jdk/src/java.base/share/classes/java/lang/foreign/package-info.java

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/**
 * <p>Provides low-level access to memory and functions outside the Java runtime.
 *
 * <h2 id="fma">Foreign memory access</h2>
 *
 * <p>
 * The main abstraction introduced to support foreign memory access is {@link java.lang.foreign.MemorySegment}, which
 * models a contiguous region of memory, residing either inside or outside the Java heap. Memory segments are
 * typically allocated using an {@link java.lang.foreign.Arena}, which controls the lifetime of the regions of memory
 * backing the segments it allocates. The contents of a memory segment can be described using a
 * {@link java.lang.foreign.MemoryLayout memory layout}, which provides basic operations to query sizes, offsets and
 * alignment constraints. Memory layouts also provide an alternate, more abstract way, to
 * <a href=MemorySegment.html#segment-deref>access memory segments</a> using
 * {@linkplain java.lang.foreign.MemoryLayout#varHandle(java.lang.foreign.MemoryLayout.PathElement...) var handles},
 * which can be computed using <a href="MemoryLayout.html#layout-paths"><em>layout paths</em></a>.
 *
 * For example, to allocate an off-heap region of memory big enough to hold 10 values of the primitive type {@code int},
 * and fill it with values ranging from {@code 0} to {@code 9}, we can use the following code:
 *
 * {@snippet lang = java:
 * try (Arena arena = Arena.ofConfined()) {
 *     MemorySegment segment = arena.allocate(10 * 4);
 *     for (int i = 0 ; i < 10 ; i++) {
 *         segment.setAtIndex(ValueLayout.JAVA_INT, i, i);
 *     }
 * }
 * }
 *
 * This code creates a <em>native</em> 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 native segment is allocated using a {@linkplain java.lang.foreign.Arena#ofConfined() confined arena}.
 * As such, access to the native segment is restricted to the current thread (the thread that created the arena).
 * Moreover, when the arena is closed, the native segment is invalidated, and its backing region of memory is
 * deallocated. Note the use of the <em>try-with-resources</em> construct: this idiom ensures that the off-heap region
 * of memory backing the native segment will be released at the end of the block, according to the semantics described
 * in Section {@jls 14.20.3} of <cite>The Java Language Specification</cite>.
 * <p>
 * Memory segments provide strong safety guarantees when it comes to memory access. First, when accessing a memory segment,
 * the access coordinates are validated (upon access), to make sure that access does not occur at any address which resides
 * <em>outside</em> the boundaries of the memory segment used by the access operation. We call this guarantee <em>spatial safety</em>;
 * in other words, access to memory segments is bounds-checked, in the same way as array access is, as described in
 * Section {@jls 15.10.4} of <cite>The Java Language Specification</cite>.
 * <p>
 * Additionally, to prevent a region of memory from being accessed <em>after</em> it has been deallocated
 * (i.e. <em>use-after-free</em>), a segment is also validated (upon access) to make sure that the arena from which it
 * has been obtained has not been closed. We call this guarantee <em>temporal safety</em>.
 * <p>
 * Together, spatial and temporal safety ensure that each memory access operation either succeeds - and accesses a valid
 * location within the region of memory backing the memory segment - or fails.
 *
 * <h2 id="ffa">Foreign function access</h2>
 * The key abstractions introduced to support foreign function access are {@link java.lang.foreign.SymbolLookup},
 * {@link java.lang.foreign.FunctionDescriptor} and {@link java.lang.foreign.Linker}. The first is used to look up symbols
 * inside libraries; the second is used to model the signature of foreign functions, while the third is used
 * to link foreign functions as {@link java.lang.invoke.MethodHandle} instances,
 * so that clients can perform foreign function calls directly in Java, without the need for intermediate layers of C/C++
 * code (as is the case with the <a href="{@docRoot}/../specs/jni/index.html">Java Native Interface (JNI)</a>).
 * <p>
 * For example, to compute the length of a string using the C standard library function {@code strlen} on a Linux/x64 platform,
 * we can use the following code:
 *
 * {@snippet lang = java:
 * Linker linker = Linker.nativeLinker();
 * SymbolLookup stdlib = linker.defaultLookup();
 * MethodHandle strlen = linker.downcallHandle(
 *     stdlib.find("strlen").orElseThrow(),
 *     FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS)
 * );
 *
 * try (Arena arena = Arena.ofConfined()) {
 *     MemorySegment cString = arena.allocateFrom("Hello");
 *     long len = (long)strlen.invokeExact(cString); // 5
 * }
 *}
 *
 * Here, we obtain a {@linkplain java.lang.foreign.Linker#nativeLinker() native linker} and we use it
 * to {@linkplain java.lang.foreign.SymbolLookup#find(java.lang.String) look up} the {@code strlen} function in the
 * standard C library; a <em>downcall method handle</em> targeting said function is subsequently
 * {@linkplain java.lang.foreign.Linker#downcallHandle(FunctionDescriptor, Linker.Option...) obtained}.
 * To complete the linking successfully, we must provide a {@link java.lang.foreign.FunctionDescriptor} instance,
 * describing the signature of the {@code strlen} function.
 * From this information, the linker will uniquely determine the sequence of steps which will turn
 * the method handle invocation (here performed using {@link java.lang.invoke.MethodHandle#invokeExact(java.lang.Object...)})
 * into a foreign function call, according to the rules specified by the ABI of the underlying platform.
 * The {@link java.lang.foreign.Arena} class also provides many useful methods for
 * interacting with foreign code, such as
 * {@linkplain java.lang.foreign.SegmentAllocator#allocateFrom(java.lang.String) converting} Java strings into
 * zero-terminated, UTF-8 strings, as demonstrated in the above example.
 *
 * <h2 id="restricted">Restricted methods</h2>
 * Some methods in this package are considered <em>restricted</em>. Restricted methods are typically used to bind native
 * foreign data and/or functions to first-class Java API elements which can then be used directly by clients. For instance
 * the restricted method {@link java.lang.foreign.MemorySegment#reinterpret(long)}
 * can be used to create a fresh segment with the same address and temporal bounds,
 * but with the provided size. This can be useful to resize memory segments obtained when interacting with native functions.
 * <p>
 * Binding foreign data and/or functions is generally unsafe and, if done incorrectly, can result in VM crashes,
 * or memory corruption when the bound Java API element is accessed. For instance, incorrectly resizing a native
 * memory sgement using {@link java.lang.foreign.MemorySegment#reinterpret(long)} can lead to a JVM crash, or, worse,
 * lead to silent memory corruption when attempting to access the resized segment. For these reasons, it is crucial for
 * code that calls a restricted method to never pass arguments that might cause incorrect binding of foreign data and/or
 * functions to a Java API.
 * <p>
 * Given the potential danger of restricted methods, the Java runtime issues a warning on the standard error stream
 * every time a restricted method is invoked. Such warnings can be disabled by granting access to restricted methods
 * to selected modules. This can be done either via implementation-specific command line options, or programmatically, e.g. by calling
 * {@link java.lang.ModuleLayer.Controller#enableNativeAccess(java.lang.Module)}.
 * <p>
 * For every class in this package, unless specified otherwise, any method arguments of reference
 * type must not be null, and any null argument will elicit a {@code NullPointerException}.  This fact is not individually
 * documented for methods of this API.
 *
 * @apiNote Usual memory model guarantees, for example stated in {@jls 6.6} and {@jls 10.4}, do not apply
 * when accessing native memory segments as these segments are backed by off-heap regions of memory.
 *
 * @implNote
 * In the reference implementation, access to restricted methods can be granted to specific modules using the command line option
 * {@code --enable-native-access=M1,M2, ... Mn}, where {@code M1}, {@code M2}, {@code ... Mn} are module names
 * (for the unnamed module, the special value {@code ALL-UNNAMED} can be used). If this option is specified, access to
 * restricted methods is only granted to the modules listed by that option. If this option is not specified,
 * access to restricted methods is enabled for all modules, but access to restricted methods will result in runtime warnings.
 *
 * @spec jni/index.html Java Native Interface Specification
 *
 * @since 22
 */
package java.lang.foreign;