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;

# 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(flavor = "current_thread")]
# 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:

# #[cfg(not(target_family = "wasm"))]
# {
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(flavor = "current_thread")]
# 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;
# }