test/jdk/java/lang/Thread/jni/AttachCurrentThread/ImplicitAttach.java

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 * version 2 for more details (a copy is included in the LICENSE file that
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 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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import java.lang.foreign.*;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.util.concurrent.CountDownLatch;

/**
 * Test native threads attaching implicitly to the VM by means of an upcall.
 */
public class ImplicitAttach {
    private static final ValueLayout.OfInt C_INT = ValueLayout.JAVA_INT;
    private static final AddressLayout C_POINTER = ValueLayout.ADDRESS;

    private static volatile CountDownLatch latch;

    public static void main(String[] args) throws Throwable {
        int threadCount;
        if (args.length > 0) {
            threadCount = Integer.parseInt(args[0]);
        } else {
            threadCount = 2;
        }
        latch = new CountDownLatch(threadCount);

        Linker abi;
        try {
            abi = Linker.nativeLinker();
        } catch (UnsupportedOperationException e) {
            System.out.println("Test skipped, no native linker on this platform");
            return;
        }

        // stub to invoke callback
        MethodHandle callback = MethodHandles.lookup()
                .findStatic(ImplicitAttach.class, "callback", MethodType.methodType(void.class));
        MemorySegment upcallStub = abi.upcallStub(callback,
                FunctionDescriptor.ofVoid(),
                Arena.global());

        // void start_threads(int count, void *(*f)(void *))
        SymbolLookup symbolLookup = SymbolLookup.loaderLookup();
        MemorySegment symbol = symbolLookup.find("start_threads").orElseThrow();
        FunctionDescriptor desc = FunctionDescriptor.ofVoid(C_INT, C_POINTER);
        MethodHandle start_threads = abi.downcallHandle(symbol, desc);

        // start the threads and wait for the threads to call home
        start_threads.invoke(threadCount, upcallStub);
        latch.await();
    }

    /**
     * Invoked from native thread.
     */
    private static void callback() {
        System.out.println(Thread.currentThread());
        latch.countDown();
    }

    static {
        System.loadLibrary("ImplicitAttach");
    }
}