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Introduction to CGI Variables

lemonade

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1. Introduction to CGI Variables

If you come to Java servlets from traditional CGI, you are probably used to the idea of "CGI Variables". These are a somewhat eclectic collection of information about the request. Some are derived from the HTTP request line and headers (e.g. the part of the URI that came after the question mark and typically contains the form data, or the Content-Length header), some are derived from the socket itself (e.g. the name and IP address of the requesting host), and some are derived from server installation parameters (e.g. the mapping of URLs to actual paths).

2. Servlet Equivalent of Standard CGI Variables

Although it probably makes more sense to think of different sources of data (request data, server data, etc.) as distinct, experienced CGI programmers may find the following table useful. Assume that request is the HttpServletRequest supplied to the doGet and doPost methods.

CGI Variable	Meaning	Accessing it from doGet or doPost
AUTH_TYPE	If an Authorization header was supplied, this is the scheme specified (basic or digest)	request.getAuthType()
CONTENT_LENGTH	For POST requests only, the number of bytes of data sent.	Technically, the equivalent is String.valueOf(request.getContentLength()) (a String) but you´ll probably want to just call request.getContentLength(), which returns an int.
CONTENT_TYPE	MIME type of attached data, if specified.	request.getContentType()
DOCUMENT_ROOT	Path to directory corresponding to http://host/	getServletContext().getRealPath("/") Note that this was request.getRealPath("/") in older servlet specifications.
HTTP_XXX_YYY	Access to arbitrary HTTP headers	request.getHeader("Xxx-Yyy")
PATH_INFO	Path information attached to the URL. Since servlets, unlike standard CGI programs, can talk to the server, they don´t need to treat this separately. Path info could be sent as part of the regular form data.	request.getPathInfo()
PATH_TRANSLATED	The path information mapped to a real path on the server. Again, with servlets there is no need to have a special case for this.	request.getPathTranslated()
QUERY_STRING	For GET requests, this is the attached data as one big string, with values still URL-encoded. You rarely want the raw data in servlets; instead use request.getParameter to access individual parameters.	request.getQueryString()
REMOTE_ADDR	The IP address of the client that made the request, as a String. E.g. "192.9.48.9".	request.getRemoteAddr()
REMOTE_HOST	The fully qualified domain name (e.g. "java.sun.com") of the client that made the request. The IP address is returned if this cannot be determined.	request.getRemoteHost()
REMOTE_USER	If an Authorization header was supplied, the user part.	request.getRemoteUser()
REQUEST_METHOD	The request type, which is usually GET or POST, but is occasionally HEAD, PUT, DELETE, OPTIONS, or TRACE.	request.getMethod()
SCRIPT_NAME	Path to servlet.	request.getServletPath()
SERVER_NAME	Web server´s name	request.getServerName()
SERVER_PORT	Port server is listening on.	Technically, the equivalent is String.valueOf(request.getServerPort()), which returns a String. You´ll usually just want request.getServerPort(), which returns an int.
SERVER_PROTOCOL	Name and version used in the request line (e.g. HTTP/1.0 or HTTP/1.1).	request.getProtocol()
SERVER_SOFTWARE	Identifying information about the Web server	getServletContext().getServerInfo()

3. Example: Reading the CGI Variables

Here´s a servlet that creates a table showing the values of all the CGI variables other than HTTP_XXX_YYY, which are just the HTTP request headers shown in the previous section.

3.1 ShowCGIVariables.java

You can also download the source or try it on-line. Note: also uses ServletUtilities.java, shown earlier. package hall; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.*; /** Creates a table showing the values of all the CGI variables. *

* Part of tutorial on servlets and JSP that appears at * http://www.apl.jhu.edu/~hall/java/Servlet-Tutorial/ * 1999 Marty Hall; may be freely used or adapted. */ public class ShowCGIVariables extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String[][] variables = { { "AUTH_TYPE", request.getAuthType() }, { "CONTENT_LENGTH", String.valueOf(request.getContentLength()) }, { "CONTENT_TYPE", request.getContentType() }, { "DOCUMENT_ROOT", getServletContext().getRealPath("/") }, { "PATH_INFO", request.getPathInfo() }, { "PATH_TRANSLATED", request.getPathTranslated() }, { "QUERY_STRING", request.getQueryString() }, { "REMOTE_ADDR", request.getRemoteAddr() }, { "REMOTE_HOST", request.getRemoteHost() }, { "REMOTE_USER", request.getRemoteUser() }, { "REQUEST_METHOD", request.getMethod() }, { "SCRIPT_NAME", request.getServletPath() }, { "SERVER_NAME", request.getServerName() }, { "SERVER_PORT", String.valueOf(request.getServerPort()) }, { "SERVER_PROTOCOL", request.getProtocol() }, { "SERVER_SOFTWARE", getServletContext().getServerInfo() } }; String title = "Servlet Example: Showing CGI Variables"; out.println(ServletUtilities.headWithTitle(title) + "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1 ALIGN=CENTER>" + title + "</H1>\n" + "<TABLE BORDER=1 ALIGN=CENTER>\n" + "<TR BGCOLOR=\"#FFAD00\">\n" + "<TH>CGI Variable Name<TH>Value"); for(int i=0; i<variables.length; i++) { String varName = variables[i][0]; String varValue = variables[i][1]; if (varValue == null) varValue = "<I>Not specified</I>"; out.println("<TR><TD>" + varName + "<TD>" + varValue); } out.println("</TABLE></BODY></HTML>"); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }

3.2 ShowCGIVariables Output

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