Hailin ZengArgo part 4 - Servlet Filter
The root of the call hierarchy of Argo is ArgoFilter. What is a Filter?
Filter
Filter is usually used as a small segment of functionality that appeared before or after the main functions in web application. For example, a filter to check if user has access right to a certain URL.
A Filter guide： http://www.avajava.com/tutorials/lessons/what-is-a-filter-and-how-do-i-use-it.html
Filter must implements the javax.servlet.Filter interface. And override the method doFilter to do the filtering.
An example from the guide: A filer to display request params in url.
public class MyFilter implements Filter {
FilterConfig filterConfig = null;
public void init(FilterConfig filterConfig) throws ServletException {
this.filterConfig = filterConfig;
}
…
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
throws IOException, ServletException {
servletResponse.setContentType("text/html");
PrintWriter out = servletResponse.getWriter();
out.println("my-param (InitParameter): " + filterConfig.getInitParameter("my-param"));
out.println("<br/><br/>Parameters:<br/>");
Enumeration<String> parameterNames = servletRequest.getParameterNames();
if (parameterNames.hasMoreElements()) {
while (parameterNames.hasMoreElements()) {
String name = parameterNames.nextElement();
String value = servletRequest.getParameter(name);
out.println("name:" + name + ", value: " + value + "<br/>");
}
} else {
out.println("---None---<br/>");
}
out.println("<br/>Start Regular Content:<br/><hr/>");
filterChain.doFilter(servletRequest, servletResponse);
out.println("<br/><hr/>End Regular Content:<br/>");
}
} It displays an extra params in the front of the regular content.
Filter Configuration
A Filter can be configured
- in web.xml
- by @WebFilter annotation
For example, you can configure Argo in web.xml
<filter>
<filter-name>mvcfilter</filter-name>
<filter-class>com.bj58.argo.servlet.ArgoFilter</filter-class>
</filter>@WebFilter
@WebFilter examples
http://www.codejava.net/java-ee/servlet/webfilter-annotation-examples
@WebFilter(
urlPatterns = {"/*"},
attribute2=value2,
...
)
public class TheFilter implements javax.servlet.Filter {
// implements Filter's methods
} The urlPatterns is a must attribute in @WebFilter. And the class annotated by @WebFilter must implements the javax.servlet.Filter interface.
Filter in Argo
/**
* Schedule by a Filter
*/
@WebFilter(urlPatterns = {"/*"},
dispatcherTypes = {DispatcherType.REQUEST},
initParams = {@WebInitParam(name = "encoding", value = "UTF-8")}
)
public class ArgoFilter implements Filter {
private ArgoDispatcher dispatcher;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
ServletContext servletContext = filterConfig.getServletContext();
try {
dispatcher = ArgoDispatcherFactory.create(servletContext);
dispatcher.init();
} catch (Exception e) {
servletContext.log("failed to argo initialize, system exit!!!", e);
System.exit(1);
}
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
HttpServletRequest httpReq = (HttpServletRequest) request;
HttpServletResponse httpResp = (HttpServletResponse) response;
dispatcher.service(httpReq, httpResp);
}
@Override
public void destroy() {
dispatcher.destroy();
}
} -
@WebFilter(urlPatterns = {“/”}* means ArgoFilter filtering on any the request URLS.
-
In init(), a ArgoDispatcher instance is instantiated，and all the request is route to the dispatcher.