IBM User Guide for Java V7 on Windows > IBM SDK for Java > The Java Native Interface (JNI) > Copying and pinning

Using the isCopy flag

The JNI Get<Type> functions specify a pass-by-reference output parameter (jboolean *isCopy) that allows the caller to determine whether a given JNI call is returning the address of a copy or the address of the pinned object in the heap.

The Get<Type> and Release<Type> functions come in pairs:

• GetStringChars and ReleaseStringChars
• GetStringCritical and ReleaseStringCritical
• GetStringUTFChars and ReleaseStringUTFChars
• Get<PrimitiveType>ArrayElements and Release<PrimitiveType>ArrayElements
• GetPrimitiveArrayCritical and ReleasePrimitiveArrayCritical

If you pass a non-null address as the isCopy parameter, the JNI function sets the jboolean value at that address to JNI_TRUE if the address returned is the address of a copy of the array elements and JNI_FALSE if the address points directly into the pinned object in the heap.

Except for the critical functions, the IBM JVM always returns a copy. Copying eases the burden on the GC, because pinned objects cannot be compacted and complicate defragmentation.

To avoid leaks, you must:

• Manage the copy memory yourself using the Get<Type>Region and Set<Type>Region functions.
• Ensure that you free copies made by a Get<Type> function by calling the corresponding Release<Type> function when the copy is no longer needed.

Parent: Copying and pinning

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