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Tracking Users with Sessions and Cookies

Feb 28, 2003

📄 Contents

␡

Using Hidden Controls
The Cookie Class
The HttpServletResponse Interface
Creating a Cookie
Reading a Cookie
Setting and Reading a Cookie in the Same Page
Using Sessions
Creating a Session
Setting Session Timeouts
Using Applications
Using Sessions, Applications, and JavaBeans
Summary
Q&A
Workshop

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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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