Interface Arena

All Superinterfaces:
AutoCloseable,SegmentAllocator
public interfaceArenaextendsSegmentAllocator,AutoCloseable
An arena controls the lifecycle of native memory segments, providing both flexible allocation and timely deallocation.

An arena has ascope - thearena scope. All the segments allocated by the arena are associated with the arena scope. As such, the arena determines the temporal bounds of all the memory segments allocated by it.

Moreover, an arena also determines whether access to memory segments allocated by it should berestricted to specific threads. An arena is aSegmentAllocator and features several allocation methods that can be used by clients to obtain native segments.

The simplest arena is theglobal arena. The global arena features anunbounded lifetime. The scope of the global arena is the global scope. As such, native segments allocated with the global arena are always accessible and their backing regions of memory are never deallocated. Moreover, memory segments allocated with the global arena can beaccessed from any thread.

 MemorySegment segment = Arena.global().allocate(100, 1); ... // segment is never deallocated!

Alternatively, clients can obtain anautomatic arena, that is an arena which features abounded lifetime that is managed, automatically, by the garbage collector. The scope of an automatic arena is an automatic scope. As such, the regions of memory backing memory segments allocated with the automatic arena are deallocated at some unspecified timeafter the automatic arena (and all the segments allocated by it) becomesunreachable, as shown below:

 MemorySegment segment = Arena.ofAuto().allocate(100, 1); ... segment = null; // the segment region becomes available for deallocation after this point
Memory segments allocated with an automatic arena can also beaccessed from any thread.

Rather than leaving deallocation in the hands of the Java runtime, clients will often wish to exercise control over the timing of deallocation for regions of memory that back memory segments. Two kinds of arenas support this, namelyconfined andshared arenas. They both feature bounded lifetimes that are managed manually. For instance, when a confined arena isclosed successfully, its scope isinvalidated. As a result, all the memory segments allocated by the arena can no longer be accessed, and their regions of memory are deallocated:

 MemorySegment segment = null; try (Arena arena = Arena.ofConfined()) {     segment = arena.allocate(100);     ... } // segment region deallocated here segment.get(ValueLayout.JAVA_BYTE, 0); // throws IllegalStateException
Memory segments allocated with aconfined arena can only be accessed (and closed) by the thread that created the arena. If access to a memory segment from multiple threads is required, clients can allocate segments in ashared arena instead.

The characteristics of the various arenas are summarized in the following table:

Arenas characteristics
KindBounded lifetimeExplicitly closeableAccessible from multiple threads
GlobalNoNoYes
AutomaticYesNoYes
ConfinedYesYesNo
SharedYesYesYes

Safety and thread-confinement

Arenas provide strong temporal safety guarantees: a memory segment allocated by an arena cannot be accessedafter the arena has been closed. The cost of providing this guarantee varies based on the number of threads that have access to the memory segments allocated by the arena. For instance, if an arena is always created and closed by one thread, and the memory segments allocated by the arena are always accessed by that same thread, then ensuring correctness is trivial.

Conversely, if an arena allocates segments that can be accessed by multiple threads, or if the arena can be closed by a thread other than the accessing thread, then ensuring correctness is much more complex. For example, a segment allocated with the arena might be accessedwhile another thread attempts, concurrently, to close the arena. To provide the strong temporal safety guarantee without forcing every client, even simple ones, to incur a performance impact, arenas are divided intothread-confined arenas, andshared arenas.

Confined arenas, support strong thread-confinement guarantees. Upon creation, they are assigned anowner thread, typically the thread which initiated the creation operation. The segments created by a confined arena can only beaccessed by the owner thread. Moreover, any attempt to close the confined arena from a thread other than the owner thread will fail with aWrongThreadException.

Shared arenas, on the other hand, have no owner thread. The segments created by a shared arena can beaccessed by any thread. This might be useful when multiple threads need to access the same memory segment concurrently (e.g. in the case of parallel processing). Moreover, a shared arena can be closed by any thread.

Custom arenas

Clients can define custom arenas to implement more efficient allocation strategies, or to have better control over when (and by whom) an arena can be closed. As an example, the following code defines aslicing arena that behaves like a confined arena (i.e., single-threaded access), but internally uses aslicing allocator to respond to allocation requests. When the slicing arena is closed, the underlying confined arena is also closed; this will invalidate all segments allocated with the slicing arena (since the scope of the slicing arena is the same as that of the underlying confined arena):
class SlicingArena implements Arena {    final Arena arena = Arena.ofConfined();    final SegmentAllocator slicingAllocator;    SlicingArena(long size) {        slicingAllocator = SegmentAllocator.slicingAllocator(arena.allocate(size));    }    public MemorySegment allocate(long byteSize, long byteAlignment) {        return slicingAllocator.allocate(byteSize, byteAlignment);    }    public MemorySegment.Scope scope() {        return arena.scope();    }    public void close() {        arena.close();    }}
In other words, a slicing arena provides a vastly more efficient and scalable allocation strategy, while still retaining the timely deallocation guarantee provided by the underlying confined arena:
try (Arena slicingArena = new SlicingArena(1000)) {    for (int i = 0; i < 10; i++) {        MemorySegment s = slicingArena.allocateFrom(JAVA_INT, 1, 2, 3, 4, 5);        ...    }} // all memory allocated is released here
Implementation Requirements:
Implementations of this interface are thread-safe.
Since:
22
See Also:

  • Method Details

    • ofAuto

      static Arena ofAuto()
      Creates a new arena that is managed, automatically, by the garbage collector.Segments allocated with the returned arena can beaccessed by any thread.Callingclose() on the returned arena will result in anUnsupportedOperationException.

      Memory segmentsallocated by the returned arenaare zero-initialized.

      Returns:
      a new arena that is managed, automatically, by the garbage collector

    • global

      static Arena global()
      Returns the global arena. Segments allocated with the global arena can beaccessed by any thread. Callingclose() on the returned arena will result in anUnsupportedOperationException.

      Memory segmentsallocated by the returned arenaare zero-initialized.

      Returns:
      the global arena

    • ofConfined

      static Arena ofConfined()
      Returns a new confined arena. Segments allocated with the confined arena can only beaccessed by the thread that created the arena, the arena'sowner thread.

      Memory segmentsallocated by the returned arenaare zero-initialized.

      Returns:
      a new confined arena

    • ofShared

      static Arena ofShared()
      Returns a new shared arena. Segments allocated with the shared arena can beaccessed by any thread.

      Memory segmentsallocated by the returned arenaare zero-initialized.

      Returns:
      a new shared arena

    • allocate

      MemorySegment allocate(long byteSize, long byteAlignment)
      Returns a native memory segment with the given size (in bytes) and alignmentconstraint (in bytes).The returned segment is associated with thisarena scope.The segment'saddress is the starting address ofthe allocated off-heap region of memory backing the segment, and the address isaligned according the provided alignment constraint.
      Specified by:
      allocate in interface SegmentAllocator
      Implementation Requirements:
      Implementations of this method must return a native segment featuring therequested size, and that is compatible with the provided alignment constraint.Furthermore, for any two segmentsS1, S2 returned by this method, thefollowing invariant must hold:
          S1.asOverlappingSlice(S2).isEmpty() == true
      Parameters:
      byteSize - the size (in bytes) of the off-heap region of memory backing the native memory segment
      byteAlignment - the alignment constraint (in bytes) of the off-heap region of memory backing the native memory segment
      Returns:
      a new native memory segment
      Throws:
      IllegalArgumentException - ifbytesSize < 0,byteAlignment <= 0, or ifbyteAlignment is not a power of 2
      IllegalStateException - if this arena has already beenclosed
      WrongThreadException - if this arena is confined, and this method is called from a thread other than the arena's owner thread

    • scope

      Returns the arena scope.
      Returns:
      the arena scope

    • close

      void close()
      Closes this arena. If this method completes normally, the arena scope is no longeralive, and all the memory segments associated with itcan no longer be accessed. Furthermore, any off-heap region of memory backing thesegments obtained from this arena are also released.
      Specified by:
      close in interface AutoCloseable
      API Note:
      This operation is not idempotent; that is, closing an already closed arenaalways results in an exception being thrown. This reflects a deliberate design choice: failure to close an arena might reveal a bug in the underlying application logic.
      Implementation Requirements:
      If this method completes normally, thenthis.scope().isAlive() == false. Implementations are allowed to throwUnsupportedOperationException if an explicit close operation is not supported.
      Throws:
      IllegalStateException - if the arena has already been closed
      IllegalStateException - if a segment associated with this arena is being accessed concurrently, e.g. by adowncall method handleRESTRICTED
      WrongThreadException - if this arena is confined, and this method is called from a thread other than the arena's owner thread
      UnsupportedOperationException - if this arena cannot be closed explicitly
      RuntimeException - if an exception is thrown while executing a custom cleanup action associated with this arena (e.g. as a result of callingMemorySegment.reinterpret(long, Arena, Consumer)RESTRICTED orMemorySegment.reinterpret(Arena, Consumer)RESTRICTED).
      See Also: