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package java.lang.foreign;
import jdk.internal.foreign.MemorySessionImpl;
import jdk.internal.ref.CleanerFactory;
import java.lang.foreign.MemorySegment.Scope;
/**
* An arena controls the lifecycle of native memory segments, providing both flexible allocation and timely deallocation.
*
* An arena has a {@linkplain MemorySegment.Scope scope} - 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
* {@linkplain MemorySegment#isAccessibleBy(Thread) restricted} to specific threads.
* An arena is a {@link SegmentAllocator} and features several allocation methods that can be used by clients
* to obtain native segments.
*
* The simplest arena is the {@linkplain Arena#global() global arena}. The global arena
* features an unbounded 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 be {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} from any thread.
* {@snippet lang = java:
* MemorySegment segment = Arena.global().allocate(100, 1); // @highlight regex='global()'
* ...
* // segment is never deallocated!
*}
*
* Alternatively, clients can obtain an {@linkplain Arena#ofAuto() automatic arena}, that is an arena
* which features a bounded 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 time
* after the automatic arena (and all the segments allocated by it) becomes
* unreachable, as shown below:
* {@snippet lang = java:
* MemorySegment segment = Arena.ofAuto().allocate(100, 1); // @highlight regex='ofAuto()'
* ...
* segment = null; // the segment region becomes available for deallocation after this point
*}
* Memory segments allocated with an automatic arena can also be {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} 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,
* namely {@linkplain #ofConfined() confined} and {@linkplain #ofShared() shared} arenas. They both feature
* bounded lifetimes that are managed manually. For instance, when a confined arena is {@linkplain #close() closed}
* successfully, its scope is {@linkplain Scope#isAlive() invalidated}. As a result, all the memory segments allocated
* by the arena can no longer be accessed, and their regions of memory are deallocated:
*
* {@snippet lang = java:
* MemorySegment segment = null;
* try (Arena arena = Arena.ofConfined()) { // @highlight regex='ofConfined()'
* segment = arena.allocate(100);
* ...
* } // segment region deallocated here
* segment.get(ValueLayout.JAVA_BYTE, 0); // throws IllegalStateException
*}
*
* Memory segments allocated with a {@linkplain #ofConfined() confined 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 a {@linkplain #ofShared() shared arena} instead.
*
* The characteristics of the various arenas are summarized in the following table:
*
*
* Arenas characteristics
*
*
* | 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 {@linkplain MemorySegment#isAccessibleBy(Thread) accessed}
* by the owner thread. Moreover, any attempt to close the confined arena from a thread other than the owner thread will
* fail with {@link WrongThreadException}.
*
* Shared arenas, on the other hand, have no owner thread. The segments created by a shared arena
* can be {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} 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
* {@linkplain SegmentAllocator#slicingAllocator(MemorySegment) slicing 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):
*
* {@snippet lang = java:
* 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:
*
* {@snippet lang = java:
* 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
* }
*
* @implSpec
* Implementations of this interface are thread-safe.
*
* @see MemorySegment
*
* @since 22
*/
public interface Arena extends SegmentAllocator, AutoCloseable {
/**
* Creates a new arena that is managed, automatically, by the garbage collector.
* Segments allocated with the returned arena can be
* {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by any thread.
* Calling {@link #close()} on the returned arena will result in an {@link UnsupportedOperationException}.
*
* @return a new arena that is managed, automatically, by the garbage collector.
*/
static Arena ofAuto() {
return MemorySessionImpl.createImplicit(CleanerFactory.cleaner()).asArena();
}
/**
* Obtains the global arena. Segments allocated with the global arena can be
* {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by any thread.
* Calling {@link #close()} on the returned arena will result in an {@link UnsupportedOperationException}.
*
* @return the global arena.
*/
static Arena global() {
class Holder {
static final Arena GLOBAL = MemorySessionImpl.GLOBAL_SESSION.asArena();
}
return Holder.GLOBAL;
}
/**
* {@return a new confined arena} Segments allocated with the confined arena can be
* {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by the thread that created the arena,
* the arena's owner thread.
*/
static Arena ofConfined() {
return MemorySessionImpl.createConfined(Thread.currentThread()).asArena();
}
/**
* {@return a new shared arena} Segments allocated with the global arena can be
* {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by any thread.
*/
static Arena ofShared() {
return MemorySessionImpl.createShared().asArena();
}
/**
* Returns a native memory segment with the given size (in bytes) and alignment constraint (in bytes).
* The returned segment is associated with this {@linkplain #scope() arena scope}.
* The segment's {@link MemorySegment#address() address} is the starting address of the
* allocated off-heap region of memory backing the segment, and the address is
* aligned according to the provided alignment constraint.
*
* @implSpec
* 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
* {@code S1, S2} returned by this method, the following invariant must hold:
*
* {@snippet lang = java:
* S1.asOverlappingSlice(S2).isEmpty() == true
* }
*
* @param byteSize the size (in bytes) of the off-heap region of memory backing the native memory segment.
* @param byteAlignment the alignment constraint (in bytes) of the off-heap region of memory backing the native memory segment.
* @return a new native memory segment.
* @throws IllegalArgumentException if {@code bytesSize < 0}, {@code byteAlignment <= 0},
* or if {@code byteAlignment} is not a power of 2
* @throws IllegalStateException if this arena has already been {@linkplain #close() closed}
* @throws WrongThreadException if this arena is confined, and this method is called from a thread
* other than the arena's owner thread
*/
@Override
MemorySegment allocate(long byteSize, long byteAlignment);
/**
* {@return the arena scope}
*/
Scope scope();
/**
* Closes this arena. If this method completes normally, the arena scope is no longer {@linkplain Scope#isAlive() alive},
* 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.
*
* @apiNote 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.
*
* @implSpec If this method completes normally, then {@code this.scope().isAlive() == false}.
* Implementations are allowed to throw {@link UnsupportedOperationException} if an explicit close operation is
* not supported.
*
* @see Scope#isAlive()
*
* @throws IllegalStateException if the arena has already been closed
* @throws IllegalStateException if a segment associated with this arena is being accessed concurrently, e.g.
* by a {@linkplain Linker#downcallHandle(FunctionDescriptor, Linker.Option...) downcall method handle}
* @throws WrongThreadException if this arena is confined, and this method is called from a thread
* other than the arena's owner thread
* @throws UnsupportedOperationException if this arena cannot be closed explicitly
*/
@Override
void close();
}