A servlet is a Java programming language class used to extend the capabilities of servers that host applications accessed by means of a request-response programming model. Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by web servers. For such applications, Java Servlet technology defines HTTP-specific servlet classes.
The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets. All servlets must implement the Servlet interface, which defines lifecycle methods. When implementing a generic service, you can use or extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet and doPost, for handling HTTP-specific services.
The lifecycle of a servlet is controlled by the container in which the servlet has been deployed. When a request is mapped to a servlet, the container performs the following steps.
init methodIf it needs to remove the servlet, the container finalizes the servlet by calling the servlet’s destroy method
You can monitor and react to events in a servlet’s lifecycle by defining listener objects whose methods get invoked when lifecycle events occur. These types of events can monitor with following classes;
| Object | Event | Listener Interface and Event Class |
|---|---|---|
| Web context | Initialization and destruction | javax.servlet.ServletContextListener and ServletContextEvent |
| Web context | Attribute added, removed, or replaced | javax.servlet.ServletContextAttributeListener and ServletContextAttributeEvent |
| Session | Creation, invalidation, activation, passivation, and timeout | javax.servlet.http.HttpSessionListener, javax.servlet.http.HttpSessionActivationListener, and HttpSessionEvent |
| Session | Attribute added, removed, or replaced | javax.servlet.http.HttpSessionAttributeListener and HttpSessionBindingEvent |
| Request | A servlet request has started being processed by web components | javax.servlet.ServletRequestListener and ServletRequestEvent |
| Request | Attribute added, removed, or replaced | javax.servlet.ServletRequestAttributeListener and ServletRequestAttributeEvent |
** Use the @WebListener annotation to define a listener to get events for various operations on the particular web application context.
Collaborating web components share information by means of objects that are maintained as attributes of four scope objects. You access these attributes by using the getAttribute and setAttribute methods of the class representing the scope. Below lists the scope objects.
| Scope Object | Class | Accessible From |
|---|---|---|
| Web context | javax.servlet.ServletContext |
Web components within a web context. |
| Session | javax.servlet.http.HttpSession |
Web components handling a request that belongs to the session. |
| Request | Subtype of javax.servlet.ServletRequest |
Web components handling the request. |
| Page | javax.servlet.jsp.JspContext |
The JSP page that creates the object. |
In a servlet never create or reassing a member variable(incuding static variales). Member variables are not thread safe. A web container will typically create a thread to handle each request. To ensure that a servlet instance handles only one request at a time, a servlet can implement the SingleThreadModel interface. If a servlet implements this interface, no two threads will execute concurrently in the servlet’s service method. A web container can implement this guarantee by synchronizing access to a single instance of the servlet or by maintaining a pool of web component instances and dispatching each new request to a free instance. This interface does not prevent synchronization problems that result from web components’ accessing shared resources, such as static class variables or external objects.
@WebServlet("/test")
public class TestServlet extends HttpServlet {
// all of same
@WebServlet("/test")
@WebServlet(value = "/test")
@WebServlet(urlPatterns = {"/test"})
@WebInitParam(name = "initParam", value = "2")
The service provided by a servlet is implemented in the service method of a GenericServlet, in the doMethod methods (where Method can take the value Get, Delete, Options, Post, Put, or Trace) of an HttpServlet object, or in any other protocol-specific methods defined by a class that implements the Servlet interface. The term service method is used for any method in a servlet class that provides a service to a client.
GenericServlet.service method called in every HTTP request. Tthis method calls doGet or doPost or .. with appriciate incoming HTTP request. If you override this doXxx methods doesnt works because you must implement these methods.
A request contains data passed between a client and the servlet. All requests implement the ServletRequest interface. This interface defines methods for accessing the following information:
You can also retrieve an input stream from the request and manually parse the data. To read character data, use the BufferedReader object returned by the request’s getReader method. To read binary data, use the ServletInputStream returned by getInputStream.
HTTP servlets are passed an HTTP request object, HttpServletRequest, which contains the request URL, HTTP headers, query string, and so on.
HttpSession session = request.getSession();
can hold information about a given user, between requests. So, if you set an object into the session object during one request, it will be available for you to read during any subsequent requests within the same session time scope.
ServletContext context = request.getSession().getServletContext();
contains meta information about the web application. For instance, you can access context parameters set in the web.xml file, you can forward the request to other servlet, you can store application wide parameters in the ServletContext
A response contains data passed between a server and the client. All responses implement the ServletResponse interface. This interface defines methods that allow you to do the following.
PrintWriter returned by the response’s getWriter method. To send binary data in a Multipurpose Internet Mail Extensions (MIME) body response, use the ServletOutputStream returned by getOutputStream. To mix binary and text data, as in a multipart response, use a ServletOutputStream and manage the character sections manually.text/html) being returned by the response with the setContentType(String) method. This method must be called before the response is committed. A registry of content type names is kept by the Internet Assigned Numbers Authority (IANA) at http://www.iana.org/assignments/media-types/.setBufferSize(int) method. By default, any content written to the output stream is immediately sent to the client. Buffering allows content to be written before anything is sent back to the client, thus providing the servlet with more time to set appropriate status codes and headers or forward to another web resource. The method must be called before any content is written or before the response is committed.HTTP response objects, javax.servlet.http.HttpServletResponse, have fields representing HTTP headers, such as the following.
A filter is an object that can transform the header and content (or both) of a request or response. The filtering API is defined by the Filter, FilterChain, and FilterConfig interfaces in the javax.servlet package. You define a filter by implementing the Filter interface.
@WebFilter(urlPatterns = {"/Login"})
public class SampleFilter implements Filter{
The most important method in the Filter interface is doFilter, which is passed request, response, and filter chain objects. This method can perform the following actions;
doFilter method on the chain object, passing in the request and response it was called with or the wrapped versions it may have created. Alternatively, the filter can choose to block the request by not making the call to invoke the next entity. In the latter case, the filter is responsible for filling out the response.To invoke a resource available on the server that is running a web component, you must first obtain a RequestDispatcher object by using the getRequestDispatcher("URL") method. You can get a RequestDispatcher object from either a request or the web context; however, the two methods have slightly different behavior. The method takes the path to the requested resource as an argument. A request can take a relative path (that is, one that does not begin with a /), but the web context requires an absolute path. If the resource is not available or if the server has not implemented a RequestDispatcher object for that type of resource, getRequestDispatcher will return null. Your servlet should be prepared to deal with this condition.
include(request, response);
To transfer control to another web component, you invoke the forward method of a RequestDispatcher. When a request is forwarded, the request URL is set to the path of the forwarded page. The original URI and its constituent parts are saved as the following request attributes:
javax.servlet.forward.request_uri javax.servlet.forward.context_path javax.servlet.forward.servlet_path javax.servlet.forward.path_info javax.servlet.forward.query_string
This text copied from https://docs.oracle.com/javaee/7/tutorial/servlets001.htm