JSP FAQs

What is a JSP ?

JavaServer Pages (JSP) is a technology that helps software developers create dynamically generated web pages based onHTML, XML, or other document types.

Basically it is html file with JAVA logic inside it with the extension of .jsp

.jsp  files (html + java code) = .php (html + php code) = .xhtml in .net (html + C# code or razor syntax)

How JAVA logic is inserted inside html markup?

<%@ page global attributes and imports %>

<%@ include file="jsp file location" %>

<%! Java functions to be exectued %>

<% Java code %>

<%=JAVA expression %>

How .jsp s are interpreted in a server ?

abc.jsp will be executed as a servelt named abc_jsp. So corresponding abc_jsp.class and abc_jsp.java file will be generated in a server folder to be executed.

Depending on the request (GET or POSt...) the following will be parsed within doGET() or doPOST() ...

• All the html content will be rendered in printwriter.println("html content") statements.
• All the Java codes will be executed as normal java  code.
• Any Java functions decalred in <%! %> will be parsed outside doGET() or doPOST() 

Then the final HTML data will be sent to browser/client.

What are implicit objects in JSP page ?

Meaning you dont have to create references to some objects. In a jsp scope they are created for you.

request.attribute --> HttpServeletRequest objects

session.attribute --> HttpSession objects

application.attribute --> ServletContext objects

What is the role of PageContext class ?

This object is intended as a means to access information about the page while avoiding most of the implementation details.

This object stores references to the request and response objects for each request. The application, config, session, and out objects are derived by accessing attributes of this object.

1. JSP Page – Scope: PAGE_CONTEXT
2. HTTP Request – Scope: REQUEST_CONTEXT
3. HTTP Session – Scope: SESSION_CONTEXT
4. Application Level – Scope: APPLICATION_CONTEXT

for example,

without 

request.setAttribute("key","value");

session.setAttribute("key", "value"),

you can 

pageContext.setAttribute("key", "value", PageContext.SESSION_SCOPE);

pageContext.setAttribute("key", "value", PageContext.REQUEST_SCOPE);

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Java Servlets FAQs

What is a servlet ?

Simply it means something that can extend a servers functionality.

(for example a servlet can be used to process DB queries or client request processing )

So servelts run in a server just like PHP and ASP.net and normally serverlets are deployed in a web container. So there can be many servlets in a web container.

What happens in a sevelts life cycle ?

3 Things actually.

1. Init() - Container intialize a servlet. (Serveltconfig object is initially passed to this method)
2. Service() - servlet handles the service. Decides doGET(),doPOST() or any appropriate method to execute. Normally servlet can handle any number of requests and each request run on a separate thread.
3. Destroy() - Once the service is performed that servlet/thread instance is killed.

What describes the first view/page to load in a Java web application unless it is not configured in the web server ?
web.xml (deplyment descripter)in WebContent/WEB-INF

How a Java class becomes a servlet ?
Two things.
1. It should extend HttpServlet class (if the servelt is a type of HTTP coz there are other types of servelets as well which inherits from GenericServelt class)
2. it should contain proper annotations (or Servelt tags in web.xml for apps before JAVA 5)

What happens in a Servlet web project when the browser executes an URL ?

1. Browser sends the url to Java server (Lets say www.example.com/webapplication1/randomservlet1 to wildfly/Jboss server or tomcat or anything)
2. Jboss at example.com has multiple web applications. 
3. Jboss get the url and sees that it belongs to webappplication 1
4. Jboss creates 2 objects
1. HttpServletRequest object - as inputs to servlet
2. HttpServletResponse object -as outputs from servlets
5. Jboss then sends these 2 objects to randomservlet1. (probably doGET() after reading web.xml)
6. Then servelt does its job.
7. If there is something to be sent back to browser it is included in the response object.
8. Jboss renders what is in the response object and sends back to browser. 

How a servelt gets a value from url prameter from ?name=RobinHood
String name = response.getPrameter("name")
If it is a GET this will be executed in doGET()
If it is a POST this will be executed in doPOST()
If it is a HEAD this will be executed in doHAED() and so on...................

How object creation happens in Servlet application ?

• HttpServletRequest, HttpServletResponse objects are created per connection (These are stateless)
• Create HttpSession objects if we want to maintain the state (getSession() from request object per client/browser) 
• Create ServletContext if you want to have persistand data across users/browser (setServeltContect() from request object)
• HTTPServelt s are cerated in threads per connection

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How to install Titanium Appcelerator on Linux (Ubuntu) ?

This question has been asked like lot of times in the internet and havent seen a solid answer or someone got this working. I had this problem for like 2 weeks and finally I managed to solve it. Meaning I have just installed Titanium 5.x on Ubuntu 15.10 successfully after like 2 weeks non-stop tedious attempts. I have seen this problem of Titanium forums and StackOverflow posts lot of times but again - NO SOLID ANSWER.

I cant guarantee the correctness of this solution but these are the steps I follow. 

Install JDK

This has to be 7 or 8 according to the official documentation.

Install Node.js

For linux node.js version has to be between 0.1x and 4.2x. These are the supported version according to the documentation. The latest version of node is higher than this and there is no gurantee that it will work. So I also downgraded to  4.0.0 version.

Download and install Titanium 

You will have to sign in first and download the titanium appcelerator setup zip file. Unzip it any where. Go in to that directory and execute as any user AppeceratorStudio file. This same user should have the ownership of .npm and .appcelerator directories.

When you execute this file for the first time it just connects to appcelerator aws servers and downloads the configuration. We just cant see it . No indication what so ever. No indication in ps aux, top or netstat. So just brave your self untii this is over. This takes a looooooooooooooong time. So dont uninterrupt this. If all goes well, you have titanium.

If uninterrupted ->

You will have to simply to do the configration again. Luckily when you are  using Titanium CLI also you can do the configuration.

// This downloads titanium CLI and configure it

sudo npm install appcelerator -g

appc setup  

After this you can run AppeceratorStudio file and start working on Titanium.

Hope this help. :)

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Build with Gradle

Recently one of my friends asked me to explain what a build tool is and what is Gradle. So I thought of explaining it here and go much as further as I can go smoothly and neatly.

What is a software build ?

You write code for an application. Those are the source codes. Lets say its an android application and your write Java code. When you have done writing the code you will have to make a .apk file if you want to run it on an android device. The process of converting source code -> something deployable is called building. So building may include

• Compiling process
• Linking related files 
• If your software depends on external .jars, programs or whateve, solve those dependecies
• Package your software in a deployable manner (in this case to an .apk file) 

Build tools are the tools that can do those things.

Some good tools are

C -- make

Java -- Ant,Maven,Gradle

.net --NAnt

Do we need tools for building ?

If you are doing indpendant small scale coding, you dont any FANCY build tools. But when you have large scale developement to handle with lot of different people coding on different parts and you should have like different versions of builds for different reasons and they depend on diff third party plugins, software, files your life can be like living in hell if you dont have a build tool with you. But this not the case with interpreted programs coz interpreters dont involve a compiling process hence the interpreters ;).

What is Gradle ?

Gradle is a also a built autoamtion tool. Meaning you can automatically build your project into several builds and at the same time you can automate some tests also. So gradle is opensource and used at enterprise level which is more cool. I think it came to enterprise level just because it is opensource. Power of often source huh :D. Gradle has lot of good features of ant and Maven. Now you can build Java,C,C++ apps with gradle.

Gradle is built with a Groovy a good OO Java like scripting language. So all the gradle tasks are in Groovy and if you wanna do stuff with Gradle, you ll have to learn Groovy a lil bit. Its not that actually hard. Gradle came to popularity with the start of Android Studio. I remember when I worked with Android Studio and Gradle for the first time in 2 years back , Gradle was a crappy part of the studio with that greenish slimy logo and build scripts. But as it turns out, gradle has become a indipensable tool for me now. ;) Never judge a book by its cover, huh :D

So if you wanna start with gradle, you should have it in your system. Get with package managers or download binaries or install with ecplise. There are so much you can do with gradle but we will see most common reoccurring ones like jar configs, dependency management and configs.

gradle tasks

This can show you what you can do with gradle and are as follows.
Build tasks 
assemble - Assembles the outputs of this project.
build - Assembles and tests this project.
buildDependents - Assembles and tests this project and all projects that depend on it.
buildNeeded - Assembles and tests this project and all projects it depends on.
classes - Assembles classes 'main'.
clean - Deletes the build directory.
jar - Assembles a jar archive containing the main classes.
testClasses - Assembles classes 'test'.

Build Setup tasks
init - Initializes a new Gradle build. [incubating]
wrapper - Generates Gradle wrapper files. [incubating]

Documentation tasks
javadoc - Generates Javadoc API documentation for the main source code.

Help tasks

components - Displays the components produced by root project 'GradeDemo'. [incubating]

dependencies - Displays all dependencies declared in root project 'GradeDemo'.

dependencyInsight - Displays the insight into a specific dependency in root project 'GradeDemo'.

help - Displays a help message.

model - Displays the configuration model of root project 'GradeDemo'. [incubating]

projects - Displays the sub-projects of root project 'GradeDemo'.

properties - Displays the properties of root project 'GradeDemo'.

tasks - Displays the tasks runnable from root project 'GradeDemo'.

Verification tasks
check - Runs all checks.
test - Runs the unit tests.

Rules
Pattern: clean<TaskName>: Cleans the output files of a task.
Pattern: build<ConfigurationName>: Assembles the artifacts of a configuration.
Pattern: upload<ConfigurationName>: Assembles and uploads the artifacts belonging to a configuration.

apply plugin: "java" 

This will tell the gradle system that this is a Java project. So when build is done
source files will be in -> src/main
testing source files -> src/test
all your builds -> builds/lib

repositories{mavenCentral()}
dependencies {
compile  'org.glassfish.jersey.core:jersey-client:2.22.1'
runtime 'orf.groovy:groovy:2.2'
}

Gradle can reslove dependecy problems just like in Maven and pom.xml does. So when a dependency is declared it just downloads the dependecies when the build happens from a repo mentioned. Luckily we have a in built Maven repo comes with gradle ie: mavenCentral which has lot of dependencies. Or you can use Ivy whihc is like maven repo or custom define repos.
You can also specify the type of the dependency a compile time or run time. It should be in the format of Group id : artifact id: version number (ex: 'org.glassfish.jersey.core:jersey-client:2.22.1)

sourceCompatibility = 1.8
version = '2.5'
jar {
  manifest { 
    attributes "Main-Class": 'com.kapal.gradle.Main',
"Implementation-title":"Build with gradle"
"Implementation version": version
  }

We can  add attributes to Manifest.mf which is an information file which comes with built jar. You can add any attribute to the file.

configurations {
  
 customCompile
 customCompile.transitive = false
 compile.extendsFrom(customCompile)
 compile.transitive = false
 testCompile.transitive = false
    
}

You can add configure your build(s) add custom configs for build(s) and hanldle how your build happens and how your testing builds happen.

This is what you get when a succussfull build happens

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Creating a Java based RESTful web service (JAX-RS)

Here we will build a Java based RESTfull web service using Jersey.


What is Jersey ?
Jersey is an open source framework that we can use to build RESTful web services. And furthermore it is based on standard JAX-RS framework.

So we can use just the standard JAX-RS provided by JDK also. But developing with Jersey can make you a handy person. ;)

As a start you can have a dynamic web project in Eclipse and then add .jars provided by Jersey. But the thing is Jersey jars are already in maven repository. So if you create a maven project instead of a dynamic web project, maven downloads and resolves any dependencies.

You can create a Maven project from Jersey archtype. If the related archtype is not defined you can create a arch type by providing the following.

• artifactId=jersey-quickstart-webapp
• archetypeGroupId=org.glassfish.jersey.archetypes
• archetypeVersion=2.22.1 // The latest stable version available.

What is happening with Jersey ?

1. Client request the REST service via a valid URL(

Lets say http://host.com/AppName/webapi/myResource 

Because we can request services via URLs we dont need to create additiona hard coded REST clients, just a browser is enough. But if you wannt do much more with URLs and requests there are very very good browser plugins and extensions for chome and FireFox.

2. Application (AppName)gets the request. Actually web.xml in WEB-INF gets and handles the request.

It decides what package to look for (com.kapal.restdemo)

<init-param>
            <param-name>jersey.config.server.provider.packages</param-name>
            <param-value>com.kapal.restdemo</param-value>
</init-param>

And also it does the mapping between URL and resource classes. Here it maps webapi/ any_url_portion with com.kapal.restdemo

<servlet-mapping>
        <servlet-name>Jersey Web Application</servlet-name>
        <url-pattern>/webapi/*</url-pattern>
</servlet-mapping>

3. So the resource we need to look for is myResource.
If there is a class annotated with @Path("myResource"), Jersey will get that class.

package com.kapal.restdemo;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;


@Path("myresource")
public class MyResource {

    
    @GET
    @Produces(MediaType.TEXT_PLAIN)
    public String getIt() {
        return "Got it! yeah";
    }
}

Here additional 2 annotations are needed.
@GET - respond to GET HTTP verb
@Produces - Means in what format we should send the response to client.
Typically it can be plain text, or HTML format. But sometime we may need to respond with XML format or JSON.

(MediaType.APPLICATION_XML)
With JDK we have JAXB(Java Architecture for XML Binding). So it is pretty straight foreward to convert Java objects to XML format and vice versa. All we have to do annotate the requires object class as @XmlRootElement


(MediaType.APPLICATION_JSON)
Unfortunately with JDK we dont have a inbuilt support. We need to have a 3rd party support. Jersey also has a JSON library called moxy. You can manually add the .jars or let Maven do it for you. All you need do is add the dependency by uncommenting following dependency portion in pom.xml

<dependency>
            <groupId>org.glassfish.jersey.media</groupId>
            <artifactId>jersey-media-moxy</artifactId>
 </dependency>

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Calling a remote SOAP WS (JAX-WS) from a local client

Lets say a some SOAP service is hosted in a remoter server. We dont want to know its implementation logic, language, platform, server or anything. Thats the beauty of WS. You can call it from any language, platform.

If you can google you can find loads of SOAP services for free. For now I will use http://www.webservicex.net/New/. This just a random choice. This site has loads of web services.

From all of those WS I will chose http://www.webservicex.net/New/Home/ServiceDetail/17. This can basically take a countries name as a string and provide its dialing code, currency, GMT like details.

So first step is to get it's WSDL.

Which is @ http://www.webservicex.net/country.asmx?WSDL

When you look at the end of the WSDL you can see this.

Which means 

Service name = country and its is binded  port = countrySoap

Before going to have a Java client we need to have all the stubs that complies with this WSDL.

we can do that via wsimport tool by JDK. Type this in a bash.

wsimport -keep http://www.webservicex.net/country.asmx?WSDL

-keep arg can keeps the .java files with .class files. What I normally do is after all the files parsed from the WSDL remove all the .class files via rm *.class 

So in our Java main method, we can have a instance of web service and get its binded interface which is at the above mentioned port and access all the web methods via the interface.

import net.webservicex.Country;
import net.webservicex.CountrySoap;

public class Main {

	public static void main(String[] args) {

		// According to WSDL service name is country
		// Create a web service instance
		Country country = new Country();
		
		// According to WSDL this service is binded to countrySoap port
		// So get this service's port
		CountrySoap cs = country.getCountrySoap();
		
		// Now we have the access to services interface
		// We can call its services 
		// We don't need to worry about in and out parameters and data types
		// They are being handled by stubs at client and server end
		// For now lets get the dialing number for any country
		String dialing = cs.getISD("Sri Lanka");
		
		System.out.println(dialing);
		
		
		
	}

}

Result

IMPORTANT

Actually this WS has 2 issues.
  1. It produces the result twice per each response.
  2. The result is not a native String type. It is wrapped around a table. we have to unwrap it first.

Any how Service implementation is beyond our duty and we can call the SOAP WS regardless.

underneath the communication media request is sent as a SOAP mag and response is also received as a SOAP msg.

We request

POST /country.asmx HTTP/1.1
Host: www.webservicex.net
Content-Type: application/soap+xml; charset=utf-8
Content-Length: length

<?xml version="1.0" encoding="utf-8"?>
<soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
  <soap12:Body>
    <GetISD xmlns="http://www.webserviceX.NET">
      <CountryName>string</CountryName>
    </GetISD>
  </soap12:Body>
</soap12:Envelope>

We get

HTTP/1.1 200 OK
Content-Type: application/soap+xml; charset=utf-8
Content-Length: length

<?xml version="1.0" encoding="utf-8"?>
<soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
  <soap12:Body>
    <GetISDResponse xmlns="http://www.webserviceX.NET">
      <GetISDResult>string</GetISDResult>
    </GetISDResponse>
  </soap12:Body>
</soap12:Envelope>

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How to consume a local SOAP web service (JAX-WS) - Implementation

Lets continue with our example from here and keep things simple and tidy.

Lets say we have a server(A local server for now. ie. localhost). It has a service to add any two numbers. So you have a client who wants to consume that ws. Meaning he frigging needs to add two numbers.

Server side implementation

Add service

You need to have the service now. So write a simple add method in dynamic web project in eclipse. Its is a good idea to add the relevant annotations coz I find it less buggy.

package controller;

import javax.jws.WebMethod;
import javax.jws.WebService;

@WebService(serviceName="MyCalc")
public class Calculator {

 
 @WebMethod
 public int add(int num1, int num2)
 {
  return num1 + num2;
 }
}

WSDL creation

Create a new web service in the same project and create as follows.You can create the WSDL from your add implementation Or you can create the WSDL manually according to add method. ;) 

Deploy and run the web service

Now run the project on the server. Im hoping you have a server created already. And BTW without a server installed you wont be able to create the WSDL even. I have used WildFly 9 to deploy. If all is ok you will be able to see your WSDL at

http://localhost:8080/CalculatorWS/MyCalc?wsdl

[Note: Port and server names can be changed as to how you have configured your sever]

Client implementation

One important thing. Under your web project you -> web content -> wsdl directory you can see your wsdl and its design. So according to following design

MyCalculalator web service is binded to Calculator interface via CalculatorSoapBinding(that little box in the middle). So if we need to access the web service we can access it via Calculator interface which acts as a proxy.

Before we access the web service we need to have our own interface to act as a stub. This stub should also comply with the WSDL also. In a real scenario you ll have to get the WSDL from a UDDI.

So to create the related stubs we will use wsimport tool by JDK.

wsimport -keep http://localhost:8080/CalculatorWS/MyCalc?wsdl

[If you dont add -keep it will generate only .class file and remove the .java files]

Import those files to a normal Java project. So now these stubs are in the controller package.

Now you can access the WS in the main method. In the command line you can see your service call has been executed.

import controller.Calculator;
import controller.MyCalc;

public class Main {

 public static void main (String args[]) 
 {
  
  // Create a web service instance
  MyCalc mycalc = new MyCalc();
  
  // Access that service with the binded port
  Calculator c = mycalc.getCalculatorPort();
  
  // Consume the service via its interface/proxy
  System.out.println(c.add(54, 87));
  
 
 }
 }

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How to consume a remote SOAP web service (JAX-WS)?

For the sake of explaining this scenario we will mix some theoretical aspects with a narration.
Disclaimer: This can be pretty stupid

Lets say this is the time where Mathematics hasn't been born yet. No cool mathematicians are born yet. But still you have numbers. So you dont know how to add or multiply or do anything with  the numbers. All of a sudden you need to add 2 numbers. Gladly there is a far far far away a machine that can add two numbers. So you are in luck.

That machine = Web service

Not so fast. You know that there is a such machine. But how the heck you can find it or frigging locate it. Just because you are such a lucky person, there is a wise old man in your villlage who knows all about every magical machinery crap that exist on this planet. So you can ask the directions to the adding machine.

Wise Old Man = UDDI

Just because you know the machine isnt mean that you can add 2 numbers. There is a specific way to present those 2 numbers to the machine and theres is a specific format that you get the result from the machine. Every machine has a manual ! So if you read the manual you can figure all about the machine.

Machine Manual = WSDL

Here you can parse the WSDL to have related stubs/proxies in 3 ways.

1. Look at the WS and write the WSDL manually. Thats gonna be preeeeeety HARD.
2. You can use an IDE like eclipse to do that. (3rd part tools)
3. Use wsimport utility provided by JDK 6+ versions. https://docs.oracle.com/javase/6/docs/technotes/tools/share/wsimport.html [Note: wsimport <wsdl link> provides class files by deafult because after compilation it removes the .java files. If you want to have .java files add relevenat arguments to wsimport]

So once you know how to read the manual and hopefully you can understand the manual, you can operate it. Meaning you can add your two numbers.

This is just the basic of it. See how it is really done here.

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Jboss hanging up in the startup ???

If you are working with Jboss 7.x and it hanging up in the startup, have no worries. You have a solution.

Reason for this is Jboss 7.x doesn't work with JDK 8.

Yeah its true. I normally work on linux and I have faced the same issue in Jboss 7.0 and 7.1.1 final.
Its a normal problem for windows users as well.
Read this https://developer.jboss.org/message/808212?_sscc=t

You can do 2 thing now.

1. Downgrade your JDK to 7 or 6 if you really want to keep JBoss 7.x AS.
2. Keep your JDK same. Upgrade your AS to wildfly. It's the new Jboss.

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