Struct `JoinHandle`

struct JoinHandle<T> { ... }

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;

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

Creation from task::spawn_blocking:

use tokio::task;

# async fn doc() {
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;

# async fn doc() {
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;

# async fn doc() {
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;

# #[tokio::main] async fn main() {
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>`

fn abort(self: &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;

# #[tokio::main(flavor = "current_thread", start_paused = true)]
# async fn main() {
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());
}
# }

fn is_finished(self: &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;

# #[tokio::main(flavor = "current_thread", start_paused = true)]
# async fn main() {
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());
# }

fn abort_handle(self: &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};

# #[tokio::main(flavor = "current_thread", start_paused = true)]
# async fn main() {
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());
}
# }

fn id(self: &Self) -> Id

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

`impl<F> IntoFuture for JoinHandle<T>`

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

`impl<T> Any for JoinHandle<T>`

fn type_id(self: &Self) -> TypeId

`impl<T> Borrow for JoinHandle<T>`

fn borrow(self: &Self) -> &T

`impl<T> BorrowMut for JoinHandle<T>`

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

`impl<T> Debug for JoinHandle<T>`

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

`impl<T> Drop for JoinHandle<T>`

fn drop(self: &mut Self)

`impl<T> Freeze for JoinHandle<T>`

`impl<T> From for JoinHandle<T>`

fn from(t: T) -> T: Returns the argument unchanged.

`impl<T> Future for JoinHandle<T>`

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

`impl<T> RefUnwindSafe for JoinHandle<T>`

`impl<T> Unpin for JoinHandle<T>`

`impl<T> UnsafeUnpin for JoinHandle<T>`

`impl<T> UnwindSafe for JoinHandle<T>`

`impl<T, U> Into for JoinHandle<T>`

fn into(self: Self) -> U

Calls U::from(self).

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

`impl<T, U> TryFrom for JoinHandle<T>`

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

`impl<T, U> TryInto for JoinHandle<T>`

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