Classes to support low-level, safe and efficient memory access. For example: + * + *
{@code
+static final VarHandle intHandle = MemoryHandles.varHandle(int.class, ByteOrder.BIG_ENDIAN);
+
+try (MemorySegment segment = MemorySegment.allocateNative(10 * 4)) {
+ MemoryAddress base = segment.baseAddress();
+ for (long i = 0 ; i < 10 ; i++) {
+ intHandle.set(base.offset(i * 4), (int)i);
+ }
+ }
+ * }
+ *
+ * Here we create a var handle, namely {@code intHandle}, to manipulate values of the primitive type {@code int}, at
+ * a given memory location. We then create a native memory segment, that is, a memory segment backed by
+ * off-heap memory; the size of the segment is 40 bytes, enough to store 10 values of the primitive type {@code int}.
+ * The segment is created inside a try-with-resources construct: this idiom ensures that all the memory resources
+ * associated with the segment will be released at the end of the block. Inside the try-with-resources block, we initialize
+ * the contents of the memory segment; more specifically, if we view the memory segment as a set of 10 adjacent slots,
+ * {@code s[i]}, where {@code 0 <= i < 10}, where the size of each slot is exactly 4 bytes, the initialization logic above will set each slot
+ * so that {@code s[i] = i}, again where {@code 0 <= i < 10}.
+ *
+ * + * The key abstractions introduced by this package are {@link jdk.incubator.foreign.MemorySegment} and {@link jdk.incubator.foreign.MemoryAddress}. + * The first models a contiguous memory region, which can reside either inside or outside the Java heap; the latter models an address - that is, + * an offset inside a given segment. A memory address represents the main access coordinate of a memory access var handle, which can be obtained + * using the combinator methods defined in the {@link jdk.incubator.foreign.MemoryHandles} class. Finally, the {@link jdk.incubator.foreign.MemoryLayout} class + * hierarchy enables description of memory layouts and basic operations such as computing the size in bytes of a given + * layout, obtain its alignment requirements, and so on. Memory layouts also provide an alternate, more abstract way, to produce + * memory access var handles, e.g. using layout paths. + * + *
+ * The deterministic deallocation model differs significantly from the implicit strategies adopted within other APIs, most + * notably the {@link java.nio.ByteBuffer} API: in that case, when a native byte buffer is created (see {@link java.nio.ByteBuffer#allocateDirect(int)}), + * the underlying memory is not released until the byte buffer reference becomes unreachable. While implicit deallocation + * models such as this can be very convenient - clients do not have to remember to close a direct buffer - such models can also make it + * hard for clients to ensure that the memory associated with a direct buffer has indeed been released. + * + *
+ * Since memory segments can be closed (see above), a memory address is also validated (upon access) to make sure that + * the segment it belongs to has not been closed prematurely. We call this guarantee temporal safety. Note that, + * in the general case, guaranteeing temporal safety can be hard, as multiple threads could attempt to access and/or close + * the same memory segment concurrently. The memory access API addresses this problem by imposing strong + * thread-confinement guarantees on memory segments: each + * memory segment is associated with an owner thread, which is the only thread that can either access or close the segment. + * A thread other than the owner thread will have to explicitly acquire a segment in order to be able to use it. + *
+ * Together, spatial and temporal safety ensure that each memory access operation either succeeds - and accesses a valid + * memory location - or fails. + */ +package jdk.incubator.foreign;