Struct JoinHandle

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

Available on crate feature rt only.

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.

It is guaranteed that the destructor of the spawned task has finished before task completion is observed via JoinHandle await, JoinHandle::is_finished or AbortHandle::is_finished.

Cancel safety

Awaiting a &mut JoinHandle<T> is cancel safe. If it is used as a branch in tokio::select! and another 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;

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

impl<T> JoinHandle<T>

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());
}

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());

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());
}

pub fn id(&self) -> Id

Returns a task ID that uniquely identifies this task relative to other currently spawned tasks.

Trait Implementations

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

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

Formats the value using the given formatter. Read more

impl<T> Drop for JoinHandle<T>

fn drop(&mut self)

Executes the destructor for this type. Read more

fn pin_drop(self: Pin<&mut Self>)

🔬This is a nightly-only experimental API. (pin_ergonomics)

Execute the destructor for this type, but different to Drop::drop, it requires self to be pinned. Read more

impl<T> Future for JoinHandle<T>

type Output = Result<T, JoinError>

The type of value produced on completion.

fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>

Attempts to resolve the future to a final value, registering the current task for wakeup if the value is not yet available. Read more

impl<T> RefUnwindSafe for JoinHandle<T>

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

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

impl<T> Unpin for JoinHandle<T>

impl<T> UnwindSafe for JoinHandle<T>

Auto Trait Implementations

impl<T> Freeze for JoinHandle<T>

impl<T> UnsafeUnpin for JoinHandle<T>

Blanket Implementations

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

fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more

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

fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more

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

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

Mutably borrows from an owned value. Read more

impl<T> From<T> for T

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> Instrument for T

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

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

fn in_current_span(self) -> Instrumented<Self> ⓘ

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

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

fn into(self) -> U

Calls U::from(self).

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

impl<F> IntoFuture for F
where 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?

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

Creates a future from a value. Read more

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

type Error = Infallible

The type returned in the event of a conversion error.

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

Performs the conversion.

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

type Error = >::Error