OnJava8-Examples/lowlevel/AttemptLocking.java

// 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
*/