About the relationship between coroutines, threads and concurrency issues

« - », ? - ? . , Android, , , Java, .

, . Android, , , . , :

// ,   10-      
someScope.launch(Dispatchers.Default) {
   val fibonacci10 = synchronousFibonacci(10)
   saveFibonacciInMemory(10, fibonacci10)
}

private fun synchronousFibonacci(n: Long): Long { /* … */ }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

(async



) , , (thread pool), , Dispatchers.Default



. - .

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:

//    4 
val executorService = Executors.newFixedThreadPool(4)

//          
executorService.execute {
   val fibonacci10 = synchronousFibonacci(10)
   saveFibonacciInMemory(10, fibonacci10)
}

      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, Android , , , Jetpack.

? , , CoroutineDispatcher ; Dispatchers.Default



.

CoroutineDispatcher . , CoroutineDispatcher



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( ) DispatchedContinuation. , CoroutineDispatcher



ContinuationInterceptor .

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resumeWith DispatchedContinuation



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, DispatchedContinuation DispatchedTask, Java, , Runnable



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CoroutineStart. , , , CoroutineStart.LAZY



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,

, CoroutineDispatcher



Executor.asCoroutineDispatcher(). (Dispatchers), .

, Dispatchers.Default



createDefaultDispatcher. DefaultScheduler. Dispatchers.IO, DefaultScheduler



64 . Dispatchers.Default



Dispatchers.IO



, , . withContext



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withContext

Android , , . ! , , . , , . , - withContext



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, , . , , , , :

class TransactionsRepository(
  private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default
) {

  private val transactionsCache = mutableMapOf<User, List<Transaction>()

  private suspend fun addTransaction(user: User, transaction: Transaction) =
   // !     .
   //   :     
   //     .
    withContext(defaultDispatcher) {
      if (transactionsCache.contains(user)) {
        val oldList = transactionsCache[user]
        val newList = oldList!!.toMutableList()
        newList.add(transaction)
        transactionsCache.put(user, newList)
      } else {
        transactionsCache.put(user, listOf(transaction))
      }
    }
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

Kotlin, « Java » - , Java. , Jetbrains .

, . , , , API . , API, , .

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/ . / (producer/consumer) . JetBrains .

Android , . , AtomicInteger. , map , ConcurrentHashMap. ConcurrentHashMap



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, , . , . , transactionCache



, , .

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, , @Volatile ! , @Synchronized , .

, (latches), (semaphores) (barriers). (locks) (mutexes).

Mutex lock unlock, . , Mutex.withLock :

class TransactionsRepository(
  private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default
) {

  // ,    
  private val cacheMutex = Mutex()
  private val transactionsCache = mutableMapOf<User, List<Transaction>()

  private suspend fun addTransaction(user: User, transaction: Transaction) =
    withContext(defaultDispatcher) {
      // ,    
      cacheMutex.withLock {
        if (transactionsCache.contains(user)) {
          val oldList = transactionsCache[user]
          val newList = oldList!!.toMutableList()
          newList.add(transaction)
          transactionsCache.put(user, newList)
        } else {
          transactionsCache.put(user, listOf(transaction))
        }
      }
    }
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, , , Mutex



, , Java, . Java , , , .

, , . , Android . . !

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