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Struct JoinSet 

pub struct JoinSet<T> { /* private fields */ }
Available on crate feature rt only.

A collection of tasks spawned on a Tokio runtime.

A JoinSet can be used to await the completion of some or all of the tasks in the set. The set is not ordered, and the tasks will be returned in the order they complete.

All of the tasks must have the same return type T.

When the JoinSet is dropped, all tasks in the JoinSet are immediately aborted.

Examples

Spawn multiple tasks and wait for them.

use tokio::task::JoinSet;

let mut set = JoinSet::new();

for i in 0..10 {
    set.spawn(async move { i });
}

let mut seen = [false; 10];
while let Some(res) = set.join_next().await {
    let idx = res.unwrap();
    seen[idx] = true;
}

for i in 0..10 {
    assert!(seen[i]);
}

Task ID guarantees

While a task is tracked in a JoinSet, that task’s ID is unique relative to all other running tasks in Tokio. For this purpose, tracking a task in a JoinSet is equivalent to holding a JoinHandle to it. See the task ID documentation for more info.

Implementations

impl<T> JoinSet<T>

pub fn new() -> Self

Create a new JoinSet.

pub fn len(&self) -> usize

Returns the number of tasks currently in the JoinSet.

pub fn is_empty(&self) -> bool

Returns whether the JoinSet is empty.

impl<T: 'static> JoinSet<T>

pub fn build_task(&mut self) -> Builder<'_, T>

Available on tokio_unstable and crate feature tracing only.

Returns a Builder that can be used to configure a task prior to spawning it on this JoinSet.

Examples
use tokio::task::JoinSet;

#[tokio::main]
async fn main() -> std::io::Result<()> {
    let mut set = JoinSet::new();

    // Use the builder to configure a task's name before spawning it.
    set.build_task()
        .name("my_task")
        .spawn(async { /* ... */ })?;

    Ok(())
}

pub fn spawn<F>(&mut self, task: F) -> AbortHandle
where F: Future<Output = T> + Send + 'static, T: Send,

Spawn the provided task on the JoinSet, returning an AbortHandle that can be used to remotely cancel the task.

The provided future will start running in the background immediately when this method is called, even if you don’t await anything on this JoinSet.

Panics

This method panics if called outside of a Tokio runtime.

pub fn spawn_on<F>(&mut self, task: F, handle: &Handle) -> AbortHandle
where F: Future<Output = T> + Send + 'static, T: Send,

Spawn the provided task on the provided runtime and store it in this JoinSet returning an AbortHandle that can be used to remotely cancel the task.

The provided future will start running in the background immediately when this method is called, even if you don’t await anything on this JoinSet.

pub fn spawn_local<F>(&mut self, task: F) -> AbortHandle
where F: Future<Output = T> + 'static,

Spawn the provided task on the current LocalSet or LocalRuntime and store it in this JoinSet, returning an AbortHandle that can be used to remotely cancel the task.

The provided future will start running in the background immediately when this method is called, even if you don’t await anything on this JoinSet.

Panics

This method panics if it is called outside of a LocalSet or LocalRuntime.

pub fn spawn_local_on<F>( &mut self, task: F, local_set: &LocalSet, ) -> AbortHandle
where F: Future<Output = T> + 'static,

Spawn the provided task on the provided LocalSet and store it in this JoinSet, returning an AbortHandle that can be used to remotely cancel the task.

Unlike the spawn_local method, this method may be used to spawn local tasks on a LocalSet that is not currently running. The provided future will start running whenever the LocalSet is next started.

pub fn spawn_blocking<F>(&mut self, f: F) -> AbortHandle
where F: FnOnce() -> T + Send + 'static, T: Send,

Spawn the blocking code on the blocking threadpool and store it in this JoinSet, returning an AbortHandle that can be used to remotely cancel the task.

Examples

Spawn multiple blocking tasks and wait for them.

use tokio::task::JoinSet;

#[tokio::main]
async fn main() {
    let mut set = JoinSet::new();

    for i in 0..10 {
        set.spawn_blocking(move || { i });
    }

    let mut seen = [false; 10];
    while let Some(res) = set.join_next().await {
        let idx = res.unwrap();
        seen[idx] = true;
    }

    for i in 0..10 {
        assert!(seen[i]);
    }
}
Panics

This method panics if called outside of a Tokio runtime.

pub fn spawn_blocking_on<F>(&mut self, f: F, handle: &Handle) -> AbortHandle
where F: FnOnce() -> T + Send + 'static, T: Send,

Spawn the blocking code on the blocking threadpool of the provided runtime and store it in this JoinSet, returning an AbortHandle that can be used to remotely cancel the task.

pub async fn join_next(&mut self) -> Option<Result<T, JoinError>>

Waits until one of the tasks in the set completes and returns its output.

Returns None if the set is empty.

Cancel safety

This method is cancel safe. If join_next is used as a branch in tokio::select! and another branch completes first, it is guaranteed that no tasks were removed from this JoinSet.

pub async fn join_next_with_id(&mut self) -> Option<Result<(Id, T), JoinError>>

Waits until one of the tasks in the set completes and returns its output, along with the task ID of the completed task.

Returns None if the set is empty.

When this method returns an error, then the id of the task that failed can be accessed using the JoinError::id method.

Cancel safety

This method is cancel safe. If join_next_with_id is used as a branch in tokio::select! and another branch completes first, it is guaranteed that no tasks were removed from this JoinSet.

pub fn try_join_next(&mut self) -> Option<Result<T, JoinError>>

Tries to join one of the tasks in the set that has completed and return its output.

Returns None if there are no completed tasks, or if the set is empty.

pub fn try_join_next_with_id(&mut self) -> Option<Result<(Id, T), JoinError>>

Tries to join one of the tasks in the set that has completed and return its output, along with the task ID of the completed task.

Returns None if there are no completed tasks, or if the set is empty.

When this method returns an error, then the id of the task that failed can be accessed using the JoinError::id method.

pub async fn shutdown(&mut self)

Aborts all tasks and waits for them to finish shutting down.

Calling this method is equivalent to calling abort_all and then calling join_next in a loop until it returns None.

This method ignores any panics in the tasks shutting down. When this call returns, the JoinSet will be empty.

pub async fn join_all(self) -> Vec<T>

Awaits the completion of all tasks in this JoinSet, returning a vector of their results.

The results will be stored in the order they completed not the order they were spawned. This is a convenience method that is equivalent to calling join_next in a loop. If any tasks on the JoinSet fail with an JoinError, then this call to join_all will panic and all remaining tasks on the JoinSet are cancelled. To handle errors in any other way, manually call join_next in a loop.

Examples

Spawn multiple tasks and join_all them.

use tokio::task::JoinSet;
use std::time::Duration;

let mut set = JoinSet::new();

for i in 0..3 {
    set.spawn(async move {
        tokio::time::sleep(Duration::from_secs(3 - i)).await;
        i
    });
}

let output = set.join_all().await;
assert_eq!(output, vec![2, 1, 0]);

Equivalent implementation of join_all, using join_next and loop.

use tokio::task::JoinSet;
use std::panic;

let mut set = JoinSet::new();

for i in 0..3 {
    set.spawn(async move {i});
}

let mut output = Vec::new();
while let Some(res) = set.join_next().await{
    match res {
        Ok(t) => output.push(t),
        Err(err) if err.is_panic() => panic::resume_unwind(err.into_panic()),
        Err(err) => panic!("{err}"),
    }
}
assert_eq!(output.len(),3);

pub fn abort_all(&mut self)

Aborts all tasks on this JoinSet.

This does not remove the tasks from the JoinSet. To wait for the tasks to complete cancellation, you should call join_next in a loop until the JoinSet is empty.

pub fn detach_all(&mut self)

Removes all tasks from this JoinSet without aborting them.

The tasks removed by this call will continue to run in the background even if the JoinSet is dropped.

pub fn poll_join_next( &mut self, cx: &mut Context<'_>, ) -> Poll<Option<Result<T, JoinError>>>

Polls for one of the tasks in the set to complete.

If this returns Poll::Ready(Some(_)), then the task that completed is removed from the set.

When the method returns Poll::Pending, the Waker in the provided Context is scheduled to receive a wakeup when a task in the JoinSet completes. Note that on multiple calls to poll_join_next, only the Waker from the Context passed to the most recent call is scheduled to receive a wakeup.

Returns

This function returns:

  • Poll::Pending if the JoinSet is not empty but there is no task whose output is available right now.
  • Poll::Ready(Some(Ok(value))) if one of the tasks in this JoinSet has completed. The value is the return value of one of the tasks that completed.
  • Poll::Ready(Some(Err(err))) if one of the tasks in this JoinSet has panicked or been aborted. The err is the JoinError from the panicked/aborted task.
  • Poll::Ready(None) if the JoinSet is empty.

Note that this method may return Poll::Pending even if one of the tasks has completed. This can happen if the coop budget is reached.

pub fn poll_join_next_with_id( &mut self, cx: &mut Context<'_>, ) -> Poll<Option<Result<(Id, T), JoinError>>>

Polls for one of the tasks in the set to complete.

If this returns Poll::Ready(Some(_)), then the task that completed is removed from the set.

When the method returns Poll::Pending, the Waker in the provided Context is scheduled to receive a wakeup when a task in the JoinSet completes. Note that on multiple calls to poll_join_next, only the Waker from the Context passed to the most recent call is scheduled to receive a wakeup.

Returns

This function returns:

  • Poll::Pending if the JoinSet is not empty but there is no task whose output is available right now.
  • Poll::Ready(Some(Ok((id, value)))) if one of the tasks in this JoinSet has completed. The value is the return value of one of the tasks that completed, and id is the task ID of that task.
  • Poll::Ready(Some(Err(err))) if one of the tasks in this JoinSet has panicked or been aborted. The err is the JoinError from the panicked/aborted task.
  • Poll::Ready(None) if the JoinSet is empty.

Note that this method may return Poll::Pending even if one of the tasks has completed. This can happen if the coop budget is reached.

Trait Implementations

impl<T> Debug for JoinSet<T>

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

Formats the value using the given formatter. Read more

impl<T> Default for JoinSet<T>

fn default() -> Self

Returns the “default value” for a type. Read more

impl<T> Drop for JoinSet<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, F> Extend<F> for JoinSet<T>
where F: Future<Output = T> + Send + 'static, T: Send + 'static,

Extend a JoinSet with futures from an iterator.

This is equivalent to calling JoinSet::spawn on each element of the iterator.

Examples

use tokio::task::JoinSet;

#[tokio::main]
async fn main() {
    let mut set: JoinSet<_> = (0..5).map(|i| async move { i }).collect();

    set.extend((5..10).map(|i| async move { i }));

    let mut seen = [false; 10];
    while let Some(res) = set.join_next().await {
        let idx = res.unwrap();
        seen[idx] = true;
    }

    for i in 0..10 {
        assert!(seen[i]);
    }
}

fn extend<I>(&mut self, iter: I)
where I: IntoIterator<Item = F>,

Extends a collection with the contents of an iterator. Read more

fn extend_one(&mut self, item: A)

🔬This is a nightly-only experimental API. (extend_one)
Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one)
Reserves capacity in a collection for the given number of additional elements. Read more

impl<T, F> FromIterator<F> for JoinSet<T>
where F: Future<Output = T> + Send + 'static, T: Send + 'static,

Collect an iterator of futures into a JoinSet.

This is equivalent to calling JoinSet::spawn on each element of the iterator.

Examples

The main example from JoinSet’s documentation can also be written using collect:

use tokio::task::JoinSet;

let mut set: JoinSet<_> = (0..10).map(|i| async move { i }).collect();

let mut seen = [false; 10];
while let Some(res) = set.join_next().await {
    let idx = res.unwrap();
    seen[idx] = true;
}

for i in 0..10 {
     assert!(seen[i]);
}

fn from_iter<I: IntoIterator<Item = F>>(iter: I) -> Self

Creates a value from an iterator. Read more

Auto Trait Implementations

impl<T> !RefUnwindSafe for JoinSet<T>

impl<T> !UnwindSafe for JoinSet<T>

impl<T> Freeze for JoinSet<T>

impl<T> Send for JoinSet<T>
where T: Send,

impl<T> Sync for JoinSet<T>
where T: Send,

impl<T> Unpin for JoinSet<T>

impl<T> UnsafeUnpin for JoinSet<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<U> 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<T, U> TryFrom<U> 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<U>>::Error>

Performs the conversion.

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

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.

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

Performs the conversion.

impl<T> WithSubscriber for T

fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self> ⓘ
where S: Into<Dispatch>,

Attaches the provided Subscriber to this type, returning a WithDispatch wrapper. Read more

fn with_current_subscriber(self) -> WithDispatch<Self> ⓘ

Attaches the current default Subscriber to this type, returning a WithDispatch wrapper. Read more

MIT License
Copyright © Tokio Contributors
https://docs.rs/tokio/1.53.1/tokio/task/join_set/struct.JoinSet.html

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