New JavaRanch Journal article: Handling Uncommon File Formats - Introducing Lexers

[Ulf Dittmer]

The latest issue of the JavaRanch Journal contains an article by yours truly on Handling Uncommon File Formats - Introducing Lexers. Go read it while it's fresh, and discuss it in this thread.

[Raghavan Muthu]

Hi Ulf,

Thanks for the nice article. It sounded interesting and I started reading through. On my way, I had downloaded the 'Properties.zip' file and ran the very first example you had demonstrated to run the 'PropertiesReader1.class' with 'example1.properties'. I face an issue here as it gives the infamous 'NoClassDefFoundError'.

I did run the command

java -classpath .:JFlex.jar PropertiesReader1 example1.properties

in the directory where I had extracted the Properties.zip file and the error I get is

Exception in thread "main" java.lang.NoClassDefFoundError: PropertiesReader1
Caused by: java.lang.ClassNotFoundException: PropertiesReader1
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClassInternal(Unknown Source)
Could not find the main class: PropertiesReader1.  Program will exit.

I could see the .class, .flex, .properties files present inside the extracted folder content. I also checked inside the .flex file to see if the PropertiesReader1.class file belongs to any package. But it does NOT. Why I am getting this error here?

Do I miss something here?

Thank you.

[Ulf Dittmer]

That is odd. I just downloaded the zip file, changed to the directory, and ran the command you quoted - it worked just fine. You could try recompiling by

java -classpath JFlex.jar JFlex.Main  PropertiesReader1.flex
javac -classpath .:JFlex.jar PropertiesReader1.java

[Ulf Dittmer]

Are you using Windows? If so, you need to use a semi-colon instead of a colon in the classpath.

[Raghavan Muthu]

Thank you Ulf. Yes I am using windows and when i tried executing the same command without having the -classpath switch itself, it did work fine .

Let me proceed further and get back for any clarifications.

[Raghavan Muthu]

Ulf,

Thank you very much. I am done with the full article and ran the complete set of examples in the zip file. It is really nice and I am happy about the last example (PropertiesReader5.flex) as it gives a provision/solution for the hierarchical properties in standard Java properties. .

It somehow gives me a hint to solve my long time problem of having a parameterized properties -- as what we do with ANT. Like we can refer a property inside the .properties file using the {} pairs to get its value so as to have a single point of change. I guess this JFlex can help me in solving this. Aint I?

To what extent should I know the 'internals' of JFlex? OR can i just go ahead with the same pace as that of your examples?

[Ulf Dittmer]

Glad you liked it.

I don't think you need to know the internals of JFlex. At least, I don't know them, and I've used it for more complex file formats than this one.

Maybe I'm misunderstanding what you mean by "parameterized properties", but Java already has that. Check out the "Messages" examples at http://www.exampledepot.com/egs/java.text/pkg.html. That sounds like what you're trying to do.

[Raghavan Muthu]

Thank you Ulf for the immediate response as usual and expected.

I am aware of the example which is shown in the page -- which has the parameters to be passed to a property using {0}, {1} etc., and at the runtime, you can pass an Object[] to it by using MessageFormat.format() method.

I guess i had termed it in an inappropriate way. All what I want is

# Sample properties file

application.name=Test

# Having other properties which refer to this one at various places

warfile.name={application.name}.war
jarfile.name={application.name}.jar

This we use it with ANT in our build.properties file by making use of "${}" to get the value of an existing property.

When we do NOT use ANT and if we write our own parser to deal with the traditional .properties file, is there any bulit in facility to deal with this OR do we have write on our own? Here is where I thought, this JFlex could be of some use to me. Aint I?

[Ulf Dittmer]

While you could write a lexer to do that, it seems overkill. A simple regular expression will extract the property name(s), and also allow you to replace them with their respective property values. That's much easier to write.

[Raghavan Muthu]

Yes, the idea of using RegEx seems to be an easy deal. Let me try. Thanks Ulf.