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The HttpServletResponse Interface

To create cookies, you need to use the addCookie method of javax.servlet.http.HttpServletResponse interface. What's a Java interface? You can think of an interface as a class that defines method names, but not their code. You can't create objects directly using an interface. Instead, you must base a class on that interface—the interface only defines what data members and methods that class will have. As we'll see in Day 11, "Creating More Powerful JavaBeans," using interfaces is as close as you can come in Java to basing a class on more than one other class (in object-oriented programming terms, interfaces represent Java's support for multiple inheritance, as we'll see in Day 13, "Creating More Powerful Servlets").

So where does that leave you? Fortunately, the server has done all the work you'll need here—one of the built-in objects available to you in your JSP code is the response object (see "The JSP Programming Environment" in Day 2, "Handling Data and Operators"), and this is a pre-built object based on the javax.servlet.http.HttpServletResponse interface.

Just as you use the request object to see what data the user has sent to you, you can use the response object to send data back to the user. In this case, you'll use this object to send cookies back to the browser, which will store them. You can see the fields of the javax.servlet.http.HttpServletResponse interface in Table 7.2, and the methods of this interface in Table 7.3. Because the response object is built on this interface, it shares all the methods of the interface, and you'll see how to put them to work in the next topic, creating your own cookies with the addCookie method. You use the various field values in Table 7.2 if you want to send a status report back to the browser, as with the sendError or setStatus methods (which we won't do here).

NOTE

The convention for fields is that if its name is in capital letters, it's a read-only constant whose value doesn't change.

Table 7.2 HttpServletResponse Fields

Method

Does This

int SC_ACCEPTED

Indicates that a request was accepted for processing, but was not completed.

int SC_BAD_GATEWAY

Indicates that the HTTP server received an invalid response from a server it consulted when acting as a proxy or gateway.

int SC_BAD_REQUEST

Indicates the request sent by the client was incorrect.

int SC_CONFLICT

Indicates that the request cannot be completed because of a conflict with the current state of the resource.

int SC_CONTINUE

Indicates the client can continue.

int SC_CREATED

Indicates the request succeeded and created a new resource on the server.

int SC_EXPECTATION_FAILED

Indicates that the server cannot meet the request in the Expect request header.

int SC_FORBIDDEN

Indicates the server cannot complete the request.

int SC_GATEWAY_TIMEOUT

Indicates that the server did not receive a response from a gateway or proxy server in time.

int SC_GONE

Indicates that the resource is no longer available.

int SC_HTTP_VERSION_NOT_SUPPORTED

Indicates that the server does not support the HTTP protocol version used in the request.

int SC_INTERNAL_SERVER_ERROR

Indicates an error inside the HTTP server.

int SC_LENGTH_REQUIRED

Indicates that the request cannot be handled without a specified content length.

int SC_METHOD_NOT_ALLOWED

Indicates that the method specified in the Request-Line is not allowed.

int SC_MOVED_PERMANENTLY

Indicates that the resource has moved to a new location.

int SC_MOVED_TEMPORARILY

Indicates that the resource has temporarily moved to another location.

int SC_MULTIPLE_CHOICES

Indicates that the resource corresponds to any one of a set.

int SC_NO_CONTENT

Indicates that the request was successful but there was no new content to return.

int SC_NON_AUTHORITATIVE_INFORMATION

Indicates that the meta information sent by the browser did not come from the server.

int SC_NOT_ACCEPTABLE

Indicates that the resource identified by the request was not acceptable.

int SC_NOT_FOUND

Indicates that the requested resource was not available.

int SC_NOT_IMPLEMENTED

Indicates the HTTP server does not support the request.

int SC_NOT_MODIFIED

Indicates that a GET operation found that the resource was available and not modified.

int SC_OK

Indicates the request succeeded.

int SC_PARTIAL_CONTENT

Indicates that the server has fulfilled the partial GET request.

int SC_PAYMENT_REQUIRED

Reserved for future use.

int SC_PRECONDITION_FAILED

Indicates that the precondition given in a request header field failed.

int SC_PROXY_AUTHENTICATION_REQUIRED

Indicates that the client must authenticate itself with the proxy.

int SC_REQUEST_ENTITY_TOO_LARGE

Indicates that the request entity is larger than the server can process.

int SC_REQUEST_TIMEOUT

Indicates that the request timed out.

int SC_REQUEST_URI_TOO_LONG

Indicates that the Request-URI field is too long.

int SC_REQUESTED_RANGE_NOT_SATISFIABLE

Indicates that the server cannot serve the requested data range.

int SC_RESET_CONTENT

Indicates that the agent will reset the display of the document.

int SC_SEE_OTHER

Indicates that the response to the request can be found under a different URI.

int SC_SERVICE_UNAVAILABLE

Indicates that the HTTP server is overloaded.

int SC_SWITCHING_PROTOCOLS

Indicates the server is switching protocols.

int SC_TEMPORARY_REDIRECT

Indicates that the requested resource is temporarily at a different URI.

int SC_UNAUTHORIZED

Indicates that the request needs HTTP authentication.

int SC_UNSUPPORTED_MEDIA_TYPE

Indicates that the request is in a format not supported by the requested resource.

int SC_USE_PROXY

Indicates that the requested resource must be accessed through the proxy.


Table 7.3 HttpServletResponse Methods

Method

Does This

void addCookie(Cookie cookie)

Adds the given cookie to the response.

void addDateHeader(java.lang.String name, long date)

Adds a response header with the given name and date.

void addHeader(java.lang.String name, java.lang.String value)

Adds a response header with the given name and value.

void addIntHeader(java.lang.String name, int value)

Adds a response header with the given name and integer value.

boolean containsHeader (java.lang.String name)

Returns a Boolean value indicating if a response header has been set.

java.lang.String encodeRedirectUrl (java.lang.String url)

Deprecated. As of servlet specification version 2.1, use encodeRedirectURL (String url) instead.

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

Encodes the specified URL for use in the sendRedirect method.

java.lang.String encodeUrl (java.lang.String url)

Deprecated. As of servlet specification version 2.1, use encodeURL(String url) instead.

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

Encodes the specified URL by including the session ID in it.

void sendError(int sc)

Sends an error response to the client using the given status field value.

void sendError(int sc, java.lang.String msg)

Sends an error response to the client using the given field value and text.

void sendRedirect (java.lang.String location)

Sends a temporary redirect response to the browser using the given URL.

void setDateHeader(java.lang.String name, long date)

Sets a response header with the given name and date.

void setHeader(java.lang.String name, java.lang.String value)

Sets a response header with the given name and value.

void setIntHeader(java.lang.String name, int value)

Sets a response header with the given name and integer value.

void setStatus(int sc)

Sets the status code for this response.

void setStatus (int sc, java.lang.String sm)

Deprecated. As of servlet specification version 2.1, use setStatus(int) to send an error with a description.


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