Java Servlets Interview Questions

61) Is there any way to determine the number of concurrent connections my servlet engine can handle?

Depends on whether or not your servlet container uses thread pooling. If you do not use a thread pool, the number of concurrent connections accepted by Tomcat 3.1, for example, is 10. This you can see for yourself by testing a servlet with the Apache JMeter tool.

However, if your servlet container uses a thread pool, you can specify the number of concurrent connections to be accepted by the container. For Tomcat 3.1, the information on how to do so is supplied with the documentation in the TOMCAT_HOME/doc/uguide directory.

62) What is a request dispatcher and how does it work?

A RequestDispatcher object can forward a client's request to a resource or include the resource itself in the response back to the client. A resource can be another servlet, or an HTML file, or a JSP file, etc.

You can also think of a RequestDispatcher object as a wrapper for the resource located at a given path that is supplied as an argument to the getRequestDispatcher method.

For constructing a RequestDispatcher object, you can use either the ServletRequest.getRequestDispatcher() method or the ServletContext.getRequestDispatcher() method. They both do the same thing, but impose slightly different constraints on the argument path. For the former, it looks for the resource in the same webapp to which the invoking servlet belongs and the pathname specified can be relative to invoking servlet. For the latter, the pathname must begin with '/' and is interpreted relative to the root of the webapp.

To illustrate, suppose you want Servlet_A to invoke Servlet_B. If they are both in the same directory, you could accomplish this by incorporating the following code fragment in either the service method or the doGet method of Servlet_A:

   RequestDispatcher dispatcher = getRequestDispatcher("Servlet_B");

   dispatcher.forward( request, response );

where request, of type HttpServletRequest, is the first parameter of the enclosing service method (or the doGet method) and response, of type HttpServletResponse, the second. You could accomplish the same by

  RequestDispatcher dispatcher=getServletContext().getRequestDispatcher( "/servlet/Servlet_B" );

   dispatcher.forward( request, response );

63) What is a Servlet Context?

A Servlet Context is a grouping under which related servlets (and JSPs and other web resources) run. They can share data, URL namespace, and other resources. There can be multiple contexts in a single servlet container.

The ServletContext object is used by an individual servlet to "call back" and obtain services from the container (such as a request dispatcher). Read the JavaDoc for javax.servlet.ServletContext for more information.

You can maintain "application global" variables by using Servlet Context Attributes.

64) Does the RequestDispatcher expect a relative URL to be relative to the originally-called servlet or to the current servlet (if different)?

Since the RequestDispatcher will be passing the control (request object and response object) from the current Servlet, the relative URL must be relative to the current servlet.

The originally called servlet has passed the control to the current servlet, and now current servlet is acting as controller to other resourses.

65) What is the difference between in-process and out-of-process servlet containers?

The in-process Servlet containers are the containers which work inside the JVM of Web server, these provides good performance but poor in scalibility.

The out-of-process containers are the containers which work in the JVM outside the web server. poor in performance but better in scalibility
In the case of out-of-process containers, web server and container talks with each other by using the some standard mechanism like IPC.

In addition to these types of containers, there is 3rd type which is stand-alone servlet containers. These are an integral part of the web server.

66) How is SingleThreadModel implemented in Tomcat? In other containers? [I would assume that Tomcat uses its connection thread pool, and creates a new instance of the servlet for each connection thread, instead of sharing one instance among all threads. Is that true?]

The question mixes together two rather independent aspects of a servlet container: "concurrency control" and "thread pooling".

Concurrency control, such as achieved by having a servlet implement the SingleThreadModel interface, addresses the issue of thread safety. A servlet will be thread-safe or thread-unsafe regardless of whether the servlet container used a thread pool. Thread pooling merely eliminates the overhead associated with the creation and destruction of threads as a servlet container tries to respond to multiple requests received simultaneously. It is for this reason that the specification document for Servlet 2.2 API is silent on the subject of thread pooling -- as it is merely an implementation detail. However, the document does indeed address the issue of thread safety and how and when to use SingleThreadModel servlets.

Section 3.3.3.1 of the Servlet 2.2 API Specification document says that if a servlet implements the SingleThreadModel it is guaranteed "that only one request thread at time will be allowed in the service method." It says further that "a servlet container may satisfy this guarantee by serializing requests on a servlet or by maintaining a pool of servlet instances."

Obviously, for superior performance you'd want the servlet container to create multiple instances of a SingleThreadModel type servlet should there be many requests received in quick succession. Whether or not a servlet container does that depends completely on the implementation. My experiments show that Tomcat 3.1 does indeed create multiple instances of a SingleThreadModel servlet, but only for the first batch of requests received concurrently. For subsequent batches of concurrent requests, it seems to use only one of those instances.

67) Which servlet containers have persistent session support? Specifically, does Tomcat 3.1?

All servlet containers that implement the Servlet 2.2 API must provide for session tracking through either the use of cookies or through URL rewriting. All Tomcat servlet containers support session tracking.

68) Can I use JAAS as the authentication technology for servlets ?

Yes, JAAS can be used as authentication technology for servlets. One important feature of JAAS is pure Java implementation. The JAAS infrastructure is divided into two main components: an authentication component and an authorization component. The JAAS authentication component provides the ability to reliably and securely determine who is currently executing Java code, regardless of whether the code is running as an application, an applet, a bean, or a servlet.

69) How can I set a servlet to load on startup of the container, rather than on the first request?

The Servlet 2.2 spec defines a load-on-startup element for just this purpose. Put it in the <servlet> section of your web.xml deployment descriptor. It is either empty (<load-on-startup/>) or contains "a positive integer indicating the order in which the servlet should be loaded. Lower integers are loaded before higher integers. If no value is specified, or if the value specified is not a positive integer, the container is free to load it at any time in the startup sequence."

For example,

<servlet>

 <servlet-name>foo</servlet-name>

 <servlet-class>com.foo.servlets.Foo</servlet-class>

 <load-on-startup>5</load-on-startup>

</servlet>

Some servlet containers also have their own techniques for configuring this; please submit feedback with information on these.

70) Is it possible to write a servlet that acts as a FTP server?

Yes. It would spawn a thread that opens a ServerSocket, then listens for incoming connections and speaks the FTP protocol.