I was starting today to look on different approaches and techniques that are used in scalable web or non web applications. One technique used in many large systems is simply called “memory cache”. It means that data are cached in memory so the data will not be queried again.

Cache and memory cache exists for a long time; even the hardware parts link hard disks cd units have some sort of memory cache.

Why memory cache becomes so important when we talk about web applications? It’s simple. Because web applications happens to fulfill thousands of requests simultaneously. Or sometimes they have to. It’s obvious that keeping the data into memory and reuse it the next time you need it it will improve the performance. And it looks very simple. At the first view simple hashmap would do the job, unless…

There a few facts we need to be consider in real world:

• What happens when the database is updated?
• In most cases a web application create a thread for each http request. The same thing happens in java of php or other languages. The creation of the threads is handled by the web server, web container, and not by the code we write.
• The scalable applications run distributed on multiple servers. If one application change the database, the cache system should be informed on all the machines.

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Using Google Guice in web applications can raise some issues. First of all when we talk about web applications we talk about servlets and we talk about managed environments. The creation of servlets in not controlled by us, when we write the application, it is controlled by the web application container. This generates some problems:
- Because we can not control the servlet creation we can not use Guice to inject servlet members in the classic way.
- Bootstrapping: we need a mechanism to bootstrap Guice into the application. The best way is to do it at start up before any http request will be invoked.
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I use windows and I hate when I have to switch on another computer, or when I have to do any change related to windows especially changing the environment variables. I like to download the tool, framework, … I use as a zip and then to do minimal operations to install it. The thing I hate the most is setting environment variables, and path in that small dialog window. I have all my java applications and frameworks in one single directory so the configuration depends only on the thing I hate most: thats right, environment variables.
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What?

There are various reasons when we have to wrap java exceptions. Often those reasons are called: Checked Exceptions. According to the definition there are 2 types of exceptions: Read the rest of this entry »

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I get tones of exceptions in my code because I tend to use Generics in the old-fashioned way. I have my reasons for that. And I have tons of warnings like those:

• HashMap is a raw type. References to generic type HashMap should be parameterized
• Type safety: The method put(Object, Object) belongs to the raw type Map. References to generic type Map should be parameterized

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There are two options to store parameters in web.xml:
- context parameters - available to the entire scope of the web application
- init parameters - available in the context of a servlet or filter in the web application

Context Parameters

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There are several options to handle the Java properties files:

To read java properties files from the classpath:

The properties files can be stored in the classpath. This way they can be put inside jar files and it’s really useful for web applications when the absolute location of the properties files is not known. When I tested this in an web application it didn’t work: Read the rest of this entry »

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The visitor pattern can be used on structures of objects which implements a specific interface defining a method called accept. In practice, in many cases, the structures are already created and we have to visit structures of already created objects. Changing hierarchies of classes for adding a new method is not a viable solution.

We need somehow to extend the structure of objects for accepting the visitors without changing them. A way of doing it would be to add a wrapper for the hierarchy classes which accepts visitors and duplicates the structure.

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I’ve installed the eclipse plugin for bazaar. For me Bazaar is installed in C:\Java(don’t ask why I put it there, because I don’t know).
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What is Google Guice

Google Guice is a light java dependency inversion framework using annotations. It is developed by Google(Bob Lee and Kevin Bourrillion) and it is used internally by Google for their applications. Google Guice is sometimes considered an Inversion of Control Container but as their authors state it’s not a container, it’s just an Dependency Injector, being too Read the rest of this entry »

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