Get Started With Java Servlets

What Is a Servlet?

A Java Servlet is simply a class that extends from one of the classes in javax.servlet.http or javax.servlet packages and is used in applications to handle network communication like HTTP request-response model.

A Container is an interface between a component (Servlet, data persistance…) and the low-level functionalities that supports the component. Web containers receive network requests and redirect those requests to a servlet object by mapping the URL path contained in the request to the servlet. A URL path contains the context root and, optionally, a URL pattern:

http://host:port/context-root[/url-pattern]

Servlet containers handle many tedious tasks like opening sockets; tranformations to convert HTTP into Java API calls; or provide a set of interfaces to write your own servlet implementation.

Note: Servlets and JSPs require Servlet Containers liket Apache Tomcat, WildFly, etc..

How a Client Request is processed by the Web Server

  1. The web server receives the HTTP request.
  2. The request is forwarded to the Servlet Container.
  3. The Servlet Container creates two objects the HttpServletRequest and HttpServletResponse.
  4. The container creates a thread for the request, passes the request and reponse objects as arguments and delegates the request to a particular Servlet chosen between the Servlets it contains.
  5. The service() method is executed and evaluates which Servlet method will be called (doGet(), doPost(), doPut(), doDelete(), doHead(), …) based on the request sent by the client (GET, POST, PUT, DELETE, HEAD, …).
  6. The Servlet handles the client request and generates content to be returned to the client.
  7. The container sends the Servlet response to the client.

Note: ServletRequest contains parameters such as name, values, attributes, and an input stream.

Web Server and Servlet Container
Web Server and Servlet Container

Servlet Lifecycle

The servlet container is responsible for controlling the Servlet lifecycle. What happen when a request comes in?

  1. If an instance of the Servlet does not exist, the web container:
    • Loads the servlet class.
    • Creates an instance of the class.
    • Calls init() method to initialize the servlet.
  2. If an instance already exists the container calls the service() and passes the request and response objects.

Servlets stay in memory waiting for other requests, and will not be unloaded unless the servlet container sees a shortage of memory. Before the servlet is ready to be garbage collected the destroy() method is called. This will happen when all servlet methods are completed or after a server-specific grace period.

Note: when destroy() method is called you release resources created by the init() method like database connections.

Servlet Component

Use the @WebServlet(name = "ConvertServlet") annotation or the deployment descriptor web.xml.

<servlet>
    <servlet-name>ConvertServlet</servlet-name>
    <servlet-class>com.sergiomartinrubio.javaservletfilters.servlet.IpAddressConverterServlet</servlet-class>
</servlet>

Both the annotated servlet or the XML servlet declarion must specify at least one URL pattern. In case of annation use the urlPatterns attribute when other attributes are also used. For XML declaration:

<servlet-mapping>
    <servlet-name>ConvertServlet</servlet-name>
    <url-pattern>/convert</url-pattern>
</servlet-mapping>
@WebServlet(urlPatterns = "/convert", name = "ConvertServlet")
public class IpAddressConverterServlet extends HttpServlet { 
    // servlet methods (doGet(), doPost(), doPut())
}

Note: Annotations require Servlet API 3.0 or higher and tomcat7 or any later version of Tomcat.

From now on most of the servlet definitions will be shown with annotations.

The servlet initialization process can be customize if you override the init() method of the Servlet interface, or if you use the initParams anotation attribute in combination with @WebInitParam annotation.

@WebServlet(urlPatterns = "/convert", name = "ConvertServlet", initParams = {
        @WebInitParam(name = "param2", value = "hello"),
        @WebInitParam(name = "param2", value = "goodbye")
})

Initializatio paramters are used to provide data that a Servlet needs. The values can be retrieve with the getInitParameter() method.

Servlet Request

Clients send data to the Servlet in the HttpServletRequest, which contains the request URL, HTTP headers, query string, and so on. Query strings contain a set of parameters and values, that can be retrieved by using the getParameter() method.

e.g.

String myParameter = request.getParameter("myParameter");

Servlet Response

Servlets return responses to clients in the HttpServletResponse. To send character data, use the PrintWriter returned by the response’s getWriter() method. You can also send binary data with the ServletOutputStream returned by getOutputStream() method. Additionally, the response object allows you to set things like content type, status codes, cookies.

PrintWriter out = response.getWriter();
response.setStatus(HttpServletResponse.SC_OK);
response.setContentType("application/json");
out.println("{\"value\":\"Hello World}\"");

Filters

A filter is used to transform headers or content of a request or reponse. Filters are attatched to web components, but they should be independent from web resources, so they can reused with multiple web components.

Common use cases:

Use the @WebFilter annotation with at least one URL pattern to define a filter in a web application. Classes annotated with the @WebFilter annotation must implement the javax.servlet.Filter interface.

@WebFilter(urlPatterns = "/convert")
public class FormatFilter implements Filter { 
    // filter methods (doFilter(), init(), destroy())
}

doFilter() is the main method in a Filter and is used to access and/or modify the request and reponse headers, invoke the next filter in the filter chain. If the current filter is the last filter in the chain that ends with the target web component or static resource. However, the filter can block the request and handle the reponse. In addition to doFilter, you must implement the init and destroy methods.

Filters can modify, add or remove data in the request and reponse by calling methods like setAttribute(). You can also override HTTP request methods by wrapping the request in an object that extends either HttpServletRequestWrapper. To override HTTP response methods, you wrap the response in an object that extends either HttpServletResponseWrapper.

@WebFilter(urlPatterns = "/convert")
public class FormatFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        log.info("The filter " + FormatFilter.class.getName() + " has been created!");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {

        // FormatRequestWrapper extends HttpServletRequestWrapper
        request = new FormatRequestWrapper((HttpServletRequest) request);
        String format = request.getParameter("format");

        if (format != null) {
            chain.doFilter(request, response);
        } else {
            throw new InputParameterException("Missing format parameter!");
        }
    }

    @Override
    public void destroy() {
        log.info("Destroy method is invoked for the servlet " + FormatFilter.class.getName());
    }
}

Note: A web container uses filter mappings to decide how to apply filters to web resources. A filter mapping matches a filter to a web component by name or to web resources by URL pattern. The filters are invoked in the order in which filter mappings appear in the filter mapping list of a WAR.

Filter constrains:

The default dispatcher type is REQUEST and multiple types can be selected as follows:

@WebFilter(urlPatterns = "/convert", dispatcherTypes = {DispatcherType.REQUEST, DispatcherType.FORWARD})
public class FormatFilter implements Filter {
    // filter methods (doFilter(), init(), destroy())
}

Event Listeners

Event Listeners are used to track events in your Web application. There are two types of servlet events:

  1. Servlet context-level catches events about ServletContext lifecycle changes.
  2. Session-level events are about requests coming into and going out of scope of a web application.

Both types can catch lifecycle and attribute changes.

You can implement ServletRequestListener, ServletRequestAttributeListener, ServletContextAttributeListener, ServletContxtListener, HttpSessionAttributeListener… and use the @WebListener annotation to define an event listener.

@WebListener("Checks for new attributes during request")
public class NewAttributeListener implements ServletRequestAttributeListener {

    @Override
    public void attributeAdded(ServletRequestAttributeEvent servletRequestAttributeEvent) {
        log.info("The attribute \"" + servletRequestAttributeEvent.getName() + "\" with value \""
                + servletRequestAttributeEvent.getValue() + "\" was added.");
    }

    @Override
    public void attributeReplaced(ServletRequestAttributeEvent servletRequestAttributeEvent) {

    }

    @Override
    public void attributeRemoved(ServletRequestAttributeEvent servletRequestAttributeEvent) {

    }
}

Exception Handling

The Servlet API provides support for custom servlet error handling. This can be configured in the web.xml file in your project.

Exceptions can be mapped to Servlets or JSP.

Mapping error to a servlet:

<error-page>
    <exception-type>com.sergiomartinrubio.javaservletfilters.exception.InputParameterException</exception-type>
    <location>/ExceptionHandler</location>
</error-page>

Mapping error to JPS:

<error-page>
    <exception-type>com.sergiomartinrubio.javaservletfilters.exception.InputParameterException</exception-type>
    <location>/WEB-INF/jsp/400.jsp</location>
</error-page>

In either case you can access the error attributes in the request, so the exception response can be customized.

Request Attribute Type
javax.servlet.error.status_code java.lang.Integer
javax.servlet.error.exception_type java.lang.Class
javax.servlet.error.message java.lang.String
javax.servlet.error.exception java.lang.Throwable
javax.servlet.error.request_uri java.lang.String
javax.servlet.error.servlet_name java.lang.String

Error codes can also be mapped as follows:

<error-page>
    <error-code>500</error-code>
    <location>/WEB-INF/jsp/400.jsp</location>
</error-page>

Source Code