SimpleCaptcha for Java 1.5
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SimpleCaptcha
SimpleCaptcha is an excellent Turing test solution for Java web applications. It allows you to generate images with scrambled text (that supposedly humans can read and machines can't) to prevent automatic form submissions by bots. We use it on this site too, on the contact page. SimpleCaptcha is configurable, extendable and comes with a Latin and Chinese-based implementation. It is distributed under an Open Source BSD license.
only for java 1.6
There is one unfortunate drawback to SimpleCaptcha as it is distributed: it's written to and compiled for Java 1.6 So you can't use it right away when you are running a Java 1.5 environment.
java 1.5 distribution
We at GeoNetwork TV have made a SimpleCaptcha implementation suitable for Java 1.5. The code changes are minimal, we basically rewrote a Java 1.6 Arrays.copyOf() to its equivalent in Java 1.5, and we also removed various @Override annotations in classes that implement interfaces (not classes that extend base classes), because Eclipse cannot handle them.
Our distribution is based on SimpleCaptcha 1.1.1, distributed on SourceForge.
You can download a ready-made jar, compiled with JDK 1.5, or you can download the full sources.
You can see the original license terms (which we have not modified) here.