Struct actix_web::rt::task::JoinHandle

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pub struct JoinHandle<T> { /* private fields */ }
Expand description

An owned permission to join on a task (await its termination).

This can be thought of as the equivalent of std::thread::JoinHandle for a Tokio task rather than a thread. Note that the background task associated with this JoinHandle started running immediately when you called spawn, even if you have not yet awaited the JoinHandle.

A JoinHandle detaches the associated task when it is dropped, which means that there is no longer any handle to the task, and no way to join on it.

This struct is created by the task::spawn and task::spawn_blocking functions.

§Cancel safety

The &mut JoinHandle<T> type is cancel safe. If it is used as the event in a tokio::select! statement and some other branch completes first, then it is guaranteed that the output of the task is not lost.

If a JoinHandle is dropped, then the task continues running in the background and its return value is lost.

§Examples

Creation from task::spawn:

use tokio::task;

let join_handle: task::JoinHandle<_> = task::spawn(async {
    // some work here
});

Creation from task::spawn_blocking:

use tokio::task;

let join_handle: task::JoinHandle<_> = task::spawn_blocking(|| {
    // some blocking work here
});

The generic parameter T in JoinHandle<T> is the return type of the spawned task. If the return value is an i32, the join handle has type JoinHandle<i32>:

use tokio::task;

let join_handle: task::JoinHandle<i32> = task::spawn(async {
    5 + 3
});

If the task does not have a return value, the join handle has type JoinHandle<()>:

use tokio::task;

let join_handle: task::JoinHandle<()> = task::spawn(async {
    println!("I return nothing.");
});

Note that handle.await doesn’t give you the return type directly. It is wrapped in a Result because panics in the spawned task are caught by Tokio. The ? operator has to be double chained to extract the returned value:

use tokio::task;
use std::io;

#[tokio::main]
async fn main() -> io::Result<()> {
    let join_handle: task::JoinHandle<Result<i32, io::Error>> = tokio::spawn(async {
        Ok(5 + 3)
    });

    let result = join_handle.await??;
    assert_eq!(result, 8);
    Ok(())
}

If the task panics, the error is a JoinError that contains the panic:

use tokio::task;
use std::io;
use std::panic;

#[tokio::main]
async fn main() -> io::Result<()> {
    let join_handle: task::JoinHandle<Result<i32, io::Error>> = tokio::spawn(async {
        panic!("boom");
    });

    let err = join_handle.await.unwrap_err();
    assert!(err.is_panic());
    Ok(())
}

Child being detached and outliving its parent:

use tokio::task;
use tokio::time;
use std::time::Duration;

let original_task = task::spawn(async {
    let _detached_task = task::spawn(async {
        // Here we sleep to make sure that the first task returns before.
        time::sleep(Duration::from_millis(10)).await;
        // This will be called, even though the JoinHandle is dropped.
        println!("♫ Still alive ♫");
    });
});

original_task.await.expect("The task being joined has panicked");
println!("Original task is joined.");

// We make sure that the new task has time to run, before the main
// task returns.

time::sleep(Duration::from_millis(1000)).await;

Implementations§

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impl<T> JoinHandle<T>

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pub fn abort(&self)

Abort the task associated with the handle.

Awaiting a cancelled task might complete as usual if the task was already completed at the time it was cancelled, but most likely it will fail with a cancelled JoinError.

Be aware that tasks spawned using spawn_blocking cannot be aborted because they are not async. If you call abort on a spawn_blocking task, then this will not have any effect, and the task will continue running normally. The exception is if the task has not started running yet; in that case, calling abort may prevent the task from starting.

See also the module level docs for more information on cancellation.

use tokio::time;

let mut handles = Vec::new();

handles.push(tokio::spawn(async {
   time::sleep(time::Duration::from_secs(10)).await;
   true
}));

handles.push(tokio::spawn(async {
   time::sleep(time::Duration::from_secs(10)).await;
   false
}));

for handle in &handles {
    handle.abort();
}

for handle in handles {
    assert!(handle.await.unwrap_err().is_cancelled());
}
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pub fn is_finished(&self) -> bool

Checks if the task associated with this JoinHandle has finished.

Please note that this method can return false even if abort has been called on the task. This is because the cancellation process may take some time, and this method does not return true until it has completed.

use tokio::time;

let handle1 = tokio::spawn(async {
    // do some stuff here
});
let handle2 = tokio::spawn(async {
    // do some other stuff here
    time::sleep(time::Duration::from_secs(10)).await;
});
// Wait for the task to finish
handle2.abort();
time::sleep(time::Duration::from_secs(1)).await;
assert!(handle1.is_finished());
assert!(handle2.is_finished());
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pub fn abort_handle(&self) -> AbortHandle

Returns a new AbortHandle that can be used to remotely abort this task.

Awaiting a task cancelled by the AbortHandle might complete as usual if the task was already completed at the time it was cancelled, but most likely it will fail with a cancelled JoinError.

use tokio::{time, task};

let mut handles = Vec::new();

handles.push(tokio::spawn(async {
   time::sleep(time::Duration::from_secs(10)).await;
   true
}));

handles.push(tokio::spawn(async {
   time::sleep(time::Duration::from_secs(10)).await;
   false
}));

let abort_handles: Vec<task::AbortHandle> = handles.iter().map(|h| h.abort_handle()).collect();

for handle in abort_handles {
    handle.abort();
}

for handle in handles {
    assert!(handle.await.unwrap_err().is_cancelled());
}

Trait Implementations§

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impl<T> Debug for JoinHandle<T>
where T: Debug,

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fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more
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impl<T> Drop for JoinHandle<T>

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fn drop(&mut self)

Executes the destructor for this type. Read more
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impl<T> Future for JoinHandle<T>

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type Output = Result<T, JoinError>

The type of value produced on completion.
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fn poll( self: Pin<&mut JoinHandle<T>>, cx: &mut Context<'_>, ) -> Poll<<JoinHandle<T> as Future>::Output>

Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available. Read more
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impl<T> RefUnwindSafe for JoinHandle<T>

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impl<T> Send for JoinHandle<T>
where T: Send,

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impl<T> Sync for JoinHandle<T>
where T: Send,

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impl<T> Unpin for JoinHandle<T>

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impl<T> UnwindSafe for JoinHandle<T>

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impl<T> Freeze for JoinHandle<T>

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impl<T> Any for T
where T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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where T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for T
where T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T> FutureExt for T
where T: Future + ?Sized,

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fn map<U, F>(self, f: F) -> Map<Self, F>
where F: FnOnce(Self::Output) -> U, Self: Sized,

Map this future’s output to a different type, returning a new future of the resulting type. Read more
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Map this future’s output to a different type, returning a new future of the resulting type. Read more
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fn then<Fut, F>(self, f: F) -> Then<Self, Fut, F>
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Chain on a computation for when a future finished, passing the result of the future to the provided closure f. Read more
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where A: Future<Output = Self::Output>, Self: Sized,

Wrap this future in an Either future, making it the right-hand variant of that Either. Read more
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where Self: Sized,

Convert this future into a single element stream. Read more
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Flatten the execution of this future when the output of this future is itself another future. Read more
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where Self::Output: Stream, Self: Sized,

Flatten the execution of this future when the successful result of this future is a stream. Read more
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where Self: Sized,

Fuse a future such that poll will never again be called once it has completed. This method can be used to turn any Future into a FusedFuture. Read more
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where Self: Sized + UnwindSafe,

Catches unwinding panics while polling the future. Read more
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where Self: Sized, Self::Output: Clone,

Create a cloneable handle to this future where all handles will resolve to the same result. Read more
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fn remote_handle(self) -> (Remote<Self>, RemoteHandle<Self::Output>)
where Self: Sized,

Turn this future into a future that yields () on completion and sends its output to another future on a separate task. Read more
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fn poll_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Self::Output>
where Self: Unpin,

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Evaluates and consumes the future, returning the resulting output if the future is ready after the first call to Future::poll. Read more
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type Ok = T

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