JSP

JSP Interview Questions 

1.What is JSP?

 Ans:- JSPs are normal HTML pages with Java code pieces embedded in them. JSP pages are saved to *.jsp files. A JSP compiler is used in the background to generate a Servlet from the JSP page.

2.Explain JSP LifeCycle?

Ans:- JSP’s life cycle can be explained in the following phases.

1. JSP Page Translation:-Jsp never executed directly,first jsp page transalated into servlet. In the translation phase, the container validates the syntactic correctness of the JSP pages and tag files. The container interprets the standard directives and actions, and the custom actions referencing tag libraries used in the page.

2. JSP Page Compilation:-The generated java servlet file is compiled into a java servlet class.

3. Class Loading:-The java servlet class that was compiled from the JSP source is loaded into the container.

4. Execution phase:-In the execution phase the container manages one or more instances of this class in response to requests and other events.

The interface JspPage contains jspInit() and jspDestroy(). The JSP specification has provided a special interface HttpJspPage for JSP pages serving HTTP requests and this interface contains _jspService().

5. Initialization:-jspInit() method is called immediately after the instance was created. It is called only once during JSP life cycle.

6. _jspService() execution:-This method is called for every request of this JSP during its life cycle. This is where it serves the purpose of creation. Oops! it has to pass through all the above steps to reach this phase. It passes the request and the response objects. _jspService() cannot be overridden.

7. jspDestroy() execution:-This method is called when this JSP is destroyed. With this call the servlet serves its purpose and submits itself to heaven (garbage collection). This is the end of jsp life cycle.
jspInit(), _jspService() and jspDestroy() are called the life cycle methods of the JSP.


 3.Explain JSP Scripting Tags?

 Ans:-We have three Types of Scripting Tags are there in JSP.
1)JSP scriptlet tag :-
JSP scriptlet lets you declare or define any java code that use into the jsp page.
scriptlet tag Start with  <% and End with %>
The Code placed inside this tag must end with a semicolon (;).
Syntax:
  <% Java Code; %>

EX.


<%
   int a = 10;
   out.print("a ="+a);
%>

2)JSP Declaration tag:-
JSP declaration lets you declare or define variables and methods (fields) that use into the jsp page.
Declaration tag Start with  <%! And End with %>
The Code placed inside this tag must end with a semicolon (;).
Syntax:    <%! Java Code; %>

EX:-
<%! private int i = 10; %>

 <%!
private int squre(int i)
      {
             i = i * i ;
             return i;
      }  
%>

3)JSP Expression tag:-
JSP expression is used to insert Java values directly into the output. JSP Expression tag is use for evaluating any expression and directly displays the output in appropriate web browser.
Expression tag Start with <%= and End with %>
The Code placed inside this tag not end with a semicolon (;).
Syntax:
  <%= Java Code %>

For example. the following shows the date/time that the page was requested:

Current time: <%= new java.util.Date() %>

4)What JSP lifecycle methods can I override?

Ans:- You cannot override the _jspService() method within a JSP page. You can however, override the jspInit() and jspDestroy() methods within a JSP page. jspInit() can be useful for allocating resources like database connections, network connections, and so forth for the JSP page. It is good programming practice to free any allocated resources within jspDestroy().
The jspInit() and jspDestroy() methods are each executed just once during the lifecycle of a JSP page and are typically declared as JSP declarations.

5)Explain about JSP Comments?

Ans:-There is only one type of JSP comment available by JSP specification.

JSP Comment Syntax:

<%-- comment --%>
This JSP comment tag tells the JSP container to ignore the comment part from compilation. That is, the commented part of source code is not considered for the content parsed for ‘response’.
Example:

<%-- This JSP comment part will not be included in the response object --%>
Note:-
 <!-- comment --> is not a JSP comment. This is HTML comment. The JSP container treats this HTML comment tag as equal as any other HTML tags. When view source is done, the content given between this tag is visible. This is not anyway related to the JSP container, it is the expected behaviour of this HTML tag.
Example
<!-- This HTML comment part is included in the response object and can be seen in view source -->


6)What are implicit objects?

Ans:- Implicit objects are objects that are created by the web container and contain information related to a particular request, page, or application. They are: request, response, pageContext, session, application, out, config, page, exception.

7)Is there a way to reference the "this" variable within a JSP page?

Ans:-Yes, there is. Under JSP 1.0, the page implicit object is equivalent to "this", and returns a reference to the Servlet generated by the JSP page.

8)How do you pass control from one JSP page to another?

Ans:-Use the following ways to pass control of a request from one servlet to another or one jsp to another.
The RequestDispatcher object ‘s forward method to pass the control.
The response.sendRedirect method.

 9) What is the difference between ServletContext and PageContext?

Ans:-ServletContext: Gives the information about the container
PageContext: Gives the information about the Request.

 10)Why is _jspService() method starting with an '_' while other life cycle methods do not?

Ans: main JSP life cycle method are jspInit() jspDestroy() and _jspService() ,bydefault whatever content we write in our jsp page will go inside the _jspService() method by the container if again will try to override this method JSP compiler will give error but we can override other two life cycle method as we have implementing this two in jsp so making this difference container use _ in jspService() method and shows that we cant override this method.

 11) Can you override jspInit() method? If yes, In which cases?

Ans:yes, we can. We do it usually when we need to initialize any members which are to be available for a servlet/JSP throughout its lifetime.

 12)Explain how JSP handle run-time exceptions?

Ans:The errorPage attribute of the page directive can be used to redirects the browser to an error processing page, when uncaught exception is encountered.
For example: <%@ page errorPage="error.jsp" %> redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during request processing.
In error.jsp, if it’s indicated that it’s an error-processing page by <%@ page isErrorPage="true" %> directive, then throwable object describing the exception may be accessed within the error page via the exception implicit object.

 13) Explain how to implement a thread-safe JSP page?

Ans:JSPs can be made thread-safe by having them implement the SingleThreadModel interface by using <%@ page isThreadSafe="false" %> directive in the page.
By doing this, one will have multiple number of instances (say N) of the servlet loaded and initialized, with the service method of each instance effectively synchronized.
SingleThreadModel is not recommended for normal use as there are many pitfalls. Hence one should try the old fashioned way, which is making them thread safe.


 14)What's the difference between forward and sendRedirect?

Ans:A forward is server side redirect while sendRedirect is client side redirect.
When a forward request is invoked, the request is sent to another resource on the server. The client is not informed that a different resource is going to process the request. This process occurs completely within the web container and then returns to the calling method.
When a sendRedirect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. As a result any object that is stored as a request attribute before the redirect occurs will be lost. Due to this extra round trip, a sendRedirect is slower than forward.

 15)What is the difference between JSP and Servlet life cycles?

Ans:In servlet life cycle, the servlet object is created first. The init() method is invoked by the servlet container and the servlet is initialized by its arguments. Servlet’s service() method is invoked next. And at the end, the destroy() method is invoked.
In case of a Java Server Page life cycle, the .jsp is converted into .class file which is a servlet and then follow the the process of the servlet. In orther words, the .jsp is translated into servlet and the functionality is same as that of the servlet.

 16) Explain the difference between JSP include directive and JSP include action?

Ans:When a JSP include directive is used, the included file's code is added into the added JSP page at page translation time. This happens before the JSP page is translated into a servlet.
However, if any page is included using action tag, the output of the page is returned back to the added page which happens at runtime.

 17) Explain JSP Actions in brief?

Ans:JSP actions can be used to print a script expression, create and store a Java Bean and for many other things. They are used for constructs in XML syntax to control the behavior of the servlet engine.
Dynamic insertion of files, reuse of JavaBeans components, forwarding a user to another page or generation of HTML for the java plug-in can be done.
Available actions include:
<jsp:include> – Includes a file at the time the page is requested.
<jsp:useBean> - Finds or instantiates a Java Bean.
<jsp:setProperty> - Sets the property of a Java Bean.
<jsp:getProperty> - Inserts the property of a Java Bean into the output.
<jsp:forward> - Forwards the requests to a new page.
jsp:plugin> - used to generate browser specific HTML to specify Java applets. This tag is replaced by either an <object> or <embed> tag depending on what is more appropriate for Client Browser.

 18) What is difference between custom JSP tags and beans?

Ans:Custom JSP tag is a user defined tag describing the way its attributes and its body are interpreted, and then the tags are grouped into collections called tag libraries that can be used in any number of JSP files. Both custom tags and beans encapsulate complex behavior into simple and accessible forms. There are several differences:
Custom tags can manipulate JSP content but beans cannot.
• Complex operations can be reduced to a much simpler form with custom tags than with beans.
• Custom tags require more work to set up than beans do.
• Custom tags usually defined in relatively self-contained behavior, whereas beans are often defined in one servlet and used in a different servlet or JSP page.
• Custom tags are available only in JSP 1.1 and later versions, but beans are available in all JSP 1.x versions.

 19) Explain how to perform browser redirection from a JSP page?

Ans:The response implicit objects can be used in the following manner:
response.sendRedirect(http://www.abc.com/path/error.html);
The Location HTTP header attribute can be physically altered, as shown below:
<%
response.setStatus(HttpServletResponse.SC_MOVED_PERMANENTLY);
String newLocn = "/newpath/index.html";
response.setHeader("Location",newLocn);
%>
You can also use the: <jsp:forward page="/newpage.jsp" />
Note: This can be used only before any output has been sent to the client. Same is the case with the response.sendRedirect() method as well. If you want to pass any paramateres then you can pass using <jsp:forward page="/servlet/login"> <jsp:param name="username" value="jsmith" /> </jsp:forward>>

 20)What is the page directive is used to prevent a JSP page from automatically creating a session?

Ans:<%@ page session="false">


 21)How do you delete a Cookie within a JSP?

 Ans:
Cookie mycook = new Cookie("name","value");
response.addCookie(mycook);
Cookie killmycook = new Cookie("mycook","value");
killmycook.setMaxAge(0);
killmycook.setPath("/");
killmycook.addCookie(killmycook);

 22)Can we implement an interface in a JSP?

 Ans:No

 23)How to pass information from JSP to included JSP?

 Ans:Using <%jsp:param> tag.

 24)How is JSP include directive different from JSP include action. ?

 Ans:When a JSP include directive is used, the included file's code is added into the added JSP page at page translation time, this happens before the JSP page is translated into a servlet. While if any page is included using action tag, the page's output is returned back to the added page. This happens at runtime.

 25) Can JSP be multi-threaded? How can I implement a thread-safe JSP page?

Ans: By default the service() method of all the JSP execute in a multithreaded fashion. You can make a page “thread-safe” and have it serve client requests in a single-threaded fashion by setting the page tag’s is Thread Safe attribute to false:        
<%@ page is ThreadSafe=”false” %>

 26) Invoking a Servlet from a JSP page? Passing data to a Servlet invoked from a JSP page?

 Ans: Use <jsp:forward page="/relativepath/YourServlet" />
(or)
            response.sendRedirect("http://path/YourServlet").

Variables also can be sent as:
<jsp:forward page=/relativepath/YourServlet>
     <jsp:param name="name1" value="value1" />
     <jsp:param name="name2" value="value2" />
</jsp:forward>
You may also pass parameters to your servlet by specifying     response.sendRedirect("http://path/YourServlet?param1=val1").

 27)How do you pass an InitParameter to a JSP?

Ans:<%!
ServletConfig cfg =null;
            public void jspInit(){
                        ServletConfig  cfg=getServletConfig();
           for (Enumeration e=cfg.getInitParameterNames(); e.hasMoreElements();)
 {
                        String name=(String)e.nextElement();
           String value = cfg.getInitParameter(name);
                        System.out.println(name+"="+value);
            }
           }
%>

 28) How to view an image stored on database with JSP?

 Ans:<%@ page language="java" import="java.sql.*,java.util.*"%>
<%
          String image_id = (String) request.getParameter("ID");
          if (image_id != null){
          try
          {
            Class.forName("interbase.interclient.Driver");
Connection con = DriverManager.getConnection("jdbc:interbase://localhost/D:/examp/Database/employee.gdb","java","java");
            Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery("SELECT * FROM IMMAGINE WHERE IMMAGINE_ID = " + image_id);
            if (rs.next())
            {
                        String dim_image = rs.getString("IMMAGINE_DIMENSIONE");
                        byte [] blocco = rs.getBytes("IMMAGINE_IMMAGINE");
                        response.setContentType("image/jpeg");
                        ServletOutputStream op = response.getOutputStream();
                        for(int i=0;i<Integer.parseInt(dim_image);i++)
                        {
                                    op.write(blocco[i]);
                        }
               }
               rs.close();
               stmt.close();
               con.close();
             } catch(Exception e) {
                        out.println("An error occurs : " + e.toString());                                        
         }                        
            }
%>

 29) What are the different scope values or what are the different scope values for   "jsp:usebean"?

Ans)
Scope Object Comment
Page PageContext Available to the handling JSP page only.
Request Request Available to the handling JSP page or Servlet and forwarded JSP page or Servlet.
Session Session Available to any JSP Page or Servlet within the same session.
Application Application Available to all the JSP pages and Servlets within the same Web Application.


 30) Is JSP variable declaration thread safe?

 Ans) No. The declaration of variables in JSP is not thread-safe, because the declared variables end up in the generated Servlet as an instance variable, not within the body of the _jspservice() method.
The following declaration is not thread safe: because these are declarations, and will only be evaluated once when the page is loaded
<%! int a = 5 %>
The following declaration is thread safe: because the variables declared inside the scriplets have the local scope and not shared.
<% int a = 5 %>;

 31)  What are custom tags? Explain how to build custom tags?

 Ans) Custom JSP tag is a tag you define. You define how a tag, its attributes and its body are interpreted, and then group your tags into collections called tag libraries that can be used in any number of JSP files. So basically it is a reusable and extensible JSP only solution. The pre-built tags also can speed up Web development.
Step 1
Create a Custom tag class using only doStartTag()
 package myTagPkg;
  public class MyTag extends TagSupport
   {
    int attr = null;
    public int setAttr(int a ttr){this.attr = a ttr}
    public int getAttr(){return attr;}
    public int doStartTag() throws JspException {
    .......
    return 0;
    }
    public void release(){.....}
   }

Step 2 The Tag library descriptor file (*.tld) maps the XML element names to the tag implementations. The code sample MyTagDesc.tld is shown below:
<taglib>
  <tag>
   <name>tag1</name>
   <tagclass>myTagPkg.MyTag</tagclass>
   <bodycontent>empty</bodycontent>
   <attribute>
    <name>attr</name>
    <required>false</required>
    <rtexprvalue>false</rtexprvalue>
   </attribute>
  </tag>
</taglib>

Step 3
The web.xml deployment descriptor maps the URI to the location of the *.tld (Tag Library Descriptor) file. The code sample web.xml file is shown below:
 <web-app>
  <taglib>
   <taglib-uri>/WEB-INF/MyTagURI</taglib-uri>
   <taglib-location>/WEB-INF/tags/MyTagDesc.tld</taglib-location>
  </taglib>
 </web-app>

STEP: 4
The JSP file declares and then uses the tag library as shown below:
<%@ taglib uri="/WEB-INF/ MyTagURI" prefix="myTag" %>
< myTag:tag1 attr=�abc� />
<taglib>
  <tag>
   <name>tag1</name>
   <tagclass>myTagPkg.MyTag</tagclass>
   <bodycontent>empty</bodycontent>
   <attribute>
    <name>attr</name>
    <required>false</required>
    <rtexprvalue>false</rtexprvalue>
   </attribute>
  </tag>
</taglib>

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