Well, if you read about the Java API, you will know that Set will only add object that does not exist in the Set itself. You must be aware of this especially when working with Hibernate! Here's my experience:
I was trying to add a few objects in a Set and set this Set. Then I found out from the database that there is always only ONE object added. I did a debug and found the size of the Set is always > 1 !!!
What happened? Here is my answer from torough checking:
Only the 1st object had been added to the Set. It is because the rest are actually "equal" to the first object so Set does not take them anymore! Bear in mind that each object differs from each other using their ID field. They do have their own ID yet when they are being added into the Set so they are rejected by the Set!
Thus, i have to save each object first before adding them into the Set. The reason is the ID will be generated by Hibernate for each object. The Set knows that they are NOT EQUAL right now then they will be added.
Thursday, May 29, 2008
Tuesday, May 13, 2008
Apache POI
I have one web application, everytime i deploy i will need to load initial reference data again and again.
The amount of these data is huge!
I used to hardcode all these data in my Java file. I organised them properly using data structure such as String array and so on. In my Java file i have a few functions that actually perform the data loading job.
The problem is, It's time consuming and error prone to hardcode each and every data in the Java file.
Therefore, today i decided to put all the data in Spreadsheet program. It is easier to maintain data at Spreadsheet rather than in the Java file itself.
Then i used Apache POI library to read me those data from the Spreadsheet (Microsoft Excel).
Just one time try and it worked fine!
So simple! I was really delighted!
The amount of these data is huge!
I used to hardcode all these data in my Java file. I organised them properly using data structure such as String array and so on. In my Java file i have a few functions that actually perform the data loading job.
The problem is, It's time consuming and error prone to hardcode each and every data in the Java file.
Therefore, today i decided to put all the data in Spreadsheet program. It is easier to maintain data at Spreadsheet rather than in the Java file itself.
Then i used Apache POI library to read me those data from the Spreadsheet (Microsoft Excel).
Just one time try and it worked fine!
So simple! I was really delighted!
Wednesday, May 7, 2008
One day effort for unknown vm crach of Eclipse during Subclipse installation
WTF,
It took me one day to get the problem fixed. The solution is so simple!
I setup Eclipse at my new laptop. As usual, i also installed the Subclipse plugin. By right it should work properly. Well, when i tried to create a new remote source repository using Subclipse Perspective in Eclipse, Eclipse just quitted suddenly followed by a prompt saying that it is some kind of memory crash.
Also, there is an error log file generated each time Eclipse exited. However the message is so so not comprehensive.
Here's the message:
With this only clue, i kept Googling for the whole day and still did not get the problem fixed! How frustated was i!
Besides trying out the solutions found online, I also tried to tackle the problem by doing a few times of trial and error, which is actually install-uninstall this kind of stuff.
We really got to be patient enough when dealing with this kind of problem.
Yet, the problems stayed. All the tries did not bring positive outcome.
Office hour ended. Today i did nothing but just configuring stupid Subversion on Eclipse. Boss, please don't get mad.
I started over the puzzle solving game again at home after coming back from dinner.
With calm mind, and of course enough of rest, i found the answer within an hour!
I came across this article online:
http://subclipse.tigris.org/faq.html#win32-crash
Here's the excerpt:
So, i uninstalled the Subversion Server built against Apache 2.2 and replaced it with the one built against Apache 2.0.
Eclipse looks fine now.
Finally!
It took me one day to get the problem fixed. The solution is so simple!
I setup Eclipse at my new laptop. As usual, i also installed the Subclipse plugin. By right it should work properly. Well, when i tried to create a new remote source repository using Subclipse Perspective in Eclipse, Eclipse just quitted suddenly followed by a prompt saying that it is some kind of memory crash.
Also, there is an error log file generated each time Eclipse exited. However the message is so so not comprehensive.
Here's the message:
## An unexpected error has been detected by Java Runtime Environment:
## EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x3b397bfc,
pid=3824, id=5160
## Java VM: Java HotSpot(TM) Client VM (10.0-b22 mixed mode
windows-x86)
# Problematic frame:# C [libapr-1.dll+0x7bfc]
## If you would like to submit a bug report, please visit:
# http://java.sun.com/webapps/bugreport/crash.jsp
# The crash happened outside the Java Virtual Machine in native code.
# See problematic frame for where to report the bug.
With this only clue, i kept Googling for the whole day and still did not get the problem fixed! How frustated was i!
Besides trying out the solutions found online, I also tried to tackle the problem by doing a few times of trial and error, which is actually install-uninstall this kind of stuff.
We really got to be patient enough when dealing with this kind of problem.
Yet, the problems stayed. All the tries did not bring positive outcome.
Office hour ended. Today i did nothing but just configuring stupid Subversion on Eclipse. Boss, please don't get mad.
I started over the puzzle solving game again at home after coming back from dinner.
With calm mind, and of course enough of rest, i found the answer within an hour!
I came across this article online:
http://subclipse.tigris.org/faq.html#win32-crash
Here's the excerpt:
As soon as I do something with Subclipse on Windows, Eclipse just
crashes. Why does this happen?
This is a fairly recent problem that is caused by a DLL incompatibility. Subversion uses a library called APR or the Apache Portable Runtime. This library has a component called APR-ICONV which is used for translating characters in path and file names to/from UTF8 and the local character set. The release of Apache 2.2 brought with it new releases of APR and APR-ICONV and these are not binary compatible with previous releases.
Subclipse ships with the Apache 2.0 version of these libraries. This crash
occurs if you install an application that installs the Apache 2.2 version of
these libraries AND ALSO sets the APR_ICONV_PATH environment variable to a path that contains the Apache 2.2 version of the APR-ICONV .so objects.
The fix is actually simple. Change the name of the environment variable to APR_ICONV1_PATH. The Apache 2.2 library will look for this environment variable name first, and only fallback to the older name if it is not found. You can safely have an APR_ICONV_PATH environment variable pointing to the Apache 2.0 version of these libraries and the APR_ICONV1_PATH environment variable pointing to the Apache 2.2 version. Subversion 1.5 has resolved this problem by discontinuing the use of APR-ICONV. Instead, Subversion will use translation routines that are provided by the Windows operating system.
So, i uninstalled the Subversion Server built against Apache 2.2 and replaced it with the one built against Apache 2.0.
Eclipse looks fine now.
Finally!
Friday, April 4, 2008
Hibernate Tips - delete existing item from children
Scenario as below:
I am at a screen now from a web application.
There is a swap-select box on the screen. I select 5 items and click Save button.
Next, the system creates 1 parent object and saves 5 items (selected from screen's swap-select box) as children(a Set) into their parent.
I leave the screen.
Upon next visit to the same screen again, i am trying to remove 2 among the 5 selected items. After that i click Save button.
How should i handle this?
What i need to do is:
1) Get the original Set from the parent object. (Let's name it 'children')
2) At the same time, create new Set(let's name it 'removedChildren'), add those objects that are meant to be removed. (In this case this new Set will have 2 objects)
3) Using removeAll method from Set, put the 'removedChildren' Set in it (children.removeAll(removedChildren))
4) Set the 'children' Set back to the parent. (parent.setChildren(children))
5) Save the parent. (saveOrUpdate(parent))
I am at a screen now from a web application.
There is a swap-select box on the screen. I select 5 items and click Save button.
Next, the system creates 1 parent object and saves 5 items (selected from screen's swap-select box) as children(a Set) into their parent.
I leave the screen.
Upon next visit to the same screen again, i am trying to remove 2 among the 5 selected items. After that i click Save button.
How should i handle this?
What i need to do is:
1) Get the original Set from the parent object. (Let's name it 'children')
2) At the same time, create new Set(let's name it 'removedChildren'), add those objects that are meant to be removed. (In this case this new Set will have 2 objects)
3) Using removeAll method from Set, put the 'removedChildren' Set in it (children.removeAll(removedChildren))
4) Set the 'children' Set back to the parent. (parent.setChildren(children))
5) Save the parent. (saveOrUpdate(parent))
How to get the last 2 decimal digits using JAVA?
Example:
To get 0.99
from
double abcd =123456789012345.99;
Solution:
double a = 12345678901234.99*100;
String f = Long.toString(((long) a)%100);
To get 0.99
from
double abcd =123456789012345.99;
Solution:
double a = 12345678901234.99*100;
String f = Long.toString(((long) a)%100);
Monday, September 17, 2007
Hibernate Notes
Here's some Hibernate notes for beginners out there. I've been facing lots of troubles dealing with Hibernate. I hope my notes would be short and clear enough so that you guys could save some time instead of looking for answers in the thick book!
1) Don't get confused with Flush Session & Commit Transaction
I used to have a wrong concept that flush session usually means data is already sync from Hibernate Session to the Database and everything cannot be roll back anymore! This is obviously wrong! Both are irrelevant with each other in fact! Even tough flush session would sync everything from Hibernate Session to the Database, however if the unit of work is within a transaction scope, the developer should trigger a commit so that everything can be committed!
2) Session Save will assign an identifier value to the corresponding saved object
Calling hibernate session save() will not only persist your pojo, but will also assign an identifier to that pojo according to the Hibernate Mapping. The identifier assignment will not be that late until database-session sync process happen.
1) Don't get confused with Flush Session & Commit Transaction
I used to have a wrong concept that flush session usually means data is already sync from Hibernate Session to the Database and everything cannot be roll back anymore! This is obviously wrong! Both are irrelevant with each other in fact! Even tough flush session would sync everything from Hibernate Session to the Database, however if the unit of work is within a transaction scope, the developer should trigger a commit so that everything can be committed!
2) Session Save will assign an identifier value to the corresponding saved object
Calling hibernate session save() will not only persist your pojo, but will also assign an identifier to that pojo according to the Hibernate Mapping. The identifier assignment will not be that late until database-session sync process happen.
Wednesday, July 25, 2007
What makes a software project fail?
Came across an article discussing the factors of why software project fails:
Take a look!
I agree with the 3 pillars, which are:
First of all, a better tool for version controlling was introduced: SVN, then he started to come out with all kinds of ant build script to basically compile the source codes, moving files, deleting files, ftp files and etc. We even have a build script to refresh the whole database schema by deleting tables, drop users, creating new tables, running data scripts and etc. Lastly, we integrated these two important features into our LDAP system. We applied LuntBuild as a better framework to control the build, we applied Trac which provides a lot of features to govern or track bugs.
With the help of all these tools, we managed to save really lots of time on the deployment! The deployment can be easy & clean! Everything is handled at one end, no other parties could interfere the integrity!
Of course, there are weaknesses too, we did not manage to deploy part of the system. For every deployment we are about to do, we got to build the whole project & deploy to the server directory & in the end run some unix scripts which basically unjar the war file & restarts the web application.
Take a look!
I agree with the 3 pillars, which are:
- Version control
- Work item tracking
- Build system
First of all, a better tool for version controlling was introduced: SVN, then he started to come out with all kinds of ant build script to basically compile the source codes, moving files, deleting files, ftp files and etc. We even have a build script to refresh the whole database schema by deleting tables, drop users, creating new tables, running data scripts and etc. Lastly, we integrated these two important features into our LDAP system. We applied LuntBuild as a better framework to control the build, we applied Trac which provides a lot of features to govern or track bugs.
With the help of all these tools, we managed to save really lots of time on the deployment! The deployment can be easy & clean! Everything is handled at one end, no other parties could interfere the integrity!
Of course, there are weaknesses too, we did not manage to deploy part of the system. For every deployment we are about to do, we got to build the whole project & deploy to the server directory & in the end run some unix scripts which basically unjar the war file & restarts the web application.
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