A method expression is used to invoke an arbitrary public method. The method expressions differ from functions in the following ways:

Function	Method Expression
A function refers to static methods that return a value.	A method expression refers to non-static, arbitrary public methods on objects.
It is identified statically at translation time.	It is identified statically at run time.
The parameters and invocations of functions are specified as part of an EL expression.	The invocation of a method expression is specified in the tag attribute definition of the attribute.

The functions of a Transfer Object are as follows:

1. It reduces network traffic by aggregating fine-grained remote objects and representing them as a single call.
2. It reduces coupling between the tiers.
3. It reduces code duplication.
4. It carries multiple data elements across a tier.
5. It is used to transport data from an enterprise bean to its client.

The functions of a Business Delegate are as follows:

1. It hides the implementation details of a business service, such as lookup and access mechanisms.
2. It provides loose coupling between client and system’s business services.
3. It provides control and protection for the business service.
4. It acts as a proxy for the client to implement the remote service’s interface.
5. It throws application level exceptions and, if needed, caches results locally.
6. It receives a request from a controller, translates it, and forwards it to the business service.
7. It receives a response back from the business service, translates it, and forwards it to the controller.

An intercepting filter is a pluggable component that is used to modify incoming requests from a client to a servlet and send the response back to the client. Many filters can be chained together, so that the output of one filter becomes an input for the next filter. The filter life cycle is controlled by the container. They are added and removed by configuring tags in the deployment descriptor.

encodeRedirectURL

public java.lang.String encodeRedirectURL(java.lang.String url)

This method encodes the specified URL for use in the sendRedirect method or, if encoding is not needed, returns the URL unchanged. The implementation of this method includes the logic to determine whether the session ID needs to be encoded in the URL. Because the rules for making this determination can differ from those used to decide whether to encode a normal link, this method is seperate from the encodeURL method.

All URLs sent to the HttpServletResponse.sendRedirect method should be run through this method. Otherwise, URL rewriting cannot be used with browsers which do not support cookies.

Here, url – the url to be encoded and it returns the encoded URL if encoding is needed; the unchanged URL otherwise.

encodeURL

public java.lang.String encodeURL(java.lang.String url)

This method encodes the specified URL by including the session ID in it, or, if encoding is not needed, returns the URL unchanged. The implementation of this method includes the logic to determine whether the session ID needs to be encoded in the URL. For example, if the browser supports cookies, or session tracking is turned off, URL encoding is unnecessary.

For robust session tracking, all URLs emitted by a servlet should be run through this method. Otherwise, URL rewriting cannot be used with browsers which do not support cookies.

Here, url – the url to be encoded and it returns the encoded URL if encoding is needed; the unchanged URL otherwise.

sendRedirect

public void sendRedirect(java.lang.String location)throws java.io.IOException

This method sends a temporary redirect response to the client using the specified redirect location URL. This method can accept relative URLs; the servlet container must convert the relative URL to an absolute URL before sending the response to the client. If the location is relative without a leading ‘/’ the container interprets it as relative to the current request URI. If the location is relative with a leading ‘/’ the container interprets it as relative to the servlet container root.
If the response has already been committed, this method throws an IllegalStateException. After using this method, the response should be considered to be committed and should not be written to.

Here, location is the redirect location URL and it throws java.io.IOException. If an input or output exception occurs
IllegalStateException – If the response was committed.

The Form-based authentication is a type of authentication mechanism. It allows the developer to control the look and feel of the login authentication screens by customizing the login screen and error pages that an HTTP browser presents to the end user. The following actions occur when form-based authentication is declared:

1. A client requests for accessing a protected resource.
2. If the client is unauthenticated, the server redirects the client to a login page.
3. The client submits the login form to the server.
4. The server attempts to authenticate the user.
5. If authentication succeeds, the authenticated user’s principal is checked to ensure it is in a role that is authorized to access the resource. If the user is authorized, the server redirects the client to the resource using the stored URL path.
6. If authentication fails, the client is forwarded or redirected to an error page.

The doTrace() method allows a servlet to handle a TRACE request. It is called by the server via the service method. If the headers sent with the TRACE request, a TRACE is always returned to the client, so that they can be used in debugging. There’s no need to override this method. The following is the general format of the doTrace() method:

protected void doTrace(HttpServletRequest rq1, HttpServletResponse rsp1) throws ServletException, java.io.IOException

Here the parameter, rq1 is the HttpServletRequest object that contains the request the client made to the servlet. The parameter rsp1 is the HttpServletResponse object that contains the response the servlet returns to the client. This method will throw java.io.IOException if an input or output error occurs while the servlet is handling the TRACE request. If the request for the TRACE cannot be handled, it will throw the ServletException exception.

The doOptions() method is called by the server via the service method. It allows a servlet to handle a OPTIONS request. The OPTIONS request determines which HTTP methods the server supports and returns an appropriate header. The following is the general form of this method:

protected void doOptions(HttpServletRequest rq1, HttpServletResponse rsp1)
throws ServletException, java.io.IOException

Here, the parameter rq1 is the HttpServletRequest object that contains the request the client made of the servlet. The rsp1 is the HttpServletResponse object that contains the response the servlet returns to the client. If an input or output error occurs while the servlet is handling the OPTIONS request, it throws java.io.IOException. If the request for the OPTIONS cannot be handled, the ServletException exception is thrown.

The getLastModified() method is used to return the time the HttpServletRequest object was last modified, in milliseconds since midnight January 1, 1970 GMT. This method returns a negative number if the time is unknown. This method will be overriden by those Servlets that support HTTP GET requests and can quickly determine their last modification time. This method allows the browser and proxy caches work more effectively, reducing the load on server and network resources. The following is the general form of the method:

protected long getLastModified(HttpServletRequest rq1)

Here, the parameter rq1 is the HttpServletRequest object that is sent to the servlet. This method returns a long integer specifying the time the HttpServletRequest object was last modified.

The doDelete() method allows a servlet to handle a DELETE request. It is called by the server via the service method. This method belongs to the HttpServlet class. 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. Those operations that are required by DELETE can have side effects for which the users can be held accountable. It may be useful to save a copy of the affected URL in the temporary storage, while using this method. The doDelete() method returns an HTTP “Bad Request” message, if the HTTP DELETE request is incorrectly formatted. The following is the general format of this method:

protected void doDelete(HttpServletRequest rq1, HttpServletResponse rsp1) throws ServletException, java.io.IOException

Here, the parameter rq1 represents the HttpServletRequest object that contains the request the client made of the servlet. The parameter rsp1 represents the HttpServletResponse object that contains the response the servlet returns to the client. It will throw java.io.IOException if an input or output error occurs while the servlet is handling the DELETE request. It will throw ServletException if the request for the DELETE cannot be handled.

The doPut() method allows a servlet to handle a PUT request. It is called by the server via the service method. This method belongs to the HttpServlet class. The PUT operation allows a client to place a file on the server and is similar to sending a file by FTP. This method does not need to be either safe or idempotent. Those operations that are performed by the doPut() method can have side effects for which the user can be held accountable. While using this method, it is useful to save a copy of the affected URL in the temporary storage. This method returns an HTTP “Bad Request” message if the HTTP PUT request is incorrectly formatted. The following is the general format of this method:

protected void doPut(HttpServletRequest rq1, HttpServletResponse rsp1) throws ServletException, java.io.IOException

Here, the parameter rq1 represents the HttpServletRequest object that contains the request the client made of the servlet. The parameter rsp1 represents the HttpServletResponse object that contains the response the servlet returns to the client. This method will throw the java.io.IOException if an input or output error occurs while the servlet is handling the PUT request. It will throw the ServletException if the request for the PUT request cannot be handled.

The following table summarizes the differences between the Get and Post requests:

Get Request	Post Request
Get is the default HTTP method.	POST is not the default HTTP method.
It is triggered when the HTML tag specifies a method=”GET”.	It is triggered when the HTML tag specifies a method=”POST”.
Its target resource can be static or dynamic.	Its target resource can be dynamic only.
The query string is appended as part of the URL after the “?” character.	The query string is passed as part of the body, at the end of the request header.
It does not have a body.	It has a body.
It is used for just retrieving data from a requested URL.	It is used to update information.
It can be bookmarked.	It cannot be bookmarked.

The POST request is used to send the data to be processed. The PUT request is used to store a resource under the request URL. The Request-URI generates the fundamental difference between the POST and PUT requests. The URI in a POST request identifies the resource that will handle the enclosed entity. The resource might be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. However, the URI in a PUT request identifies the entity enclosed with the request and the user agent knows what URI is intended and the server MUST NOT attempt to apply the request to some other resource.

HEAD is a request method that is used when the client wants to see only the header information of a response. The header information can be the content type, the content length, the date, etc. It is identical to the GET method, except that the server does not return a message-body in the response. The information contained in the HTTP headers in response to a HEAD request is identical to the information sent in response to a GET request. This allows a client to obtain the information about a resource without actually transferring the resource itself.

HTTP 1.1 defines eight request methods as follows:

Each of the above XXX request methods (except CONNECT) has a corresponding doXXX method in the HttpServlet class.

GET is the default method of an Http request. In this method, the request parameters are included in the URL along with the resource path. This method is used when it is required to get a small amount of information. The server limits the number of characters in this method. The information sent in the URL is exposed and hence cannot be used to send sensitive information such as passwords. This method is very fast and is used in cases where it is required to get a small amount of information. This method is invoked when a user clicks on a hyperlink, when a form with method=GET or with no method name is submitted, or when the query parameters are appended with the resource path.

The element is used to modify the XML declaration property of the output of a JSP document or tag file. This action can be used only in the JSP documents and in tag files that are in XML syntax. The XML declaration property in JSP2.0 can only be modified. All attributes of the are optional but some of them depend upon other attributes. This attribute contains the following attributes:

1. omit-xml-declaration: It is used to specify a value of true or yes and omits the XML declaration.
2. doctype-root-element: It indicates the root element of the XML document in the DOCTYPE.
3. doctype-system: It specifies that a DOCTYPE is generated in output and gives SYSTEM literal.
4. doctype-public: It specifies the value for the Public ID of the generated DOCTYPE. It can be specified if the doctype-system is specified.

The requestDestroyed() method of the ServletRequestListener interface takes a ServletRequestEvent object and notifies that the request is about to go out of scope of the Web application.

The syntax of the requestDestroyed() method is as follows:

requestDestroyed(ServletRequestEvent sre)

The following are the main parts of the pluggable API for resolving expressions:

1. ValueExpression Class: It defines a value expression.
2. MethodExpression Class: It defines a method expression.
3. ELResolver Class: It defines a mechanism for resolving expressions.
4. Set of ELResolver implementations: Each implementation is responsible for resolving expressions that reference a particular type of object or property
5. ELContext Object: It saves state relating to EL resolution, holds references to EL resolvers, and contains context objects needed by the underlying technology to resolve expressions