Struct tokio::task::JoinHandle

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

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 task rather than a thread.

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.

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

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 complete with a Err(JoinError::Cancelled).

use tokio::time;

#[tokio::main]
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());
   }
}

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.

impl<T> Drop for JoinHandle<T>

fn drop(&mut self)

Executes the destructor for this type.

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>

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

impl<T: Send> Send for JoinHandle<T>

impl<T: Send> Sync for JoinHandle<T>

impl<T> Unpin for JoinHandle<T>