LOCAL AND GLOBAL REFERENCES in Java

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Alternatively, the Push/PopLocalFrame functions allow programmers to create nested scopes of local references For example, we may also rewrite the same example as follows:
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#define N_REFS /* the maximum number of local references used in each iteration */ for (i = 0; i < len; i++) { if ((*env)->PushLocalFrame(env, N_REFS) < 0) { /* out of memory */ } jstr = (*env)->GetObjectArrayElement(env, arr, i); /* process jstr */ (*env)->PopLocalFrame(env, NULL); } PushLocalFrame creates a new scope for speci c PopLocalFrame destroys the topmost scope, freeing
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number of local references all local references in that
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scope The advantage of using the Push/PopLocalFrame functions is that they make it possible to manage the lifetime of local references without having to worry about every single local reference that might be created during execution In the above example, if the computation that processes jstr creates additional local references, these local references will be freed after PopLocalFrame returns The NewLocalRef function is useful when you write utility functions that are expected to return a local reference We will demonstrate the use of the NewLocalRef function in Section 53 The native code may create local references beyond the default capacity of 16 or the capacity reserved in a PushLocalFrame or EnsureLocalCapacity call The virtual machine implementation will try to allocate the memory needed for the local reference There is no guarantee, however, that memory will be available The virtual machine exits if it fails to allocate the memory You should reserve enough memory for local references and free local references promptly to avoid such unexpected virtual machine exits Java 2 SDK release 12 supports a command-line option -verbose:jni When this option is enabled, the virtual machine implementation reports excessive local reference creation beyond the reserved capacity 523 Freeing Global References You should call DeleteGlobalRef when your native code no longer needs access to a global reference If you fail to call this function the Java virtual machine will not garbage collect the corresponding object, even when the object is no longer used anywhere else in the system 69
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You should call DeleteWeakGlobalRef when your native code no longer needs access to a weak global reference If you fail to call this function the Java virtual machine will still be able to garbage collect the underlying object, but will not be able to reclaim the memory consumed by the weak global reference itself
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53 Rules for Managing References
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We are now ready to go through the rules for managing JNI references in native code, based on what we have covered in previous sections The objective is to eliminate unnecessary memory usage and object retention There are, in general, two kinds of native code: functions that directly implement native methods and utility functions that are used in arbitrary contexts When writing functions that directly implement native methods, you need to be careful about excessive local reference creation in loops and unnecessary local reference creation caused by native methods that do not return It is acceptable to leave up to 16 local references in use for the virtual machine to delete after the native method returns Native method calls must not cause global or weak global references to accumulate because global and weak global references are not freed automatically after native methods return When writing native utility functions you must be careful not to leak any local references on any execution path throughout the function Because a utility function may be called repeatedly from an unanticipated context, any unnecessary reference creation may cause memory over ow When a utility function that returns a primitive type is called, it must not have the side effect of accumulating additional local, global, or weak global references When a utility function that returns a reference type is called, it must not accumulate extra local, global, or weak global references, other than the reference it returns as result It is acceptable for a utility function to create some global or weak global references for the purpose of caching because only the very rst call creates these references If a utility function returns a reference, you should make the kind of returned reference part of the function speci cation It should not return a local reference some of the time and a global reference at other times The caller needs to know the type of the reference returned by a utility function in order to manage its own JNI references correctly For example, the following code calls a utility function GetInfoString repeatedly We need to know the type of reference returned by 70
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