Struct tokio::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. You do not need to .await the
JoinHandle to make the task execute — it will start running in the
background immediately.
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
sourceimpl<T> JoinHandle<T>
impl<T> JoinHandle<T>
sourcepub fn abort(&self)
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;
#[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());
}
}sourcepub fn is_finished(&self) -> bool
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());