# Interface Arena

All Superinterfaces:
  `AutoCloseable`, `SegmentAllocator`

public interface Arena extends SegmentAllocator, AutoCloseable
An arena controls the lifecycle of native memory segments, providing both flexible allocation and timely deallocation.

An arena has a 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 restricted to specific threads. An arena is a `SegmentAllocator` 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. 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 accessed 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. 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:

     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 be 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 confined and shared arenas. They both feature bounded lifetimes that are managed manually. For instance, when a confined arena is closed successfully, its scope is invalidated. 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 a 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 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 accessed by the owner thread. Moreover, any attempt to close the confined arena from a thread other than the owner thread will fail with a `WrongThreadException`.

Shared arenas, on the other hand, have no owner thread. The segments created by a shared arena can be 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 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):

    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:
  - `MemorySegment`

## Method Summary

Modifier and Type
Method
Description
`MemorySegment`
`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.
`static Arena`
`global()`
Returns the global arena.
`static Arena`
`ofAuto()`
Creates a new arena that is managed, automatically, by the garbage collector.
`static Arena`
`ofConfined()`
Returns a new confined arena.
`static Arena`
`ofShared()`
Returns a new shared arena.
`MemorySegment.Scope`
`scope()`
Returns the arena scope.

### Methods declared in interface java.lang.foreign.SegmentAllocator

`allocate, allocate, allocate, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom, allocateFrom`

## 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 be accessed by any thread. Calling `close()` on the returned arena will result in an `UnsupportedOperationException`.

Memory segments allocated by the returned arena are 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 be accessed by any thread. Calling `close()` on the returned arena will result in an `UnsupportedOperationException`.

Memory segments allocated by the returned arena are zero-initialized.

Returns:
  the global arena

### ofConfined

static Arena ofConfined()
Returns a new confined arena. Segments allocated with the confined arena can be accessed by the thread that created the arena, the arena's owner thread.

Memory segments allocated by the returned arena are zero-initialized.

Returns:
  a new confined arena

### ofShared

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

Memory segments allocated by the returned arena are 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 alignment constraint (in bytes). The returned segment is associated with this arena scope. The segment's `address` 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 interface `SegmentAllocator`
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` - if `bytesSize < 0`, `byteAlignment <= 0`, or if `byteAlignment` 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.Scope 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 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.

Specified by:
  `close` in interface `AutoCloseable`
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 throw `UnsupportedOperationException` 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 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 calling `MemorySegment.reinterpret(long, Arena, Consumer)`RESTRICTED or `MemorySegment.reinterpret(Arena, Consumer)`RESTRICTED).
See Also:
  - `MemorySegment.Scope.isAlive()`