12.2.3 Handling JNI code

Because a JVM only has a single address space and native code can only be loaded once per address space, the JVM specification states that native code may only be loaded by one class loader in a JVM.

This may cause a problem if, for example, you have an application (EAR file) with two Web modules that both need to load the same native code through a Java Native Interface (JNI). Only the Web module that first loads the library will succeed.

To solve this problem, you can break out just the few lines of Java code that load the native code into a class on its own and place this file on WebSphere's application class loader (in a utility JAR). However, if you would deploy multiple such applications (EAR files) to the same appserver (JVM), you would have to place the class file on the WebSphere extensions class loader instead to ensure the native code is only loaded once per JVM.

If the native code is placed on a reloadable class loader (such as the application class loader or the WAR class loader), it is important that the native code can properly unload itself should the Java code have to reload. WebSphere has no control over the native code and if it does not unload and load properly the application may fail.

If one native library depends on another one, things become even more complicated. Search for Dependent native library in the Information Center for more details.