JAXB - Java Architecture for XML Binding - The OXM tool !

JAXB is  OXM - Object XML Mapping  :) ! it is what ORM is to relational databases.
Hibernate and othe ORM tools is to Databases what JAXB is to XML.
 
This simple tutorial is for people who are :
1)  Sick and tired of parsing XML with/without using any library.
2)  Who have never used JAXB and want to get started.

I used to work for a company where we used to un-marshall (parse)  XML using a library written in Delphi after installing it into Borland C++ Builder. My manager told me "It is easier to write XML data out than to read it in. If i were you i would have written the code myself". He was right to a certain extent. I wrote the code for writing XML data out to another system and I must say the whole process was quite simple except for the initial few mistakes i made for the tiny XML Writer library.

I used the same XML writer library i built there for a couple of other small hobby projects and it has worked well with not changes needed to the core library. Here's the header file for the simple XML writer library i wrote then.

#ifndef BlueXmlWriterH
#define BlueXmlWriterH

#define START_ANGLE_BRACKET "<"
#define CLOSE_ANGLE_BRACKET ">"
#define BACK_SLASH_MARK "/"
#define SPACE " "
#define EQUAL "="
#define DOUBLE_QUOTE "\""

#include<string>
using namespace std;

class BlueXmlWriter
{
    public:
            BlueXmlWriter(string &XmlBuffer);
            BlueXmlWriter(char *XmlBufferToWriteTo);
            bool AddOpeningTag(const char *StartTag);
            bool AddOpeningTag(const char *StartTag, const char **attr);
            bool AddNode( const char *ElementName, const char *ElementData);
            bool AddNode( const char *ElementName, int IntElementData);
            bool AddClosingTag(const char *EndTag);
            bool AddData(const char *Data);
            bool AddData(int Data);
            inline int GetBufferSize() { return mXmlBytesWritten; }
            char *  GetXmlBuffer() { return mXmlBuffer; }
    private:
            char *mXmlBuffer;
            int mXmlBufferSize;
            int mXmlBytesWritten;

};

It was used like :
lXmlWriter = new BlueXmlWriter(lBuffer); 
lXmlWriter->AddOpeningTag(BLUEMANTRA_START_XML);
lXmlWriter->AddNode(BLUEMANTRA_EVENT,BLUEMANTRA_EV_PRESENCE);

Later on, when i moved on to another company there was this requirement to send and receive XML data to and from a Billing server for charging prepaid mobile customers. Then, i used the DocumentBuilder class to parse the XML , writing XML was still handwritten code (My previous managers advice except that the code was now ported to Java). It worked fine and I found it to have more fine grained control for writing XML when you have your own library.
http://download.oracle.com/javase/1.4.2/docs/api/javax/xml/parsers/DocumentBuilder.html  was used for parsing the incoming XML stream. This worked fine too. But it was later on in life that i found JAXB.

I have used JAXB for a web application project and i found it to be immensely helpfull in projects which have a lot of XML marshalling and unmarshalling. The bad news was it does marshalling too - so it was time to say goodbye to my XML Writer library.

The basic steps involved in using JAXB is as follows : (I have compared it to ORM in bold just to help reader relate to it better, if you do not know or have not used any ORM tools before please skip the sentences in bold)

1) You create a XSD (XML Schema definition file) and write your XML Schema. XML Schemas define the structure of your XML. I found this link (XSD Tutorial)  to be particularly usefull for writing the XSD myself. I had no clue how to write one before. To relate this to ORM (Object Relational mapping with Hibernate)  you create the .hbm.xml file to map classes to tables and columns in the database.

2) Use the binding compiler which comes with JAXB to convert this XML Schema to POJO's which represent your XML Schema definitions.You could relate this to some ORM tools creating the POJO's from the XML file which describes how your classes map to the database table columns. 

3) Create the JAXB Context which will be used to create the Marshaller and UnMarshaller. This is equivalent to the creation of a SessionFactory with the database when it comes to ORM.


4) Use the POJO's generated by the binding compiler to marshall and unmarshall the XML.This is similar to retreiving (Reading) and saving (Writing)  objects to the databse in ORM.

Ok, lets get started then. For this tutorial we will use the example of a course and the subjects which are part of the course. Let us first try to write the XSD for this. Please read up on this link XML Schema Tutorial if you have no prior experience with XML Schema Definitions.

XSD can be used to describe the XML structure in detail. It can be used to apply constraints to the data in the XML. For example if there is an element called age as <Age>45</Age> we could write the XSD so that the value of Age cannot be outside the range say 0 to 120. And if the XML you are unmarshalling is outside this range you get an exception raised by JAXB. Just imagine the extra code you would have to write to check for such errors.

Moving to our XSD for the Course. A course can have one or more subjects as part of it. We will write the XSD for this relationship as below. 

First we describe the Subject. A Subject has a name. 

<xs:simpleType name="Subject" >
<xs:restriction base="xs:string" >
<xs:length value="15"/>
</xs:restriction>
</xs:simpleType>

This describes the Subject as a simple type with the data type restricted to a string and the length ( number of characters ) limited to 15.  Example could be Geography, Social Studies, length to be less than 15 chars.


Now let us describe the Course in XSD. 


<xs:complexType name="Course">
<xs:sequence>
<xs:element name="Name" type="xs:string" minOccurs="1" maxOccurs="1" />
<xs:element name="CSubject" type="Subject" minOccurs="1" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>

The above definition of the XML Schema for Course says "Every Course has  a name and one or more Subjects".
It has a minimum of one and maximum can be any number, we could have easily restricted the maximum number to 5 with maxOccurs="5". This is how our application.xsd file looks now :


<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" >


<!-- XML Elements  - This is an XML Comment like /* */ in  C/ C++ / Java  -->
<xs:element name="CSubject" type="Subject"/>
<xs:element name="CCourse" type="Course"/>

<!-- Defining Subject -->
<xs:simpleType name="Subject" >
<xs:restriction base="xs:string" >
<xs:length value="15"/>
</xs:restriction>
</xs:simpleType>

<!-- Defining Course -->
<xs:complexType name="Course">
<xs:sequence>
<xs:element name="Name" type="xs:string" minOccurs="1" maxOccurs="1" />
<xs:element name="CSubject" type="Subject" minOccurs="1" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
 
 </xs:schema>

Now, all that is left is to generate classes from the XSD and then use them in our application. Let us now use the Binding Compiler to generate the classes for us. The Binding  compiler is called xjc and is used as follows.
I found it in my jdk installation directory C:\Program Files\Java\jdk1.6.0_05\bin , I do not remember whether it comes with the standard jdk installation or i copied it from somewhere, in anyways it is quite easy to find this on the internet with a search. So copy it to the bin directory of your jdk installation so that it can be used from anywhere assuming you have the PATH variable set properly.
It takes the .xsd file as input and generates POJO's (Plain Old Java Objects)  reperesenting your domain objects plus an ObjectFactory which you would use to create the objects.

C:\eclipseprojects\workspace\JAXB_TEST\src> xjc -nv application.xsd  -p jaxb

 



The command instructs xjc to create the POJO's in a package called jaxb, you could use any package structure like com.xyz.xml.jaxb the compiler will create the directory structure matching your package name.In this case all the generated files go into the jaxb directory.


Here's the xjc generate Course.java file for reference.


/**
 * <p>Java class for Course complex type.
 *
 

public class Course {

    @XmlElement(name = "Name", required = true)
    protected String name;
    @XmlElement(name = "CSubject", required = true)
    protected List<String> cSubject;

    /**
     * Gets the value of the name property.
     *
     * @return
     *     possible object is
     *     {@link String }
     *    
     */
    public String getName() {
        return name;
    }

    /**
     * Sets the value of the name property.
     *
     * @param value
     *     allowed object is
     *     {@link String }
     *    
     */
    public void setName(String value) {
        this.name = value;
    }

    /**
     * Gets the value of the cSubject property.
      * For example, to add a new item, do as follows:
     * <pre>
     *    getCSubject().add(newItem);
     * </pre>
     *
     *
     * <p>
     * Objects of the following type(s) are allowed in the list
     * {@link String }
     *
     *
     */
    public List<String> getCSubject() {
        if (cSubject == null) {
            cSubject = new ArrayList<String>();
        }
        return this.cSubject;
    }
}


Here's ObjectFactory.java 

/**
 * This object contains factory methods for each
 * Java content interface and Java element interface
 * generated in the jaxb package.
 * <p>An ObjectFactory allows you to programatically
 * construct new instances of the Java representation
 * for XML content. The Java representation of XML
 * content can consist of schema derived interfaces
 * and classes representing the binding of schema
 * type definitions, element declarations and model
 * groups.  Factory methods for each of these are
 * provided in this class.
 *
 */
@XmlRegistry
public class ObjectFactory {

    private final static QName _CSubject_QNAME = new QName("", "CSubject");
    private final static QName _CCourse_QNAME = new QName("", "CCourse");

    /**
     * Create a new ObjectFactory that can be used to create new instances of schema derived classes for package: jaxb
     *
     */
    public ObjectFactory() {
    }

    /**
     * Create an instance of {@link Course }
     *
     */
    public Course createCourse() {
        return new Course();
    }

    /**
     * Create an instance of {@link JAXBElement }{@code <}{@link String }{@code >}}
     *
     */
    @XmlElementDecl(namespace = "", name = "CSubject")
    public JAXBElement<String> createCSubject(String value) {
        return new JAXBElement<String>(_CSubject_QNAME, String.class, null, value);
    }

    /**
     * Create an instance of {@link JAXBElement }{@code <}{@link Course }{@code >}}
     *
     */
    @XmlElementDecl(namespace = "", name = "CCourse")
    public JAXBElement<Course> createCCourse(Course value) {
        return new JAXBElement<Course>(_CCourse_QNAME, Course.class, null, value);
    }

}


Now that we have the domain objects and the factory, all that is left is to use JAXBContext to create a marshaller and marshall some XML data from the domain objects. Back to our example we will create a Course object named "Software Design Patterns" and add subjects called 
"Design Principles" , "Creational Patterns" , "Behavioral patterns" ,"Structural Patterns"

MARSHALLING  ->  XML FROM DOMAIN OBJECTS  :

Before we start, as mentioned above we need to create a Marshaller to marshal XML Content from our domain objects using the JAXBContext class.

The JAXBContext class provides the client's entry point to the JAXB API. It provides an abstraction for managing the XML/Java binding information necessary to implement the JAXB binding framework operations: unmarshal, marshal and validate. 

// jaxb is the package (directory) where all our xjc generated files reside  

JAXBContext mJAXBContextInstance = JAXBContext.newInstance( "jaxb" );

// create the Marshaller 

Marshaller lMarshaller = mJAXBContextInstance.createMarshaller();

/* Create the domain object factory */

ObjectFactory lFactory = new ObjectFactory();

/* Create the Course object */

Course lCourse = lFactory.createCourse();  /* We can use new Course() instead */

lCourse.setCName("Software Design Patterns");   /* Name of the course */

lCourse.getCSubject().add("Design Principles");   
lCourse.getCSubject().add("Creational Patterns");  /* Subjects in the course */

lCourse.getCSubject().add("Structural Patterns");

lCourse.getCSubject().add("Behavioral Patterns");

/* The following line of code is needed to marshall the domain object into XML format, without this

the Marshalling fails */

JAXBElement<Course> lCourse1 = lFactory.createCCourse(lCourse);
try {
            lMarshaller.marshal(lCourse1, System.out);
} catch (JAXBException e) {
            e.printStackTrace();

}

The output from running this code is :

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

<CCourse>

<Name>Software Design Patterns</Name>

<CSubject>Design Principles</CSubject>

<CSubject>Creational Patterns</CSubject>

<CSubject>Structural Patterns</CSubject>

<CSubject>Behavioral Patterns</CSubject>

</CCourse>

Isn't this cool ? We have NOT written a line of code to build the XML Content we are seeing here. It is all taken care by JAXB. 


UNMARSHALING -> DOMAIN OBJECTS FROM XML  :
 
Similarly unmarshalling XML to Domain objects works the same way you create an UnMarshaller using the JAXBContext class. 

We have our Course.xml here as below :

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

<CCourse>

<Name>Software Design Patterns</Name>

<CSubject>Design Principles</CSubject>

<CSubject>Creational Patterns</CSubject>

<CSubject>Structural Patterns</CSubject>

<CSubject>Behavioral Patterns</CSubject>

</CCourse>

We will create an UnMarshaller and parse this XML file to domain objects. Code follows :

JAXBContext lContext = null; 

Unmarshaller lUnMarshaller = null;
        try {
                lContext = JAXBContext.newInstance("jaxb");
                lUnMarshaller = lContext.createUnmarshaller();
            } catch (JAXBException e) {
                  e.printStackTrace();
            }
       
 FileReader lReader = null;
  try {
            lReader = new FileReader(new File("Course.xml"));
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
       
        try {
            JAXBElement<Course> lCourse2 = (JAXBElement<Course>)  lUnMarshaller.unmarshal(lReader);
            Course lCourse3 = lCourse2.getValue();
            System.out.println("Get the name of the course : " + lCourse3.getName());
            System.out.println("Get the Subjects count " + lCourse3.getCSubject().size());
            Iterator<String> lIter = lCourse3.getCSubject().iterator();
            while(lIter.hasNext()) {
                System.out.println("Subject is " + lIter.next());
            }
        }

}

The output from the code is below :

Get the name of the course : Software Design Patterns
Get the Subjects count 4
Subject is Design Principles
Subject is Creational Patterns
Subject is Structural Patterns
Subject is Behavioral Patterns

That's it we UnMarshalled (parsed) the XML file into our domain object Course and we saw that the properties of the Course object automagically populated with the contents by JAXB.

FINAL THOUGHTS : 

JAXB is a very handy tool to manipulate XML Content use it in large projects and when the XML structure changes you do not need to write any code, just run xjc again on the new XSD. We have just seen some parts of JAXB, there are features to handle errors dynamically. We saw how to Marshall and UnMarshal XML , there are options to validate the XML as well . Hope this blog helped you with JAXB. 

-  Voice~Streams

HOWTO - Create an executable jar file.

This blog explains how to create an executable jar file.I have often run my java programs using the dot separated package structure. For example : java com.devicemantra.apps.blueapp.Main, it looked akward and complicated. Wanted to run it like  java -jar blueapp.jar.  Finally had some time to check this out. It is a simple process and involves creating a Manifest file and familiarity with the jar command. Both are simple.


This is the directory structure of my java project in Eclipse IDE.

src
|_ com.san.app
                       |_LoggerServer.java  ---> main() is defined here.
                       |_NetworkFileLogger.java
|_com.san.logger
                       |_Logger.java 
                       |_LoggerImpl.java
                       |_LoggerFactory.java
                       |_SanLoggerException.java
|_com.san.network
                      |_NetworkListener.java
                      |_NetworkListenerImpl.java
                      |_NetworkListenerFactory.java
                      |_SanNetworkException.java

The directory structure is shown. A sanpshot from my Eclipse IDE.

Assuming all your class files are in the same directory structure as your source files. Snapshot of my directory structure.

bin
|_ com
           |_san
                  |_app
                          |_LoggerServer.class
                          |_NetworkFileLogger.class (etc)

                  |_logger
                  |_network



Now assuming that you are at the top level directory bin of your class files. Create a new file MANIFEST.MF in  the bin directory as follows using your favorite text editor. I used gvim for windows.

Notice the marking. It is going to indicate the class which has main() defined among all the classes and the class file to run when the jar file is executed. I have added the Main-Class property.Now let us create the jar file.This is how your directory structure should look like  now. bin directory looks like below.

Now, create the jar file by including the Manifest.mf file which we created using the following command.


jar -cvfm LoggerServer.jar  MANIFEST.MF com

Let me explain the jar command. jar command is similar to tar command used generally on UNIX like systems . It creates an archive. In this case it says - create an archive be verbose, name the file LoggerServer.jar and include the manifest file which is MANIFEST.MF , include all the class files under the com directory and it's sub directories while creating this jar.

'c' -  create new archive

'v' - verbose

'f' - file name  -  LoggerServe.jar in this case.

'm' - include manifest information from specified manifest file.

The ServerLogger.jar is now executable from the command line as follows :
java  -jar  LoggerServer.jar

This is possible only because we have added a MANIFEST.MF file indicates which is the main class in the jar.

Any arguments can be passed to main as below :
java  -jar  LoggerServer.jar  8888

That's it. Hope this blog helped someone somewhere create their first executable jar.

Java: Enhanced for loop and Array puzzle.

I have done a lot of programming related puzzles as a student and professional for fun or for taking tests. But this seemingly innocent and straightforward looking piece of code stumped me !  I had to actually try this out in Eclipse running in debug mode to Watch the variables. Now when i figured what is happening it looks straightforward.

The reason why i could not figure this out is first of all  :
1) It looked straightforward ( overconfidence stepped in )
2) I was used to the for(int counter = 0; counter< ...; counter++)  way of writing for loops and indexing arrays.

Whatever said, this one was a good puzzle. And i will NOT forget this one all my life :) ! 

public class Main {
   
    public static void main(String... pArgv) {
        int []arr = {1,2,3,4};
        for ( int i : arr )
        {
           arr[i] = 0;
        }
        for ( int i : arr )
        {
           System.out.print(i);
        }
    }
}

By the way the output is : 0030

Confusing Stack Iterator.

I was using the Stack<E> class from the Java Collections framework for the first time. I think the Iterator returned by this Collection is confusing. Consider the following code :

import java.util.Iterator;
import java.util.Stack;

public class Main {
   
    public static void main(String[] argv) {
        Stack<String> lStack = new Stack<String>();
        lStack.add("ONE");
        lStack.add("TWO");
        lStack.add("THREE");
       
        Iterator<String> lIterator = lStack.iterator();
        while ( lIterator.hasNext() ) {
            System.out.println( lIterator.next());
        }
     }
}

I was hoping that the output from the above code would be :
THREE
TWO
ONE

But, the output was :
ONE
TWO
THREE

The reason why i expect it to be like the first output shown above is - It is a Stack and a Stack orders elements in a Last In First Out (LIFO) manner. Hence the Iterator returned  from the call lStack.Iterator() should preserve this ordering. Am I missing something obvious ?

blueipd - The public IP Address finder daemon

There are times when I had to figure out the public IP address of the network i was in.This blog describes a daemon which i had developed for personal use. This daemon can be installed to run at startup on Linux. I called it blueipd.  blueipd periodically sends a request to the server with date and time in xml format. The server looks at the HTTP headers of the request and figures out the Public Ip address of my office router.

There is php script on the server which formats and displays this in a presentable format. The php script can be enhanced to send an SMS/Email to a precofigured mobile number/Email Address when there is a change in Ip address. In short, you could look up a particular server url and figure out the public ip address of the network where blueipd is running from.

The code for blueipd , the initscript and the Makefile is available for download from www.devicemantra.com at this url Download

A use case for blueipd

I had an application server running on my PC at office on port 8888. This PC was in a LAN. I wanted to devise a way to access this server from home by typing in a url like www.devicemantra.com/myappserver. This script looks up the database and finds out the current public ip address of my office which was updated by blueipd. It then redirects (HTTP 302 Status code) me to the public ip and port number 8888. I had mapped (port mapped) my office routers port 8888 to an internal ip address and port number ( 192.168.1.2222 and port 8888). So all connections coming on 8888 are now redirected to my office PC's application server. And now, i can reach it with one permanent url !

Otherwise i had no way of finding out my office public ip, blueipd is really a very helpfull utility. It could also be used to do ssh logins by looking up the ip address and of course adding a port mapping on your office router.

blueipd - Directory structure.


The code is split into the following directory structure:
ipaddressdaemon
- initscript
-src
       - app
       - bluehttp
       - bluelogger
       - bluexml
       - include
       - Makefile

First let us look at the main code for the daemon and go through it step by step.
blueapp.cpp

The main loop for the daemon

#include<string>
#include<iostream>
using namespace std;

#include<stdlib.h>
#include<unistd.h>
#include<signal.h>
#include<syslog.h>


/* Application includes */
#include<bluemantra.h>
#include<bluehttppost.h>
#include<bluexml/bluexmlbuilder.h>

#define BLUEMANTRA_POST_URL     "http://www.devicemantra.com/bluemantra/time_update.php"

#define BLUEMANTRA_FORM_NAME     "time"
#define BLUEMANTRA_FORM_CONTENT_TYPE "application/xml"
#define BLUEMANTRA_INTERVAL     3600

bool bDoExit = false;


void  daemonize()
{
    /* Start logging to the syslog daemon */
    openlog(" blueipd  ",LOG_PID,LOG_DAEMON);
    syslog(LOG_INFO,"Started .....");


    /* First check whether we are a daemon already ? */
    if( 1 == getpid() )
    {
        //"Already a daemon pid = "<<getpid()<<endl;
        return;
    }
   

    pid_t pid = fork();
   
    /* Handle fork failure */
    if( pid < 0 )
    {
        syslog(LOG_INFO,"Fork failed exiting ..");
        exit(-1);
    }

    if( pid > 0 )
    {
        syslog(LOG_INFO,"Parent process exiting ..");
        exit(0);
    }

    /* From now on it is the child which is executing */
   
    /* Let us install a SIGNAL handler */

    signal(SIGTERM,SIG_IGN);   
    signal(SIGHUP,SIG_IGN);   
   
    /* First let us close all the open file descriptors */
    for (int count = getdtablesize();count >=0 ; count--)
    {
        close(count); /* close all descriptors */
     }

    /* Change the session id for this process */

    setsid();


    /* Start logging to the syslog daemon */
    openlog(" blueipd  ",LOG_PID,LOG_DAEMON);
    BLUEMANTRA_RESULT lResult;
    int lCount=1;
    while( false == bDoExit )
    {
        syslog(LOG_INFO,"Executing Main  loop");
        BlueXmlBuilder *lXmlBuilder = new BlueXmlBuilder();
        string lXmlStr = "";
        BlueHttpPost *lPoster = new BlueHttpPost();
        lPoster->SetUrl(BLUEMANTRA_POST_URL);

        lXmlBuilder->GetTimeInXml(lXmlStr);

        if(BLUEMANTRA_SUCCESS == lPoster->Post(BLUEMANTRA_FORM_NAME, lXmlStr, BLUEMANTRA_FORM_CONTENT_TYPE))

        {
            syslog(LOG_INFO, "[SUCCESS] Posting to www.devicemantra.com");
        }
        else
        {
            syslog(LOG_ERR, "[ERROR]  Posting to www.devicemantra.com");
        }
               
        delete lXmlBuilder;
        delete lPoster;
        syslog(LOG_INFO,"Finished executing loop");

        /* Let us sleep for some time now */
        sleep(BLUEMANTRA_INTERVAL);
    }
}
   
int main()
{
    daemonize();
}

blueapp.cpp - Code walkthrough 

The code for the main loop is quite straight forward. The important parts are highlighted. The daemonize function does all the work.

1. It first forks to create a child process.
2. Installs some signal handlers to handle ignore SIGTERM, SIGHUP signals delivered to it. 
3. Closes all file descriptors for the child process. 
4. setsid() call detached the process from the parent and controlling terminal.
5. Create a BlueXmlBuilder and BlueHttpPost (Helper classes for building XML and HTTP request objects, please download and check the code for more) objects.
6. Post to the server url. 
7. Sleep for a predefined interval.

Note: Steps 1 to  4 can be used to write any simple daemon.

Installing blueipd on your system.

1) Download the C++ source + init scripts + php script together from this link - Download.

2) Unzip it to the directory ipaddressdaemon in your home directory.

3)  run the make command in the src directory.

4)  If everything went fine you will find blueipd (executable in the src/app directory).

5)  Copy blueipd to /usr/bin as root. 

6)  Copy ipaddressdaemon/initscript/blueipd to /etc/rc.d/init.d/ directory. 

7)  chkconfig --add blueipd  adds the blueipd daemon to services.

8)  chkconfig --level 345  blueipd on   - enables blueipd in runlevels 3,4 and 5.

9)  service blueipd start  - should start the blueipd daemon on your system.

The only thing that is left to be done is uploading the script to the web server and pointing to that url from the blueipd.

Installing the php script on the server

<?
  $time_xml = $_POST['time'];
  $pa_public_ip = $_SERVER['REMOTE_ADDR'];
  $domain = GetHostByName($REMOTE_ADDR);

  if (isset($_SERVER["HTTP_X_FORWARDED_FOR"]))
  {
   $pa_public_ip =  $_SERVER["HTTP_X_FORWARDED_FOR"];
  }
  else
  {
   $pa_public_ip =  $_SERVER["REMOTE_ADDR"];
  }

 
 

 The above php script logs the public ip address in the database. The code is self explanatory but not well structured !  I feel such code does not need undue attention !

1. The code retreives the the Public ip address by looking at the incoming post request $pa_public_ip = $_SERVER['REMOTE_ADDR'];
     $domain = GetHostByName($REMOTE_ADDR);
   
     if (isset($_SERVER["HTTP_X_FORWARDED_FOR"]))
     {
      $pa_public_ip =  $_SERVER["HTTP_X_FORWARDED_FOR"];
     }
     else
     {
      $pa_public_ip =  $_SERVER["REMOTE_ADDR"];
     }                           

That is all there is to it. It is a very simple script the actual script does more like logging it to the database and updating the time etc. That part can be added by anyone with a little knowledge of php or any scripting language for that matter and mysql.

 The blueipd init script. ( I modified vsftpd init script for this one ).

#!/bin/bash
#
### BEGIN INIT INFO
# Provides: blueipd
# Short-Description: blueipd Daemon
# Description: blueipd is a public ip address finder daemon.

### END INIT INFO


# Source function library.
. /etc/rc.d/init.d/functions

# Source networking configuration.
. /etc/sysconfig/network

RETVAL=0
prog="blueipd"

start() {
        # Start daemons.

    # Check that networking is up.
    [ ${NETWORKING} = "no" ] && exit 1


        if [ -f /usr/bin/blueipd ] ; then
                        echo -n $"Starting $prog for $site: "
                        daemon /usr/bin/blueipd
                        RETVAL=$?
                        [ $RETVAL -eq 0 ] && touch /var/lock/subsys/$prog
                        echo
                done
        else
                RETVAL=1
        fi
        return $RETVAL
}

stop() {
        # Stop daemons.
        echo -n $"Shutting down $prog: "
        killproc $prog
        RETVAL=$?
        echo
        [ $RETVAL -eq 0 ] && rm -f /var/lock/subsys/$prog
        return $RETVAL
}

# See how we were called.
case "$1" in
  start)
        start
        ;;
  stop)
        stop
        ;;
  restart|reload)
        stop
        start
        RETVAL=$?
        ;;
  condrestart)
        if [ -f /var/lock/subsys/$prog ]; then
            stop
            start
            RETVAL=$?
        fi
        ;;
  status)
        status $prog
        RETVAL=$?
        ;;
  *)
        echo $"Usage: $0 {start|stop|restart|condrestart|status}"
        exit 1
esac

exit $RETVAL

The redirect script
Name : myappserver.php 
This script redirects to my appserver when you access this url from a browser. 
Upload this script to your server. There has to be a port mapping on port 4444 on your office router to your appserver / Other Applications  ip address and port.

<?php
 /* Open connection to the database */
   include '../library/config_bm.php'; 
 include '../library/opendb.php';
 
 header("Cache-Control: no-cache, must-revalidate"); // HTTP/1.1
 header("Expires: Sat, 26 Jul 1997 05:00:00 GMT"); // Date in the past

 $select_sql = "SELECT * from public_ip_table LIMIT 1;";
 $result = mysql_query($select_sql) or die('Query failed.'. mysql_error());

 $num_rows = mysql_num_rows($result);
 if( $num_rows > 0 )
       {
         /* Check the details */ 
           $row = mysql_fetch_row($result);
         $redirect_address = "Location:"."http://".$row[1].":4444";
         header($redirect_address);       }else {

echo "FAILED: to fetch public ip "; 

}    

?>

The script queries the database for the ip address blueipd has updated. Then redirects (302 HTTP response)  the page to that ip and port number.

I hope there is enough information to implement this Public Ip Address daemon and have it working for you. Please use this link to download the code and see it for yourself. Good luck !

For any questions or feedback please send an email to amithmit (at) yahoo (dot) com.

- Voice~Streams

J2EE: This hack on Google Appengine lets you create a HSSF Workbook.

The Problem : Not able to create a HSSFWorkbook on Google App Engine.

But, I was able to receive an Excel sheet uploaded from a browser , parse it into something meaningfull using the POI API http://poi.apache.org/ (Which i should say is really cool, considering that i'm a newbie to web application development, i never knew it was that easy to parse an Excel file).

Now, when i try to create an Excel sheet and send it back to the browser dynamically - google appengine sent back a 500 Internal server error. On further reading i realised that POI Api was not supported on app engine. But half my work was already done, the creation part had to work otherwise the whole effort would be a waste of time. Being an Open Source Software user,  i downloaded POI Sources from the link mentioned above. There was good documentation on how to build it. The idea was i would remove the unnecessary parts if any and rebuild it (Which , i later on figured out was NOT needed ) with the white listed classes , if possible.

First things first. I went and looked at the logs to figure of what exactly went wrong. I saw it to find out which line caused the exception, so that i can start from there. The logs on appengine said :

/GetExcelFile
java.lang.NullPointerException

at org.apache.poi.hssf.record.WriteAccessRecord.setUsername(WriteAccessRecord.java:101)

at org.apache.poi.hssf.model.Workbook.createWriteAccess(Workbook.java:1071)
at org.apache.poi.hssf.model.Workbook.createWorkbook(Workbook.java:335)
at org.apache.poi.hssf.usermodel.HSSFWorkbook.(HSSFWorkbook.java:170)
at com.devicemantra.coordinate.util.CExcelWriter.(CExcelWriter.java:48)
at com.devicemantra.coordinate.ds.tests.GetExcelFile.doGet(GetExcelFile.java:27)

Now,  i fired up vim and opened Workbook.java  +1071. Following is the line of code which caused the exception :


 private static WriteAccessRecord createWriteAccess() {
        WriteAccessRecord retval = new WriteAccessRecord();

        try {
            retval.setUsername(System.getProperty("user.name"));
        } catch (AccessControlException e) {
                // AccessControlException can occur in a restricted context
                // (client applet/jws application or restricted security server)
                retval.setUsername("POI");
        }
        return retval;


The code below tried to get System.getProperty("user.name").  And used this as a parameter to setUserName(). Since this system variable was not set, it sent null and hence the NullPointerException.

I thought that the probable solution was to set this property in appengine-web.xml. So that this call  System.getProperty("user.name") would return something and not raise a Null Pointer Exception. 
I modified appengine-web.xml to have this : 

Now, i presumed that the call System.getProperty("user.name")  would return voicestreams and i would see some other exception. So i searched the files for System.getProperty( to see whether any other calls were made, so that i could add entries in appengine-web.xml. But, i got lucky there were no other call except for this one.  

I built my project and deployed it on appengine. I got my excel file back to the browser, i opened it and i saw what i expected it to have :) !. 

Conclusion: 

A call to the Constructor  new HSSFWorkbook() fails on appengine production, but will not raise an exception on Development server. The above hack solved it. Hope this post helps someone, somewhere. As an aside, the call to HSSFWorkbook(POIFSFileSystem fs) never failed. I had used this as below for receving uploaded excel files.

HSSFWorkbook lExcelWorkBook = new HSSFWorkbook(new POIFSFileSystem(mInputStream)); 

and it worked. But, the call to new HSSFWorkbook() fails on GAE production. 

The other alternative is to upload an Excel file to GAE and store it as a blob, when you need to create an Excel file read the blob into an InputStream and use  new HSSFWorkbook(new POIFSFileSystem(mInputStream));  to create the  workbook. I have not tried this but i think it will work, since it worked for me while reading the uploaded Excel sheet.