responseString += CRLF;
        responseLength = responseString.length();
        resp.setContentType("message/http");
        resp.setContentLength(responseLength);
        ServletOutputStream out = resp.getOutputStream();
        out.print(responseString);        
        out.close();
        return;
}

service

 protected long getLastModified(HttpServletRequest req) {
        return -1;
}

HttpServletRequest

Servlets that support HTTP GET requests and can quickly determine their last modification time should override this method. This makes browser and proxy caches work more effectively, reducing the load on server and network resources.

 protected  void service(HttpServletRequest req,
    HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                doGet(req, resp);
            } else {
                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                if (ifModifiedSince <  (lastModified / 1000 * 1000)) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }
        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);
        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);
        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);        
        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);
        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);
        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);
        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
}

service

do

XXX

javax.servlet.Servlet#service

 public  void service(ServletRequest req,
    ServletResponse res) throws ServletException, IOException {
        HttpServletRequest  request;
        HttpServletResponse response;
        try {
            request = (HttpServletRequest) req;
            response = (HttpServletResponse) res;
        } catch (ClassCastException e) {
            throw new ServletException("non-HTTP request or response");
        }
        service(request, response);
}

service