learning java coding: JSP

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Monday, March 30, 2020

Sunday, March 29, 2020

Difference Betwixt Genericservlet Vs Httpservlet Inwards Servlet Jsp - J2ee Question

Difference betwixt GenericServlet together with HttpServlet is 1 of the classic Servlet Interview Question, asked on many Servlet together with JSP Interviews on ii to four years sense developers. Since both GenericServlet together with HttpServlet forms footing of Servlets its of import to know What are they together with What is principal divergence betwixt them. From mutual sense together with at that spot names, its obvious that GenericServlet is a generic together with protocol independent implementation of Servlet interface piece HttpServlet implements HTTP protocol specifics. If you lot are working inwards Java spider web application or J2EE projects, you lot are around probable to bargain amongst HttpServlet all fourth dimension every bit HTTP is principal communication protocol of web. In this Servlet JSP article nosotros volition outline around of import difference betwixt HttpServlet together with GenericServlet which is worth knowing together with remembering.

GenericServlet vs HttpServlet

Here is my listing of divergence betwixt HttpServlet together with GenericServlet inwards Java Servlet API :

1) GenericServlet provides abstract service(ServletRequest, ServletResponse) method to implement which gets called past times container whenever it receives asking for processing, On the other manus HttpServlet overrides service method together with provides callback on doGet(HttpServletRequest request, HttpServletResponse) together with doPost(HttpServletRequest request, HttpServletResponse response) whenever it receives HTTP asking from GET or POST method. It too provides several other method based upon diverse HTTP methods of sending asking e.g. doPut() or doDelete() to conduct maintain HTTP PUT together with HTTP DELETE request.

2) Another difference betwixt GenericServlet together with HttpServlet is that afterward is a subclass of GenericServlet together with inherit properties of GenericServlet.

3) Generic servlet provides a rather easier agency to extend Servlet, its plenty to override service method to implement GenericServlet. Apart from extending Servlet interface, it too implements ServletConfig interface together with provides agency to stimulate got initialization parameter passed to Servlet from web.xml e.g. past times using getInitParamter().

forms footing of Servlets its of import to know What are they together with What is principal divergence b Difference betwixt GenericServlet vs HttpServlet inwards Servlet JSP - J2EE question

That's all on difference betwixt GenericServlet together with HttpServlet inwards Servlet API. Its real rare you lot are going to run GenericServlet simply its practiced to know basics every bit its generally asked inwards diverse Servlet interviews every bit well. HttpServlet is primary Servlet flat which is used inwards Java spider web application from treatment customer request.

Further Learning
Difference betwixt URL Encoding together with URL rewriting

Difference betwixt include directive together with include activity inwards JSP

What is JSESSIONID inwards Java spider web application?

How to escape HTML particular characters inwards JSP page

Difference betwixt forrard together with sendredirect inwards JSP

Sunday, November 24, 2019

6 Divergence Betwixt Include Directive Too Include Activeness Inwards Jsp

Difference betwixt include directive too include action
Even though both include directive too include activeness allows y'all to include response of 1 JSP into another, they are quite dissimilar from each other e.g. include directive is processed at translation time too used to include the static resources e.g. HTML files, images too CSS etc, spell the <jsp:include> activeness is processed at request time too used to include to a greater extent than dynamic resources e.g. JSP or Servlet. What is the deviation betwixt include directive too include activeness is too 1 of the close pop JSP interview questions, generally asked either at telephonic circular or origin few circular of Java spider web developer interviews? Let's run across a duet of to a greater extent than deviation betwixt these ii to respond this interrogation inwards to a greater extent than detail. Btw, if y'all are novel inwards JSP too merely learning too preparing for interviews at the same time, too then y'all should too await at Head First Servlet too JSP, 1 of the best books to larn too arrive at for Servlet JSP interviews too certification at the same time.

JSP include directive vs include action

The include directive is 1 the 3 directive supported past times JSP e.g. @taglib, @include too @page, spell the include activeness is 1 of the many criterion actions supported past times JSP i.e. <jsp:include>, <jsp:forward>, <jsp:useBean>, <jsp:setProperty>, <jsp:getProperty>, <jsp:param> too <jsp:plugin>.

Both are used to include the response of 1 resources into merely about other e.g. composing JSP page alongside header component subdivision coming shape header.jsp, footer coming from footer.jsp too sidebar coming from sidebar.jsp. But every bit I said, they are rattling dissimilar from each other.

Here are merely about important deviation betwixt include directive too include activeness inwards JSP:

1) The close cirtical deviation betwixt @include too <jsp:include> is that the include directive is processed at the translation fourth dimension but include activeness i.e. <jsp:include> is processed at the asking fourth dimension i.e. when the asking comes for processing.

2) Second of import deviation betwixt include directive too include activeness is thtat an include directive tin purpose both relative too absolute path but the include activeness ever uses relative path.

3) Third key diference betwixt include activeness too include directive is that @include tin exclusively include contents of resources e.g. HTML or CSS file but it volition non procedure the dynamic resource, spell the include activeness i.e. <jsp:include> procedure the dynamic resources too upshot volition move added to calling JSP.

4) One to a greater extent than practical deviation betwixt include directive too include activeness is that alongside include directive, y'all tin non overstep whatever other parameter e.g request, response or whatever configuraiton, but alongside include action, nosotros tin overstep merely about other parameter too using <jsp:param> tags.

5) Fifth deviation is extension of previous deviation i.e. alongside include directive, y'all cannot overstep the request or response object to calling jsp to included file or JSP or vice versa, but alongside include action, it's possible to overstep request/response object.

6) Last but non the to the lowest degree deviation betwixt include directive too include activeness is that onetime uses the file attribute to specify the resources to move included too that's why know every bit file include every bit well. On the other manus include activeness uses page attribute to specify the resources to move included, thence too known every bit page include inwards JSP.

Here is a overnice summary of all the differences betwixt file include too page include i.e. include activeness inwards JSP:

answer)

How to upload a File inwards JSP too Servlet? (answer)

Difference betwixt URL rewriting too URL encoding inwards JSP? (answer)

What is JSESSIONID inwards J2EE Web Application? (answer)

Difference betwixt sendRedirect() too forward() inwards Servlet? (answer)

Difference betwixt GET too POST method inwards HTTP Protocol? (answer)

Further Learning
Java Web Fundamentals By Kevin Jones
Spring Framework 5: Beginner to Guru
JSP, Servlets too JDBC for Beginners: Build a Database App

Java.Lang.Illegalstateexception: Getwriter() Has Already Been Called For This Response

This mistake comes when a Servlet calls the getOutputStream() method on response object for writing something afterwards calling the include() method. Suppose a Servlet calls the include() method to charge the response of a JSP. Since JSP has already written the response on it thence ane time again opening OutputStream on response object is illegal, you lot acquire the java.lang.IllegalStateException: getWriter() has already been called for this response error. This mistake besides comes when you lot endeavour to include response of but about other Servlet together with than tries to write something on output stream again. In short, your Servlet should never write anything on response object afterwards calling the include() method.

Here is the detailed stack draw of exception :
java.lang.IllegalStateException: getWriter() has already been called for this response
org.apache.catalina.connector.Response.getOutputStream(Response.java:580)
org.apache.catalina.connector.ResponseFacade.getOutputStream(ResponseFacade.java:183)
HelloServlet.doGet(HelloServlet.java:20)
javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)

This a perfect beginner mistake who tries to larn Servlet together with JSP past times case together with error. If you lot know the right agency to role the include() together with forward() method so you lot won't aspect upward this type of problem. That ane time again highlights the of import of learning novel technology from a expert mass similar Head First Servlet together with JSP. The mass volition no doubtfulness instruct you lot the basics, which is quite of import inwards the long journeying of learning novel characteristic every day.

java.lang.IllegalStateException: getWriter() has already been called for this response Solution

If you lot expect at the mistake carefully, you lot tin encounter it pointing to HelloServlet.doGet() method at work 20, let's encounter the HelloServlet degree to respect out what's happening at work 20.

import java.io.IOException;  import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  public class HelloServlet extends HttpServlet {     private static in conclusion long serialVersionUID = 1L;      public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         String userAgent = req.getHeader("user-agent");         String clientBrowser = "Not known!";         if (userAgent != null)             clientBrowser = userAgent;         req.setAttribute("client.browser", clientBrowser);         req.getRequestDispatcher("/hello.jsp").include(req, resp);         resp.getOutputStream().println("This is written from Servlet");     }  }

You tin encounter that at work twenty nosotros are calling the getOutputStream() afterwards calling the include() method of RequestDispather, this is causing the problem. Remove this work together with your programme volition run fine. The bottom work is you lot should endeavour to write anything on output stream of Servlet afterwards calling the include() together with forward() method because the response is already committed.

Further Learning
Java Web Fundamentals By Kevin Jones
Spring Framework 5: Beginner to Guru
JSP, Servlets together with JDBC for Beginners: Build a Database App

Difference Betwixt Jsp:Include Too Jsp:Forward Action- Example

JSP provides measure actions to create things without using Java within the scriptlet. Two of such measure actions, which help JSP to interact alongside other server resources e.g. approximately other JSP, Servlet or HTML files are, include too frontward actions. The <jsp:forward> activeness enables yous to frontward an HTTP asking to a static HTML file, a servlet, or approximately other JSP. It has an attribute called page, which accepts the URL of approximately other resources equally shown below:

<jsp:forward page="URL" />

Let's tell yous direct maintain 2 JSP files, hello.jsp too header.jsp inwards your Java spider web application too if yous desire to forward an HTTP request from hello.jsp to header.jsp, yous tin add together next lines into hello.jsp:

<jsp:forward page="header.jsp"></jsp:forward>

The path to the resources is relative, equally asking itself direct maintain cognition of electrical flow URL. Btw, what is the departure betwixt include too frontward activeness is ane of the mutual JSP interview questions, too if yous are preparing for Java or JEE developer interviews too hence yous should live aware of many such oft asked questions. One agency to laid upward good is to refer a expert mass similar Java Programming Interviews exposed, which contains questions too answers from all of import Java topics too frameworks, including Servlet too JSP.

JSP provides measure actions to create things without using Java within the scriptlet Difference betwixt jsp:include too jsp:forward action- Example

Worth noting is that the JSP that contains the <jsp:forward> activeness stops processing, clears its buffer, too forwards the asking to the target resource. Note that the calling JSP should non write anything to the response prior to the <jsp:forward> action. Anything written would acquire away too non seem inwards lastly response which comes from the header.jsp now. Remember, include activeness is unlike than include directive, which too includes response from approximately other resources merely at translation time, non at the asking time.

You tin too exceed additional parameters to the target resources using the <jsp:param> tag.

<jsp:forward page="footer.jsp" >     <jsp:param name="copyright" value="2016" />     <jsp:param name="credit" value="java" />  </jsp:forward>

In this example, the footer.jsp tin access the value of copyright too credit parameter past times calling the request.getParameter("copyright").

On the other paw <jsp:include> activeness is used to charge the content of approximately other JSP, Servlet or HTML file into electrical flow JSP. <jsp:include> too execute the code too forcefulness a level of the buffer inwards the output stream.
If a.jsp has the code like

<jsp:include page="header.jsp" flush="true" >    <jsp:param name="key" value="value" />  </jsp:include>

too hence the header.jsp execute too the output is placed inwards the a.jsp

Here is a amount listing of supported measure activeness past times JSP, yous tin too read Head First JSP too Servlet to acquire to a greater extent than almost Java Server pages:

That's all almost difference betwixt <jsp:include> too <jsp:forward> activeness inwards JSP. In curt include action is used to include contents of approximately other Servlet, JSP or HTML files, piece the forward action is used to frontward the electrical flow HTTP asking to approximately other Servlet or JSP for farther processing. They are precisely similar to include() too forward() method of RequestDispatcher too Servlet.

Further Learning
Java Web Fundamentals By Kevin Jones
Spring Framework 5: Beginner to Guru
JSP, Servlets too JDBC for Beginners: Build a Database App

Saturday, November 23, 2019

6 Divergence Betwixt Forward() Together With Sendredirect() Inwards Servlet - Java

Servlet inwards JEE platform provides 2 methods forward() as well as sendRedirect() to road an HTTP asking to about other Servlet for processing. Though, both are used for forwarding HTTP requests for farther processing at that spot are many differences betwixt forward() as well as sendRedirect() method e.g. frontward is performed internally past times Servlet, but a redirection is a two-step process, where Servlet instruct the spider web browser (client) to become as well as fetch about other URL, which is unlike from the original. That's why forward() is every bit good known every bit a server-side redirect as well as sendRedirect() is known every bit the client-side redirect. Becuase of their usefulness, the departure betwixt forward() as well as sendRedirect is every bit good a oft asked Servlet interview question. Let's run into a twain of to a greater extent than differences to answer this query better.

While working betwixt Servlet as well as JSP, yous volition often utilisation these asking re-direction methods similar include(), forward(), or sendRedirect(). Sometimes, yous volition include the response from other Servlet as well as sometimes it volition merely frontward the asking for processing to about other Servlet.

Btw, if yous are preparing for Java JEE interviews as well as learning this every bit utilisation of your interview preparation, yous should every bit good banking concern jibe the Java Programming Interview Exposed, 1 of the rare majority which covers all of import topics for Java JEE interviews. It covers kernel Java, Spring, Hibernate, JDBC as well as other advanced topics expected from both beginner as well as experienced Java programmers.

Servlet inwards JEE platform provides 2 methods vi Difference betwixt forward() as well as sendRedirect() inwards Servlet - Java

Forward vs SendRedirect inwards Servlet

Now, let's run into about of import differences betwixt sendRedirect() as well as forward() method of Servlet API:

1) First as well as most of import departure betwixt the forward() as well as sendRedirect() method is that inwards instance of former, redirect happens at server destination as well as non visible to client, but inwards instance of later, redirection happens at customer destination as well as it's visible to client.

2) Another key departure betwixt forward() as well as sendRedirect() is that frontward is marginally faster than the redirect. The sendRedirect() is marginally slower than a forward, since it requires 2 browser requests, non one. See Head First Servlet as well as JSP for to a greater extent than details.

3) Third departure betwixt forward() as well as sendRedirect() method is that inwards instance of forward() master URL remains intact, piece inwards instance of sendRedirect() browser knows that it's making a novel request, as well as then master URL changes.

context (Some containers brand provisions for cross-context communication but this tends non to last real portable). But the sendRedirect() method tin give the sack last used to redirect users to resources that are non utilisation of the electrical flow context, or fifty-fifty inwards the same domain. See Head First Servlet as well as JSP for to a greater extent than details.

6) In the instance of forward, Since both resources are utilisation of the same context, the master asking context is retained. On the other hand, since post redirect involves a novel request, the previous asking orbit objects, amongst all of its parameters as well as attributes are no longer available after a redirect. (Variables volition demand to last passed past times via the session object).

That's all virtually the difference betwixt forward() as well as sendRedirect() inwards Servlet. It's 1 of the of import concepts to larn for Java spider web developers. It's non merely of import from the interview signal of thought but every bit good from writing real-world Java spider web application using Servlet, JSP, jQuery as well as other JEE technologies. Practically, yous volition convey several choices for redirecting HTTP asking e.g. yous tin give the sack produce as well as then past times using jQuery every bit well, but yous demand to utilisation sendRedirect() as well as forward() if yous desire to produce as well as then from Servlet.

Further Learning
Java Web Fundamentals By Kevin Jones
Spring Framework 5: Beginner to Guru
JSP, Servlets as well as JDBC for Beginners: Build a Database App

Java.Lang.Illegalstateexception: Getoutputstream() Has Already Been Called For This Response

This fault comes when y'all telephone vociferation upwards include() or forward() method afterward calling the getOutputStream() from ServletResponse object as well as writing into it. This fault is like to java.lang.IllegalStateException: getWriter() has already been called for this answer error, which nosotros accept seen inwards the earlier article.

This is the exception:
org.apache.jasper.JasperException: java.lang.IllegalStateException: getOutputStream() has already been called for this response
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:502)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:424)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
HelloServlet.doGet(HelloServlet.java:25)
javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)

as well as hither is the root cause :

java.lang.IllegalStateException: getOutputStream() has already been called for this response
org.apache.catalina.connector.Response.getWriter(Response.java:611)
org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
javax.servlet.ServletResponseWrapper.getWriter(ServletResponseWrapper.java:112)
org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125)
org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118)
org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:180)
org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:118)
org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:77)
org.apache.jsp.hello_jsp._jspService(hello_jsp.java:80)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:388)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
HelloServlet.doGet(HelloServlet.java:25)
javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)

If y'all expect at the stack describe it points to HelloServlet class, which is our Servlet, the business 25 of HelloServlet seems to live on causing the problem.

You tin dismiss run into that nosotros are calling the include() method afterward calling the getOutputStream() on Servlet object. Just comment out the telephone vociferation upwards to getOutputStream() and everything volition live on Ok. To acquire to a greater extent than close Servlet as well as JSP, read Head First Servlet as well as JSP, ane of the best books from concluding 10 years for learning JSP as well as Servlet.

Here is the consummate Java Servlet for your testing, y'all tin dismiss run this Servlet into Tomcat correct from your Eclipse IDE.

import java.io.IOException;  import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;  public class HelloServlet extends HttpServlet {      public void doGet(HttpServletRequest req, HttpServletResponse resp)             throws ServletException, IOException {         String userAgent = req.getHeader("user-agent");         String clientBrowser = "Not known!";         if (userAgent != null) {             clientBrowser = userAgent;         }         req.setAttribute("client.browser", clientBrowser);         //req.getRequestDispatcher("/hello.jsp").forward(req, resp);          resp.getOutputStream().println("This is written from Servlet");         req.getRequestDispatcher("/hello.jsp").include(req, resp);     }  }

That's all about java.lang.IllegalStateException: getOutputStream() has already been called for this response fault inwards Servlet. You accept learned what crusade this fault as well as how to laid upwards this. In short, this fault comes when y'all telephone vociferation upwards the include() or forward() method afterward committing the answer or past times calling the getOutputStream() on the answer object.

Further Learning
Java Web Fundamentals By Kevin Jones
Spring Framework 5: Beginner to Guru
JSP, Servlets as well as JDBC for Beginners: Build a Database App

Difference Betwixt Include() In Addition To Forward() Methods Of Requestdispatcher Inwards Servlert

What is the departure betwixt include together with forrard methods of RequestDispatcher interface is 1 of the frequently asked Servlet questions from Java EE interviews together with we'll run into how yous tin reply this inquiry on your interview. You larn the RequestDispatcher reference either from ServletContext or ServletRequest interface together with fifty-fifty though both include() together with forward() method allow a Servlet to interact amongst closed to other servlet, primary departure betwixt include() together with forward is that include() method is used to charge the contents of the specified resources (could locomote a Servlet, JSP, or static resources e.g. HTML files) straight into the Servlet's response, equally if it is purpose of the calling Servlet. On the other hand, forward() method is used for server side redirection, where an HTTP asking for 1 servlet is routed to closed to other resources (Servlet, JSP file or HTML file) for processing.

This inquiry is often asked equally a follow-up of to a greater extent than pop Servlet Question, the difference betwixt forward() together with sendRedirect() inwards Servlet. Now, nosotros know the key departure betwixt these 2 methods, let's run into a twain of to a greater extent than differences to reply this inquiry a fiddling chip better.

Just recollect that, fifty-fifty though yous tin larn the RequestDispatcher reference from ServletContext.getRequestDispatcher() method or ServletRequest.getRequestDispatcher() method only at that topographic point is a slight difference, the pathname of Servlet must get amongst a / and is interpreted equally relative to the electrical flow context root, piece inwards illustration of ServletRequest, path tin locomote relative.

Btw, if yous are preparing for Java spider web or Java EE developer interview, yous tin likewise run into the Java Programming Interview exposed for to a greater extent than questions on Servlet, JSP, Spring, together with Hibernate, key skills for Java spider web developers.

include() vs forrard inwards Servlet

Before looking at the differences betwixt include() together with forward() method inwards Servlet, let's run into closed to similarities betwixt them.

1) Both include() together with forward() methods are purpose of RequestDispatcher interface of Servlet API

2) Both methods stimulate got objects of ServletRequest together with ServletResponse interface.

3) Both include() together with forward() tin interact amongst static together with dynamic resources e.g. closed to other Servlet, JSP or HTML files.

Now, let's run into twain of differences betwixt include together with forward() method from Servlet API:

1) First together with inaugural departure is that include() method includes the content of a resources inwards the response, the resources could locomote closed to other Servlet, JSP or HTML file. While forward() method is used to forrard the asking to closed to other resource.

2) The minute departure betwixt include() together with forward() from Servlet API is that If yous include a servlet or JSP document, the included resources must non endeavor to alter the response condition code or HTTP headers, whatever such asking volition locomote ignored. On the other hand, If yous include a Servlet or JSP document, the included resources must non endeavor to alter the response condition code or HTTP headers, whatever such asking volition locomote ignored.

3) Third together with a useful departure betwixt forward() together with include() method is that quondam is often used to include mutual boilerplate text of template markup which mightiness locomote included yesteryear many Servlets e.g. header or footer. While, forward() method is often used where a servlet is taking a controller role; processing closed to input together with deciding the number yesteryear returning a item response page.

Here are 2 diagrams which volition help yous sympathize how both include together with forrard methods of RequestDispatcher works inwards Servlet application:

How include method of RequestDispatcher works
You tin run into that the asking is for DispatcherServlet which calls include() method to include the response of Hello.jsp page, afterwards the response of DispatcherServlet together with Hello.jsp is combined together with sent to the client.

What is the departure betwixt include together with forrard methods of Difference betwixt include() together with forward() methods of RequestDispatcher inwards Servlert

How forrard method of RequestDispatcher works
You tin run into the asking is for Servlet1 which forwards it to Servlet2, whose response is together with hence sent to the client. The response of Servlet1 is non used or non sent to the client.

That's all virtually the difference betwixt include() together with forward() inwards Servlet API. You should usage include() method to charge a resources which could locomote a JSP page or closed to other Servlet, together with usage forward() to redirect the asking to closed to other resources for farther processing, 1 time to a greater extent than yous tin forrard the asking to closed to other Servlet or JSP page.

Related Servlet together with JSP interview questions for Java programmers

Difference betwixt ServletContext together with ServletConfig inwards Servlet? (answer)
Can yous declare Constructor within Servlet class? (answer)
Difference betwixt GenericServlet together with HttpServlet inwards Servlet API? (answer)
Difference betwixt the spider web server, application server together with Servlet container? (answer)
How to grapple customer session inwards Servlet application? (answer)
Difference betwixt include directive together with include activity inwards JSP? (answer)
Difference betwixt jsp:include together with jsp:forward activity inwards JSP? (answer)

Further Learning
Java Web Fundamentals By Kevin Jones
Spring Framework 5: Beginner to Guru
JSP, Servlets together with JDBC for Beginners: Build a Database App

Saturday, November 9, 2019

Top Xl Advanced Java/J2ee Interview Questions Answers For Experienced - Iii To V Years

I have got shared many posts most Java interview query inward the past times but nothing, especially for back upwardly engineer perspective. Since at that spot are a lot of L1 as well as L2 back upwardly task exists inward BFSI (Banking, Finance, as well as Insurance Industry) as well as they are also for the longer term, I decided to write this post. When you lot move for a back upwardly engineer role, especially for Java application live on it to heart as well as person Java or Java EE application, Interviewer expects you lot know basics of Java as well as tin start/stop the application, come across log files, warning developers, as well as other aspects of the back upwardly e.g. opening conference pair to solve production issues, communicating to stakeholders, as well as involving correct peoples. In this article, I volition percentage questions to assist with technical aspects e.g. the essential skills you lot demand to clear a back upwardly engineer interview.

Even though you lot volition live on interviewed to back upwardly Java application, it's expected from you lot to know SQL, UNIX, XML as well as other key technologies which you lot volition exercise inward your day-to-day job. In this post, you lot volition notice questions which are asked to banking concern represent whether a candidate has required skills to back upwardly Java application or not.

39 Interview Questions for Java JEE Support Engineers

Here is my collection of some of the best interview questions for Java as well as JEE back upwardly professionals. If you lot are working inward production services or supporting Java application, these interview query volition assist you lot to create good on your interview. If you lot moving from evolution to back upwardly role so also these questions volition give you lot a proficient thought most what is expected from a Java JEE back upwardly engineers.

1) How create you lot banking concern represent your Java procedure is running on Linux? (example)
You tin banking concern represent past times using "ps" ascendancy as well as "grep" ascendancy e.g. ps -ef | grep "myprocess". The keyword which you lot exercise with grep for search tin live on anything unique to your process, something which appears inward its ascendancy draw e.g. get upwardly of the shape which implements the primary method. You tin also create "ps -ef | grep java" to listing all Java process.

2) How create you lot banking concern represent how much retention as well as CPU your Java procedure is consuming?
First, you lot demand to notice the PID of your process, which you lot tin notice past times using the "ps" ascendancy equally shown inward the previous question. Once you lot notice the PID you lot tin exercise the "top" ascendancy to notice the CPU as well as retention usage. Alternatively, you lot tin also exercise the prstat ascendancy equally shown here.

3) What does -Xmx as well as -Xms parameters mean?
These are parameters to specify heap size inward Java. The -Xms defines the size of the heap when JVM starts upwardly as well as -Xmx is used to specify the maximum heap size for Java application i.e. your heap cannot grow beyond that as well as JVM volition croak past times throwing OutOfMemoryError if your heap doesn't have got plenty infinite to create novel objects. See here to acquire to a greater extent than most heap retention inward Java.

4) What is the departure betwixt JVM as well as JRE?
The JRE stands for Java Runtime Environment as well as JVM stands for Java Virtual Machine. You install JRE to run Java application e.g. Applet or Core Java application or Web server similar Tomcat. The JVM is business office of JRE. See here to acquire to a greater extent than differences betwixt JVM as well as JRE.

5) What is the departure betwixt JVM as well as JIT? (answer)
The JVM stands for Java Virtual machine spell JIT stands for Just inward fourth dimension Compiler. The JIT is business office of JVM as well as used to convert the Java bytecode into native machine code which runs faster. There is some threshold gear upwardly if a code runs to a greater extent than than the threshold it becomes the candidate of only inward fourth dimension compilation past times JIT.

6) How create you lot accept the heap dump of a Java process?
There are many ways to accept the heap dump of a Java procedure e.g. Tomcat, but most mutual is past times using tools available inward JDK e.g. jVisualVM, jCmd, as well as jmap. Here is the ascendancy you lot tin exercise to accept the heap dump of Java process:

$ jmap -dump:live, file=/location/of/heap_dump.hprof  PID

The heap dump volition comprise all alive objects as well as they are stored inward heap_dump.hprof file. Remember, you lot also demand PID of Java procedure which you lot tin notice past times using "ps" as well as "grep" ascendancy equally discussed inward the source question. You tin come across Java Performance Companion past times Charlie Hunt to acquire to a greater extent than most taking as well as analyzing heap dump inward Java to notice retention leak as well as other retention related errors.

7) How create you lot accept a thread dump of a Java process?
Taking thread dump is easier than taking heap dump because you lot don't demand to call back tool. In Linux, you lot tin only exercise the kill ascendancy to accept the thread dump e.g.

$ kill -3 PID

volition impress the thread dump inward the log file or where System.out is redirected. Similarly, inward Windows, you lot tin exercise Ctrl + Break from the ascendancy prompt. Alternatively, you lot tin also exercise jConsole as well as VisualVM to accept the thread dump of Java application inward both Windows as well as Linux. You tin also read Java Performance The Definitive Guide By Scott Oaks to acquire to a greater extent than most thread dump as well as heap dump.

8) What is OutOfMemoryError inward Java? How create you lot bargain with that?
The Java virtual machine throws java.lang.OutOfMemoryError when at that spot is non plenty retention to run the application e.g. no to a greater extent than retention to create novel objects, no to a greater extent than retention to create novel threads etc. The most mutual OutOfMemoryError is the java.lang.OutOfMemoryError: coffee heap space, which comes when at that spot is no to a greater extent than retention left to create a novel object.

9) What is the departure betwixt 32-bit as well as 64-bit JVM? (answer)
The primary differences betwixt 32-bit as well as 64-bit JVM are that afterward is designed for 64-bit operating arrangement e.g. Windows 8 or afterward versions of Linux. From Java developer's perspective, the primary departure betwixt them comes from heap size. Influenza A virus subtype H5N1 64-bit JVM virtually has unlimited heap memory equally compared to 4GB of the theoretical bound of 32-bit JVM. If your plan needs to a greater extent than memory, meliorate run it on 64-bit JVM with large heap space. See hither to acquire to a greater extent than most 32-bit as well as 64-bit JVM.

13) What is Garbage Collector? (answer)
The garbage collector is a element of Java virtual machine which is responsible for reclaiming retention from dead objects. It's i of the key components as well as allows an application developer to focus on application evolution rather than doing retention management. Some of the pop garbage collectors are a Concurrent Mark-Sweep garbage collector as well as G1 garbage collector inward recent times.

14) How create you lot analyze a heap dump?
There are many tools to analyze heap dump inward Java e.g. you lot tin exercise the jhat tool which comes along with JDK. You tin also exercise Eclipse Memory Analyzer to analyze heap dump to notice out whatever retention leak inward Java spell dealing with OutOfMemoryError inward Java. See Java Performance The Definitive Guide By Scott Oaks to acquire to a greater extent than about analyzing Java Heap dump.

I have got shared many posts most Java interview query inward the past times but zero Top xl Advanced Java/J2EE Interview Questions Answers for Experienced - iii to five Years

15) What is the departure betwixt Web Server as well as Application Server? (answer)
The primary departure betwixt Web as well as Application Server comes from the fact that you lot cannot run EJB on the Web server similar Tomcat or Jetty. The application server similar WebLogic as well as WebSphere provides the runtime surroundings for EJB as well as other advanced services required past times Java EE or J2EE specification.

16) What is the departure betwixt Apache httpd as well as Tomcat?
Though both httpd as well as Tomcat are products of Apache software foundation, the httpd is to a greater extent than pop as well as used across the spider web as well as non only inward Java world. The httpd is a spider web server which tin serve static HTML files as well as dynamic content using PHP, Perl or Python, spell Tomcat is a Servlet container which provides the runtime surroundings for Servlet as well as JSP. You tin also exercise both Apache httpd as well as Tomcat together inward Java world.

17) Can you lot exercise Apache as well as Tomcat together?
Yes, you lot tin exercise Apache as well as Tomcat together. There are many Java spider web application which is fronted past times Apache spider web server to deliver static resources e.g. HTML files, images, configuration files etc. You tin so configure Apache to forwards the asking to Tomcat.

18) How create you lot start as well as halt Tomcat inward Linux?
When you lot install Tomcat inward Linux past times unzipping the downloaded package, you lot tin come across that at that spot is a startup.sh as well as shutdown.sh file inward the tomcat/bin directory. These scripts are used to start as well as halt Tomcat inward Linux. These scripts internally telephone phone Catalina.sh, the primary script to start Tomcat engine.

19) How create you lot start as well as halt Apache inward Linux?
The Apache server runs equally httpd daemon inward Linux as well as you lot tin either exercise kill command after finding the PID of httpd procedure equally shown hither or you lot tin exercise the apachectl script equally shown below to start as well as halt Apache spider web server inward Linux

Starting Apache spider web server inward Linux

$ apachectl start

Stopping Apache web server inward Linux

$ apachectl stop

Restarting Apache spider web server inward Linux

$ apachectl restart
or:
/sbin/service httpd restart

You mightiness demand root access to create if Apache spider web server is non running on your application account.

20) What is JDBC?
The JDBC stands for Java Database connectivity as well as it provides API to connect a database from Java programs. Different Java vendors e.g. MySQL, Oracle, Microsoft SQL Server, PostgreSQL they implement the API as well as provides their driver implementation to allow Java to connect to their database. Since almost every venture application has a database, JDBC is real of import for whatever existent globe projects.

21) What is the departure betwixt JDBC as well as JNDI?
As I said inward the previous answer, the JDBC stands for Java Database Connectivity as well as provides APIs as well as guidelines to connect a database from Java, spell JNDI stands for Java Naming as well as Directory Interface as well as provides a logical construction for retrieving resources e.g. database, messaging queues, venture Java beans without knowing their physical house e.g. host or port. You tin register a resources with JNDI as well as so residuum of your application element tin access them using JNDI name. The database connexion puddle is the most mutual resources accessed via JNDI inward web servers similar Tomcat or WebLogic.

22) What is database connexion pool? (answer)
As it get upwardly suggests, it only a puddle of database connections. Since creating a novel database connexion inward real-time is an expensive procedure as well as tin potentially deadening downwards the response time, many application maintains a puddle of active database connection. When a request comes inward they retrieve a connexion from the pool, acquire the information from the database as well as so render the connexion dorsum to puddle so that it tin live on reused. This way, response fourth dimension is improved inward most of the Java spider web application. See here to setup a database connexion puddle inward Tomcat using Spring framework.

23) Your Java application is connected to a Database via a connexion pool. Suddenly your database goes down? Is that an number with your Java application? Do you lot demand to restart your Java application?

hint: Since your Java application is using a connexion pool, it has active connections to database which volition acquire disconnected i time DB goes down. If you lot attempt to execute a query, you lot volition have Socket errors.

24) How create you lot shipping spider web service asking from Linux? (answer)
There are some commands available inward Linux e.g. curl as well as wget which allows you lot to shipping HTTP commands, which you lot tin exercise to telephone phone as well as essay your spider web services from Linux. Particular, Curl is used extensively to essay RESTful Web Services because it tin shipping POST request, GET request, asking with headers as well as also authenticate using both basic as well as digest authentication.

If you lot are working with REST API, I advise learning curl, it's real convenient to banking concern represent if your API is working properly using curl from the ascendancy line. You tin fifty-fifty write a script to automate those stuff.

25) What is the departure betwixt HTTP as well as HTTPS? (answer)
This is i of the mutual as well as like shooting fish in a barrel questions. Of course, you lot know the departure betwixt HTTP as well as HTTPS right? well, HTTP is insecure but HTTPS is secure, the extra "s" is for security, which agency it non exclusively encode as well as encrypt the message earlier sending but also verify the identity of the server past times using SSL certificates provided past times global certificate government e.g. GoDaddy, Thawte, VeriSign, Digicert, GeoTrust, as well as Comodo. See here to acquire to a greater extent than how SSL as well as Certifications run inward Java spider web application.

26) What is the departure betwixt the GET as well as POST method? (answer)
Another uncomplicated but ofttimes asked query on Java back upwardly interviews. The primary departure betwixt the GET as well as POST is that GET is both idempotent and rubber but POST is not. You tin burn GET asking multiple fourth dimension as well as it volition give the same number but multiple POST submission should ever live on avoided. GET is also used to read information from server as well as POST is used to shipping information to the server.

27) What is the departure betwixt Servlet as well as JSP? (answer)
This is some other interesting Java JEE back upwardly Interview Question. Even though both Servlet as well as JSP is used to create dynamic HTML the key departure betwixt them is their purpose. Servlet is meant for Java developers as well as you lot write to a greater extent than Java than HTML as well as that's why it serves equally Controller inward many pop MVC framework e.g. DispatcherServlet inward Spring MVC.

On the other hand, JSP is designed for HTML developers as well as it's to a greater extent than HTML than Java as well as that's why it is used equally "view" inward MVC frameworks similar Spring as well as Struts. See the link inward the reply for to a greater extent than detailed comparison.

28) What is the departure betwixt Struts as well as Spring MVC?
This is i time again a pop as well as ofttimes asked query on Java JEE Interviews. Even though both are pop spider web MVC framework for Java applications, the key departure is that Spring brings dependency injection source as well as Struts brings it afterward using Struts 2.0.

Spring is also a suite of libraries e.g. you lot acquire Spring Security to implement safety inward your application, so at that spot is Spring Boot, Spring Data, Spring Cloud as well as many to a greater extent than useful libraries nether Spring umbrella.

answer)
There are many differences betwixt JDBC as well as Hibernate but the most of import i is that JDBC provides an API to connect to the database as well as execute the query but Hibernate is an ORM (Object Relational modeling) framework, which agency it allows you lot to run with objects spell it takes aid of saving as well as retrieving object from database.

In short, inward JDBC you lot demand to write SQL queries to acquire as well as shop information from database but inward Hibernate you lot only bargain with objects, Hibernate takes aid of issuing SQL queries to the database.

30) What is the departure betwixt Stack retention as well as heap? Which i is faster? (answer)
These are only 2 dissimilar retention areas used to shop dissimilar kinds of variables. The stack is local to every thread spell heap retention is shared alongside all threads. Since Stack is closer to thread as well as unremarkably implemented using CPU registers they are faster than heap memory. The stack is used to shop local variables equally good method telephone phone frames spell heap is used to shop objects as well as shape metadata. See the detailed reply for to a greater extent than points.

31) What is the departure betwixt TCP/IP as well as UDP protocol? (answer)
There are several differences betwixt TCP as well as UDP protocol but the most of import of them is that TCP provides a guarantee of message delivery spell UDP doesn't give whatever guarantee. Because of this guarantee TCP sends to a greater extent than messages than UDP as well as thus eat to a greater extent than bandwidth as well as slower than UDP. TCP is also connexion oriented protocol spell UDP is connectionless. See the detailed reply for to a greater extent than points.

32) Your Java application is connecting to some other Java application (server) running on the remote host as well as listening on port 17865. Now, you lot don't have got access to that remote host to move as well as come across if the procedure is running. How create notice if the server is upwardly as well as running?
hint: you tin exercise the telnet command

33) You are supporting a Java spider web application which connects to many heart as well as person Java procedure as well as gets information from them to the distribution to clients? One customer complains that the asking for information is taking also long as well as it's almost similar hung? How create you lot troubleshoot this problem?

hint: banking concern represent the log, banking concern represent database, banking concern represent the file system

34) The Java application you lot back upwardly connects to a database via a DNS, which automatically switches to the secondary database server when the primary goes down. You notice that inward the trial of a failover, your Java application is non connecting to secondary DB server? How create you lot troubleshoot this problem?

hint: banking concern represent which version your Java application is running. Some JRE caches the DNS e.g. JRE 1.6

answer)
The deadlock is a status which tin occur betwixt 2 or multiple threads. In this case, each of the thread waits for each other as well as cannot progress. In Java, this unremarkably happens when thread 1 holds the lock required past times thread 2 as well as thread 2 holds the lock required past times thread 1. If your Java plan is hung so it could live on a deadlock. You tin accept a thread dump as well as notice out if whatever thread is waiting for the lock concord past times other as well as vice-versa. You tin also exercise jConsole tool to notice deadlock.

36) What is the race condition? (answer)
The race status is some other multithreading as well as concurrency põrnikas which happens due to racing betwixt 2 threads, for example, if i thread is updating a variable as well as minute thread tries to read the value earlier it finished. You tin avoid race weather condition past times properly synchronizing your code.

37) What is the departure betwixt PATH as well as CLASSPATH? (answer)
Both are key surroundings variable used past times Java platform, but the key departure betwixt them is that PATH points to the JDK binaries or native libraries e.g. java.exe, spell CLASSPATH points to Java binaries e.g. JAR files, which contains bytecode. PATH is also arrangement marker concept independent of Java but CLASSPATH is purely Java concept as well as used past times JVM to charge classes required past times Java application you lot are running.

38) How create you lot notice the Java version used past times your application? (answer)
You tin run the coffee -version ascendancy inward the ascendancy prompt to notice out the version of Java used past times your application. If you lot have got multiple JDK or JRE installed so brand certain you lot exercise the i which is used past times your application.

39) What is JAVA_HOME surroundings variable? (answer)
The JAVA_HOME is zero but a user defined surroundings variable which points to the installation directory of JDK. For example, if you lot have got installed JDK inward /opt/jdk_1.6_20 inward Linux so the JAVA_HOME variable volition betoken to this directory. Same goes for Windows or whatever other operating system.

That's all most 39 essential questions fro Java JEE Support Engineer. It's only a pocket-size gear upwardly of questions you lot tin hold off inward whatever service based companionship or investment banks but does call back to create key skills required for back upwardly engineers e.g. UNIX, SQL, Java, as well as others. In the adjacent business office of this article, I volition percentage some of the ofttimes asked questions from Java Support Engineer on SQL, UNIX, as well as other supporting technologies.

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This list of Java interview questions for Java, JEE, as well as J2EE back upwardly engineers should live on plenty to create for whatever Java interview. You tin crevice both fresher as well as sense marker interview past times preparing these Java interview questions, at to the lowest degree source few rounds. If you lot human face upwardly whatever other interesting query which is non inward this list, so delight permit me know via comments as well as I'll add together it.

Further Learning
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