Correctly implementing wait and notify in Kotlin

Question

According to this document, using wait and notify is discouraged in Kotlin: https://kotlinlang.org/docs/reference/java-interop.html

wait()/notify()

Effective Java Item 69 kindly suggests to prefer concurrency utilities to wait() and notify(). Thus, these methods are not available on references of type Any.

However the document does not propose any correct way of doing it.

Basically, I would like to implement a service, which would read the input data and process them. If there were no input data, it would suspend itself until someone notifies that there are new input data. Something like

while (true) {     val data = fetchData()     processData(data)     if (data.isEmpty()) {         wait()     } } 

EDIT:

I don't want to use these not recommended methods (antipatterns), I really want to find out how to do this properly.

In my case fetchData reads data from the database, so queues in my case cannot be used.

Answer 1

In general you should use higher-level concurrency utilities when possible.

However, if none of the higher-level constructs work in your case, the direct replacement is to use a ReentrantLock and a single Condition on that lock.

For example, if your Java code was something like:

private Object lock = new Object();  ...  synchronized(lock) {     ...     lock.wait();     ...     lock.notify();     ...     lock.notifyAll();     ... } 

You can change it to the following Kotlin:

private val lock = ReentrantLock() private val condition = lock.newCondition()  lock.withLock {           // like synchronized(lock)     ...     condition.await()     // like wait()     ...     condition.signal()    // like notify()     ...     condition.signalAll() // like notifyAll()     ... } 

While this is slightly more verbose, conditions do provide some extra flexibility, as you can have multiple conditions on a single lock, and there are also other kinds of locks (notably ReentrantReadWriteLock.ReadLock and ReentrantReadWriteLock.WriteLock).

Note that withLock is a Kotlin-provided extension function that takes care of calling Lock.lock()/Lock.unlock() before/after invoking the supplied lambda.