Module java.base
Package java.lang.foreign

Interface MemoryLayout

All Known Implementing Classes:
GroupLayoutPREVIEW, SequenceLayoutPREVIEW, ValueLayoutPREVIEW, ValueLayout.OfAddressPREVIEW, ValueLayout.OfBooleanPREVIEW, ValueLayout.OfBytePREVIEW, ValueLayout.OfCharPREVIEW, ValueLayout.OfDoublePREVIEW, ValueLayout.OfFloatPREVIEW, ValueLayout.OfIntPREVIEW, ValueLayout.OfLongPREVIEW, ValueLayout.OfShortPREVIEW
public sealed interface MemoryLayout permits SequenceLayoutPREVIEW, GroupLayoutPREVIEW, ValueLayoutPREVIEW (not exhaustive)
MemoryLayout is a preview API of the Java platform.
Programs can only use MemoryLayout when preview features are enabled.
Preview features may be removed in a future release, or upgraded to permanent features of the Java platform.
A memory layout can be used to describe the contents of a memory segment. There are two leaves in the layout hierarchy, value layouts, which are used to represent values of given size and kind (see ValueLayoutPREVIEW) and padding layouts which are used, as the name suggests, to represent a portion of a memory segment whose contents should be ignored, and which are primarily present for alignment reasons (see paddingLayout(long)). Some common value layout constants are defined in the ValueLayoutPREVIEW class.

More complex layouts can be derived from simpler ones: a sequence layout denotes a repetition of one or more element layout (see SequenceLayoutPREVIEW); a group layout denotes an aggregation of (typically) heterogeneous member layouts (see GroupLayoutPREVIEW).

Layouts can be optionally associated with a name. A layout name can be referred to when constructing layout paths.

Consider the following struct declaration in C: 

typedef struct {
    char kind;
    int value;
} TaggedValues[5];
The above declaration can be modelled using a layout object, as follows: 
SequenceLayout taggedValues = MemoryLayout.sequenceLayout(5,
    MemoryLayout.structLayout(
        ValueLayout.JAVA_BYTE.withName("kind"),
        MemoryLayout.paddingLayout(24),
        ValueLayout.JAVA_INT.withName("value")
    )
).withName("TaggedValues");

Size, alignment and byte order

All layouts have a size; layout size for value and padding layouts is always explicitly denoted; this means that a layout description always has the same size in bits, regardless of the platform in which it is used. For derived layouts, the size is computed as follows:
  • for a sequence layout S whose element layout is E and size is L, the size of S is that of E, multiplied by L
  • for a group layout G with member layouts M1, M2, ... Mn whose sizes are S1, S2, ... Sn, respectively, the size of G is either S1 + S2 + ... + Sn or max(S1, S2, ... Sn) depending on whether the group is a struct or an union, respectively

Furthermore, all layouts feature a natural alignment which can be inferred as follows:

  • for a padding layout L, the natural alignment is 1, regardless of its size; that is, in the absence of an explicit alignment constraint, a padding layout should not affect the alignment constraint of the group layout it is nested into
  • for a value layout L whose size is N, the natural alignment of L is N
  • for a sequence layout S whose element layout is E, the natural alignment of S is that of E
  • for a group layout G with member layouts M1, M2, ... Mn whose alignments are A1, A2, ... An, respectively, the natural alignment of G is max(A1, A2 ... An)
A layout's natural alignment can be overridden if needed (see withBitAlignment(long)), which can be useful to describe hyper-aligned layouts.

All value layouts have an explicit byte order (see ByteOrder) which is set when the layout is created.

Layout paths

A layout path originates from a root layout (typically a group or a sequence layout) and terminates at a layout nested within the root layout - this is the layout selected by the layout path. Layout paths are typically expressed as a sequence of one or more MemoryLayout.PathElementPREVIEW instances.

Layout paths are for example useful in order to obtain offsets of arbitrarily nested layouts inside another layout, to quickly obtain a memory access handle corresponding to the selected layout, or to select an arbitrarily nested layout inside another layout.

Such layout paths can be constructed programmatically using the methods in this class. For instance, given the taggedValues layout instance constructed as above, we can obtain the offset, in bits, of the member layout named value in the first sequence element, as follows: 

long valueOffset = taggedValues.bitOffset(PathElement.sequenceElement(0),
                                          PathElement.groupElement("value")); // yields 32
Similarly, we can select the member layout named value, as follows: 
MemoryLayout value = taggedValues.select(PathElement.sequenceElement(),
                                         PathElement.groupElement("value"));
Layout paths can feature one or more free dimensions. For instance, a layout path traversing an unspecified sequence element (that is, where one of the path component was obtained with the MemoryLayout.PathElement.sequenceElement()PREVIEW method) features an additional free dimension, which will have to be bound at runtime. This is important when obtaining a memory segment view var handlePREVIEW from layouts, as in the following code: 
VarHandle valueHandle = taggedValues.varHandle(PathElement.sequenceElement(),
                                               PathElement.groupElement("value"));
Since the layout path constructed in the above example features exactly one free dimension (as it doesn't specify which member layout named value should be selected from the enclosing sequence layout), it follows that the var handle valueHandle will feature an additional long access coordinate.

A layout path with free dimensions can also be used to create an offset-computing method handle, using the bitOffset(PathElement...) or byteOffsetHandle(PathElement...) method. Again, free dimensions are translated into long parameters of the created method handle. The method handle can be used to compute the offsets of elements of a sequence at different indices, by supplying these indices when invoking the method handle. For instance: 

MethodHandle offsetHandle = taggedValues.byteOffsetHandle(PathElement.sequenceElement(),
                                                          PathElement.groupElement("kind"));
long offset1 = (long) offsetHandle.invokeExact(1L); // 8
long offset2 = (long) offsetHandle.invokeExact(2L); // 16
Implementation Requirements:
Implementations of this interface are immutable, thread-safe and value-based.
Since:
19

  • Method Details

    • bitSize

      long bitSize()
      Returns the layout size, in bits.
      Returns:
      the layout size, in bits

    • byteSize

      long byteSize()
      Returns the layout size, in bytes.
      Returns:
      the layout size, in bytes
      Throws:
      UnsupportedOperationException - if bitSize() is not a multiple of 8.

    • name

      Optional<String> name()
      Returns the name (if any) associated with this layout.
      Returns:
      the name (if any) associated with this layout
      See Also:

    • withName

      MemoryLayoutPREVIEW withName(String name)
      Returns a memory layout with the same size and alignment constraints as this layout, but with the specified name.
      Parameters:
      name - the layout name.
      Returns:
      a memory layout with the given name.
      See Also:

    • bitAlignment

      long bitAlignment()
      Returns the alignment constraint associated with this layout, expressed in bits. Layout alignment defines a power of two A which is the bit-wise alignment of the layout. If A <= 8 then A/8 is the number of bytes that must be aligned for any pointer that correctly points to this layout. Thus:
      • A=8 means unaligned (in the usual sense), which is common in packets.
      • A=64 means word aligned (on LP64), A=32 int aligned, A=16 short aligned, etc.
      • A=512 is the most strict alignment required by the x86/SV ABI (for AVX-512 data).
      If no explicit alignment constraint was set on this layout (see withBitAlignment(long)), then this method returns the natural alignment constraint (in bits) associated with this layout.
      Returns:
      the layout alignment constraint, in bits.

    • byteAlignment

      default long byteAlignment()
      Returns the alignment constraint associated with this layout, expressed in bytes. Layout alignment defines a power of two A which is the byte-wise alignment of the layout, where A is the number of bytes that must be aligned for any pointer that correctly points to this layout. Thus:
      • A=1 means unaligned (in the usual sense), which is common in packets.
      • A=8 means word aligned (on LP64), A=4 int aligned, A=2 short aligned, etc.
      • A=64 is the most strict alignment required by the x86/SV ABI (for AVX-512 data).
      If no explicit alignment constraint was set on this layout (see withBitAlignment(long)), then this method returns the natural alignment constraint (in bytes) associated with this layout.
      Returns:
      the layout alignment constraint, in bytes.
      Throws:
      UnsupportedOperationException - if bitAlignment() is not a multiple of 8.

    • withBitAlignment

      MemoryLayoutPREVIEW withBitAlignment(long bitAlignment)
      Returns a memory layout with the same size and name as this layout, but with the specified alignment constraints (in bits).
      Parameters:
      bitAlignment - the layout alignment constraint, expressed in bits.
      Returns:
      a memory layout with the given alignment constraints.
      Throws:
      IllegalArgumentException - if bitAlignment is not a power of two, or if it's less than 8.

    • bitOffset

      default long bitOffset(MemoryLayout.PathElementPREVIEW... elements)
      Computes the offset, in bits, of the layout selected by the given layout path, where the path is considered rooted in this layout.
      Parameters:
      elements - the layout path elements.
      Returns:
      The offset, in bits, of the layout selected by the layout path in elements.
      Throws:
      IllegalArgumentException - if the layout path does not select any layout nested in this layout, or if the layout path contains one or more path elements that select multiple sequence element indices (see MemoryLayout.PathElement.sequenceElement()PREVIEW and MemoryLayout.PathElement.sequenceElement(long, long)PREVIEW).
      NullPointerException - if either elements == null, or if any of the elements in elements is null.

    • bitOffsetHandle

      default MethodHandle bitOffsetHandle(MemoryLayout.PathElementPREVIEW... elements)
      Creates a method handle that can be used to compute the offset, in bits, of the layout selected by the given layout path, where the path is considered rooted in this layout.

      The returned method handle has a return type of long, and features as many long parameter types as there are free dimensions in the provided layout path (see MemoryLayout.PathElement.sequenceElement()PREVIEW), where the order of the parameters corresponds to the order of the path elements. The returned method handle can be used to compute a layout offset similar to bitOffset(PathElement...), but where some sequence indices are specified only when invoking the method handle.

      The final offset returned by the method handle is computed as follows: 

      
 offset = c_1 + c_2 + ... + c_m + (x_1 * s_1) + (x_2 * s_2) + ... + (x_n * s_n)
      where x_1, x_2, ... x_n are dynamic values provided as long arguments, whereas c_1, c_2, ... c_m are static offset constants and s_0, s_1, ... s_n are static stride constants which are derived from the layout path.
      Parameters:
      elements - the layout path elements.
      Returns:
      a method handle that can be used to compute the bit offset of the layout element specified by the given layout path elements, when supplied with the missing sequence element indices.
      Throws:
      IllegalArgumentException - if the layout path contains one or more path elements that select multiple sequence element indices (see MemoryLayout.PathElement.sequenceElement(long, long)PREVIEW).

    • byteOffset

      default long byteOffset(MemoryLayout.PathElementPREVIEW... elements)
      Computes the offset, in bytes, of the layout selected by the given layout path, where the path is considered rooted in this layout.
      Parameters:
      elements - the layout path elements.
      Returns:
      The offset, in bytes, of the layout selected by the layout path in elements.
      Throws:
      IllegalArgumentException - if the layout path does not select any layout nested in this layout, or if the layout path contains one or more path elements that select multiple sequence element indices (see MemoryLayout.PathElement.sequenceElement()PREVIEW and MemoryLayout.PathElement.sequenceElement(long, long)PREVIEW).
      UnsupportedOperationException - if bitOffset(elements) is not a multiple of 8.
      NullPointerException - if either elements == null, or if any of the elements in elements is null.

    • byteOffsetHandle

      default MethodHandle byteOffsetHandle(MemoryLayout.PathElementPREVIEW... elements)
      Creates a method handle that can be used to compute the offset, in bytes, of the layout selected by the given layout path, where the path is considered rooted in this layout.

      The returned method handle has a return type of long, and features as many long parameter types as there are free dimensions in the provided layout path (see MemoryLayout.PathElement.sequenceElement()PREVIEW), where the order of the parameters corresponds to the order of the path elements. The returned method handle can be used to compute a layout offset similar to byteOffset(PathElement...), but where some sequence indices are specified only when invoking the method handle.

      The final offset returned by the method handle is computed as follows: 

      
 bitOffset = c_1 + c_2 + ... + c_m + (x_1 * s_1) + (x_2 * s_2) + ... + (x_n * s_n)
 offset = bitOffset / 8
      where x_1, x_2, ... x_n are dynamic values provided as long arguments, whereas c_1, c_2, ... c_m are static offset constants and s_0, s_1, ... s_n are static stride constants which are derived from the layout path.

      The method handle will throw an UnsupportedOperationException if the computed offset in bits is not a multiple of 8.

      Parameters:
      elements - the layout path elements.
      Returns:
      a method handle that can be used to compute the byte offset of the layout element specified by the given layout path elements, when supplied with the missing sequence element indices.
      Throws:
      IllegalArgumentException - if the layout path contains one or more path elements that select multiple sequence element indices (see MemoryLayout.PathElement.sequenceElement(long, long)PREVIEW).

    • varHandle

      default VarHandle varHandle(MemoryLayout.PathElementPREVIEW... elements)
      Creates an access var handle that can be used to dereference memory at the layout selected by the given layout path, where the path is considered rooted in this layout.

      The final memory location accessed by the returned var handle can be computed as follows: 

      
 address = base + offset
      where base denotes the base address expressed by the MemorySegmentPREVIEW access coordinate (see MemorySegment.address()PREVIEW and MemoryAddress.toRawLongValue()PREVIEW) and offset can be expressed in the following form: 
      
 offset = c_1 + c_2 + ... + c_m + (x_1 * s_1) + (x_2 * s_2) + ... + (x_n * s_n)
      where x_1, x_2, ... x_n are dynamic values provided as long arguments, whereas c_1, c_2, ... c_m are static offset constants and s_1, s_2, ... s_n are static stride constants which are derived from the layout path.

      Additionally, the provided dynamic values must conform to some bound which is derived from the layout path, that is, 0 <= x_i < b_i, where 1 <= i <= n, or IndexOutOfBoundsException is thrown.

      API Note:
      the resulting var handle will feature an additional long access coordinate for every unspecified sequence access component contained in this layout path. Moreover, the resulting var handle features certain access mode restrictions, which are common to all memory segment view handles.
      Parameters:
      elements - the layout path elements.
      Returns:
      a var handle which can be used to dereference memory at the (possibly nested) layout selected by the layout path in elements.
      Throws:
      UnsupportedOperationException - if the layout path has one or more elements with incompatible alignment constraints.
      IllegalArgumentException - if the layout path in elements does not select a value layout (see ValueLayoutPREVIEW).
      See Also:

    • sliceHandle

      default MethodHandle sliceHandle(MemoryLayout.PathElementPREVIEW... elements)
      Creates a method handle which, given a memory segment, returns a slicePREVIEW corresponding to the layout selected by the given layout path, where the path is considered rooted in this layout.

      The returned method handle has a return type of MemorySegment, features a MemorySegment parameter as leading parameter representing the segment to be sliced, and features as many trailing long parameter types as there are free dimensions in the provided layout path (see MemoryLayout.PathElement.sequenceElement()PREVIEW), where the order of the parameters corresponds to the order of the path elements. The returned method handle can be used to create a slice similar to using MemorySegment.asSlice(long, long)PREVIEW, but where the offset argument is dynamically compute based on indices specified when invoking the method handle.

      The offset of the returned segment is computed as follows: 

      
 bitOffset = c_1 + c_2 + ... + c_m + (x_1 * s_1) + (x_2 * s_2) + ... + (x_n * s_n)
 offset = bitOffset / 8
      where x_1, x_2, ... x_n are dynamic values provided as long arguments, whereas c_1, c_2, ... c_m are static offset constants and s_1, s_2, ... s_n are static stride constants which are derived from the layout path.

      After the offset is computed, the returned segment is created as if by calling: 

      segment.asSlice(offset, layout.byteSize());
      where segment is the segment to be sliced, and where layout is the layout selected by the given layout path, as per select(PathElement...).

      The method handle will throw an UnsupportedOperationException if the computed offset in bits is not a multiple of 8.

      Parameters:
      elements - the layout path elements.
      Returns:
      a method handle which can be used to create a slice of the selected layout element, given a segment.
      Throws:
      UnsupportedOperationException - if the size of the selected layout in bits is not a multiple of 8.

    • select

      default MemoryLayoutPREVIEW select(MemoryLayout.PathElementPREVIEW... elements)
      Selects the layout from a path rooted in this layout.
      Parameters:
      elements - the layout path elements.
      Returns:
      the layout selected by the layout path in elements.
      Throws:
      IllegalArgumentException - if the layout path does not select any layout nested in this layout, or if the layout path contains one or more path elements that select one or more sequence element indices (see MemoryLayout.PathElement.sequenceElement(long)PREVIEW and MemoryLayout.PathElement.sequenceElement(long, long)PREVIEW).

    • isPadding

      boolean isPadding()
      Returns true, if this layout is a padding layout.
      Returns:
      true, if this layout is a padding layout

    • equals

      boolean equals(Object other)
      Compares the specified object with this layout for equality. Returns true if and only if the specified object is also a layout, and it is equal to this layout. Two layouts are considered equal if they are of the same kind, have the same size, name and alignment constraints. Furthermore, depending on the layout kind, additional conditions must be satisfied:
      Overrides:
      equals in class Object
      Parameters:
      other - the object to be compared for equality with this layout.
      Returns:
      true if the specified object is equal to this layout.
      See Also:

    • hashCode

      int hashCode()
      Returns the hash code value for this layout.
      Overrides:
      hashCode in class Object
      Returns:
      the hash code value for this layout
      See Also:

    • toString

      String toString()
      Returns the string representation of this layout.
      Overrides:
      toString in class Object
      Returns:
      the string representation of this layout

    • paddingLayout

      static MemoryLayoutPREVIEW paddingLayout(long size)
      Creates a padding layout with the given size.
      Parameters:
      size - the padding size in bits.
      Returns:
      the new selector layout.
      Throws:
      IllegalArgumentException - if size <= 0.

    • valueLayout

      static ValueLayoutPREVIEW valueLayout(Class<?> carrier, ByteOrder order)
      Creates a value layout of given Java carrier and byte order. The type of resulting value layout is determined by the carrier provided:
      Parameters:
      carrier - the value layout carrier.
      order - the value layout's byte order.
      Returns:
      a value layout with the given Java carrier and byte-order.
      Throws:
      IllegalArgumentException - if the carrier type is not supported.

    • sequenceLayout

      static SequenceLayoutPREVIEW sequenceLayout(long elementCount, MemoryLayoutPREVIEW elementLayout)
      Creates a sequence layout with the given element layout and element count. If the element count has the special value -1, the element count is inferred to be the biggest possible count such that the sequence layout size does not overflow, using the following formula: 
      
 inferredElementCount = Long.MAX_VALUE / elementLayout.bitSize();
      Parameters:
      elementCount - the sequence element count; if set to -1, the sequence element count is inferred.
      elementLayout - the sequence element layout.
      Returns:
      the new sequence layout with the given element layout and size.
      Throws:
      IllegalArgumentException - if elementCount < -1.
      IllegalArgumentException - if elementCount != -1 and the computation elementCount * elementLayout.bitSize() overflows.

    • structLayout

      static GroupLayoutPREVIEW structLayout(MemoryLayoutPREVIEW... elements)
      Creates a struct layout with the given member layouts.
      Parameters:
      elements - The member layouts of the struct layout.
      Returns:
      a struct layout with the given member layouts.
      Throws:
      IllegalArgumentException - if the sum of the bit sizes of the member layouts overflows.

    • unionLayout

      static GroupLayoutPREVIEW unionLayout(MemoryLayoutPREVIEW... elements)
      Creates a union layout with the given member layouts.
      Parameters:
      elements - The member layouts of the union layout.
      Returns:
      a union layout with the given member layouts.