Java Beans dot Asia - xml category

Redefining web applications with AJAX, Servlets and JSON

Sun, 13 Sep 2009 04:00:00 GMT

In this article I would like to show how JSON (JavaScript Object Notation) and Java servlet can be used together in a little AJAX (Asynchronous JavaScript and XML) application.

To give brief description to those who are not closely familiar with JSON -
JSON is a lightweight syntax for representing data, which makes working with it much more pleasant than with XML and makes AJAX applications faster. Also, when working with JSON, there is no need for an XML parsing.

In the following example, I am going to create a callback servlet that fetches and parses an RSS feed. Then the parsed feed data is passed to the client side in a form of JSON. The data then formatted and presented to the user. The client uses AJAX call to query the servlet.

For this application, I used three third-party libraries:

JSON library provided by JSON.org and extended by JSON-RPC-Java which allows to create and easily parse JSON data through Java code. This library can run in a Servlet container such as Tomcat, JBoss and other J2EE Application servers.
Project ROME
ROME is an set of open source Java tools for parsing, generating and publishing RSS and Atom feeds.
JDOM XML parser
JDOM is a Java-based "document object model" for XML files. JDOM serves the same purpose as DOM, but is easier to use

The libraries are included in the source code which accompanies this article. This application example is also included as a WAR archive, ready to be deployed on Tomcat.

The following is my servlet implementation. The servlet fetches and parses feed data. The JSON library mentioned previously allows me easily to create and populate JSON object.

package asia.javabeans.json;

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.List;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.json.JSONArray;
import org.json.JSONObject;

import com.sun.syndication.feed.synd.SyndEntry;
import com.sun.syndication.feed.synd.SyndFeed;
import com.sun.syndication.fetcher.FeedFetcher;
import com.sun.syndication.fetcher.FetcherException;
import com.sun.syndication.fetcher.impl.FeedFetcherCache;
import 
com.sun.syndication.fetcher.impl.HashMapFeedInfoCache;
import com.sun.syndication.fetcher.impl.HttpURLFeedFetcher;
import com.sun.syndication.io.FeedException;

/**
 * @author Alexander Zagniotov (http://javabeans.asia)
 * 
 */
public class JsonServlet extends HttpServlet {

private static final long serialVersionUID = 1L;
private static final String BLOG_URL = 
		"http://javabeans.asia/rss.xml";
private static final String CONTENT_TYPE = 
			"application/json";
private FeedFetcherCache feedInfoCache = null;
private FeedFetcher feedFetcher = null;

public void init(ServletConfig config) 
			throws ServletException {
	super.init(config);

	feedInfoCache = 
		HashMapFeedInfoCache.getInstance();
	feedFetcher = 
		new HttpURLFeedFetcher(feedInfoCache);
}

public void doGet(HttpServletRequest request, 
			HttpServletResponse response)
		throws ServletException, IOException {

	SyndFeed feed = this.feedFethcer(BLOG_URL);

  if (feed != null) {
	String json = this.feedToJSON(feed);
	response.setContentType(CONTENT_TYPE);
	response.setHeader("Cache-Control", "no-cache");
	response.getWriter().write(json);
  } 
}

private SyndFeed feedFethcer(String url) {
	SyndFeed feed = null;
	try {
		feed = feedFetcher.retrieveFeed(
		new URL(BLOG_URL));
	} catch (IllegalArgumentException e) {
		e.printStackTrace();
	} catch (FeedException e) {
		e.printStackTrace();
	} catch (FetcherException e) {
		e.printStackTrace();
	} catch (MalformedURLException e) {
		e.printStackTrace();
	} catch (IOException e) {
		e.printStackTrace();
	}

	return feed;
}

private String feedToJSON(SyndFeed feed) {

	JSONObject jsonObj = new JSONObject();
	JSONArray jsonEntryTitles = new JSONArray();

	jsonObj.put("blogtitle", feed.getTitle());
	jsonObj.put("blogdescription", feed.getDescription());
	jsonObj.put("bloglanguage", feed.getLanguage());
	jsonObj.put("bloglink", feed.getLink());
	jsonObj.put("author", feed.getCopyright());

	List<?> feedEntries = feed.getEntries();

	for (Object c : feedEntries) {
	     SyndEntry syndicateEntry = (SyndEntry) c;
	     jsonEntryTitles.put(syndicateEntry.getTitle());
	}

	jsonObj.put("blogentrytitles", jsonEntryTitles);
	return jsonObj.toString();
  }
}

As you can see it is very easy to construct JSON objects and arrays on the server side and pass them to the client. For the purpose of this example I am getting my data from RSS feed, but the data can also be coming from DB etc.

The following is my client implementation. The client queries the servlet using AJAX call. When an AJAX call returns a response from the servlet in a form of JSON object, the object data is formatted and information about the RSS feed is presented to the client:

<html>
<head>

<title>Java Beans dot Asia</title>

<script language="JavaScript" type="text/javascript">
		
	var httpRequest = null;
	
function getDescriptionAsJSON() {

	var description = 
		document.getElementById('description');
	description.innerHTML = "Loading, please wait ...";

	var url = "http://localhost:8080/json/json";
	if(window.XMLHttpRequest){
		httpRequest = new XMLHttpRequest();
	}
	else if(window.ActiveXObject){
		httpRequest = 
			new ActiveXObject("Microsoft.XMLHTTP");
	}

	httpRequest.open("GET", url, true);
	httpRequest.onreadystatechange = handler;
	httpRequest.send(null);
}

function handler() {
	if (httpRequest.readyState == 4) {
		if (httpRequest.status == 200) {
		    processJSON(httpRequest.responseText);
		}
	}
}

function processJSON(jsonObjectString) {

	var description = 
		document.getElementById('description');
	
      //Since JSON is a subset of JavaScript, I am using 
      //JavaScript's own compiler to parse JSON in one line!
      var jsonObject = eval('(' + jsonObjectString + ')')

	var text = "";
	
	text += "Author: " + jsonObject.author + "< br />";
	text += "Blog Name: " + 
		jsonObject.blogtitle + "< br />";
	text += "Blog URL: " + 
		jsonObject.bloglink + "< br />";
	text += "Blog Description: " + 
		jsonObject.blogdescription + "< br />";
	text += "Blog Language: " + 
		jsonObject.bloglanguage + "< br />";
	
	description.innerHTML = text;
	var entries = "Last " + 
                  jsonObject.blogentrytitles.length 
	+ " blog entries are:\n\n";

	for (var index = 0; 
		index < jsonObject.blogentrytitles.length; 
							index ++) {
		entries += (index + 1) + ": " + 
			jsonObject.blogentrytitles[index] + "\n";
	} 
	alert(entries);
}
	
</script>

</head>
<body>
<img src="images/javabeansmugshot_120x120.jpg" border="1" />
< br />
< br />
<div id="description"></div>
< br />
< br />
<a href="javascript:void(0)" 
             onclick="return getDescriptionAsJSON();">
Click to get description!</a>
</body>
</html>

As you could see, JSON data can be easily parsed on the client side with the help of Java script eval() function. To remind - JSON is a subset of Java script, therefore eval() will produce a valid object.

Keep in mind, that there is a need for extra care when using eval. The problem is that eval will compile and execute Java script code that coming back from the response. This could cause a security risk if the response data is coming from an untrusted source.

That's it. I hope this example was clear and helpful :)

Please note that this example was tested by me and its working fine. The source code as mentioned previously is included as Eclipse project. You can simply create a new Java project from the existing Ant build.xml file.

Comments/flames are appreciated :)

Cheers

Social Bookmarks :

Related Posts
There are no related posts for this blog entry

JBoss Security - JMX Console

Thu, 27 Aug 2009 02:46:00 GMT

I was reading about JBoss security today, and came across the following page:
JBoss Security vulnerability JMX Management Console. The guy tells how common is to come across servers on the Internet that run completely unsecured JMX-Console.

I gave it a try, typed few keywords in Google, and voilà -
around one third of the Google results on the first page were links to various servers that run completely unsecured JBoss and JMX-console. Having said that, I would like to point out that I did not attempt anything naughty or malicious.

Its still amazes me how careless some people are - leaving an open door for anyone to come in. Come on people, surely in this day and age we all know how important is to keep web applications secured. Don't be lazy, its not going to take much of your time since securing JMX-console its such a trivial task.

Social Bookmarks :

Hibernate Event Interceptor

Sun, 08 Mar 2009 05:42:00 GMT

Its quite common when you create an application, there is a need to create an audit trail on the application level where all entity insert, update and delete events are logged.

In this post, I would like to describe a simple approach that can help you to avoid littering with unnecessary statements in your application code. The solution is to register a class as a listener on Hibernate events. Once class is triggered, you will be able to write audit information to a database or log file.

The following shows a Hibernate event interceptor class that is triggered when persistent entity is inserted, deleted or updated.

public class HibernateEventInterceptor
	implements	
	        PostInsertEventListener, 
		PostUpdateEventListener,
		PostDeleteEventListener, 
				Initializable {

  public HibernateEventInterceptor() {
		
  }

  public void initialize(Configuration cfg) {

  }
	
  public void onPostInsert(PostInsertEvent event) {
   String entityName = event.getPersister().
                                   getEntityName();
   System.out.println("Inserted entity: " + entityName);
  }

  public void onPostUpdate(PostUpdateEvent event) {
   String entityName = event.getPersister().
                                   getEntityName();
   System.out.println("Updated entity: " + entityName);
  }

  public void onPostDelete(PostDeleteEvent event) {
    String entityName = event.getPersister().
                                    getEntityName();
    System.out.println("Deleted entity: " + entityName);
  }
}

The following shows extra configuration that must be added to persistence.xml, in order for the interceptor class to be triggered:

<persistence>
  <persistence-unit name="org.example.demo">
    <jta-data-source>java:/test</jta-data-source>
      <properties>
	.
	.
	.
	.
	<property name="hibernate.ejb.event.post-insert"
	value="org.example.demo.HibernateEventInterceptor"/>
		
	<property name="hibernate.ejb.event.post-update"
	value="org.example.demo.HibernateEventInterceptor"/>
		
	<property name="hibernate.ejb.event.post-delete"
	value="org.example.demo.HibernateEventInterceptor"/>
			
    </properties>
  </persistence-unit>
 </persistence>

Social Bookmarks :

Using template to deploy a JBoss queue

Thu, 01 May 2008 00:48:00 GMT

Currently I am involved in a project, where I have to use Velocity template engine to deploy queues and message-driven beans to JBoss. Velocity is a template engine that can be used for many purposes, and in my case I am using it to generate XML descriptor for a JBoss queue, hence: some-queue-name-service.xml. Its a simple scripting language and the XML descriptor template looks quite similar to the resulting XML descriptor.

Why do we use a template for deployment?
Because some of our deployments occur at runtime, where properties for the queues and MDB's are generated on the fly and we need a templating engine to be able generate the deployments.

So i decided to describe a process of deploying a JBoss queue. I am hoping that I will be clear enough, since i am quite new to this my self.

The setup of my system:

/opt/mitto/jboss/server/mitto/

Root directory for my JBoss instance. (the [.])

My Velocity templates sit under the root, inside dynamic-templates directory:

./dynamic-templates/

JBoss deployment service will look under the root of my JBoss instance for the dynamic-templates directory.

JBoss deployment service depends on Velocity library (velocity.jar) that must be included under the root of:

./deploy/deployment-service.sar/

XML descriptor of JBoss deployment service (jboss-service.xml) sits under:

./deploy/deployment-service.sar/META-INF/

You can see below the jboss-service.xml:

<mbean
   code="org.jboss.services.deployment.DeploymentService"
     name="jboss:service=DeploymentService">
	<attribute name="TemplateDir">
		dynamic-templates
	</attribute>
	<attribute name="DeployDir">
		dynamic-deploy
	</attribute>
	<attribute name="UndeployDir">
		dynamic-undeploy
	</attribute>
</mbean>

The three directories mentioned above in jboss-service.xml, sit under the root of my JBoss instance.
TemplateDir - where my deployment generation templates can be found.
UndeployDir - the generated modules go there.
DeployDir - the directory to use when I ask to deploy a generated module.

Now, inside the:

./dynamic-templates/

I have following structure:

jbossmessaging-queue/
		    /vm/
		       jbossmessaging-queue.xml.vm
		    template-config.xml

jbossmessaging-queue - directory that contains Velocity XML descriptor template for my queue. Also it represents the name of the deployment template that should be used, when createModule() method of deployment service will be called.

template-config.xml specifies the properties for Velocity template XML descriptor for our queue. Deployment service will be able to access those properties when rendering the Velocity template and to retrieve the values of JNDIName and QueueName properties.

Below you can see the descriptor for the template-config.xml:

<template-config
     template="vm/jbossmessaging-queue.xml.vm"
		       extension="-service.xml">
   <property-list>
  	<property name="QueueName"
	    type="java.lang.String" optional="false"/>
   </property-list>
</template-config>

jbossmessaging-queue.xml.vm mentioned above, is the name of the actual Velocity template XML descriptor for the queue, which you can see below:

<?xml version="1.0" encoding="UTF-8"?>
<!--
  ==============================================
  QueueName          - $QueueName
  ==============================================
-->
<server>
<mbean
   code="org.jboss.jms.server.destination.QueueService"
   name="jboss.messaging.destination:service=Queue,
   name=$QueueName" xmbean-dd="xmdesc/Queue-xmbean.xml">
    
   <depends optional-attribute-name="ServerPeer">
	jboss.messaging:service=ServerPeer</depends>
   <attribute name="Clustered">true</attribute>
   <depends>jboss.messaging:service=PostOffice</depends>     
 
</mbean>
</server>

Implementation:
I created a service bean that will acquire proxies for the JBoss MainDeployerMBean and JBoss DeploymentServiceMBean. I will use JBoss deployment service to construct a queue module in the file system, and I will use JBoss main deployer to deploy the queue, by pointing it to the module in the file system. My implementation of my service bean as follows:

public class DeploymentService 
	extends ServiceMBeanSupport 
		implements DeploymentServiceMBean {

private static final String 
	JBOSS_MAIN_DEPLOYER_SERVICE = "jboss.system:service=MainDeployer";
private static final String 
	JBOSS_DEPLOYMENT_SERVICE = "jboss:service=DeploymentService";

private MainDeployerMBean mainDeployerService = null;
private DeploymentServiceMBean jbossDeploymentService = null;

public void startService() throws Exception {

	try {
                //get proxy for main deployer service bean
		mainDeployerService = getJBossMainDeployer();
	}
	catch (MBeanProxyCreationException e)  {
		e.printStackTrace();
	}

	try {
		
		//get proxy for deployemnt service bean		
		jbossDeploymentService = getJBossDeploymentService();
	}
	catch (MBeanProxyCreationException e)  {
		e.printStackTrace();
	}
		
	deploy();
}

public void stopService() {

}


private void deploy()  {

//hashmap contains properties for the queue template
HashMap<String, Object> properties = new HashMap<String, Object>();

//short file name of my queue
String destination = "myqueue-service.xml";


//directory name where the queue Velocity template is kept, 
//inside 'dynamic-templates' dir 
String template = "jbossmessaging-queue";


//property that holds a queue name for the Velocity template
properties.put("QueueName", "myqueue");


String resultingFileName = "";

//create a queue module in a filesystem, at this stage queue 
//is not deployed yet.
try {
	resultingFileName = jbossDeploymentService
			.createModule(destination, template, properties);
} catch (Exception e) {
		e.printStackTrace();
}

String path = constructPath(destination);

try {
	//check whether deployment exists for the given 
	//module queue in the file system
	if (!mainDeployerService.isDeployed(path)) {

		mainDeployerService.deploy(path);
	}
} catch (MalformedURLException e) {
	e.printStackTrace();
}
	
}

//construct an absolute path to the queue module in a filesystem 
//for a given name.
private String constructPath(String shortName) {
String path = ServerConfigLocator.locate().getServerHomeDir()
			.getAbsolutePath()
			+ "/"
			+ jbossDeploymentService.getUndeployDir()
			+ "/"
			+ shortName;
	return path;
	}

//Constructs and returns a proxy for MainDeployerMBean
private MainDeployerMBean getJBossMainDeployer() 
					throws MBeanProxyCreationException
{
	MBeanServer server = getServer();

	ObjectName serviceName = new ObjectName(JBOSS_MAIN_DEPLOYER_SERVICE);

//'false' -> dont make the returned proxy implement NotificationEmitter interface
	MainDeployerMBean mainDeployer =
	(MainDeployerMBean) MBeanServerInvocationHandler
                                .newProxyInstance(server, serviceName,
                                        MainDeployerMBean.class, false);

  return mainDeployer;
}


//Constructs and returns a proxy for DeploymentServiceMBean
private DeploymentServiceMBean getJBossDeploymentService() 
					throws MBeanProxyCreationException
{
	MBeanServer server = getServer();

	ObjectName serviceName = new ObjectName(JBOSS_DEPLOYMENT_SERVICE);

//'false' -> dont make the returned proxy implement NotificationEmitter interface
	DeploymentServiceMBean deploymentService =
	(DeploymentServiceMBean) MBeanServerInvocationHandler
                                .newProxyInstance(server, serviceName,
                                        DeploymentServiceMBean.class, false);

  return deploymentService;
}

}

Two main points that i want to explain here are:
createModule() method of JBoss deployment service and
deploy() of JBoss main deployer.

createModule() - accepts three arguments: resulting file name (in this case is myqueue-service.xml), the directory name where the Velocity template is kept and a HashMap object which contains properties for the template.

JBoss deployment service will use the Velocity template to generate myqueue-service.xml module under dynamic-undeploy directory.

deploy() - accepts a path to the module in the system and deploys the module. In this case the path to the module is:

/opt/mitto/jboss/server/mitto/
                dynamic-undeploy/myqueue-service.xml

Thats about it :) You can use Velocity templates to deploy queues, message-driven and other types of beans. Basically anything that has XML descriptors.
I really hope I was clear enough in this example, since it is quite consized.

Social Bookmarks :

Serialize POJO to XML

Wed, 30 Apr 2008 00:18:00 GMT

Today my colleague, Chandana was trying to convert POJO to an XML file. In our company we are working with dom4j library, but unfortunately it does not provide the capabilities to convert a Java object on the fly to an XML.

Off course Chandana could always manually create new Document, root element, sub elements and attributes and that would work fine. But this still does not solves Chandana's problem where she needed to generate XML on the fly.

After some time doing a research on the Internet she came across this very nice library call XStream. Its a very nice library which allows to serialize objects to XML and back again. It doesn't require mapping nor modifications to the object itself. They also have a short tutorial which converts a POJO to an XML in few simple steps. Very nice and useful

Social Bookmarks :