是否存在带超时的Java Lock实现

Question

I have a spring application which can run in a clustered environment. In that environment I use Redis (and Redisson) as a distributed lock-service.

Many of the locks are used to eg protect certain tasks which can only run once at a time, or which may only be started every X seconds.

The application is also capable of running in standalone mode (without redis).

However for this case I need a different implementation of the lockservice. I thought this would be extremely simple because I only need to create a Lock instance locally with a certain timeout (eg for the "only run action at most ever 2 minutes"). However when looking around I could not find any implementation of the Java Lock interface which supports setting a timeout for the lock (so that it automatically unlocks itself after that time).

Is there such a thing, or is there an extremely simple (in terms of lines-of-code) way how I can implement this myself, which I'm just missing?

How that lock impl should behave:

• Other threads are not able to lock it while it's active (as any other lock)
• Ideally, the owning thread should be able to call lock(long timoutMs) again to extend the lock (set the timeout to the given time again)

EDIT: It seems that a concrete example could help understand what I am looking for:

• Imagine the server has an HTTP action "doExpesiveTask"
• whenever this task is called, my application goes to its ockService" and calls .tryAcquireLock("expensiveTaskId", 10, TimeUnit.Minutes) and gets back a boolean if it got the lock or not.
• if it got the lock it starts the task
• if it didn't get the lock it doesn't and shows the user "you have to be more patient"

In a distributed setup the implementation of the lockService uses redis (and the Redisson library) distributed locks (this already works great)! To have a very simple switch between distributed and standalone mode, I simply want to have an implementation of lockService which doesn't rely on any external service. Therefore I would simply need an implementation of a Lock which supports a timeout. With that I could simply have a ConcurrentHashMap inside the lockservice which maps lock-ids to these lock instances.

Why not simply use a Map that maps lock-ids to time-objects: because I also need to prevent other threads from re-locking (extending the lifetime) of a lock which was acquired by another thread.

Answer 1

Your description is a little bit ambiguous, as you are talking about locks, but you are not actually locking a resource (or did not provide example). I feel your problem relates to scheduling.

Since you already use Spring, you could have a look at its scheduling options. Recent versions allow you to use @Scheduled annotation to trigger that. @EnableScheduling fires up background task executor. You could combo that with Spring profiles, to ensure these kick in only when you pass a profile, for example as a JVM parameter.

Copied from docs:

package hello;

import java.text.SimpleDateFormat;
import java.util.Date;

import org.springframework.scheduling.annotation.Scheduled;
import org.springframework.stereotype.Component;

@Component
public class ScheduledTasks {

    private static final SimpleDateFormat dateFormat = new SimpleDateFormat("HH:mm:ss");

    @Scheduled(fixedRate = 5000)
    public void reportCurrentTime() {
        System.out.println("The time is now " + dateFormat.format(new Date()));
    }
}

and to enable:

package hello;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.scheduling.annotation.EnableScheduling;

@SpringBootApplication
@EnableScheduling
public class Application {

    public static void main(String[] args) throws Exception {
        SpringApplication.run(Application.class);
    }
}

There is a quick guide here:

Spring Docs

Service code (you would want to go with enumerators, used strings for clarity):

import org.apache.commons.collections4.map.PassiveExpiringMap;

public class StandAloneLockService {
    private Map ordinaryLocks;
    private Map expiringLocks;

    public StandAloneLockService() {
        this.ordinaryLocks = new HashMap<String, Long>();
        this.expiringLocks = new PassiveExpiringMap<String, Long>(2L,
                TimeUnit.MINUTES);
    }

    public synchronized boolean accquireLock(String task) {
        if (ordinaryLocks.containsKey("task")
                || expiringLocks.containsKey("task")) {
            return false;
        } else {
            return handle("task");
        }
    }

    private boolean handle(String jdk7) {
        switch (jdk7) { // logic 
        }
    }
    private void releaseLock(String task) {
        switch (task) { // logic 
        }
    }
}

Answer 2

There is a method in the Object class: public final void wait(long timeout) . See http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#wait(long)

You can only call it in a synchronized block.

As the example (from the javadoc):

synchronized (obj) {
         while (<condition does not hold>)
             obj.wait(timeout);
         ... // Perform action appropriate to condition
     }

Answer 3

If anyone is interested, this is the LockService implementation which I came up with for "clusterless" mode:

import com.google.common.collect.Maps;
import org.slf4j.Logger;

import java.lang.management.ThreadInfo;
import java.time.LocalDateTime;
import java.time.temporal.ChronoUnit;
import java.time.temporal.TemporalUnit;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

import static org.slf4j.LoggerFactory.getLogger;

public class LocalLockServiceImpl implements LockService {
    private static final Logger LOG = getLogger(ClusterLockServiceLocalImpl.class);

    private ConcurrentMap<String, TimeoutLock> lockMap = Maps.newConcurrentMap();
    private Object sync = new Object();

    @Override
    public boolean tryLockOrRelock(String lockName, long lockTimeMillis) {
        synchronized (sync) {
            TimeoutLock lock = lockMap.getOrDefault(lockName, new TimeoutLock());
            lockMap.put(lockName, lock);
            if (!lock.isExpired()) {
                if (!lock.isHeldByCurrentThread()) {
                    LOG.debug("cannot lock " + lockName + " because it is held by a different thread");
                    return false;
                }
            }
            lock.setExpiry(lockTimeMillis);
            return true;
        }
    }

    @Override
    public void unlock(String lockName) {
        synchronized (sync) {
            TimeoutLock lock = lockMap.getOrDefault(lockName, null);
            if (lock != null && lock.isHeldByCurrentThread()) {
                lockMap.remove(lockName);
            }
        }
    }

    private static class TimeoutLock {
        private LocalDateTime expiresAt;
        private long threadId;

        public TimeoutLock() {
            expiresAt = LocalDateTime.now();
            threadId = Thread.currentThread().getId();
        }

        public void setExpiry(long millisFromNow) {
            expiresAt = LocalDateTime.now().plus(millisFromNow, ChronoUnit.MILLIS);
        }

        public boolean isHeldByCurrentThread() {
            return threadId == Thread.currentThread().getId();
        }

        public boolean isExpired() {
            return expiresAt.isBefore(LocalDateTime.now());
        }
    }

}

So no real Locks used, the locking happens via the service and the TimeoutLock objects simply keep track of owning thread-id and timeouts. Wrote a few tests for it and so far everything looks good.

Answer 4

You can try putting the timeout in the ReentrantLock's await call for example:

public class MessageUtil {
  private static final Lock lock = new ReentrantLock();

  public enum Conditions {
     BAR_INIT(lock.newCondition()),
     TEST_DELAY(lock.newCondition());

     Condition condition;

     private Conditions(Condition condition) {
         this.condition = condition;
     }
  }

  public static void await(Conditions condition, int timeout) throws Interrupted Exception {
     lock.lock();
     condition.condition.await(timeout, TimeUnit.SECONDS);
     lock.unlock();
  }

  public static void signalAll(Conditions condtition) {
     lock.lock();
     condition.condition.signalAll();
     lock.unlock();
  }
}

This utility class allows you to use the await method to wait for a certain condition, say waiting for the Bar class to finish initializing or waiting for a certain step in a test then use the signalAll method to end the wait condition and resume normal operations.