How can I use JUnit to test a method that takes a complex parameter - like an HttpServletRequest?

Say you want to test this method:

public Vector getEmployeeList(HttpServletRequest req) {
    Vector list = new Vector();
    String department = req.getParameter("departmentID");
    // get the employees in the specified department...
    return list;
}

We need to pass in something that implements the HttpServletRequest interface. We don't want to go digging into the Tomcat source code or somewhere else to try to find a class that already implements this interface.

One answer is to use a variation on the NullObject pattern. This allows you to implement an interface to do only what you need to do - and everything else on the interface simply returns null, false, or 0. For example, here's a NullObject implementation of HttpServletRequest:

public class NullHttpServletRequest implements HttpServletRequest
{
	public Hashtable parameters = new Hashtable();
		
	public void setParameter(String key, String value) {
		parameters.put(key, value);
	}
	
	public String getParameter(String key)	{
		return (String)this.parameters.get(key);
	}

	public Enumeration getParameterNames(){
		return this.parameters.elements();
	}
	
	public Cookie[] getCookies() {return null;}
	public String getMethod(){return null;}
	public String getRequestURI(){return null;}
	public String getServletPath(){return null;}
	public String getPathInfo(){return null;}
	public String getPathTranslated(){return null;}
	public String getQueryString(){return null;}
	public String getRemoteUser(){return null;}
	public String getAuthType(){return null;}
	public String getHeader(String name){return null;}
	public int getIntHeader(String name){return 0;}
	public long getDateHeader(String name){return 0;}
	public Enumeration getHeaderNames(){return null;}
	public HttpSession getSession(boolean create){return null;}
	public String getRequestedSessionId(){return null;}
	public boolean isRequestedSessionIdValid(){return false;}
	public boolean isRequestedSessionIdFromCookie(){return false;}
	public boolean isRequestedSessionIdFromUrl(){return false;}
	public int getContentLength(){return 0;}
	public String getContentType(){return null;}
	public String getProtocol(){return null;}
	public String getScheme(){return null;}
	public String getServerName(){return null;}
	public int getServerPort(){return 0;}
	public String getRemoteAddr(){return null;}
	public String getRemoteHost(){return null;}
	public String getRealPath(String path){return null;}
	public ServletInputStream getInputStream() throws IOException{return null;}
	public String[] getParameterValues(String name){return null;}
	public Enumeration getAttributeNames(){return null;}
	public Object getAttribute(String name){return null;}
	public HttpSession getSession(){return null;}
	public BufferedReader getReader() throws IOException{return null;}
	public String getCharacterEncoding(){return null;}
	public void setAttribute(String name, Object o) {}
	public boolean isRequestedSessionIdFromURL() {return false;}
}

Note that some methods have been added to allow you to set some initial parameters in the request.

So you can use this class in a JUnit test case like this:

	public void testGetEmployeeList() {
		AnObject o = new AnObject();
		NullHttpServletRequest req = new NullHttpServletRequest();
		req.setParameter("department", "2");
		Vector list = o.getEmployeeList(req);
                // code to ensure employees are in department 2...
	}