Module java.base
Package java.lang.foreign

Interface MemoryLayout

All Known Subinterfaces:
AddressLayoutPREVIEW, GroupLayoutPREVIEW, PaddingLayoutPREVIEW, SequenceLayoutPREVIEW, StructLayoutPREVIEW, UnionLayoutPREVIEW, ValueLayoutPREVIEW, ValueLayout.OfBooleanPREVIEW, ValueLayout.OfBytePREVIEW, ValueLayout.OfCharPREVIEW, ValueLayout.OfDoublePREVIEW, ValueLayout.OfFloatPREVIEW, ValueLayout.OfIntPREVIEW, ValueLayout.OfLongPREVIEW, ValueLayout.OfShortPREVIEW
public sealed interface MemoryLayout permits SequenceLayoutPREVIEW, GroupLayoutPREVIEW, PaddingLayoutPREVIEW, ValueLayoutPREVIEW
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 describes the contents of a memory segment.

There are two leaves in the layout hierarchy, value layoutsPREVIEW, which are used to represent values of given size and kind (see and padding layoutsPREVIEW 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. Some common value layout constants, such as ValueLayout.JAVA_INTPREVIEW and ValueLayout.JAVA_FLOAT_UNALIGNEDPREVIEW are defined in the ValueLayoutPREVIEW class. A special kind of value layout, namely an address layoutPREVIEW, is used to model values that denote the address of a region of memory.

More complex layouts can be derived from simpler ones: a sequence layoutPREVIEW denotes a homogeneous repetition of zero or more occurrences of an element layout; a group layoutPREVIEW denotes a heterogeneous aggregation of zero or more member layouts. Group layouts come in two flavors: struct layoutsPREVIEW, where member layouts are laid out one after the other, and union layoutsPREVIEW where member layouts are laid out at the same starting offset.

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(3),
        ValueLayout.JAVA_INT.withName("value")
    )
).withName("TaggedValues");

Characteristics of memory layouts

All layouts have a size (expressed in bytes), which is defined as follows:
  • The size of a value layout is determined by the ValueLayout.carrier()PREVIEW associated with the value layout. That is, the constant ValueLayout.JAVA_INTPREVIEW has carrier int, and size of 4 bytes;
  • The size of an address layout is platform-dependent. That is, the constant ValueLayout.ADDRESSPREVIEW has size of 8 bytes on a 64-bit platform;
  • The size of a padding layout is always provided explicitly, on construction;
  • The size of a sequence layout whose element layout is E and element count is L, is the size of E, multiplied by L;
  • The size of a struct layout with member layouts M1, M2, ... Mn whose sizes are S1, S2, ... Sn, respectively, is S1 + S2 + ... + Sn;
  • The size of a union layout U with member layouts M1, M2, ... Mn whose sizes are S1, S2, ... Sn, respectively, is max(S1, S2, ... Sn).

Furthermore, all layouts have a natural alignment (expressed in bytes) which is defined as follows:

  • The natural alignment of a padding layout is 1;
  • The natural alignment of a value layout whose size is N is N;
  • The natural alignment of a sequence layout whose element layout is E is the alignment of E;
  • The natural alignment of a group layout with member layouts M1, M2, ... Mn whose alignments are A1, A2, ... An, respectively, is max(A1, A2 ... An).
A layout's alignment can be overridden if needed (see withByteAlignment(long)), which can be useful to describe layouts with weaker or stronger alignment constraints.

Layout paths

A layout path is used to unambiguously select a layout that is nested in some other layout. Layout paths are typically expressed as a sequence of one or more path elementsPREVIEW. (A more formal definition of layout paths is provided below).

Layout paths can be used to:

  • obtain offsets of arbitrarily nested layouts;
  • obtain a var handle that can be used to access the value corresponding to the selected layout;
  • select an arbitrarily nested layout.

For instance, given the taggedValues sequence layout constructed above, we can obtain the offset, in bytes, of the member layout named value in the first sequence element, as follows: 

long valueOffset = taggedValues.byteOffset(PathElement.sequenceElement(0),
                                          PathElement.groupElement("value")); // yields 4
Similarly, we can select the member layout named value, as follows: 
MemoryLayout value = taggedValues.select(PathElement.sequenceElement(),
                                         PathElement.groupElement("value"));

Open path elements

Some layout path elements, said open path elements, can select multiple layouts at once. For instance, the open path elements MemoryLayout.PathElement.sequenceElement()PREVIEW, MemoryLayout.PathElement.sequenceElement(long, long)PREVIEW select an unspecified element in a sequence layout. A var handle derived from a layout path containing one or more open path element features additional coordinates of type long, which can be used by clients to bind the open elements in the path: 
VarHandle valueHandle = taggedValues.varHandle(PathElement.sequenceElement(),
                                               PathElement.groupElement("value"));
MemorySegment valuesSegment = ...
int val = (int) valueHandle.get(valuesSegment, 2); // reads the "value" field of the third struct in the array

Open path elements also affects the creation of offset-computing method handles. Each open path element becomes an additional long parameter in the obtained method handle. This parameter can be used to specify the index of the sequence element whose offset is to be computed: 

MethodHandle offsetHandle = taggedValues.byteOffsetHandle(PathElement.sequenceElement(),
                                                          PathElement.groupElement("kind"));
long offset1 = (long) offsetHandle.invokeExact(1L); // 8
long offset2 = (long) offsetHandle.invokeExact(2L); // 16

Dereference path elements

A special kind of path element, called dereference path element, allows var handles obtained from memory layouts to follow pointers. Consider the following layout: 
StructLayout RECTANGLE = MemoryLayout.structLayout(
        ValueLayout.ADDRESS.withTargetLayout(
                MemoryLayout.sequenceLayout(4,
                        MemoryLayout.structLayout(
                                ValueLayout.JAVA_INT.withName("x"),
                                ValueLayout.JAVA_INT.withName("y")
                        ).withName("point")
                 )
         ).withName("points")
);
This layout is a struct layout which describe a rectangle. It contains a single field, namely points, an address layout whose target layoutPREVIEW is a sequence layout of four struct layouts. Each struct layout describes a two-dimensional point, and is defined as a pair or ValueLayout.JAVA_INTPREVIEW coordinates, with names x and y, respectively.

With dereference path elements, we can obtain a var handle which accesses the y coordinate of one of the point in the rectangle, as follows: 

VarHandle rectPointYs = RECTANGLE.varHandle(
        PathElement.groupElement("points"),
        PathElement.dereferenceElement(),
        PathElement.sequenceElement(),
        PathElement.groupElement("y")
);

MemorySegment rect = ...
int rect_y_4 = (int) rectPointYs.get(rect, 2); // rect.points[2]->y

Layout path well-formedness

A layout path is applied to a layout C_0, also called the initial layout. Each path element in a layout path can be thought of as a function which updates the current layout C_i-1 to some other layout C_i. That is, for each path element E1, E2, ... En, in a layout path P, we compute C_i = f_i(C_i-1), where f_i is the selection function associated with the path element under consideration, denoted as E_i. The final layout C_i is also called the selected layout.

A layout path P is considered well-formed for an initial layout C_0 if all its path elements E1, E2, ... En are well-formed for their corresponding input layouts C_0, C_1, ... C_n-1. A path element E is considered well-formed for a layout L if any of the following is true:

Any attempt to provide a layout path P that is not well-formed for an initial layout C_0 will result in an IllegalArgumentException.
Implementation Requirements:
Implementations of this interface are immutable, thread-safe and value-based.
Sealed Class Hierarchy Graph:
Sealed class hierarchy graph for MemoryLayoutSealed class hierarchy graph for MemoryLayout
Since:
19

  • Method Details

    • byteSize

      long byteSize()
      Returns the layout size, in bytes.
      Returns:
      the layout size, in bytes

    • 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 characteristics as this layout, but with the given name.
      Parameters:
      name - the layout name.
      Returns:
      a memory layout with the same characteristics as this layout, but with the given name
      See Also:

    • withoutName

      MemoryLayoutPREVIEW withoutName()
      Returns a memory layout with the same characteristics as this layout, but with no name.
      API Note:
      This can be useful to compare two layouts that have different names, but are otherwise equal.
      Returns:
      a memory layout with the same characteristics as this layout, but with no name
      See Also:

    • byteAlignment

      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 withByteAlignment(long)), then this method returns the natural alignment constraint (in bytes) associated with this layout.
      Returns:
      the alignment constraint associated with this layout, expressed in bytes

    • withByteAlignment

      MemoryLayoutPREVIEW withByteAlignment(long byteAlignment)
      Returns a memory layout with the same characteristics as this layout, but with the given alignment constraint (in bytes).
      Parameters:
      byteAlignment - the layout alignment constraint, expressed in bytes.
      Returns:
      a memory layout with the same characteristics as this layout, but with the given alignment constraint (in bytes)
      Throws:
      IllegalArgumentException - if byteAlignment is not a power of two.

    • byteOffset

      default long byteOffset(MemoryLayout.PathElementPREVIEW... elements)
      Computes the offset, in bytes, of the layout selected by the given layout path, where the initial layout in the path is 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 is not well-formed for this layout.
      IllegalArgumentException - if the layout path contains one or more open path elements.
      IllegalArgumentException - if the layout path contains one or more dereference path elements.

    • byteOffsetHandle

      default MethodHandle byteOffsetHandle(MemoryLayout.PathElementPREVIEW... elements)
      Creates a method handle that computes the offset, in bytes, of the layout selected by the given layout path, where the initial layout in the path is this layout.

      The returned method handle has the following characteristics:

      • its return type is long;
      • it has as zero or more parameters of type long, one for each open path element in the provided layout path. The order of these parameters corresponds to the order in which the open path elements occur in the provided layout path.

      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.
      API Note:
      The returned method handle can be used to compute a layout offset, similarly to byteOffset(PathElement...), but more flexibly, as some indices can be specified when invoking the method handle.
      Parameters:
      elements - the layout path elements.
      Returns:
      a method handle that computes the offset, in bytes, of the layout selected by the given layout path.
      Throws:
      IllegalArgumentException - if the layout path is not well-formed for this layout.
      IllegalArgumentException - if the layout path contains one or more dereference path elements.

    • varHandle

      default VarHandle varHandle(MemoryLayout.PathElementPREVIEW... elements)
      Creates a var handle that accesses a memory segment at the offset selected by the given layout path, where the initial layout in the path is this layout.

      The returned var handle has the following characteristics:

      • its type is derived from the carrierPREVIEW of the selected value layout;
      • it has as zero or more access coordinates of type long, one for each open path element in the provided layout path. The order of these access coordinates corresponds to the order in which the open path elements occur in the provided layout path.

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

      
 address = base(segment) + offset
      Where base(segment) denotes a function that returns the physical base address of the accessed memory segment. For native segments, this function just returns the native segment's addressPREVIEW. For heap segments, this function is more complex, as the address of heap segments is virtualized. The offset value 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 bounds which are derived from the layout path, that is, 0 <= x_i < b_i, where 1 <= i <= n, or IndexOutOfBoundsException is thrown.

      The base address must be aligned according to the alignment constraint of the root layout (this layout). Note that this can be more strict (but not less) than the alignment constraint of the selected value layout.

      Multiple paths can be chained, with dereference path elements. A dereference path element constructs a fresh native memory segment whose base address is the address value read obtained by accessing a memory segment at the offset determined by the layout path elements immediately preceding the dereference path element. In other words, if a layout path contains one or more dereference path elements, the final address accessed by the returned var handle can be computed as follows: 

      
 address_1 = base(segment) + offset_1
 address_2 = base(segment_1) + offset_2
 ...
 address_k = base(segment_k-1) + offset_k
      where k is the number of dereference path elements in a layout path, segment is the input segment, segment_1, ... segment_k-1 are the segments obtained by dereferencing the address associated with a given dereference path element (e.g. segment_1 is a native segment whose base address is address_1), and offset_1, offset_2, ... offset_k are the offsets computed by evaluating the path elements after a given dereference operation (these offsets are obtained using the computation described above). In these more complex access operations, all memory accesses immediately preceding a dereference operation (e.g. those at addresses address_1, address_2, ..., address_k-1 are performed using the VarHandle.AccessMode.GET access mode.
      API Note:
      The resulting var handle features certain access mode restrictions, which are common to all memory segment view handlesPREVIEW.
      Parameters:
      elements - the layout path elements.
      Returns:
      a var handle that accesses a memory segment at the offset selected by the given layout path.
      Throws:
      IllegalArgumentException - if the layout path is not well-formed for this layout.
      IllegalArgumentException - if the layout selected by the provided path is not a value layoutPREVIEW.
      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 initial layout in the path is this layout.

      The returned method handle has the following characteristics:

      • its return type is MemorySegment;
      • it has a leading parameter of type MemorySegment, corresponding to the memory segment to be sliced;
      • it has as zero or more parameters of type long, one for each open path element in the provided layout path. The order of these parameters corresponds to the order in which the open path elements occur in the provided layout path.

      The offset of the returned segment is computed as follows: 

      long offset = byteOffset(elements);
long size = select(elements).byteSize();
MemorySegment slice = segment.asSlice(offset, size);

      The segment to be sliced must be aligned according to the alignment constraint of the root layout (this layout). Note that this can be more strict (but not less) than the alignment constraint of the selected value layout.

      API Note:
      The returned method handle can be used to obtain a memory segment slice, similarly to MemorySegment.asSlice(long, long)PREVIEW, but more flexibly, as some indices can be specified when invoking the method handle.
      Parameters:
      elements - the layout path elements.
      Returns:
      a method handle which is used to slice a memory segment at the offset selected by the given layout path.
      Throws:
      IllegalArgumentException - if the layout path is not well-formed for this layout.
      IllegalArgumentException - if the layout path contains one or more dereference path elements.

    • select

      default MemoryLayoutPREVIEW select(MemoryLayout.PathElementPREVIEW... elements)
      Returns the layout selected from the provided path, where the initial layout in the path is this layout.
      Parameters:
      elements - the layout path elements.
      Returns:
      the layout selected by the layout path in elements.
      Throws:
      IllegalArgumentException - if the layout path is not well-formed for this layout.
      IllegalArgumentException - if the layout path contains one or more dereference path elements.
      IllegalArgumentException - if the layout path contains one or more path elements that select one or more sequence element indices, such as MemoryLayout.PathElement.sequenceElement(long)PREVIEW and MemoryLayout.PathElement.sequenceElement(long, long)PREVIEW).

    • 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 constraint. 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 PaddingLayoutPREVIEW paddingLayout(long byteSize)
      Creates a padding layout with the given byte size. The alignment constraint of the returned layout is 1. As such, regardless of its size, in the absence of an explicit alignment constraint, a padding layout does not affect the natural alignment of the group or sequence layout it is nested into.
      Parameters:
      byteSize - the padding size (expressed in bytes).
      Returns:
      the new selector layout.
      Throws:
      IllegalArgumentException - if byteSize <= 0.

    • sequenceLayout

      static SequenceLayoutPREVIEW sequenceLayout(long elementCount, MemoryLayoutPREVIEW elementLayout)
      Creates a sequence layout with the given element layout and element count.
      Parameters:
      elementCount - the sequence element count.
      elementLayout - the sequence element layout.
      Returns:
      the new sequence layout with the given element layout and size.
      Throws:
      IllegalArgumentException - if elementCount is negative.
      IllegalArgumentException - if elementLayout.byteSize() * elementCount overflows.
      IllegalArgumentException - if elementLayout.byteSize() % elementLayout.byteAlignment() != 0.

    • sequenceLayout

      static SequenceLayoutPREVIEW sequenceLayout(MemoryLayoutPREVIEW elementLayout)
      Creates a sequence layout with the given element layout and the maximum element count such that it does not overflow a long. This is equivalent to the following code: 
      sequenceLayout(Long.MAX_VALUE / elementLayout.byteSize(), elementLayout);
      Parameters:
      elementLayout - the sequence element layout.
      Returns:
      a new sequence layout with the given element layout and maximum element count.
      Throws:
      IllegalArgumentException - if elementLayout.byteSize() % elementLayout.byteAlignment() != 0.

    • structLayout

      static StructLayoutPREVIEW structLayout(MemoryLayoutPREVIEW... elements)
      Creates a struct layout with the given member layouts.
      API Note:
      This factory does not automatically align element layouts, by inserting additional padding layoutPREVIEW elements. As such, the following struct layout creation will fail with an exception: 
      structLayout(JAVA_SHORT, JAVA_INT);
      To avoid the exception, clients can either insert additional padding layout elements: 
      structLayout(JAVA_SHORT, MemoryLayout.paddingLayout(2), JAVA_INT);
      Or, alternatively, they can use a member layout which features a smaller alignment constraint. This will result in a packed struct layout: 
      structLayout(JAVA_SHORT, JAVA_INT.withByteAlignment(2));
      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 byte sizes of the member layouts overflows.
      IllegalArgumentException - if a member layout in elements occurs at an offset (relative to the start of the struct layout) which is not compatible with its alignment constraint.

    • unionLayout

      static UnionLayoutPREVIEW 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.