Module java.base
Package java.lang.foreign

Interface SegmentAllocator

All Known Subinterfaces:
MemorySessionPREVIEW
Functional Interface:
This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
@FunctionalInterface public interface SegmentAllocator
SegmentAllocator is a preview API of the Java platform.
Programs can only use SegmentAllocator when preview features are enabled.
Preview features may be removed in a future release, or upgraded to permanent features of the Java platform.
An object that may be used to allocate memory segmentsPREVIEW. Clients implementing this interface must implement the allocate(long, long) method. This interface defines several default methods which can be useful to create segments from several kinds of Java values such as primitives and arrays. This interface is a functional interface: clients can easily obtain a new segment allocator by using either a lambda expression or a method reference.

This interface also defines factories for commonly used allocators:

Passing a segment allocator to an API can be especially useful in circumstances where a client wants to communicate where the results of a certain operation (performed by the API) should be stored, as a memory segment. For instance, downcall method handlesPREVIEW can accept an additional SegmentAllocatorPREVIEW parameter if the underlying foreign function is known to return a struct by-value. Effectively, the allocator parameter tells the linker runtime where to store the return value of the foreign function.

  • Method Details

    • allocateUtf8String

      default MemorySegmentPREVIEW allocateUtf8String(String str)
      Converts a Java string into a UTF-8 encoded, null-terminated C string, storing the result into a memory segment.

      This method always replaces malformed-input and unmappable-character sequences with this charset's default replacement byte array. The CharsetEncoder class should be used when more control over the encoding process is required.

      If the given string contains any '\0' characters, they will be copied as well. This means that, depending on the method used to read the string, such as MemorySegment.getUtf8String(long)PREVIEW, the string will appear truncated when read again.

      Implementation Requirements:
      the default implementation for this method copies the contents of the provided Java string into a new memory segment obtained by calling this.allocate(str.length() + 1).
      Parameters:
      str - the Java string to be converted into a C string.
      Returns:
      a new native memory segment containing the converted C string.

    • allocate

      default MemorySegmentPREVIEW allocate(ValueLayout.OfBytePREVIEW layout, byte value)
      Allocates a memory segment with the given layout and initializes it with the given byte value.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(layout).
      Parameters:
      layout - the layout of the block of memory to be allocated.
      value - the value to be set on the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocate

      default MemorySegmentPREVIEW allocate(ValueLayout.OfCharPREVIEW layout, char value)
      Allocates a memory segment with the given layout and initializes it with the given char value.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(layout).
      Parameters:
      layout - the layout of the block of memory to be allocated.
      value - the value to be set on the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocate

      default MemorySegmentPREVIEW allocate(ValueLayout.OfShortPREVIEW layout, short value)
      Allocates a memory segment with the given layout and initializes it with the given short value.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(layout).
      Parameters:
      layout - the layout of the block of memory to be allocated.
      value - the value to be set on the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocate

      default MemorySegmentPREVIEW allocate(ValueLayout.OfIntPREVIEW layout, int value)
      Allocates a memory segment with the given layout and initializes it with the given int value.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(layout).
      Parameters:
      layout - the layout of the block of memory to be allocated.
      value - the value to be set on the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocate

      default MemorySegmentPREVIEW allocate(ValueLayout.OfFloatPREVIEW layout, float value)
      Allocates a memory segment with the given layout and initializes it with the given float value.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(layout).
      Parameters:
      layout - the layout of the block of memory to be allocated.
      value - the value to be set on the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocate

      default MemorySegmentPREVIEW allocate(ValueLayout.OfLongPREVIEW layout, long value)
      Allocates a memory segment with the given layout and initializes it with the given long value.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(layout).
      Parameters:
      layout - the layout of the block of memory to be allocated.
      value - the value to be set on the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocate

      default MemorySegmentPREVIEW allocate(ValueLayout.OfDoublePREVIEW layout, double value)
      Allocates a memory segment with the given layout and initializes it with the given double value.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(layout).
      Parameters:
      layout - the layout of the block of memory to be allocated.
      value - the value to be set on the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocate

      default MemorySegmentPREVIEW allocate(ValueLayout.OfAddressPREVIEW layout, AddressablePREVIEW value)
      Allocates a memory segment with the given layout and initializes it with the given address value. The address value might be narrowed according to the platform address size (see ValueLayout.ADDRESSPREVIEW).
      Implementation Requirements:
      the default implementation for this method calls this.allocate(layout).
      Parameters:
      layout - the layout of the block of memory to be allocated.
      value - the value to be set on the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocateArray

      default MemorySegmentPREVIEW allocateArray(ValueLayout.OfBytePREVIEW elementLayout, byte... elements)
      Allocates a memory segment with the given layout and initializes it with the given byte elements.
      Implementation Requirements:
      the default implementation for this method calls this.allocateArray(layout, array.length).
      Parameters:
      elementLayout - the element layout of the array to be allocated.
      elements - the byte elements to be copied to the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocateArray

      default MemorySegmentPREVIEW allocateArray(ValueLayout.OfShortPREVIEW elementLayout, short... elements)
      Allocates a memory segment with the given layout and initializes it with the given short elements.
      Implementation Requirements:
      the default implementation for this method calls this.allocateArray(layout, array.length).
      Parameters:
      elementLayout - the element layout of the array to be allocated.
      elements - the short elements to be copied to the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocateArray

      default MemorySegmentPREVIEW allocateArray(ValueLayout.OfCharPREVIEW elementLayout, char... elements)
      Allocates a memory segment with the given layout and initializes it with the given char elements.
      Implementation Requirements:
      the default implementation for this method calls this.allocateArray(layout, array.length).
      Parameters:
      elementLayout - the element layout of the array to be allocated.
      elements - the char elements to be copied to the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocateArray

      default MemorySegmentPREVIEW allocateArray(ValueLayout.OfIntPREVIEW elementLayout, int... elements)
      Allocates a memory segment with the given layout and initializes it with the given int elements.
      Implementation Requirements:
      the default implementation for this method calls this.allocateArray(layout, array.length).
      Parameters:
      elementLayout - the element layout of the array to be allocated.
      elements - the int elements to be copied to the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocateArray

      default MemorySegmentPREVIEW allocateArray(ValueLayout.OfFloatPREVIEW elementLayout, float... elements)
      Allocates a memory segment with the given layout and initializes it with the given float elements.
      Implementation Requirements:
      the default implementation for this method calls this.allocateArray(layout, array.length).
      Parameters:
      elementLayout - the element layout of the array to be allocated.
      elements - the float elements to be copied to the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocateArray

      default MemorySegmentPREVIEW allocateArray(ValueLayout.OfLongPREVIEW elementLayout, long... elements)
      Allocates a memory segment with the given layout and initializes it with the given long elements.
      Implementation Requirements:
      the default implementation for this method calls this.allocateArray(layout, array.length).
      Parameters:
      elementLayout - the element layout of the array to be allocated.
      elements - the long elements to be copied to the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocateArray

      default MemorySegmentPREVIEW allocateArray(ValueLayout.OfDoublePREVIEW elementLayout, double... elements)
      Allocates a memory segment with the given layout and initializes it with the given double elements.
      Implementation Requirements:
      the default implementation for this method calls this.allocateArray(layout, array.length).
      Parameters:
      elementLayout - the element layout of the array to be allocated.
      elements - the double elements to be copied to the newly allocated memory block.
      Returns:
      a segment for the newly allocated memory block.

    • allocate

      default MemorySegmentPREVIEW allocate(MemoryLayoutPREVIEW layout)
      Allocates a memory segment with the given layout.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(layout.byteSize(), layout.byteAlignment()).
      Parameters:
      layout - the layout of the block of memory to be allocated.
      Returns:
      a segment for the newly allocated memory block.

    • allocateArray

      default MemorySegmentPREVIEW allocateArray(MemoryLayoutPREVIEW elementLayout, long count)
      Allocates a memory segment with the given element layout and size.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(MemoryLayout.sequenceLayout(count, elementLayout)).
      Parameters:
      elementLayout - the array element layout.
      count - the array element count.
      Returns:
      a segment for the newly allocated memory block.
      Throws:
      IllegalArgumentException - if count < 0.

    • allocate

      default MemorySegmentPREVIEW allocate(long bytesSize)
      Allocates a memory segment with the given size.
      Implementation Requirements:
      the default implementation for this method calls this.allocate(bytesSize, 1).
      Parameters:
      bytesSize - the size (in bytes) of the block of memory to be allocated.
      Returns:
      a segment for the newly allocated memory block.
      Throws:
      IllegalArgumentException - if bytesSize < 0

    • allocate

      MemorySegmentPREVIEW allocate(long bytesSize, long bytesAlignment)
      Allocates a memory segment with the given size and alignment constraints.
      Parameters:
      bytesSize - the size (in bytes) of the block of memory to be allocated.
      bytesAlignment - the alignment (in bytes) of the block of memory to be allocated.
      Returns:
      a segment for the newly allocated memory block.
      Throws:
      IllegalArgumentException - if bytesSize < 0, alignmentBytes <= 0, or if alignmentBytes is not a power of 2.

    • newNativeArena

      static SegmentAllocatorPREVIEW newNativeArena(MemorySessionPREVIEW session)
      Creates an unbounded arena-based allocator used to allocate native memory segments. The returned allocator features a predefined block size and maximum arena size, and the segments it allocates are associated with the provided memory session. Equivalent to the following code: 
      SegmentAllocator.newNativeArena(Long.MAX_VALUE, predefinedBlockSize, session);
      Parameters:
      session - the memory session associated with the segments allocated by the arena-based allocator.
      Returns:
      a new unbounded arena-based allocator
      Throws:
      IllegalStateException - if session is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owningPREVIEW session.

    • newNativeArena

      static SegmentAllocatorPREVIEW newNativeArena(long arenaSize, MemorySessionPREVIEW session)
      Creates an arena-based allocator used to allocate native memory segments. The returned allocator features a block size set to the specified arena size, and the native segments it allocates are associated with the provided memory session. Equivalent to the following code: 
      SegmentAllocator.newNativeArena(arenaSize, arenaSize, session);
      Parameters:
      arenaSize - the size (in bytes) of the allocation arena.
      session - the memory session associated with the segments allocated by the arena-based allocator.
      Returns:
      a new unbounded arena-based allocator
      Throws:
      IllegalArgumentException - if arenaSize <= 0.
      IllegalStateException - if session is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owningPREVIEW session.

    • newNativeArena

      static SegmentAllocatorPREVIEW newNativeArena(long arenaSize, long blockSize, MemorySessionPREVIEW session)
      Creates an arena-based allocator used to allocate native memory segments. The returned allocator features the given block size B and the given arena size A, and the native segments it allocates are associated with the provided memory session.

      The allocator arena is first initialized by allocatingPREVIEW a native memory segment S of size B. The allocator then responds to allocation requests in one of the following ways:

      • if the size of the allocation requests is smaller than the size of S, and S has a free slice S' which fits that allocation request, return that S'.
      • if the size of the allocation requests is smaller than the size of S, and S has no free slices which fits that allocation request, allocate a new segment S', with size B, and set S = S'; the allocator then tries to respond to the same allocation request again.
      • if the size of the allocation requests is bigger than the size of S, allocate a new segment S', which has a sufficient size to satisfy the allocation request, and return S'.

      This segment allocator can be useful when clients want to perform multiple allocation requests while avoiding the cost associated with allocating a new off-heap memory region upon each allocation request.

      The returned allocator might throw an OutOfMemoryError if the total memory allocated with this allocator exceeds the arena size A, or the system capacity. Furthermore, the returned allocator is not thread safe. Concurrent allocation needs to be guarded with synchronization primitives.

      Parameters:
      arenaSize - the size (in bytes) of the allocation arena.
      blockSize - the block size associated with the arena-based allocator.
      session - the memory session associated with the segments returned by the arena-based allocator.
      Returns:
      a new unbounded arena-based allocator
      Throws:
      IllegalArgumentException - if blockSize <= 0, if arenaSize <= 0 or if arenaSize < blockSize.
      IllegalStateException - if session is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owningPREVIEW session.

    • prefixAllocator

      static SegmentAllocatorPREVIEW prefixAllocator(MemorySegmentPREVIEW segment)
      Returns a segment allocator which responds to allocation requests by recycling a single segment. Each new allocation request will return a new slice starting at the segment offset 0 (alignment constraints are ignored by this allocator), hence the name prefix allocator. Equivalent to (but likely more efficient than) the following code: 
      MemorySegment segment = ...
SegmentAllocator prefixAllocator = (size, align) -> segment.asSlice(0, size);

      This allocator can be useful to limit allocation requests in case a client knows that they have fully processed the contents of the allocated segment before the subsequent allocation request takes place.

      While the allocator returned by this method is thread-safe, concurrent access on the same recycling allocator might cause a thread to overwrite contents written to the underlying segment by a different thread.

      Parameters:
      segment - the memory segment to be recycled by the returned allocator.
      Returns:
      an allocator which recycles an existing segment upon each new allocation request.

    • implicitAllocator

      static SegmentAllocatorPREVIEW implicitAllocator()
      Returns an allocator which allocates native segments in independent implicit memory sessionsPREVIEW. Equivalent to (but likely more efficient than) the following code: 
      SegmentAllocator implicitAllocator = (size, align) -> MemorySegment.allocateNative(size, align, MemorySession.openImplicit());
      Returns:
      an allocator which allocates native segments in independent implicit memory sessionsPREVIEW.