What does the suspend function mean in a Kotlin Coroutine?

Question

I'm reading Kotlin Coroutine and know that it is based on suspend function. But what does suspend mean?

Coroutine or function gets suspended?

From https://kotlinlang.org/docs/reference/coroutines.html

Basically, coroutines are computations that can be suspended without blocking a thread

I heard people often say "suspend function". But I think it is the coroutine who gets suspended because it is waiting for the function to finished? "suspend" usually means "cease operation", in this case the coroutine is idle.

Should we say the coroutine is suspended ?

Which coroutine gets suspended?

From https://kotlinlang.org/docs/reference/coroutines.html

To continue the analogy, await() can be a suspending function (hence also callable from within an async {} block) that suspends a coroutine until some computation is done and returns its result:

async { // Here I call it the outer async coroutine
    ...
    // Here I call computation the inner coroutine
    val result = computation.await()
    ...
}

It says "that suspends a coroutine until some computation is done", but coroutine is like a lightweight thread. So if the coroutine is suspended, how can the computation is done ?

We see await is called on computation, so it might be async that returns Deferred, which means it can start another coroutine

fun computation(): Deferred<Boolean> {
    return async {
        true
    }
}

The quote say that suspends a coroutine. Does it mean suspend the outer async coroutine, or suspend the inner computation coroutine?

Does suspend mean that while outer async coroutine is waiting (await) for the inner computation coroutine to finish, it (the outer async coroutine) idles (hence the name suspend) and returns thread to the thread pool, and when the child computation coroutine finishes, it (the outer async coroutine) wakes up, takes another thread from the pool and continues?

The reason I mention the thread is because of https://kotlinlang.org/docs/tutorials/coroutines-basic-jvm.html

The thread is returned to the pool while the coroutine is waiting, and when the waiting is done, the coroutine resumes on a free thread in the pool

Answer 1

Suspending functions are at the center of everything coroutines. A suspending function is simply a function that can be paused and resumed at a later time. They can execute a long running operation and wait for it to complete without blocking.

The syntax of a suspending function is similar to that of a regular function except for the addition of the suspend keyword. It can take a parameter and have a return type. However, suspending functions can only be invoked by another suspending function or within a coroutine.

suspend fun backgroundTask(param: Int): Int {
     // long running operation
}

Under the hood, suspend functions are converted by the compiler to another function without the suspend keyword, that takes an addition parameter of type Continuation<T>. The function above for example, will be converted by the compiler to this:

fun backgroundTask(param: Int, callback: Continuation<Int>): Int {
   // long running operation
}

Continuation<T> is an interface that contains two functions that are invoked to resume the coroutine with a return value or with an exception if an error had occurred while the function was suspended.

interface Continuation<in T> {
   val context: CoroutineContext
   fun resume(value: T)
   fun resumeWithException(exception: Throwable)
}

Answer 2

First of all, the best source to understand this IMO is the talk "Deep Dive into Coroutines" by Roman Elizarov.

Coroutine or function gets suspended?

Calling a suspending function suspends the coroutine, meaning the current thread can start executing another coroutine. So, the coroutine is said to be suspended rather than the function.

In fact, call sites of suspending functions are called "suspension points" for this reason.

Which coroutine gets suspended?

Let's look at your code and break down what happens:

// 1. this call starts a new coroutine (let's call it C1).
//    If there were code after it, it would be executed concurrently with
//    the body of this async
async {
    ...
    // 2. this is a regular function call, so we go to computation()'s body
    val deferred = computation()
    // 4. because await() is suspendING, it suspends coroutine C1.
    //    This means that if we had a single thread in our dispatcher, 
    //    it would now be free to go execute C2
    // 7. once C2 completes, C1 is resumed with the result `true` of C2's async
    val result = deferred.await() 
    ...
    // 8. C1 can now keep going in the current thread until it gets 
    //    suspended again (or not)
}

fun computation(): Deferred<Boolean> {
    // 3. this async call starts a second coroutine (C2). Depending on the 
    //    dispatcher you're using, you may have one or more threads.
    // 3.a. If you have multiple threads, the block of this async could be
    //      executed in parallel of C1 in another thread. The control flow 
    //      of the current thread returns to the caller of computation().
    // 3.b. If you have only one thread, the block is sort of "queued" but 
    //      not executed right away, and the control flow returns to the 
    //      caller of computation(). (unless a special dispatcher or 
    //      coroutine start argument is used, but let's keep it simple).
    //    In both cases, we say that this block executes "concurrently"
    //    with C1.
    return async {
        // 5. this may now be executed
        true
        // 6. C2 is now completed, so the thread can go back to executing 
        //    another coroutine (e.g. C1 here)
    }
}

The outer async starts a coroutine. When it calls computation(), the inner async starts a second coroutine. Then, the call to await() suspends the execution of the outer async coroutine, until the execution of the inner async's coroutine is over.

You can even see that with a single thread: the thread will execute the outer async's beginning, then call computation() and reach the inner async. At this point, the body of the inner async is skipped, and the thread continues executing the outer async until it reaches await(). await() is a "suspension point", because await is a suspending function. This means that the outer coroutine is suspended, and thus the thread starts executing the inner one. When it is done, it comes back to execute the end of the outer async.

Does suspend mean that while outer async coroutine is waiting (await) for the inner computation coroutine to finish, it (the outer async coroutine) idles (hence the name suspend) and returns thread to the thread pool, and when the child computation coroutine finishes, it (the outer async coroutine) wakes up, takes another thread from the pool and continues?

Yes, precisely.

The way this is actually achieved is by turning every suspending function into a state machine, where each "state" corresponds to a suspension point inside this suspend function. Under the hood, the function can be called multiple times, with the information about which suspension point it should start executing from (you should really watch the video I linked for more info about that).

Answer 3

To understand what exactly it means to suspend a coroutine, I suggest you go through this code:

import kotlinx.coroutines.Dispatchers.Unconfined
import kotlinx.coroutines.launch
import kotlinx.coroutines.runBlocking
import kotlin.coroutines.Continuation
import kotlin.coroutines.resume
import kotlin.coroutines.suspendCoroutine

var continuation: Continuation<Int>? = null

fun main() = runBlocking {
    launch(Unconfined) {
        val a = a()
        println("Result is $a")
    }
    10.downTo(0).forEach {
        continuation!!.resume(it)
    }
}

suspend fun a(): Int {
    return b()
}

suspend fun b(): Int {
    while (true) {
        val i = suspendCoroutine<Int> { cont -> continuation = cont }
        if (i == 0) {
            return 0
        }
    }
}

The Unconfined coroutine dispatcher eliminates the magic of coroutine dispatching and allows us to focus directly on bare coroutines.

The code inside the launch block starts executing right away on the current thread, as a part of the launch call. What happens is as follows:

1. Evaluate val a = a()
2. This chains to b(), reaching suspendCoroutine.
3. Function b() executes the block passed to suspendCoroutine and then returns a special COROUTINE_SUSPENDED value. This value is not observable through the Kotlin programming model, but that's what the compiled Java method does.
4. Function a(), seeing this return value, itself also returns it.
5. The launch block does the same and control now returns to the line after the launch invocation: 10.downTo(0)...

Note that, at this point, you have the same effect as if the code inside the launch block and your fun main code are executing concurrently. It just happens that all this is happening on a single native thread so the launch block is "suspended".

Now, inside the forEach looping code, the program reads the continuation that the b() function wrote and resumes it with the value of 10. resume() is implemented in such a way that it will be as if the suspendCoroutine call returned with the value you passed in. So you suddenly find yourself in the middle of executing b(). The value you passed to resume() gets assigned to i and checked against 0. If it's not zero, the while (true) loop goes on inside b(), again reaching suspendCoroutine, at which point your resume() call returns, and now you go through another looping step in forEach(). This goes on until finally you resume with 0, then the println statement runs and the program completes.

The above analysis should give you the important intuition that "suspending a coroutine" means returning the control back to the innermost launch invocation (or, more generally, coroutine builder). If a coroutine suspends again after resuming, the resume() call ends and control returns to the caller of resume().

The presence of a coroutine dispatcher makes this reasoning less clear-cut because most of them immediately submit your code to another thread. In that case the above story happens in that other thread, and the coroutine dispatcher also manages the continuation object so it can resume it when the return value is available.

Answer 4

As many good answers are already there, I would like to post a simpler example for others.

runBlocking use case :

• myMethod() is suspend function
• runBlocking { } starts a Coroutine in blocking way. It is similar to how we were blocking normal threads with Thread class and notifying blocked threads after certain events.

• runBlocking { } does block the current executing thread, until the coroutine (body between {}) gets completed

   override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.main_activity)
      Log.i(TAG,"Outer code started on Thread : " + Thread.currentThread().name);
      runBlocking {
          Log.d(TAG,"Inner code started  on Thread : " + Thread.currentThread().name + " making outer code suspend");
          myMethod();
      }
      Log.i(TAG,"Outer code resumed on Thread : " + Thread.currentThread().name);
  }
  
  private suspend fun myMethod() {
      withContext(Dispatchers.Default) {
      for(i in 1..5) {
          Log.d(TAG,"Inner code i : $i on Thread : " + Thread.currentThread().name);
      }
  }
  

This outputs :

I/TAG: Outer code started on Thread : main
D/TAG: Inner code started  on Thread : main making outer code suspend
// ---- main thread blocked here, it will wait until coroutine gets completed ----
D/TAG: Inner code i : 1 on Thread : DefaultDispatcher-worker-2
D/TAG: Inner code i : 2 on Thread : DefaultDispatcher-worker-2
D/TAG: Inner code i : 3 on Thread : DefaultDispatcher-worker-2
D/TAG: Inner code i : 4 on Thread : DefaultDispatcher-worker-2
D/TAG: Inner code i : 5 on Thread : DefaultDispatcher-worker-2
// ---- main thread resumes as coroutine is completed ----
I/TAG: Outer code resumed on Thread : main

launch use case :

• launch { } starts a coroutine concurrently.
• This means that when we specify launch, a coroutine starts execution on worker thread.
• The worker thread and outer thread (from which we called launch { }) both runs concurrently. Internally, JVM may perform Preemptive Threading

• When we require multiple tasks to run in parallel, we can use this. There are scopes which specify lifetime of coroutine. If we specify GlobalScope, the coroutine will work until application lifetime ends.

  override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.main_activity)
      Log.i(TAG,"Outer code started on Thread : " + Thread.currentThread().name);
  
      GlobalScope.launch(Dispatchers.Default) {
          Log.d(TAG,"Inner code started  on Thread : " + Thread.currentThread().name + " making outer code suspend");
          myMethod();
      }
      Log.i(TAG,"Outer code resumed on Thread : " + Thread.currentThread().name);
  }
  
  private suspend fun myMethod() {
      withContext(Dispatchers.Default) {
          for(i in 1..5) {
              Log.d(TAG,"Inner code i : $i on Thread : " + Thread.currentThread().name);
          }
      }
  }
  

This Outputs :

10806-10806/com.example.viewmodelapp I/TAG: Outer code started on Thread : main
10806-10806/com.example.viewmodelapp I/TAG: Outer code resumed on Thread : main
// ---- In this example, main had only 2 lines to execute. So, worker thread logs start only after main thread logs complete
// ---- In some cases, where main has more work to do, the worker thread logs get overlap with main thread logs
10806-10858/com.example.viewmodelapp D/TAG: Inner code started  on Thread : DefaultDispatcher-worker-1 making outer code suspend
10806-10858/com.example.viewmodelapp D/TAG: Inner code i : 1 on Thread : DefaultDispatcher-worker-1
10806-10858/com.example.viewmodelapp D/TAG: Inner code i : 2 on Thread : DefaultDispatcher-worker-1
10806-10858/com.example.viewmodelapp D/TAG: Inner code i : 3 on Thread : DefaultDispatcher-worker-1
10806-10858/com.example.viewmodelapp D/TAG: Inner code i : 4 on Thread : DefaultDispatcher-worker-1
10806-10858/com.example.viewmodelapp D/TAG: Inner code i : 5 on Thread : DefaultDispatcher-worker-1

async and await use case :

• When we have multiple tasks to do and they depend on other's completion, async and await would help.
• For example, in below code, there are 2 suspend functions myMethod() and myMethod2(). myMethod2() should get executed only after full completion of myMethod() OR myMethod2() depends on result of myMethod(), we can use async and await
• async starts a coroutine in parallel similar to launch. But, it provides a way to wait for one coroutine before starting another coroutine in parallel.

• That way is await(). async returns an instance of Deffered<T>. T would be Unit for default. When we need to wait for any async's completion, we need to call .await() on Deffered<T> instance of that async. Like in below example, we called innerAsync.await() which implies that the execution would get suspended until innerAsync gets completed. We can observe the same in output. The innerAsync gets completed first, which calls myMethod(). And then next async innerAsync2 starts, which calls myMethod2()

  override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.main_activity)
      Log.i(TAG,"Outer code started on Thread : " + Thread.currentThread().name);
  
       job = GlobalScope.launch(Dispatchers.Default) {
           innerAsync = async {
               Log.d(TAG, "Inner code started  on Thread : " + Thread.currentThread().name + " making outer code suspend");
               myMethod();
           }
           innerAsync.await()
  
           innerAsync2 = async {
               Log.w(TAG, "Inner code started  on Thread : " + Thread.currentThread().name + " making outer code suspend");
               myMethod2();
           }
      }
  
      Log.i(TAG,"Outer code resumed on Thread : " + Thread.currentThread().name);
      }
  
  private suspend fun myMethod() {
      withContext(Dispatchers.Default) {
          for(i in 1..5) {
              Log.d(TAG,"Inner code i : $i on Thread : " + Thread.currentThread().name);
          }
      }
  }
  
  private suspend fun myMethod2() {
      withContext(Dispatchers.Default) {
          for(i in 1..10) {
              Log.w(TAG,"Inner code i : $i on Thread : " + Thread.currentThread().name);
          }
      }
  }
  

This outputs :

11814-11814/? I/TAG: Outer code started on Thread : main
11814-11814/? I/TAG: Outer code resumed on Thread : main
11814-11845/? D/TAG: Inner code started  on Thread : DefaultDispatcher-worker-2 making outer code suspend
11814-11845/? D/TAG: Inner code i : 1 on Thread : DefaultDispatcher-worker-2
11814-11845/? D/TAG: Inner code i : 2 on Thread : DefaultDispatcher-worker-2
11814-11845/? D/TAG: Inner code i : 3 on Thread : DefaultDispatcher-worker-2
11814-11845/? D/TAG: Inner code i : 4 on Thread : DefaultDispatcher-worker-2
11814-11845/? D/TAG: Inner code i : 5 on Thread : DefaultDispatcher-worker-2
// ---- Due to await() call, innerAsync2 will start only after innerAsync gets completed
11814-11848/? W/TAG: Inner code started  on Thread : DefaultDispatcher-worker-4 making outer code suspend
11814-11848/? W/TAG: Inner code i : 1 on Thread : DefaultDispatcher-worker-4
11814-11848/? W/TAG: Inner code i : 2 on Thread : DefaultDispatcher-worker-4
11814-11848/? W/TAG: Inner code i : 3 on Thread : DefaultDispatcher-worker-4
11814-11848/? W/TAG: Inner code i : 4 on Thread : DefaultDispatcher-worker-4
11814-11848/? W/TAG: Inner code i : 5 on Thread : DefaultDispatcher-worker-4
11814-11848/? W/TAG: Inner code i : 6 on Thread : DefaultDispatcher-worker-4
11814-11848/? W/TAG: Inner code i : 7 on Thread : DefaultDispatcher-worker-4
11814-11848/? W/TAG: Inner code i : 8 on Thread : DefaultDispatcher-worker-4
11814-11848/? W/TAG: Inner code i : 9 on Thread : DefaultDispatcher-worker-4
11814-11848/? W/TAG: Inner code i : 10 on Thread : DefaultDispatcher-worker-4

Answer 5

I've found that the best way to understand suspend is to make an analogy between this keyword and coroutineContext property.

Kotlin functions can be declared as local or global. Local functions magically have access to this keyword while global don't.

Kotlin functions can be declared as suspend or blocking. suspend functions magically have access to coroutineContext property while blocking functions don't.

The thing is: coroutineContext property is declared like a "normal" property in Kotlin stdlib but this declaration is just a stub for documentation/navigation purposes. In fact coroutineContext is builtin intrinsic property that means under the hood compiler magic aware of this property like it aware of language keywords.

What this keyword does for local functions is what coroutineContext property does for suspend functions: it gives access to current context of execution.

So, you need suspend to get an access to coroutineContext property - the instance of currently executed coroutine context

Answer 6

I wanted to give you a simple example of the concept of continuation. This is what a suspend function does, it can freeze/suspend and then it continues/resumes. Stop thinking of coroutine in terms of threads and Semaphore. Think of it in terms of continuation and even callback hooks.

To be clear, a coroutine can be paused by using a suspend function. lets investigate this:

In android we could do this for example :

var TAG = "myTAG:"
        fun myMethod() { // function A in image
            viewModelScope.launch(Dispatchers.Default) {
                for (i in 10..15) {
                    if (i == 10) { //on first iteration, we will completely FREEZE this coroutine (just for loop here gets 'suspended`)
                        println("$TAG im a tired coroutine - let someone else print the numbers async. i'll suspend until your done")
                        freezePleaseIAmDoingHeavyWork()
                    } else
                        println("$TAG $i")
                    }
            }

            //this area is not suspended, you can continue doing work
        }


        suspend fun freezePleaseIAmDoingHeavyWork() { // function B in image
            withContext(Dispatchers.Default) {
                async {
                    //pretend this is a big network call
                    for (i in 1..10) {
                        println("$TAG $i")
                        delay(1_000)//delay pauses coroutine, NOT the thread. use  Thread.sleep if you want to pause a thread. 
                    }
                    println("$TAG phwww finished printing those numbers async now im tired, thank you for freezing, you may resume")
                }
            }
        }

Above code prints the following :

I: myTAG: my coroutine is frozen but i can carry on to do other things

I: myTAG: im a tired coroutine - let someone else print the numbers async. i'll suspend until your done

I: myTAG: 1
I: myTAG: 2
I: myTAG: 3
I: myTAG: 4
I: myTAG: 5
I: myTAG: 6
I: myTAG: 7
I: myTAG: 8
I: myTAG: 9
I: myTAG: 10

I: myTAG: phwww finished printing those numbers async now im tired, thank you for freezing, you may resume

I: myTAG: 11
I: myTAG: 12
I: myTAG: 13
I: myTAG: 14
I: myTAG: 15

imagine it working like this:

So the current function you launched from does not stop, just a coroutine would suspend while it continues. The thread is not paused by running a suspend function.

I think this site can help you straight things out and is my reference.

Let's do something cool and freeze our suspend function in the middle of an iteration. We will resume it later in onResume

Store a variable called continuation and we'll load it with the coroutines continuation object for us :

var continuation: CancellableContinuation<String>? = null

suspend fun freezeHere() = suspendCancellableCoroutine<String> {
            continuation = it
        }

 fun unFreeze() {
            continuation?.resume("im resuming") {}
        }

Now, let's return to our suspended function and make it freeze in middle of iteration :

 suspend fun freezePleaseIAmDoingHeavyWork() {
        withContext(Dispatchers.Default) {
            async {
                //pretend this is a big network call
                for (i in 1..10) {
                    println("$TAG $i")
                    delay(1_000)
                    if(i == 3)
                        freezeHere() //dead pause, do not go any further
                }
            }
        }
    }

Then somewhere else like in onResume (for example):

override fun onResume() {
        super.onResume()
        unFreeze()
    }

And the loop will continue. Its pretty neat to know we can freeze a suspend function at any point and resume it after some time has beeb passed. You can also look into channels

Answer 7

There are a lot of great answers here, but I think there are two additional things that are important to note.

launch / withContext / runBlocking and a lot of other things in the examples are from the coroutines library. which actually have nothing to do with suspend. you don't need the coroutines library to use coroutines. Coroutines are a compiler "trick". Yes, the library sure makes things easier, but the compiler is doing the magic of suspending & resuming things.

The second thing, is the compiler is just taking code that looks procedural and turning it into callbacks under the hood.

Take the following minimal coroutine that suspends that does not use the coroutine library :

lateinit var context: Continuation<Unit>

    suspend {
        val extra="extra"
        println("before suspend $extra")
        suspendCoroutine<Unit> { context = it }
        println("after suspend $extra")
    }.startCoroutine(
        object : Continuation<Unit> {
            override val context: CoroutineContext = EmptyCoroutineContext
            // called when a coroutine ends. do nothing.
            override fun resumeWith(result: Result<Unit>) {
                result.onFailure { ex : Throwable -> throw ex }
            }
        }
    )

    println("kick it")
    context.resume(Unit)

I think an important way to understand it is to look at what the compiler does with this code. effectively it creates a class for the lambda. it creates a property in the class for the "extra" string, then it creates two function, one that prints the "before" and another the prints the "after".

Effectively the compiler took what looks like procedural code and turned it into callbacks.

So what does the suspend keyword do? It tell the compiler how far back to look for context that the generated callbacks will need. The compiler needs to know which variables are used in which "callbacks", and the suspend keyword helps it. In this example the "extra" variable is used both before and after the suspend. So it needs to be pulled out to a property of the class containing the callbacks the compiler makes.

It also tells the compiler that this is the "beginning" of state and to prepare to split up the following code into callbacks. The startCoroutine only exists on suspend lambda.

The actual Java code generated by the Kotlin compiler is here. It's a switch statement instead of callbacks, but it's effectively the same thing. Called first w/ case 0, then w/ case 1 after the resume.

            @Nullable
            public final Object invokeSuspend(@NotNull Object $result) {
                var10_2 = IntrinsicsKt.getCOROUTINE_SUSPENDED();
                switch (this.label) {
                    case 0: {
                        ResultKt.throwOnFailure((Object)$result);
                        extra = "extra";
                        var3_4 = "before delay " + extra;
                        var4_9 = false;
                        System.out.println((Object)var3_4);
                        var3_5 = this;
                        var4_9 = false;
                        var5_10 = false;
                        this.L$0 = extra;
                        this.L$1 = var3_5;
                        this.label = 1;
                        var5_11 = var3_5;
                        var6_12 = false;
                        var7_13 = new SafeContinuation(IntrinsicsKt.intercepted((Continuation)var5_11));
                        it = (Continuation)var7_13;
                        $i$a$-suspendCoroutine-AppKt$main$1$1 = false;
                        this.$context.element = it;
                        v0 = var7_13.getOrThrow();
                        if (v0 == IntrinsicsKt.getCOROUTINE_SUSPENDED()) {
                            DebugProbesKt.probeCoroutineSuspended((Continuation)var3_5);
                        }
                        v1 = v0;
                        if (v0 == var10_2) {
                            return var10_2;
                        }
                        ** GOTO lbl33
                    }
                    case 1: {
                        var3_6 = this.L$1;
                        extra = (String)this.L$0;
                        ResultKt.throwOnFailure((Object)$result);
                        v1 = $result;
lbl33:
                        // 2 sources

                        var3_8 = "after suspend " + extra;
                        var4_9 = false;
                        System.out.println((Object)var3_8);
                        return Unit.INSTANCE;
                    }
                }
                throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");
            }

Answer 8

Anyone still stumbling upon this question, I would recommend taking a quick look at this. I've read a lot of misleading answers to this question, even some top rated. This cleared a lot of doubts I had.

https://youtu.be/BOHK_w09pVA?t=577