I was surprised at first, because the services were working well on Tomcat. So was wandering that what went wrong with the JBoss. I searched and went through some articles over the internet and gathered some sequential steps to get my work done. While following the solutions , i came across few new errors/exceptions, so here goes the list and their solutions:

Only One JAX-RS application allowed

When my war file failed to deploy in JBoss, I ran an enquiry on logs and came across this line in the Server log, “Only One JAX-RS application allowed”. Its ambiguous message. It seems like it depicts that only one war file with JAX-RS application can be deployed, but this can become a major flaw of JBoss if it is unable to run multiple JAX-RS applications.

But the real issue was something else. The issue was the REST-EASY implementation already provided by JBoss. I was creating the Rest Services using jersey and hence deploying them on JBoss, but JBoss was already shipped with REST-EASY packages. So I need to tell JBoss by some way that use jersey and ignore the REST-EASY implementation. So for doing that, I added  following lines in my web.xml file .

<context-param>    <param-name>resteasy.scan</param-name>    <param-value>false</param-value> </context-param> <context-param>    <param-name>resteasy.scan.providers</param-name>    <param-value>false</param-value> </context-param> <context-param>    <param-name>resteasy.scan.resources</param-name>    <param-value>false</param-value> </context-param>

By adding so, JBoss was atleast understanding that which implementation is needed now, but a new issue was awaiting.

java : Unsupported major.minor version 52.0

Now what?, I said to myself. One gone another came. But in an instant I asked myself, is there any difference between the JRE of client machine and the machine on which the webservices were compiled. And there was the resolution of this issue as well. I was compiling the services in JDK 1.8 and deploying them on a system having java 1.6. I recompiled my services and deployed them, and the error was gone. But this was not the end yet.

Package Scanning is not possible

As I progressed,  I came across another issue in server log. There was no way the server is going to package scan my Java packages to know that which service classes I have implemented. In other words,for everything to work as expected We have to annotate our classes with some special annotations to let servlet container know that which are web service implementation classes. I had already done that. But this new issue came up along with a suggestion in the logs. The suggestion was to manually implement an application class by myself and register the Resource classes in it rather then depending on annotations. So I added something like following into my existing package.

and the error was gone.

Issue with the Filters

As explained in the above issue that I had to register my rest resources(classes) in application class itself, In similar way I registered my filters there and added them in the existing set. However the filters were not getting executed even after doing so. Hence I removed the filter from application class and mentioned them in web.xml of my project. Magically they started executing.

No more..........

After getting these issues and resolving them one by one,my half a day was spent but I was able to get them working and deliver the build on time. The more exciting thing about this chain of issues was that; this was my first rest deployment project and was an immediate requirement by the client. So, I was happy that I managed to get that in time and so I am sharing the my experience, so that if any of you may come across these will be able to solve them.

So if you will ever make up your mind to learn about the JAX-RS  , believe me whatever book or tutorial you will pick, the first chapter will have that bold dark line in it. Annotations , customisation and each feature you will  go through while learning will be fun stuff and will never bore you,(unless you are not practising the small code examples along the way).  

JAX-RS utilises the already existing HTTP specifications, which are the backbone of all web interactions happening over internet all over the world to create and share Resources on servers.So you know its easy for someone who wants to learn how to ride a a bike who already knows how to ride a simple scooter(atleast easier than learning some out of the world new vehicle).

You just need to stick to the basics, the accelerator is still there,the  brakes are there but the clutch is added, You can learn it with practise right. 

I liked the idea when developers of the Android framework picked up JAVA as there primary language, (although under the hood there is no JVM , or byte code is involved. ) as JAVA community is one of the largest developer community across the globe. Developers learned the skills quickly and now Android Developer community has grown in number  considerably.

Back to the Road, REST is so easy in comparison to SOAP, and implementations like Jersey, RestEasy and RestLet are the way to go with it. Application endpoints are  easy to develop and manage.  

Whenever I see some Job posting in Android or any other  profile, JAX-RS is one of the requirements.  Web is prevalent, and hence web services are. JAX-RS also provides cross points called as filters and Interceptors, that will make life easy for you when you think of reusing some code across your applications.

BUT...

The one and only pain I found in the API implementation, was the long running arguments in the methods.For example see the syntax of following abstract method of MessageBodyWriter

public void writeTo(TableData data, Class<?> type, Type genericType, Annotation[] annotations,MediaType mediaType, MultivaluedMap<String, Object> headers, OutputStream out)

Cmmon Guys, you know Robert Martin is gona hate you for this, he clearly mentioned in his book that avoid these long running argument scenarios, However, Lets just compromise for now.

I hope you will also love to learn it, Do some code practices along the way and you will master it easily. I am polishing myself too.

Dear Java provide one of the famous keyword called as final and all of the Developer community working with the language must be aware of it and what does it do. Lets revise this a bit....

final keyword is applied to the following........

Variable: You simply cannot assign the variable a new value (rendering it a constant, of course)

Method: You cannot re-implement (i.e., override) this method in a subclass

Class: You cannot define a subclass

In short, you want to prevent the change or alteration to above java constructs  use the final ..

so in the above java code, the variable ‘yourConstant’ can’t be changed through out the execution of the program. That was something you must be aware of.

Consider the following example now,

Now  we have declared a constant array having values 12,13 and 14 on the corresponding zeroth, first and second indices respectively. So now one would believe that the array values can’t be altered anymore(of course array is declared as final), but here lies the Twist for some of you who are not already aware of the spoiler of the ongoing episode.

Even the array is declared as final, still the array values of this final declared array can be changed at any point of time.

The above code will compile perfectly without any issue assigning a value of 100 to the zeroth index of the array. and the array will contain following values : 100, 13, 14.

Why did that worked?

final implementation  : it implies to Array object reference which means once initiated the reference can never change but the array its self can be populated. final only guarantees immutability of primitives. And also guarantees that a variable is assigned only once. If an object is mutable you can change the content of it even if  it is defined as final. 

What I meant in the above statement is explained in the following example:

Why I discussed this?

Mr. Joshua Bloch  (a technology author, formerly employed at Sun Microsystems and Google.) emphasised the above use of final in programs as big loop hole in one of his books. Hence I found it a worth sharing concept.

Lets create a table  named as ‘Cousins’ which is holding the information of some people who are cousins, with there names and are given a unique ID. Let The create Statement is

CREATE TABLE COUSINS (ID INT PRIMARY KEY, NAME VARCHAR(10),AGE VARCHAR(2))

I inserted some data in the table, You know its not a big deal with Rich Gui’s present around us to insert data in the tables. The table looked like this after inserting the small piece of data.

Easy stuff till now,

Lets start scratching this small piece of data, Let that I want to fetch the records stored in the table on the basis of ID in the ascending order(Note that the ID is entered manually in the table).Hence I will write my select query like this

SELECT ID,NAME FROM COUSINS ORDER BY ID;

And the resultset will be as expected( note that ‘order by’ uses ascending order by default)

What if I want the data in the order of age, Lets do it

SELECT AGE,NAME FROM COUSINS ORDER BY AGE;

So, the data fetched is the same resultset as was desired by me, Lets add some twist to the tale.

TWIST:

Add another row for  another cousin who is youngest of all having age of 9,and then the table will look like this(I know the story is getting long but we are close to the climax, Hold on)

Now lets again try to fetch the results by applying an order by clause on age and see the surprising results

SELECT AGE,NAME FROM COUSINS ORDER BY AGE;

John who is the youngest is the ‘oldest’ in the fetched results. What? But he is the youngest of all Dudes. I thought the Sql Engine was drunk.

But wait, The SQL engine was not drunk, May be we have to go through the story  again. Check out the ‘create table’ statement of the table. Haven’t you noticed something fishy there. Something silly. If you found it than congrats ,If you haven’t, then lets figure it out.

The Data Type of the age is varchar(), and

‘Any numbers in the varchar field are treated like the words in a dictionary irrespective of their magnitudes’

Hence the sql engine is taking the data entered in that column as a String and ordering out the data accordingly.(just like alphabets) i.e lexically

Just change the datatype of the AGE column to Integer, and everything will be back on track again. I know that it was kind of weird to declare Age as a varchar, but anyhow, we came across a pretty useful fact because of it.

“ Flipkart to move to app-only format within a year, says a top executive ”

I was stammering, saying to myself,”wha,wha,what?But why?”

Flipkart is India’s reputed eCommerce website,also available on mobile platforms.But why are they switching to Application only format. A lot of questions surrounded me which was proving that this is not one of the smart  move.Either they will have to take this decision down,or they may have to face problems in their business.There may be some other face of the story too,but the clear and very first view of the strategy was not acceptable to me.I can tell you why it was so,

If everybody on the web will start moving to mobile App format,then users will have to install applications of each one of them whose services was accessed by the user from the website.Imagine your phone’s App Drawer filled up of such apps,and your slow and hanging phone.Not less than a nightmare,Is n’t it?

Also let that you want to buy a shoes, you opened some website,There are numerous varieties there.You scrolled using your mouse through all of them from your desktop.Imagine the same when you will access the products from your mobile phone when there will be no website available.Will you be able to go through all of them,the variety etc.

Now the Google’s push notification service from websites,directly to devices,It is in implementation phase and clearly is having a lot advantages.Without a website,you will not be able to access theses services.

And the question list in my mind went on and on,and on

Now few days back I was just doing some work  ,The databases in particular and I was left wandering.

Any silly reason for it may be, But I would like to share, so that whenever you will came across such thing, you will not have to panick.

I created a database named ‘MyDatabase’ and a table ‘MyTable’  in android programmatically and inserted some records. I decided to pull the file from the EMULATOR through DDMS to know that what was being saved in the database.There were three records,Like the following:

I decided to increase the number of records in the database, so I thought of adding records from the SQLITE browser.You know its easy to go and click the New Record button and start adding fields. I just clicked the NEW Record button and then decided that,”Oh man, i will do this insertion later,lets do something else” and then saved that file and pushed it on emulator through DDMS and left my System.

When I came back,I wrote a method to calculate the number of records in the table and display it through a toast(A type of message) in the emulator’s screen.I was surprised by the output,It was saying four records.I was amazed,and then I thought that there may be some problem in mycode which is fetching the data ,may be some increment-decrement variable issue,but altering my code by any means was never changing anything.

  Then i thought of rechecking the database file(pulling it from emulator) in the SQLITE browser.I pulled it and was still seeing three  records there..

I was talking to myself that where is the mistake buddy,where is it?

Then I just enlarged my eyes and found an empty insertion,see the image the 4th empty record.

I said to me “What?Really?An empty record?”

The 4 at the left of the empty record is representing the number of records in the table and is not inserted by me but was generated by the sqlite Browser.

Then I just recalled the moments, When I was just leaving my system

I was going to insert some record

I clicked on ‘New Record’

But added no record

Saved the file as it is

Pushed it on the emulator

And phew...Left the System

Then I recognized the problem,

FACT IS:

whenever you click the New Record button in the SQLITE browser,A new row with null values is getting inserted everytime.Like this

The SQL create statement for the table is

As you can see that there is no primary key specified in the create table statement.That was the reason of ambiguity in this case. However when I tried it the same thing by making the ID field in mytable as a PrimaryKey like the following

Then on the click of ‘New record’ button the id was getting inserted by itself(with some value ofcourse) which clarified me that the records were getting inserted with null values,and the primary key is automatically getting incremented.

I hope next time whenever you will  came across such an issue you will not panick as you will be already knowing upfront that what is happening and you can have your coffee peacefully.

@ Or any other scenario you can take into account where you want to go on a different lane other than following the normal rules which are generally followed while developing android applications

You will be glad to know as an Android developer that all these can be overrided,and you can make each of the component to perform differently  from there normal behavior.

AS AN ANDROID USER

Now let us consider this from the point of view of the Android user using some application on his device,It may happen that user may have installed some Bad application on his device and when he/she is using it,suddenly  finds out that all the common normal functionalities of the device have been overrided by the application.He can't go out of the application by any means because the developer of the app has overided everything.Shutting down the device by taking out the battery is the only option left for you.Thats the worst part. Wait! wait! wait!...STOP.This paragraph was a bad dream .Isn't it.Let me pinch you and wake you up.

Fortunately, the Android  OS developers have something that is not in the hands of even the Android application developers. And it is your one and only HOME BUTTON.Yes,It is the only button which can't be overrided. As we all know that minimizing the application is the normal functionality of the HOME button. So even if you got stuck in your application,you can come out of it by pressing the home button anytime,and then handle that Bad application from the Application Manager easliy(disable or uninstall it,whatever you want).

AS AN ANDROID DEVELOPER

So, some words for my Android Mates ,What if you still wants to override the home button.Unfortunately No is the answer,but except in one case.If you are thinking of some application that may act as the home screen of the android device.Then doing so will help you to override the HOME button only.Otherwise you can't even think of it.This is a security feature which was embedded just to avoid android applications to take away complete access from the User of the device.

If you are already aware of the above things than you will be knowing that how to start an AsyncTask.

Just make an object of the class extending the AsyncTask class and call .execute on it. like

MyAsycTask.execute();

Now this was all a happy going,but lets just consider a tedious situation where you want to call up a URL,get the response and according to the received response you want to call another URL.That is we are looking to make two httpPosts requests,but to make it more modular and save ourselves from tedious Exception Handling,we can perform these tasks in two different AsyncTasks.

Let the first AsyncTask is MyAsyncTask1 and the second one is MyAsyncTask2. Now what we want is.

Execute the MyAsyncTask1 from with in the activity

Take the response received

Now on successfull response,execute the MyAsyncTask2

what about calling them in sequence like:

MyAsyncTask1.execute();

MyAsyncTask2.execute();

The problem with the above code can be understood by the fact that calling execute on the AsyncTask object will create a  new thread to run in different memory space,and immediately the next AyncTask will be started with in its own memory space.But that was not what we want.We want that the first thread must finish its working and only then the second one will start to work.How this can be achieved?

The answer lies in the onPostExecute().This is the method which will help you to get rid of this situation.While you call execute on MyAsyncTask1 then after doInBackground() of the first AsyncTask its,onPostExecute() will be called .Place a check in that onPostExecute() that if response is null, if received response is not equal to null,and is a valid response then call execute on the MyAsyncTask2(with respective parameters if specified).