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

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

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

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

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.allocateFrom(JAVA_INT, 1, 2, 3, 4, 5);
        ...
    }
} // all memory allocated is released here