Collection Framework:List of classess and Interfaces

How many ways we can set java class path

How to set Java classpath? List as many ways as you can. This can be an interesting Java job interview question. Here are what I came up with:

1. Use -classpath JVM option:
   

   java -classpath C:\hello\build\classes com.javahowto.test.HelloWorld

2. Use -cp JVM option, a short form of -classpath:
   

   java -cp C:\hello\build\classes com.javahowto.test.HelloWorld

3. Use -Djava.class.path system property:
   

   java -Djava.class.path=C:\hello\build\classes com.javahowto.test.HelloWorld

4. Use CLASSPATH environment variable:
   

   set CLASSPATH=C:\hello\build\classes;
   java com.javahowto.test.HelloWorld

5. Use current directory as the default classpath:
   

   cd C:\hello\build\classes;
   java com.javahowto.test.HelloWorld

6. Package all classes into a self-containing jar file that has this in its META-INF/MANIFEST.MF.
   Main-Class: com.javahowto.test.HelloWorld
   

   java -jar hello-world.jar

   Note: when you run java with -jar option, any -classpath, or -cp options are ignored, as JVM thinks all classes are already contained inside the jar file.

Among all those options, I usually use option 1, and occasionally 6. I don't like my Java apps have dependency on environment settings like CLASSPATH, which is totally unpredictable. Using current directory as the default classpath is not a good idea either, since your classes may be scattered in several directories, folders and jar files.

Can I set classpath at java runtime dynamically and programmatically? No. Although you can set the system property java.class.path in your application, but its new value doesn't affect the system classloader. If you need to reset classpath, it's time to consider using a custom classloader as the child loader of the system classloader.

With a custom classloader, you have full control where to load class files, from local file system, remote url, or even database. Classes loaded by such a custom loader are only visible by this loader and its child loaders, but not to its parent loader nor the system classloader.

For the same reason, you can't dynamically set minimum/maximum heap size of a JVM. These JVM options are all set once when JVM is started and unchanged throughout its life.

JDK 6 now support * in classpath, so you can include all jar files in a directory to the classpath easily. But you may run into problems described in http://javahowto.blogspot.com/2006/07/jdk-6-supports-in-classpath-but-be.html

While using Glassfish v3.1 with JSF 2.0 I ended up getting this very annoying warning. WARNING: PWC4011: Unable to set request character encoding to UTF-8 from context /, because request parameters have already been read, or ServletRequest.getReader() has already been called

This seems to be very common with a very hard to find solution on google.
Finnally after a lot of search I found this link which had the solution.


Place the following in the glassfish-web.xml of the web application.

<parameter-encoding default-charset="UTF-8"/>

<!DOCTYPE glassfish-web-app PUBLIC "-//GlassFish.org//DTD GlassFish Application Server 3.1 

Servlet 3.0//EN" "http://glassfish.org/dtds/glassfish-web-app_3_0-1.dtd">

 <glassfish-web-app error-url="">
 <parameter-encoding default-charset="UTF-8"/>

...
</glassfish-web-app>

Getting values From the url in jsf

1. activation.xhtml











Your account is successfully activated!
You will in 3 seconds be redirected to home page


Activation failed! Please enter your email address to try once again.

...




Backing Bean:


/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package com.example;

import java.util.Map;
import javax.annotation.PostConstruct;
import javax.faces.bean.ManagedBean;
import javax.faces.bean.ManagedProperty;
import javax.faces.bean.RequestScoped;
import javax.faces.context.FacesContext;

/**
*
* @author Pujya Sri
*/
@ManagedBean
@RequestScoped
public class Activation {
private boolean valid;

public boolean isValid() {
return valid;
}

public void setValid(boolean valid) {
this.valid = valid;
}

@PostConstruct
public void init() {
Map requestMap = FacesContext.getCurrentInstance().getExternalContext().getRequestParameterMap();
String key = (String) requestMap.get("key");
System.out.println("hello from url" + key);

// Get User based on activation key.
// Delete activation key from database.
// Login user.
}
// ...
}

counting the number of times a user has visited a page in HTML 5

<script type="text/javascript">
if (localStorage.pagecount)
{
localStorage.pagecount=Number(localStorage.pagecount) +1;
}
else
{
localStorage.pagecount=1;
}
document.write("Visits "+ localStorage.pagecount + " time(s).");
</script>

Creating our own Captcha in Java server faces

step 1:Create a servlet

//This can be located at a package called servlets(next to entities and managed beans)
public class MyCaptcha extends HttpServlet{

      private int height = 0;

    private int width = 0;

    public static final String CAPTCHA_KEY = "captcha_key_name";

   @Override

    public void init(ServletConfig config) throws ServletException {

        super.init(config);

        height = Integer.parseInt(getServletConfig().getInitParameter("height"));

        width = Integer.parseInt(getServletConfig().getInitParameter("width"));

    }

    @Override

    protected void doGet(HttpServletRequest req, HttpServletResponse response) throws IOException, ServletException {

        //Expire response

        response.setHeader("Cache-Control", "no-cache");

        response.setDateHeader("Expires", 0);

        response.setHeader("Pragma", "no-cache");

        response.setDateHeader("Max-Age", 0);

        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

        Graphics2D graphics2D = image.createGraphics();

        Hashtable<TextAttribute, Object> map = new Hashtable<TextAttribute, Object>();

        Random r = new Random();

        String token = Long.toString(Math.abs(r.nextLong()), 36);

        String ch = token.substring(0, 6);

        Color c = new Color(0.6662f, 0.4569f, 0.3232f);

       GradientPaint gp = new GradientPaint(30, 30, c, 15, 25, Color.white, true);

        graphics2D.setPaint(gp);

        Font font = new Font("Verdana", Font.CENTER_BASELINE, 26);

        graphics2D.setFont(font);

        graphics2D.drawString(ch, 2, 20);

        graphics2D.dispose();

        HttpSession session = req.getSession(true);

        session.setAttribute(CAPTCHA_KEY, ch);

        OutputStream outputStream = response.getOutputStream();

            ImageIO.write(image, "jpeg", outputStream);

        outputStream.close();

    }

}

Step 2: configuring web.xml

<servlet>

            <servlet-name>Captcha</servlet-name>

            <servlet-class>servlets.MyCaptcha</servlet-class>

            <init-param>

                  <description>passing height</description>

                  <param-name>height</param-name>

                  <param-value>30</param-value>

            </init-param>

            <init-param>

                  <description>passing height</description>

                  <param-name>width</param-name>

                  <param-value>120</param-value>

            </init-param>

      </servlet>

      <servlet-mapping>

            <servlet-name>Captcha</servlet-name>

            <url-pattern>/Captcha.jpg</url-pattern>

      </servlet-mapping>

step 3;Add the captcha markup at the front page

<h:graphicImage id="capimg"

                                    value="#{facesContext.externalContext.requestContextPath}/../Captcha.jpg" />

                              <br />

                              <h:outputText value="*Napisite text koj vidite u slici " />

                              <h:inputText id="captchaSellerInput"

                                    value="#{registrationControllerSeller.captchaSellerInput}" />

Step 4:Add the backing bean logic of the captha:

HttpServletRequest request = (HttpServletRequest) FacesContext

            .getCurrentInstance().getExternalContext().getRequest();

            Boolean isResponseCorrect = Boolean.FALSE;

            javax.servlet.http.HttpSession session = request.getSession();

            String parm = captchaSellerInput;

            String c = (String) session.getAttribute(MyCaptcha.CAPTCHA_KEY);

            if (parm.equals(c)) {
                 //CAPTCHA CORRECT INPUT
            }
            else {
                //CAPTCHA INCORRECT INPUT  

            }

JSF with AJAX Example for handling selectOneMenu

The index.xhtml will contains following tags to display two dropdown list and to link then using Ajax.

<?xml version='1.0' encoding='UTF-8' ?>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<!-- index.xhtml -->
<html xmlns="http://www.w3.org/1999/xhtml"

xmlns:h="http://java.sun.com/jsf/html" 
xmlns:f="http://java.sun.com/jsf/core">
 <h:head>

<title>JSF AJAX</title>

</h:head>

<h:body>

<h3>Select Mobile</h3>  <h:form>

Company : <h:selectOneMenu value="#{jsfBean.company_name}">

<f:selectItems value="#{jsfBean.company}"/>

<f:ajax event="change" render="model" listener="#{jsfBean.setModel}" />

</h:selectOneMenu><br></br><br></br>

Model :

<h:selectOneMenu value="#{jsfBean.model_no}" id="model" style="width: 120px;">

<f:selectItems value="#{jsfBean.model}"/>

</h:selectOneMenu>

</h:form>

</h:body>


</html>
This application can be more effective if the given application can be implemented with database, where the dropdown lists will be loaded from database records.
Outputs :

/*

* To change this template, choose Tools | Templates

* and open the template in the editor.

*/package beans; import javax.faces.bean.ManagedBean;
import javax.faces.bean.RequestScoped;
import javax.faces.event.AjaxBehaviorEvent;
import javax.faces.model.SelectItem;
/**
*
* @author Prashant
*/
@ManagedBean
@RequestScoped
public class JsfBean {
/** Creates a new instance of JsfBean */
private SelectItem[] company=new SelectItem[]{
new SelectItem(“nokia”, “Nokia”),
new SelectItem(“samsung”, “Samsung”),
new SelectItem(“sony ericson”, “Sony Ericsson”),
new SelectItem(“black berry”, “Black Berry”)
};
private SelectItem[] nokia=new SelectItem[]{
new SelectItem(“8800 Carbon”,”8800 Carbon”),
new SelectItem(“E90 Communicator”, “E90 Communicator”),
new SelectItem(“N97″, “N97″),
new SelectItem(“N96″, “N96″),
new SelectItem(“N95″, “N95″),
new SelectItem(“6260 Slide”, “6260 Slide”),
new SelectItem(“5800 Express music”, “5800 Express music”),
new SelectItem(“N73 Music Edition”, “N73 Music Edition”)
};
private SelectItem[] samsung=new SelectItem[]{
new SelectItem(“OMNIA i900″,”OMNIA i900″),
new SelectItem(“S8330 Ultra TOUCH”, “S8330 Ultra TOUCH”),
new SelectItem(“M8800 Pixon”, “M8800 Pixon”),
new SelectItem(“Jet S8003″, “Jet S8003″),
new SelectItem(“Jet U900 Soul”, “U900 Soul”),
new SelectItem(“DJ M7603″, “DJ M7603″),
new SelectItem(“U800 Soul B”, “U800 Soul B”),
new SelectItem(“Beat M3510″, “Beat M3510″),
new SelectItem(“D780″, “D780″),
new SelectItem(“E251″, “E251″),
};
private SelectItem[] sony=new SelectItem[]{
new SelectItem(“Xperia X1″,”Xperia X1″),
new SelectItem(“C905″, “C905″),
new SelectItem(“W995″, “W995″),
new SelectItem(“W980i”, “W980i”),
new SelectItem(“K850i”, “K850i”),
new SelectItem(“C902″, “C902″),
new SelectItem(“W705″, “W705″)
};
private SelectItem[] blackberry=new SelectItem[]{
new SelectItem(“Bold 9000″,”Bold 9000″),
new SelectItem(“8820″, “8820″),
new SelectItem(“8830″, “8830″),
new SelectItem(“8800″, “8800″),
new SelectItem(“Curve 8900″, “Curve 8900″),
new SelectItem(“Curve 8310″, “Curve 8310″),
new SelectItem(“Pearl 8110″, “Pearl 8110″),
new SelectItem(“Pearl Flip 8220″, “Pearl Flip 8220″)
};
private SelectItem model[]=nokia;
private String company_name=”company_name”;
private String model_no;
public JsfBean() {
}
public SelectItem[] getCompany() {
return company;
}
public void setCompany(SelectItem[] company) {
this.company = company;
}
public String getCompany_name() {
return company_name;
}
public void setCompany_name(String company_name) {
this.company_name = company_name;
}
public String getModel_no() {
return model_no;
}
public void setModel_no(String model_no) {
this.model_no = model_no;
}
public SelectItem[] getNokia() {
return nokia;
}
public void setNokia(SelectItem[] nokia) {
this.nokia = nokia;
}
public SelectItem[] getSamsung() {
return samsung;
}
public void setSamsung(SelectItem[] samsung) {
this.samsung = samsung;
}
public SelectItem[] getModel() {
//
// this.model = nokia;
return model;
}
public void setModel(AjaxBehaviorEvent evt) {
if(company_name.equalsIgnoreCase(“nokia”))
this.model =nokia;
else if(company_name.equalsIgnoreCase(“samsung”))
this.model =samsung;
else if(company_name.equalsIgnoreCase(“sony ericson”))
this.model =sony;
else if(company_name.equalsIgnoreCase(“black berry”))
this.model =blackberry;
}
}