Module java.base

Package java.lang.foreign

package java.lang.foreign

Provides low-level access to memory and functions outside the Java runtime.

Foreign memory access

The main abstraction introduced to support foreign memory access is MemorySegmentPREVIEW, which models a contiguous region of memory, residing either inside or outside the Java heap. Memory segments are typically allocated using an ArenaPREVIEW, 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 memory layoutPREVIEW, which provides basic operations to query sizes, offsets and alignment constraints. Memory layouts also provide an alternate, more abstract way, to access memory segments using var handlesPREVIEW, which can be computed using layout paths. For example, to allocate an off-heap region of memory big enough to hold 10 values of the primitive type int, and fill it with values ranging from 0 to 9, we can use the following code: 

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 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 int. The native segment is allocated using a confined arenaPREVIEW. 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 try-with-resources 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 14.20.3 of The Java Language Specification.

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 outside the boundaries of the memory segment used by the access operation. We call this guarantee spatial safety; in other words, access to memory segments is bounds-checked, in the same way as array access is, as described in Section 15.10.4 of The Java Language Specification.

Additionally, to prevent a region of memory from being accessed after it has been deallocated (i.e. use-after-free), 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 temporal safety.

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.

Foreign function access

The key abstractions introduced to support foreign function access are SymbolLookupPREVIEW, FunctionDescriptorPREVIEW and LinkerPREVIEW. The first is used to look up symbols inside libraries; the second is used to model the signature of foreign functions, while the third provides linking capabilities which allows modelling foreign functions as 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 Java Native Interface (JNI)).

For example, to compute the length of a string using the C standard library function strlen on a Linux/x64 platform, we can use the following code: 

 Linker linker = Linker.nativeLinker();
 SymbolLookup stdlib = linker.defaultLookup();
 MethodHandle strlen = linker.downcallHandle(
     stdlib.find("strlen").get(),
     FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS)
 );

 try (Arena arena = Arena.ofConfined()) {
     MemorySegment cString = arena.allocateUtf8String("Hello");
     long len = (long)strlen.invokeExact(cString); // 5
 }
Here, we obtain a native linkerPREVIEW and we use it to look upPREVIEW the strlen function in the standard C library; a downcall method handle targeting said function is subsequently obtainedPREVIEW. To complete the linking successfully, we must provide a FunctionDescriptorPREVIEW instance, describing the signature of the strlen function. From this information, the linker will uniquely determine the sequence of steps which will turn the method handle invocation (here performed using MethodHandle.invokeExact(java.lang.Object...)) into a foreign function call, according to the rules specified by the ABI of the underlying platform. The ArenaPREVIEW class also provides many useful methods for interacting with foreign code, such as convertingPREVIEW Java strings into zero-terminated, UTF-8 strings, as demonstrated in the above example.

Restricted methods

Some methods in this package are considered restricted. 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 MemorySegment.reinterpret(long)PREVIEW ()} 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.

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 MemorySegment.reinterpret(long)PREVIEW 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.

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 ModuleLayer.Controller.enableNativeAccess(java.lang.Module)PREVIEW.

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 NullPointerException. This fact is not individually documented for methods of this API.

API Note:
Usual memory model guarantees, for example stated in 6.6 and 10.4, do not apply when accessing native memory segments as these segments are backed by off-heap regions of memory.
Implementation Note:
In the reference implementation, access to restricted methods can be granted to specific modules using the command line option --enable-native-access=M1,M2, ... Mn, where M1, M2, ... Mn are module names (for the unnamed module, the special value 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.
External Specifications