Struct actix_web::rt::task::JoinHandle

source ·
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§

source§

impl<T> JoinHandle<T>

source

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.

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());
}
source

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());
source

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§

source§

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

source§

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

Formats the value using the given formatter. Read more
source§

impl<T> Drop for JoinHandle<T>

source§

fn drop(&mut self)

Executes the destructor for this type. Read more
source§

impl<T> Future for JoinHandle<T>

§

type Output = Result<T, JoinError>

The type of value produced on completion.
source§

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
source§

impl<T> RefUnwindSafe for JoinHandle<T>

source§

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

source§

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

source§

impl<T> Unpin for JoinHandle<T>

source§

impl<T> UnwindSafe for JoinHandle<T>

Blanket Implementations§

source§

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

source§

fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
source§

impl<T> Borrow<T> for Twhere T: ?Sized,

source§

fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source§

impl<T> BorrowMut<T> for Twhere T: ?Sized,

source§

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

Mutably borrows from an owned value. Read more
source§

impl<T> From<T> for T

source§

fn from(t: T) -> T

Returns the argument unchanged.

source§

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

source§

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
source§

fn map_into<U>(self) -> MapInto<Self, U>where Self::Output: Into<U>, Self: Sized,

Map this future’s output to a different type, returning a new future of the resulting type. Read more
source§

fn then<Fut, F>(self, f: F) -> Then<Self, Fut, F>where F: FnOnce(Self::Output) -> Fut, Fut: Future, Self: Sized,

Chain on a computation for when a future finished, passing the result of the future to the provided closure f. Read more
source§

fn left_future<B>(self) -> Either<Self, B>where B: Future<Output = Self::Output>, Self: Sized,

Wrap this future in an Either future, making it the left-hand variant of that Either. Read more
source§

fn right_future<A>(self) -> Either<A, Self>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
source§

fn into_stream(self) -> IntoStream<Self>where Self: Sized,

Convert this future into a single element stream. Read more
source§

fn flatten(self) -> Flatten<Self>where Self::Output: Future, Self: Sized,

Flatten the execution of this future when the output of this future is itself another future. Read more
source§

fn flatten_stream(self) -> FlattenStream<Self>where Self::Output: Stream, Self: Sized,

Flatten the execution of this future when the successful result of this future is a stream. Read more
source§

fn fuse(self) -> Fuse<Self>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
source§

fn inspect<F>(self, f: F) -> Inspect<Self, F>where F: FnOnce(&Self::Output), Self: Sized,

Do something with the output of a future before passing it on. Read more
source§

fn catch_unwind(self) -> CatchUnwind<Self>where Self: Sized + UnwindSafe,

Catches unwinding panics while polling the future. Read more
source§

fn shared(self) -> Shared<Self>where Self: Sized, Self::Output: Clone,

Create a cloneable handle to this future where all handles will resolve to the same result. Read more
source§

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
source§

fn boxed<'a>( self ) -> Pin<Box<dyn Future<Output = Self::Output> + Send + 'a, Global>>where Self: Sized + Send + 'a,

Wrap the future in a Box, pinning it. Read more
source§

fn boxed_local<'a>( self ) -> Pin<Box<dyn Future<Output = Self::Output> + 'a, Global>>where Self: Sized + 'a,

Wrap the future in a Box, pinning it. Read more
source§

fn unit_error(self) -> UnitError<Self>where Self: Sized,

source§

fn never_error(self) -> NeverError<Self>where Self: Sized,

source§

fn poll_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Self::Output>where Self: Unpin,

A convenience for calling Future::poll on Unpin future types.
source§

fn now_or_never(self) -> Option<Self::Output>where Self: Sized,

Evaluates and consumes the future, returning the resulting output if the future is ready after the first call to Future::poll. Read more
source§

impl<T> Instrument for T

source§

fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided Span, returning an Instrumented wrapper. Read more
source§

fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current Span, returning an Instrumented wrapper. Read more
source§

impl<T, U> Into<U> for Twhere U: From<T>,

source§

fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

source§

impl<F> IntoFuture for Fwhere F: Future,

§

type Output = <F as Future>::Output

The output that the future will produce on completion.
§

type IntoFuture = F

Which kind of future are we turning this into?
source§

fn into_future(self) -> <F as IntoFuture>::IntoFuture

Creates a future from a value. Read more
source§

impl<T> Same<T> for T

§

type Output = T

Should always be Self
source§

impl<T, U> TryFrom<U> for Twhere U: Into<T>,

§

type Error = Infallible

The type returned in the event of a conversion error.
source§

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
source§

impl<F, T, E> TryFuture for Fwhere F: Future<Output = Result<T, E>> + ?Sized,

§

type Ok = T

The type of successful values yielded by this future
§

type Error = E

The type of failures yielded by this future
source§

fn try_poll( self: Pin<&mut F>, cx: &mut Context<'_> ) -> Poll<<F as Future>::Output>

Poll this TryFuture as if it were a Future. Read more
source§

impl<Fut> TryFutureExt for Futwhere Fut: TryFuture + ?Sized,

source§

fn flatten_sink<Item>(self) -> FlattenSink<Self, Self::Ok>where Self::Ok: Sink<Item, Error = Self::Error>, Self: Sized,

Flattens the execution of this future when the successful result of this future is a Sink. Read more
source§

fn map_ok<T, F>(self, f: F) -> MapOk<Self, F>where F: FnOnce(Self::Ok) -> T, Self: Sized,

Maps this future’s success value to a different value. Read more
source§

fn map_ok_or_else<T, E, F>(self, e: E, f: F) -> MapOkOrElse<Self, F, E>where F: FnOnce(Self::Ok) -> T, E: FnOnce(Self::Error) -> T, Self: Sized,

Maps this future’s success value to a different value, and permits for error handling resulting in the same type. Read more
source§

fn map_err<E, F>(self, f: F) -> MapErr<Self, F>where F: FnOnce(Self::Error) -> E, Self: Sized,

Maps this future’s error value to a different value. Read more
source§

fn err_into<E>(self) -> ErrInto<Self, E>where Self: Sized, Self::Error: Into<E>,

Maps this future’s Error to a new error type using the Into trait. Read more
source§

fn ok_into<U>(self) -> OkInto<Self, U>where Self: Sized, Self::Ok: Into<U>,

Maps this future’s Ok to a new type using the Into trait.
source§

fn and_then<Fut, F>(self, f: F) -> AndThen<Self, Fut, F>where F: FnOnce(Self::Ok) -> Fut, Fut: TryFuture<Error = Self::Error>, Self: Sized,

Executes another future after this one resolves successfully. The success value is passed to a closure to create this subsequent future. Read more
source§

fn or_else<Fut, F>(self, f: F) -> OrElse<Self, Fut, F>where F: FnOnce(Self::Error) -> Fut, Fut: TryFuture<Ok = Self::Ok>, Self: Sized,

Executes another future if this one resolves to an error. The error value is passed to a closure to create this subsequent future. Read more
source§

fn inspect_ok<F>(self, f: F) -> InspectOk<Self, F>where F: FnOnce(&Self::Ok), Self: Sized,

Do something with the success value of a future before passing it on. Read more
source§

fn inspect_err<F>(self, f: F) -> InspectErr<Self, F>where F: FnOnce(&Self::Error), Self: Sized,

Do something with the error value of a future before passing it on. Read more
source§

fn try_flatten(self) -> TryFlatten<Self, Self::Ok>where Self::Ok: TryFuture<Error = Self::Error>, Self: Sized,

Flatten the execution of this future when the successful result of this future is another future. Read more
source§

fn try_flatten_stream(self) -> TryFlattenStream<Self>where Self::Ok: TryStream<Error = Self::Error>, Self: Sized,

Flatten the execution of this future when the successful result of this future is a stream. Read more
source§

fn unwrap_or_else<F>(self, f: F) -> UnwrapOrElse<Self, F>where Self: Sized, F: FnOnce(Self::Error) -> Self::Ok,

Unwraps this future’s output, producing a future with this future’s Ok type as its Output type. Read more
source§

fn into_future(self) -> IntoFuture<Self>where Self: Sized,

Wraps a TryFuture into a type that implements Future. Read more
source§

fn try_poll_unpin( &mut self, cx: &mut Context<'_> ) -> Poll<Result<Self::Ok, Self::Error>>where Self: Unpin,

A convenience method for calling TryFuture::try_poll on Unpin future types.
source§

impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

§

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
source§

fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.
source§

impl<V, T> VZip<V> for Twhere V: MultiLane<T>,

source§

fn vzip(self) -> V

source§

impl<T> WithSubscriber for T

source§

fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>where S: Into<Dispatch>,

Attaches the provided Subscriber to this type, returning a WithDispatch wrapper. Read more
source§

fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default Subscriber to this type, returning a WithDispatch wrapper. Read more