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Using Unsafe to access memory is extremely efficient: All memory access operations are defined as HotSpot JVM intrinsics, so memory access operations are routinely optimized by the HotSpot JIT compiler. Unsafe exposes many memory access operations (e.g., Unsafe::getInt and putInt) which work for on-heap as well as off-heap access thanks to a relatively general addressing model.
#JDK 6 API CODE#
Many of these limitations stem from the fact that the ByteBuffer API was designed not only for off-heap memory access but also for producer/consumer exchanges of bulk data in areas such as charset encoding/decoding and partial I/O operations.Īnother common avenue by which developers can access foreign memory from Java code is the Unsafe API. Many requests for enhancement have been filed over the years in order to overcome these and other limitations (e.g., 4496703, 6558368, 48375431). Moreover, working with direct buffers can be cumbersome since deallocation of the memory associated with them is left to the garbage collector that is, only after a direct buffer is deemed unreachable by the garbage collector can the associated memory be released. For example, it is not possible to create a direct buffer larger than two gigabytes since the ByteBuffer API uses an int-based indexing scheme. The ByteBuffer API, introduced in Java 1.4, allows the creation of direct byte buffers, which are allocated off-heap and therefore allow users to manipulate off-heap memory directly from Java. Unfortunately, the Java API does not provide a satisfactory solution for accessing foreign memory:
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Usability: For programs that need to access foreign memory, the API should be a compelling alternative to legacy Java APIs such as.
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Safety: It should not be possible for the API to undermine the safety of the JVM, regardless of the kind of memory being operated upon.Ĭontrol: Clients should have options as to how memory segments are to be deallocated: either explicitly (via a method call) or implicitly (when the segment is no longer in use). Generality: A single API should be able to operate on various kinds of foreign memory (e.g., native memory, persistent memory, managed heap memory, etc.). The ability to register segments with a Cleaner.A new interface, MemoryAccess, which provides common static memory accessors so as to minimize the need for the VarHandle API in simple cases.A clearer separation of roles between the MemorySegment and MemoryAddress interfaces.
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The following changes are included in this API refresh: This JEP proposes to incorporate refinements based on feedback, and to re-incubate the API in Java 16. The Foreign-Memory Access API was first proposed by JEP 370 and targeted to Java 14 in late 2019 as an incubating API, and later re-incubated by JEP 383 which was targeted to Java 15 in mid 2020. Introduce an API to allow Java programs to safely and efficiently access foreign memory outside of the Java heap.