Foreign function and memory API in Java 16

24 Oct 2023

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Java 16, released in March 2021, introduced a new feature called the Foreign Function and Memory API. This API allows Java developers to interact with native code and access native memory directly. In this article, we will explore the Foreign Function and Memory API and see how it can be used in Java 16.

What is the Foreign Function and Memory API?

The Foreign Function and Memory API (FFMA) in Java 16 provides a standardized way to interact with native code and manipulate native memory. This API allows developers to call functions defined in native libraries and perform low-level memory operations in a type-safe manner.

Prior to Java 16, developers had to rely on external libraries or use JNI (Java Native Interface) to achieve similar functionality. The FFMA simplifies the process and provides a native interoperability mechanism directly within the Java language.

Key Features of the Foreign Function and Memory API

1. Direct memory access

The FFMA allows direct access to native memory without the need for any intermediary data structures or buffers. Developers can allocate and deallocate memory, read and write values directly to memory, and manipulate memory regions.

2. Type-safe binding to native functions

The FFMA provides a way to bind Java method signatures to native function declarations, enabling direct invocation of native functions from Java code. This binding is type-safe and provides automatic mapping of native data types to equivalent Java types.

3. Interoperability with native code

The FFMA enables seamless integration with existing native code by providing mechanisms to represent native types in Java and invoke native functions directly.

How to Use the Foreign Function and Memory API

To use the Foreign Function and Memory API in Java 16, you need to follow these steps:

Import the necessary FFMA classes from the jdk.incubator.foreign package.
Use the MemorySegment class to allocate native memory regions.
Use the MemoryLayout class to define the layout of native data structures.
Bind Java method signatures to native function declarations using the CLinker class.
Invoke native functions using the bound Java methods.

Example Usage

Let’s look at a simple example that demonstrates how to use the Foreign Function and Memory API in Java 16.

import jdk.incubator.foreign.*;

public class FFMAExample {
    public static void main(String[] args) throws Throwable {
        // Allocate a native memory region of 4 bytes
        try (MemorySegment segment = MemorySegment.allocateNative(4)) {
            // Write an int value to the native memory
            segment.asIntBuffer().put(42);

            // Bind Java method signature to printf function in C
            var printf = CLinker.getInstance().downcallHandle(
                    CLinker.systemLookup().lookup("printf").get(),
                    MethodType.getMethodType(int.class, MemoryAddress.class),
                    FunctionDescriptor.of(CLinker.C_INT, CLinker.C_POINTER));

            // Invoke printf with the address of the memory segment
            printf.invokeExact(segment.address());

            // Read the value from the native memory
            int value = segment.asIntBuffer().get();
            System.out.println("Value from native memory: " + value);
        }
    }
}

In this example, we allocate a native memory region of 4 bytes using the MemorySegment.allocateNative() method. We then write the value 42 to the native memory using the asIntBuffer() method.

Next, we bind the printf function from the C library to a Java method using the downcallHandle() method of CLinker. We define the method signature and the function descriptor specifying the return type and argument type.

Finally, we invoke the printf function with the address of the memory segment using the invokeExact() method. We read the value from the native memory and print it out.

Conclusion

The Foreign Function and Memory API in Java 16 provides a powerful mechanism for Java developers to interact with native code and manipulate native memory directly. It simplifies the process of calling native functions and accessing native memory, eliminating the need for external libraries or JNI.

The FFMA opens up new possibilities for Java applications that require low-level access to native code or demand high-performance memory operations. It enables seamless integration with existing native code and empowers developers to leverage the full power of the underlying system.

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