Top Servlet interview Questions Part5

1.Which HTTP method are non-idempotent?

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.

2.What is MIME Type?

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 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.

3.What is difference between PrintWriter and ServletOutputStream?

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.

4.Can we get PrintWriter and ServletOutputStream both in a 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.

We can use ServletResponse getWriter() to get the PrintWriter instance whereas we can use ServletResponse getOutputStream() method to get the ServletOutputStream object reference.

5.can we create deadlock situation in servlet?

We can create deadlock in servlet by  call doPost() method from doGet() method and doGet() method to doPost() method to create deadlock situation in servlet.

6.What is the use of servlet wrapper classes?

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.

7.What is SingleThreadModel interface?

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 of servlets, thats why this interface is Deprecated

8.How can we invoke another servlet in a different application?

We can’t use RequestDispatcher to invoke servlet from another application because it’s specific for the application. If we have to forward the request to a resource in another application, we can use ServletResponse sendRedirect() method and provide complete URL of another servlet. This sends the response to client with response code as 302 to forward the request to another URL. If we have to send some data also, we can use cookies that will be part of the servlet response and sent in the request to another servlet

9.Is it good idea to create servlet constructor?

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.

10. Servlets are  Thread Safe? How to achieve thread safety in servlets?

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 we have a shared resource then we have to provide  thread safety in servlets.

Synchronization is the easiest and most widely used tool for thread safety in java.

Use of Atomic Wrapper classes from java.util.concurrent.atomic packageUse of locks from java.util.concurrent.locks package.

Using volatile keyword with variables to make every thread read the data from memory, not read from thread cache

11.How do we call one servlet from another servlet?

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.

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