// lowlevel/AttemptLocking.java

// (c)2021 MindView LLC: see Copyright.txt

// We make no guarantees that this code is fit for any purpose.

// Visit http://OnJava8.com for more book information.

// Locks in the concurrent library allow you

// to give up on trying to acquire a lock

import java.util.concurrent.*;

import java.util.concurrent.locks.*;

import onjava.Nap;

public class AttemptLocking {

private ReentrantLock lock = new ReentrantLock();

public void untimed() {

boolean captured = lock.tryLock();

try {

System.out.println("tryLock(): " + captured);

} finally {

if(captured)

lock.unlock();

}

}

public void timed() {

boolean captured = false;

try {

captured = lock.tryLock(2, TimeUnit.SECONDS);

} catch(InterruptedException e) {

throw new RuntimeException(e);

}

try {

System.out.println(

"tryLock(2, TimeUnit.SECONDS): " + captured);

} finally {

if(captured)

lock.unlock();

}

}

public static void main(String[] args) {

final AttemptLocking al = new AttemptLocking();

al.untimed(); // True -- lock is available

al.timed(); // True -- lock is available

// Now create a second task to grab the lock:

CompletableFuture.runAsync( () -> {

al.lock.lock();

System.out.println("acquired");

});

new Nap(0.1); // Give the second task a chance

al.untimed(); // False -- lock grabbed by task

al.timed(); // False -- lock grabbed by task

}

}

/* Output:

tryLock(): true

tryLock(2, TimeUnit.SECONDS): true

acquired

tryLock(): false

tryLock(2, TimeUnit.SECONDS): false

*/