What is the purpose of wrapping an HttpServletRequest using an HttpServletRequestWrapper ? What benefits do we gain from doing this ?
Class HttpServletRequestWrapper. Provides a convenient implementation of the HttpServletRequest interface that can be subclassed by developers wishing to adapt the request to a Servlet. This class implements the Wrapper or Decorator pattern. Methods default to calling through to the wrapped request object.
ServletRequest provides basic setter and getter methods for requesting a Servlet, but it doesn't specify how to communicate. HttpServletRequest extends the Interface with getters for HTTP-communication (which is of course the most common way for communicating since Servlets mostly generate HTML).
getParameter() − You call request. getParameter() method to get the value of a form parameter.
Then, we created a new implementation of the HttpServletRequestWrapper class. We overrode the getInputStream() method to return an object of ServletInputStream class. Finally, we created a new filter to pass the request wrapper object to the filter chain. So, we were able to read the request multiple times.
HttpServletRequest
is an interface for a HTTP specific servlet request. Typically you get instances of this interface in servlet filters or servlets.
Sometimes you want to adjust the original request at some point. With a HttpServletRequestWrapper
you can wrap the original request and overwrite some methods so that it behaves slightly different.
Example:
You have a bunch of servlets and JSPs which expect some request parameters in a certain format. E.g. dates in format yyyy-MM-dd
.
Now it is required to support the dates also in a different format, like dd.MM.yyyy
with the same functionality. Assuming there is no central string to date function (it's an inherited legacy application), you have to find all places in the servlets and JSPs.
As an alternative you can implement a servlet filter. You map the filter so that all requests to your servlets and JSPs will go through this filter.
The filter's purpose is to check the date parameters' format and reformat them to the old format if necessary. The servlets and JSPs get the date fields always in the expected old format. No need to change them.
This is the skeleton of your filter:
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
HttpServletRequest adjustedRequest = adjustParamDates((HttpServletRequest) request);
chain.doFilter(adjustedRequest, response);
}
We take the original request and in method adjustParamDates()
we manipulate the request and pass it down the filter chain.
Now, how would we implement adjustParamDates()
?
private HttpServletRequest adjustParamDates(HttpServletRequest req) {
// ???
}
We need a new instance of interface HttpServletRequest
which behaves exactly like the original instance req
. But the four methods getParameter()
, getParameterMap()
, getParameterNames()
, getParameterValues()
shouldn't work on the original parameters but on the adjusted parameter set. All other methods of interface HttpServletRequest
should behave like the original methods.
So we can do something like that. We create an instance of HttpServletRequest
and implement all methods. Most method implementations are very simple by calling the corresponding method of the original request instance:
private HttpServletRequest adjustParamDates(final HttpServletRequest req) {
final Map<String, String[]> adjustedParams = reformatDates(req.getParameterMap());
return new HttpServletRequest() {
public boolean authenticate(HttpServletResponse response) {
return req.authenticate(response);
}
public String changeSessionId() {
return req.changeSessionId();
}
public String getContextPath() {
return req.getContextPath();
}
// Implement >50 other wrapper methods
// ...
// Now the methods with different behaviour:
public String getParameter(String name) {
return adjustedParams.get(name) == null ? null : adjustedParams.get(name)[0];
}
public Map<String, String[]> getParameterMap() {
return adjustedParams;
}
public Enumeration<String> getParameterNames() {
return Collections.enumeration(adjustedParams.keySet());
}
public String[] getParameterValues(String name) {
return adjustedParams.get(name);
}
});
}
There are more than 50 methods to implement. Most of them are only wrapper implementations to the original request. We need only four custom implementations. But we have to write down all these methods.
So here comes the class HttpServletRequestWrapper
into account. This is a default wrapper implementation which takes the original request instance and implements all methods of interface HttpServletRequest
as simple wrapper methods calling the corresponding method of the original request, just as we did above.
By subclassing HttpServletRequestWrapper
we only have to overwrite the four param methods with custom behaviour.
private HttpServletRequest adjustParamDates(final HttpServletRequest req) {
final Map<String, String[]> adjustedParams = reformatDates(req.getParameterMap());
return new HttpServletRequestWrapper(req) {
public String getParameter(String name) {
return adjustedParams.get(name) == null ? null : adjustedParams.get(name)[0];
}
public Map<String, String[]> getParameterMap() {
return adjustedParams;
}
public Enumeration<String> getParameterNames() {
return Collections.enumeration(adjustedParams.keySet());
}
public String[] getParameterValues(String name) {
return adjustedParams.get(name);
}
});
}
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