- All Superinterfaces:
AutoCloseable
,SegmentAllocatorPREVIEW
Arena
is a preview API of the Java platform.
An arena has a scopePREVIEW - the arena 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 be
restrictedPREVIEW to specific threads.
An arena is a SegmentAllocator
PREVIEW and features several allocation methods that can be used by clients
to obtain native segments.
The simplest arena is the global arena. The global arena features an unbounded lifetime. 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 be accessedPREVIEW from any thread.
MemorySegment segment = Arena.global().allocate(100, 1);
...
// segment is never deallocated!
Alternatively, clients can obtain an automatic arena, that is an arena which features a bounded lifetime that is managed, automatically, by the garbage collector. As such, the regions of memory backing memory segments allocated with the automatic arena are deallocated at some unspecified time after the automatic arena (and all the segments allocated by it) becomes unreachable, as shown below:
MemorySegment segment = Arena.ofAuto().allocate(100, 1);
...
segment = null; // the segment region becomes available for deallocation after this point
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,
namely confined and shared arenas. They both feature
bounded lifetimes that are managed manually. For instance, the lifetime of a confined arena starts when the confined
arena is created, and ends when the confined arena is closed. As a result, the regions of memory
backing memory segments allocated with a confined arena are deallocated when the confined arena is closed.
When this happens, all the segments allocated with the confined arena are invalidated, and subsequent access
operations on these segments will fail IllegalStateException
:
MemorySegment segment = null;
try (Arena arena = Arena.ofConfined()) {
segment = arena.allocate(100);
...
} // segment region deallocated here
segment.get(ValueLayout.JAVA_BYTE, 0); // throws IllegalStateException
The characteristics of the various arenas are summarized in the following table:
Kind Bounded lifetime Explicitly closeable Accessible from multiple threads Global No No Yes Automatic Yes No Yes Confined Yes Yes No Shared Yes Yes Yes
Safety and thread-confinement
Arenas provide strong temporal safety guarantees: a memory segment allocated by an arena cannot be accessed after 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 accessed while 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 into thread-confined arenas, and shared arenas.
Confined arenas, support strong thread-confinement guarantees. Upon creation, they are assigned an
owner thread, typically the thread which initiated the creation operation.
The segments created by a confined arena can only be accessedPREVIEW
by the owner thread. Moreover, any attempt to close the confined arena from a thread other than the owner thread will
fail with WrongThreadException
.
Shared arenas, on the other hand, have no owner thread. The segments created by a shared arena can be accessedPREVIEW 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 a slicing arena that behaves like a confined arena (i.e., single-threaded access), but internally uses a slicing allocatorPREVIEW 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();
}
}
try (Arena slicingArena = new SlicingArena(1000)) {
for (int i = 0; i < 10; i++) {
MemorySegment s = slicingArena.allocateArray(JAVA_INT, 1, 2, 3, 4, 5);
...
}
} // all memory allocated is released here
- Implementation Requirements:
- Implementations of this interface are thread-safe.
- Since:
- 20
- See Also:
-
Method Summary
Modifier and TypeMethodDescriptiondefault MemorySegmentPREVIEW
allocate
(long byteSize, long byteAlignment) Returns a native memory segment with the given size (in bytes) and alignment constraint (in bytes).void
close()
Closes this arena.global()
Obtains the global arena.ofAuto()
Creates a new arena that is managed, automatically, by the garbage collector.Returns a new confined arena.ofShared()
Returns a new shared arena.scope()
Returns the arena scope.Methods declared in interface java.lang.foreign.SegmentAllocatorPREVIEW
allocate, allocate, allocate, allocate, allocate, allocate, allocate, allocate, allocate, allocate, allocateArray, allocateArray, allocateArray, allocateArray, allocateArray, allocateArray, allocateArray, allocateArray, allocateUtf8String
-
Method Details
-
ofAuto
Creates a new arena that is managed, automatically, by the garbage collector. Segments allocated with the returned arena can be accessedPREVIEW by any thread. Callingclose()
on the returned arena will result in anUnsupportedOperationException
.- Returns:
- a new arena that is managed, automatically, by the garbage collector.
-
global
Obtains the global arena. Segments allocated with the global arena can be accessedPREVIEW by any thread. Callingclose()
on the returned arena will result in anUnsupportedOperationException
.- Returns:
- the global arena.
-
ofConfined
Returns a new confined arena. Segments allocated with the confined arena can be accessedPREVIEW by the thread that created the arena, the arena's owner thread.- Returns:
- a new confined arena
-
allocate
Returns a native memory segment with the given size (in bytes) and alignment constraint (in bytes). The returned segment is associated with this arena scope. The segment'saddress
PREVIEW is the starting address of the allocated off-heap region of memory backing the segment, and the address is aligned according the provided alignment constraint.- Specified by:
allocate
in interfaceSegmentAllocatorPREVIEW
- Implementation Requirements:
- Implementations of this method must return a native segment featuring the requested size,
and that is compatible with the provided alignment constraint. Furthermore, for any two segments
S1, S2
returned by this method, the following 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 been closed.WrongThreadException
- if this arena is confined, and this method is called from a thread other than the arena's owner thread.
-
scope
MemorySegment.ScopePREVIEW 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 longer alivePREVIEW, and all the memory segments associated with it can no longer be accessed. Furthermore, any off-heap region of memory backing the segments obtained from this arena are also released.- Specified by:
close
in interfaceAutoCloseable
- API Note:
- This operation is not idempotent; that is, closing an already closed arena always 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, then
this.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 a downcall method handlePREVIEW.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.- See Also:
-
Arena
when preview features are enabled.