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LiveDB

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Revision as of 15:47, 29 July 2010 by JaroslavTulach (Talk | contribs)
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Another use of AnnotationProcessors (in addition to generate CompileTimeCaches) is to provide type-safe, compile-time view of a live database structure. Something that might be called JavaOnRails.

For a while languages like Ruby or Groovy have been blessed for being able to easily access structures in databases without all the hassle associated with traditional object relational mapping needed in Java. Maybe there was something more in the whole rails hype, but for me, rails mean exactly this. Be able to see content of LiveDB.

This page documents my experiment showing how to do something similar in good old Java. Well, it is not old good Java, but Java6. Java6 has been around for few years already, but it is common that good ideas take time. As a result we are only slowly recognizing how useful AnnotationProcessors can be.

Contents

Screen Cast

As visual impression is usually much stronger than thousands of words, you can now watch a screen cast recorded to demonstrate the functionality of this project. Just press play!

Thanks Geertjan for recording this screen cast with me!

Get the sources

The sources are now available and their execution is fully automated (even the Derby database is started automatically via the Ant build script). To get the sources follow these steps:

$ hg clone -r livedb http://source.apidesign.org/hg/apidesign/
$ cd apidesign
$ hg pull
$ hg update

The right project is located in samples/livedb directory. You can either play with it directly from command line using Ant or you can open the project directory inside NetBeans IDE 6.9 and later:

$ cd samples/livedb
$ ant test

Playing with the Sample

Tables in the database
Tables in the database

The first thing you need to do to really play with the sample is to create your own database. Maybe you have one, maybe you need to create one. If you have NetBeans IDE then one is ready for you. Just start the Derby database as explained for example at JavaDB tutorial. Create a table called AGE with two columns NAME and AGE.

Use Code Completion

Now open the samples/livedb project in NetBeans IDE 6.9. Version 6.9 is the necessary minimum, as it is the first version that provides superior support for AnnotationProcessors.

Open for example LiveDBTest.java. It uses class Age. That class is dynamically generated during compile time to reflect structure of the AGE table created before. But try for example code completion on:

age.<ctrl+space>

You will correctly see fields representing individual rows in the t able. Moreover these fields have proper types (e.g. NAME is String, AGE is BigInteger). All of this is a result of simple use of the @LiveDB annotation in the package-info.java file:

@LiveDB(
    classname="Age", password="j1", user="j1",
    query="select * from APP.AGE", 
    url="jdbc:derby://localhost:1527/livedb"
)
package org.apidesign.livedb.example;
 
import org.apidesign.livedb.LiveDB;

Modify the Table

Now add new column to the table. Use it in the code. See if code completion sees it (probably not). If everything compiles (probably yes).

tbd.

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