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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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package org.openjdk.jol.samples;

import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.vm.VM;

import java.util.concurrent.TimeUnit;

import static java.lang.System.out;

/**
 * @author Aleksey Shipilev
 */
public class JOLSample_14_FatLocking {

    /*
     * This is the example of fat locking.
     *
     * If VM detects contention on thread, it needs to delegate the
     * access arbitrage to OS. That involves associating the object
     * with the native lock, i.e. "inflating" the lock.
     *
     * In this example, we need to simulate the contention, this is
     * why we have the additional thread. You can see the fresh object
     * has the default mark word, the object before the lock was already
     * acquired by the auxiliary thread, and when the lock was finally
     * acquired by main thread, it had been inflated. The inflation stays
     * there after the lock is released.
     *
     * You can also see the lock is "deflated" after the GC on some JDKs.
     * Prior to JDK 15, the lock cleanup proceeds in safepoints, so any
     * GC would enter that code. Since JDK 15, monitors are deflated
     * asynchronously, once unused monitor population is large enough.
     */

    public static void main(String[] args) throws Exception {
        out.println(VM.current().details());

        final A a = new A();

        ClassLayout layout = ClassLayout.parseInstance(a);

        out.println("**** Fresh object");
        out.println(layout.toPrintable());

        Thread t = new Thread(() -> {
            synchronized (a) {
                try {
                    TimeUnit.SECONDS.sleep(10);
                } catch (InterruptedException e) {
                    // Do nothing
                }
            }
        });

        t.start();

        TimeUnit.SECONDS.sleep(1);

        out.println("**** Before the lock");
        out.println(layout.toPrintable());

        synchronized (a) {
            out.println("**** With the lock");
            out.println(layout.toPrintable());
        }

        out.println("**** After the lock");
        out.println(layout.toPrintable());

        System.gc();

        out.println("**** After System.gc()");
        out.println(layout.toPrintable());
    }

    public static class A {
        // no fields
    }

}
