JPA persist() Vs merge()

Last week I was working in our project at office and I came across this confusion between these two buddies. Even I have been working with hibernate, I found that its bit difficult to think about this in hibernate aspect. Because JPA has its own set of methods to save objects which are not same as in hibernate. Lets find out how these methods works using an simpleexample. 

Create a new student object
Get an entity manager instance from the persistence.xml file
Save a student object using persist()
Output:
name -> "Saman"
age -> 24

When we call persist on that new student instance, It will do following things.
1. Object added to persistence context as new entity(Make an instance managed)
2. New entity inserted into database at flush/commit

Save a student object using merge()
Output:
name -> "Saman"
age -> 12

When we call persist on that new student instance, It will do following things.
1. State copied to new entity.
2. New entity added to persistence context
3. New entity inserted into database at flush/commit
4. New entity returned

So if we want to update new entity using merge(), we can do something like this.
Output:
name -> "Saman"
age -> 24

Git url for demo project: https://github.com/arlahiru/JPADemo

Following links helped me to understand this difference and all the credits should go to its authors.
http://stackoverflow.com/questions/1069992/jpa-entitymanager-why-use-persist-over-merge
http://java.boot.by/scbcd5-guide/ch06.html
http://planet.jboss.org/post/get_started_quickly_with_hibernate_annotations_and_jpa_2
http://www.noway.es/JPA-eclipselink-helloworld

 Hope this helps you. See you next time. Bye! :D

JTable and bean binding

Hello everyone! After a long pause, here I am again :) Thought to power the blog again with the experience which I have gained past couple of years and hope to keep this as a future reference to me as well to remember what I have learned/gained during my work/study.

Okay! Let me begin the post. I have come across this scenario while I was working on a Java Swing application project at my office.

The problem: How to populate JTable with list of objects.

Typical and painful solution: Loop through the list and populate data column by column.
But it's a real pain when it comes to retrieve data as objects again from the table. We have to extract each property from each column in the table and create a new object with that data.

An easy and less complex/code solution: Bind beans to the table as rows.
If we can bind objects as it is to the table then its really easy to retrieve them as well. No need to iterate through columns anymore. I found this solution from this blog and total credit goes to the author.

This is how it works. There are two ways to bind beans to JTable.

• Bean Table Model
• Row Table Model

I have included both ways in the sample project please find the url for git repo at the end of this post. But here I only explain first method. Its really easy to understand second method once you learn the first. In the sample project, its just a matter of changing table model implementation in the source file to switch between those two methods.I have included all the source files in one package.

Following steps explain how to do it.

Step 1: Create a bean class
Step 2: Create the bean table model
Step 3: Bind the table model with JTable
Step 4: Retrieve data from the table
Screenshot of the demo application:

Yep. That's all. Its simple as that. Okay, time to go to bed :) I'll see you in another post. Triple gem bless you!

Git repository URL: JTableBeanDemo

P.S: You can also get the original source files from the blog link i have mentioned in the post.