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If you are running code that makes calls to a native library in Java, what is the usual way of freeing memory allocated by these libraries when the memory allocation should last for the lifetime of the object? In C++, I would use destructors, but Java never really had those and has them even less now.
The specific case I'm most interested in is JOCL, where I have an object that wraps a compiled OpenCL kernel and all of the arguments thereto that are always the same. Structures representing the compiled kernel and the arguments are all allocated on the library side, and JOCL provides a method clReleaseMemObject that you call to decrement a reference counter indicating when the object should be deleted (note that this is a bit different from directly freeing the memory, but I don't think substantially so in this case).
I presume that if the object is still around when the program terminates, everything is cleaned up by the OS, but I'm not so sure about about objects created in a thread. So:
If you want the native memory deallocated when the object is garbage collected, is there a proper place to call the method that releases this memory?
If the object is one that will last for the duration of a thread, is there a proper place to make this call, or is this even necessary?
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The Memory allocation in java is divided into parts, namely Heap, Stack, Code, and Static.
Native memory is the memory provided to the application process by the operating system. The memory is used for heap storage and other purposes. The native memory information available in the native memory perspective view varies by platform but typically includes the following information: Table 1.
A native library is a library that contains "native" code. That is, code that has been compiled for a specific hardware architecture or operating system such as x86 or windows. Including such native library in your project may break the platform-independence of you application.
Java does memory management automatically. Java uses an automatic memory management system called a garbage collector. Thus, we are not required to implement memory management logic in our application.
What you can do is use a Cleaner. This is a more official API in Java 9 but is available in Java 1.4+.
Essentially you give it a Runnable to execute when the resource is cleaned up.
One advantage of using a Cleaner is you can call it to clean up deterministically, but if you forget or fail to do so, the GC will call it after it runs.
There isn't a safe way to clean up an object when a thread dies as the Thread object can live for the life of the program even if dead. A simpler approach is to clean up as you know it is not needed or after the GC determines it is not required.
Another approach is to use a reference queue and a background thread. It's not as elegant but works across Java 8 and later versions.
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