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SCWCD : HttpServletRequest Interface
Using the HttpServletRequest interface, write code
to retrieve HTML form parameters from
the request, retrieve HTTP request header information, or retrieve cookies from the request.
HTTP Protocol Parameters.
Request parameters for the servlet are the strings sent by the client to a servlet
container as part of its request. When the request is an
HttpServletRequest object, the container populates the parameters
from the URI query string and POST-ed data.
The parameters are stored as a set of name-value pairs. Multiple parameter
values CAN exist for any given parameter name. The following methods of the
ServletRequest interface are available to access parameters:
getParameter
Returns the value of a request parameter as a String,
or null if the parameter does not exist. Request
parameters are extra information sent with the request. For HTTP
servlets, parameters are contained in the query string or
posted form data.
You should only use this method when you are sure the parameter has only
ONE value. If the parameter might have MORE than one value, use
getParameterValues(String).
If you use this method with a multivalued parameter, the value returned is
equal to the FIRST value in the array returned by
getParameterValues.
If the parameter data was sent in the request body, such as occurs with an
HTTP POST request, then reading the body directly
via getInputStream() or getReader()
can interfere with the execution of this method.
getParameterNames
Returns an Enumeration of String
objects containing the names of the parameters contained in this request.
If the request has no parameters, the method returns an
EMPTY Enumeration.
getParameterValues
Returns an array of String objects containing all
of the values the given request parameter has, or null
if the parameter does not exist. If the parameter has a single
value, the array has a length of 1.
getParameterMap
Returns an immutable java.util.Map containing parameter
names as keys and parameter values as map values. The keys in the parameter
map are of type String. The values in the parameter
map are of type String array.
public interface ServletRequest {
public java.lang.String getParameter(java.lang.String name);
public java.util.Enumeration getParameterNames();
public java.lang.String[] getParameterValues(java.lang.String name);
public java.util.Map getParameterMap();
}
The getParameterValues method returns an array of
String objects containing all the parameter values associated
with a parameter name. The value returned from the getParameter
method must be the FIRST value in the array of String objects
returned by getParameterValues. The getParameterMap
method returns a java.util.Map of the parameter of the request,
which contains names as keys and parameter values as map values.
public void doPost(HttpServletRequest request, HttpServletResponse res)
throws IOException, ServletException {
Enumeration e = request.getParameterNames();
PrintWriter out = res.getWriter ();
while (e.hasMoreElements()) {
String name = (String)e.nextElement();
String value = request.getParameter(name);
out.println(name + " = " + value);
}
}
Data from the query string and the post body are aggregated into the request
parameter set. Query string data is presented BEFORE post body data. For example,
if a request is made with a query string of a=hello and a post body of
a=goodbye&a=world, the resulting parameter set would be
ordered a=(hello, goodbye, world).
The following are the conditions that must be met before post FORM data will
be populated to the parameter set:
The request is an HTTP or HTTPS request.
The HTTP method is POST.
The content type is application/x-www-form-urlencoded.
The servlet has made an initial call of any of the 'getParameter' family
of methods on the request object.
Headers.
A servlet can access the headers of an HTTP request through the following methods
of the HttpServletRequest interface:
getHeader
Returns the value of the specified request header as a
String. If the request did not include a header of
the specified name, this method returns null. If
there are multiple headers with the same name, this method returns the first
head in the request. The header name is case insensitive. You can use this
method with any request header.
getHeaders
Returns all the values of the specified request header as an
Enumeration of String objects.
Some headers, such as Accept-Language can be sent
by clients as several headers each with a different value rather than
sending the header as a comma separated list. If the request did not
include any headers of the specified name, this method returns an
EMPTY Enumeration. The header name is case INSENSITIVE.
You can use this method with any request header.
getHeaderNames
Returns an enumeration of all the header names this request contains. If the
request has no headers, this method returns an empty enumeration.
public interface HttpServletRequest {
public java.lang.String getHeader(java.lang.String name);
public java.util.Enumeration getHeaders(java.lang.String name);
public java.util.Enumeration getHeaderNames();
}
Headers may contain String representations of
int or Date data. The
following convenience methods of the HttpServletRequest
interface provide access to header data in a one of these formats:
getIntHeader
Returns the value of the specified request header as an int. If the request
does not have a header of the specified name, this method returns -1. If the
header cannot be converted to an integer, this method throws a
NumberFormatException.
The header name is case INSENSITIVE.
getDateHeader
Returns the value of the specified request header as a long value that represents
a Date object. Use this method with headers that contain
dates, such as If-Modified-Since.
The date is returned as the number of milliseconds since January 1, 1970
GMT. The header name is case insensitive.
If the request did not have a header of the specified name, this method returns
-1. If the header can't be converted to a date, the method throws an
IllegalArgumentException.
public interface HttpServletRequest {
public int getIntHeader(java.lang.String name);
public long getDateHeader(java.lang.String name);
}
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
Enumeration e = request.getHeaderNames();
while (e.hasMoreElements()) {
String name = (String)e.nextElement();
String value = request.getHeader(name);
out.println(name + " = " + value);
}
}
Cookies.
The HttpServletRequest interface provides the
getCookies method to obtain an array of cookies that are
present in the request. This method returns null if no
cookies were sent.
The cookies are data sent from the client to the server
on every request that the client makes. Typically, the only information that the
client sends back as part of a cookie is the cookie name and the cookie value.
Other cookie attributes that can be set when the cookie is sent to the browser,
such as comments, are not typically returned. Several cookies might have the
same name but different path attributes.
public interface HttpServletRequest {
public Cookie[] getCookies();
}
package javax.servlet.http;
public class Cookie implements java.lang.Cloneable {
...
public Cookie(java.lang.String name, java.lang.String value);
public java.lang.String getName();
public java.lang.String getPath();
public java.lang.String getValue();
...
}
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