Java Servlet is server side technologies to extend the capability of web servers by providing support for dynamic response and data persistence.
The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing our own servlets. All servlets must implement the javax.servlet.Servlet interface, which defines servlet lifecycle methods. When implementing a generic service, we can 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.
Most of the times, web applications are accessed using HTTP protocol and thats why we mostly extend HttpServlet class. Servlet API hierarchy is shown in below image
The “Content-Type” response header is known as MIME Type. Server sends MIME type to client to let them know the kind of data it’s sending. It helps client in rendering the data for user. Some of the mostly used mime types are text/html, text/xml, application/xml etc.
We can use ServletContext getMimeType() method to get the correct MIME type of the file and use it to set the response content type. It’s very useful in downloading file through servlet from server.
Web Applications are modules that run on server to provide both static and dynamic content to the client browser. Apache web server supports PHP and we can create web application using PHP. Java provides web application support through Servlets and JSPs that can run in a servlet container and provide dynamic content to client browser.
Java Web Applications are packaged as Web Archive (WAR) and it has a defined structure like below image.
A web server responsibility is to handler HTTP requests from client browsers and respond with HTML response. A web server understands HTTP language and runs on HTTP protocol. Apache Web Server is kind of a web server and then we have specific containers that can execute servlets and JSPs known as servlet container, for example Tomcat. Application Servers provide additional features such as Enterprise JavaBeans support, JMS Messaging support, Transaction Management etc. So we can say that Application server is a web server with additional functionalities to help developers with enterprise applications.
A HTTP method is said to be idempotent if it returns the same result every time. HTTP methods GET, PUT, DELETE, HEAD, and OPTIONS are idempotent method and we should implement our application to make sure these methods always return same result. HTTP method POST is non-idempotent method and we should use post method when implementing something that changes with every request.
For example, to access an HTML page or image, we should use GET because it will always return the same object but if we have to save customer information to database, we should use POST method. Idempotent methods are also known as safe methods and we don’t care about the repetitive request from the client for safe methods.
Servlet technology was introduced to overcome the shortcomings of CGI technology.
Servlets provide better performance that CGI in terms of processing time, memory utilization because servlets uses benefits of multithreading and for each request a new thread is created, that is faster than loading creating new Object for each request with CGI.
Servlets and platform and system independent, the web application developed with Servlet can be run on any standard web container such as Tomcat, JBoss, Glassfish servers and on operating systems such as Windows, Linux, Unix, Solaris, Mac etc.
Servlets are robust because container takes care of life cycle of servlet and we don’t need to worry about memory leaks, security, garbage collection etc.
Servlets are maintainable and learning curve is small because all we need to take care is business logic for our application.
Servlet containers are also known as web container, for example Tomcat. Some of the important tasks of servlet container are:
Servlet Container provides easy way of communication between web client (Browsers) and the servlets and JSPs. Because of container, we don’t need to build a server socket to listen for any request from web client, parse the request and generate response. All these important and complex tasks are done by container and all we need to focus is on business logic for the applications.
Servlet Container takes care of managing the life cycle of servlet. From the loading of servlets into memory, initializing servlets, invoking servlet methods and to destroy them. Container also provides utility like JNDI for resource pooling and management.
Container creates new thread for every request to the servlet and provide them request and response objects to process. So servlets are not initialized for each request and saves time and memory.
JSPs doesn’t look like normal java classes but every JSP in the application is compiled by container and converted to Servlet and then container manages them like other servlets.
Servlet container manages the resource pool, perform memory optimizations, execute garbage collector, provides security configurations, support for multiple applications, hot deployment and several other tasks behind the scene that makes a developer life easier.
GET is a safe method (idempotent) where POST is non-idempotent method.
We can send limited data with GET method and it’s sent in the header request URL whereas we can send large amount of data with POST because it’s part of the body.
GET method is not secure because data is exposed in the URL and we can easily bookmark it and send similar request again, POST is secure because data is sent in request body and we can’t bookmark it.
GET is the default HTTP method whereas we need to specify method as POST to send request with POST method.
Hyperlinks in a page uses GET method.
PrintWriter is a character-stream class whereas ServletOutputStream is a byte-stream class. We can use PrintWriter to write character based information such as character array and String to the response whereas we can use ServletOutputStream to write byte array data to the response.
We can use ServletResponse getWriter() to get the PrintWriter instance whereas we can use ServletResponse getOutputStream() method to get the ServletOutputStream object reference.
javax.servlet.ServletConfig is used to pass configuration information to Servlet. Every servlet has it’s own ServletConfig object and servlet container is responsible for instantiating this object. We can provide servlet init parameters in web.xml file or through use of WebInitParam annotation. We can use getServletConfig() method to get the ServletConfig object of the servlet.
We can’t get instances of both PrintWriter and ServletOutputStream in a single servlet method, if we invoke both the methods; getWriter() and getOutputStream() on response; we will get java.lang.IllegalStateException at runtime with message as other method has already been called for this response.
javax.servlet.ServletContext interface provides access to web application parameters to the servlet. The ServletContext is unique object and available to all the servlets in the web application. When we want some init parameters to be available to multiple or all of the servlets in the web application, we can use ServletContext object and define parameters in web.xml using element. We can get the ServletContext object via the getServletContext() method of ServletConfig. Servlet containers may also provide context objects that are unique to a group of servlets and which is tied to a specific portion of the URL path namespace of the host.
ServletContext is enhanced in Servlet Specs 3 to introduce methods through which we can programmatically add Listeners and Filters and Servlet to the application. It also provides some utility methods such as getMimeType(), getResourceAsStream() etc.
JIf we have to initialize some resource before we want our servlet to process client requests, we should override init() method. If we override init (ServletConfig config) method, then the first statement should be super(config) to make sure superclass init(ServletConfig config) method is invoked first. That’s why GenericServlet provides another helper init() method without argument that get’s called at the end of init(ServletConfig config) method. We should always utilize this method for overriding init() method to avoid any issues as we may forget to add super() call in overriding init method with ServletConfig argument.
Some of the differences between ServletConfig and ServletContext are:
ServletConfig is a unique object per servlet whereas ServletContext is a unique object for complete application.
ServletConfig is used to provide init parameters to the servlet whereas ServletContext is used to provide application level init parameters that all other servlets can use.
We can’t set attributes in ServletConfig object whereas we can set attributes in ServletContext that other servlets can use in their implementation.
RequestDispatcher interface is used to forward the request to another resource that can be HTML, JSP or another servlet in same application. We can also use this to include the content of another resource to the response. This interface is used for inter-servlet communication in the same context.
There are two methods defined in this interface:
A:void forward(ServletRequest request, ServletResponse response) – forwards the request from a servlet to another resource (servlet, JSP file, or HTML file) on the server.
B:void include(ServletRequest request, ServletResponse response) – includes the content of a resource (servlet, JSP page, HTML file) in the response.
We can get RequestDispatcher in a servlet using ServletContext getRequestDispatcher(String path) method. The path must begin with a / and is interpreted as relative to the current context root.
Servlet Life Cycle consists of three methods:
A:public void init(ServletConfig config) – This method is used by container to initialize the servlet, this method is invoked only once in the lifecycle of servlet.
B:public void service(ServletRequest request, ServletResponse response) – This method is called once for every request, container can’t invoke service() method until unless init() method is executed.
C:public void destroy() – This method is invoked once when servlet is unloaded from memory.
We know that Servlet Container manages the life cycle of Servlet, there are four phases of servlet life cycle.
A:Servlet Class Loading – When container receives request for a servlet, it first loads the class into memory and calls it’s default no-args constructor.
B:Servlet Class Initialization – Once the servlet class is loaded, container initializes the ServletContext object for the servlet and then invoke it’s init method by passing servlet config object. This is the place where a servlet class transforms from normal class to servlet.
C:Request Handling – Once servlet is initialized, its ready to handle the client requests. For every client request, servlet container spawns a new thread and invokes the service() method by passing the request and response object reference.
D:Removal from Service – When container stops or we stop the application, servlet container destroys the servlet class by invoking it’s destroy() method.
We can create deadlock in servlet by making a loop of method invocation, just call doPost() method from doGet() method and doGet() method to doPost() method to create deadlock situation in servlet.
HttpServlet class provide HTTP protocol implementation of servlet but it’s left abstract because there is no implementation logic in service methods such as doGet() and doPost() and we should override at least one of the service methods. That’s why there is no point in having an instance of HttpServlet and is declared abstract class.
RequestDispatcher forward() is used to forward the same request to another resource whereas ServletResponse sendRedirect() is a two step process. In sendRedirect(), web application returns the response to client with status code 302 (redirect) with URL to send the request. The request sent is a completely new request.
forward() is handled internally by the container whereas sednRedirect() is handled by browser.
We should use forward() when accessing resources in the same application because it’s faster than sendRedirect() method that required an extra network call.
In forward() browser is unaware of the actual processing resource and the URL in address bar remains same whereas in sendRedirect() URL in address bar change to the forwarded resource.
forward() can’t be used to invoke a servlet in another context, we can only use sendRedirect() in this case.
Servlet HTTP API provides two wrapper classes – HttpServletRequestWrapper and HttpServletResponseWrapper. These wrapper classes are provided to help developers with custom implementation of servlet request and response types. We can extend these classes and override only specific methods we need to implement for custom request and response objects. These classes are not used in normal servlet programming.
Servlet attributes are used for inter-servlet communication, we can set, get and remove attributes in web application. There are three scopes for servlet attributes – request scope, session scope and application scope.
ServletRequest, HttpSession and ServletContext interfaces provide methods to get/set/remove attributes from request, session and application scope respectively.
Servlet attributes are different from init parameters defined in web.xml for ServletConfig or ServletContext.
We can use RequestDispatcher forward() method to forward the processing of a request to another servlet. If we want to include the another servlet output to the response, we can use RequestDispatcher include() method.
SingleThreadModel interface was provided for thread safety and it guarantees that no two threads will execute concurrently in the servlet’s service method. However SingleThreadModel does not solve all thread safety issues. For example, session attributes and static variables can still be accessed by multiple requests on multiple threads at the same time, even when SingleThreadModel servlets are used. Also it takes out all the benefits of multithreading support of servlets, thats why this interface is Deprecated in Servlet 2.4.
HttpServlet init() method and destroy() method are called only once in servlet life cycle, so we don’t need to worry about their synchronization. But service methods such as doGet() or doPost() are getting called in every client request and since servlet uses multithreading, we should provide thread safety in these methods.
If there are any local variables in service methods, we don’t need to worry about their thread safety because they are specific to each thread but if we have a shared resource then we can use synchronization to achieve thread safety in servlets when working with shared resources.
When we want to invoke another servlet from a servlet service methods, we use inter-servlet communication mechanisms. We can invoke another servlet using RequestDispatcher forward() and include() methods and provide additional attributes in request for other servlet use.
When Servlet container receives client request, it invokes the service() method which in turn invokes the doGet(), doPost() methods based on the HTTP method of request. I don’t see any use case where we would like to override service() method. The whole purpose of service() method is to forward to request to corresponding HTTP method implementations. If we have to do some pre-processing of request, we can always use servlet filters and listeners.
GenericServlet is protocol independent implementation of Servlet interface whereas HttpServlet is HTTP protocol specific implementation. Most of the times we use servlet for creating web application and that’s why we extend HttpServlet class. HttpServlet class extends GenericServlet and also provide some other methods specific to HTTP protocol.[sociallocker id=”2713″]
We can define a constructor for servlet but I don’t think its of any use because we won’t be having access to the ServletConfig object until unless servlet is initialized by container. Ideally if we have to initialize any resource for servlet, we should override init() method where we can access servlet init parameters using ServletConfig object.