Friday, March 13, 2009

JNI memory leak, briefly

Java has a problem with accessing resources outside the JVM, such as directly accessing hardware. Java solves this with native methods (JNI) that allows calls to functions written in another language (currently only C and C++ are supported). ...

There are performance overhead in JNI, especially for large messages, due to copying of the data from the JVM’s heap onto the system buffer. JNI also may lead to memory leaks because in C the programmer is responsible for allocating and freeing the memory. GC can't go beyond jvm heap to visit the malloc free store. See post on wholesale/retail.

Even regular java objects on java heap may become memory leak when you add JNI. http://www.iam.ubc.ca/guides/javatut99/native1.1/implementing/array.html says (using int array for example) that ReleaseIntArrayElements will "unpin" the Java array if it has been pinned in memory. I believe anything pinned by JNI will not be garbage collected. If JNI programmer forgets to release, it's similar to a java programmer forgetting to clear a static hashmap

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