JNI is short for Java Native Interface. It allows you to call native methods written in C, C++ or assembly language. JNI is useful for integration with existing programs or to make parts of your code speedier. You application will become unusable on other platforms unless you port that static library. Before you dive into JNI, think about the implications. There is a tradeoff, even though a native method will always be more performant than an equivalent Java one, there is still the call to the native method to consider. For some good general tips, check out Mindprod's JNI, The Java Native Interface page.
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