Devops Monk

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <configuration>
    <release>17</release>
    <compilerArgs>
      <arg>--add-modules</arg>
      <arg>jdk.incubator.foreign</arg>
    </compilerArgs>
  </configuration>
</plugin>
<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-surefire-plugin</artifactId>
  <configuration>
    <argLine>--add-modules jdk.incubator.foreign</argLine>
  </configuration>
</plugin>

javac --add-modules jdk.incubator.foreign Hello.java
java --add-modules jdk.incubator.foreign Hello

import jdk.incubator.foreign.*;

// Allocate off-heap memory: 1024 bytes
try (MemorySession session = MemorySession.openConfined()) {
    MemorySegment segment = MemorySegment.allocateNative(1024, session);
    // Write an int at offset 0
    segment.set(ValueLayout.JAVA_INT, 0, 42);
    // Read it back
    int value = segment.get(ValueLayout.JAVA_INT, 0);
    System.out.println(value); // 42
} // memory is freed when session closes

// Confined session: owned by one thread, freed when closed
try (MemorySession session = MemorySession.openConfined()) {
    MemorySegment seg = MemorySegment.allocateNative(64, session);
    // ...
} // freed here

// Shared session: can be used by multiple threads
try (MemorySession session = MemorySession.openShared()) {
    MemorySegment seg = MemorySegment.allocateNative(64, session);
    // Can be passed to other threads
}

// Implicit session: GC-managed (use with caution)
MemorySession session = MemorySession.openImplicit();

ValueLayout.JAVA_BYTE    // 1 byte
ValueLayout.JAVA_SHORT   // 2 bytes
ValueLayout.JAVA_INT     // 4 bytes
ValueLayout.JAVA_LONG    // 8 bytes
ValueLayout.JAVA_FLOAT   // 4 bytes
ValueLayout.JAVA_DOUBLE  // 8 bytes
ValueLayout.ADDRESS      // pointer-sized (4 or 8 bytes)

try (MemorySession session = MemorySession.openConfined()) {
    // Allocate 8 bytes
    MemorySegment seg = MemorySegment.allocateNative(8, session);

    // Write a long
    seg.set(ValueLayout.JAVA_LONG, 0, 123456789L);

    // Read it back
    long val = seg.get(ValueLayout.JAVA_LONG, 0);
    System.out.println(val); // 123456789

    // Write adjacent ints (at offsets 0 and 4)
    seg.set(ValueLayout.JAVA_INT, 0, 100);
    seg.set(ValueLayout.JAVA_INT, 4, 200);
    System.out.println(seg.get(ValueLayout.JAVA_INT, 0)); // 100
    System.out.println(seg.get(ValueLayout.JAVA_INT, 4)); // 200
}

// C struct: struct Point { int x; int y; }
MemoryLayout pointLayout = MemoryLayout.structLayout(
    ValueLayout.JAVA_INT.withName("x"),
    ValueLayout.JAVA_INT.withName("y")
);

// Allocate and access
try (MemorySession session = MemorySession.openConfined()) {
    MemorySegment point = MemorySegment.allocateNative(pointLayout, session);

    // Access via VarHandle derived from layout
    VarHandle xHandle = pointLayout.varHandle(MemoryLayout.PathElement.groupElement("x"));
    VarHandle yHandle = pointLayout.varHandle(MemoryLayout.PathElement.groupElement("y"));

    xHandle.set(point, 3);
    yHandle.set(point, 4);

    System.out.println(xHandle.get(point)); // 3
    System.out.println(yHandle.get(point)); // 4
}

import jdk.incubator.foreign.*;

// Find the C standard library
Linker linker = Linker.nativeLinker();
SymbolLookup stdlib = Linker.nativeLinker().defaultLookup();

// Find strlen: size_t strlen(const char *s)
MethodHandle strlen = linker.downcallHandle(
    stdlib.lookup("strlen").get(),
    FunctionDescriptor.of(
        ValueLayout.JAVA_LONG,   // return type: size_t (long)
        ValueLayout.ADDRESS      // parameter: const char*
    )
);

// Call strlen with a Java string
try (MemorySession session = MemorySession.openConfined()) {
    // Allocate a null-terminated C string
    MemorySegment cString = session.allocateUtf8String("Hello, Panama!");

    long length = (long) strlen.invoke(cString);
    System.out.println(length); // 14
}

MethodHandle abs = linker.downcallHandle(
    stdlib.lookup("abs").get(),
    FunctionDescriptor.of(ValueLayout.JAVA_INT, ValueLayout.JAVA_INT)
);

int result = (int) abs.invoke(-42);
System.out.println(result); // 42

// JNI approach: requires a .h file and .c file
// Hello.h (generated by javah)
JNIEXPORT jstring JNICALL Java_Hello_greeting(JNIEnv *, jclass, jstring);

// Hello.c
JNIEXPORT jstring JNICALL Java_Hello_greeting(JNIEnv *env, jclass cls, jstring name) {
    const char *nativeName = (*env)->GetStringUTFChars(env, name, 0);
    // ...
    (*env)->ReleaseStringUTFChars(env, name, nativeName);
    return result;
}

// FFM API approach: pure Java, no C code needed
MethodHandle greet = linker.downcallHandle(
    myLib.lookup("greeting").get(),
    FunctionDescriptor.of(ValueLayout.ADDRESS, ValueLayout.ADDRESS)
);