# [−] [src] 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>` [src]

#### `pub fn abort(&self)` [src]

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,` [src]

#### `fn fmt(&self, fmt: &mut Formatter<'_>) -> Result` [src]

Formats the value using the given formatter. Read more

### `impl<T> Drop for JoinHandle<T>` [src]

#### `fn drop(&mut self)` [src]

Executes the destructor for this type. Read more

### `impl<T> Future for JoinHandle<T>` [src]

#### `type Output = Result<T, JoinError>`

The type of value produced on completion.

#### `fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>` [src]

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

### `impl<T: Send> Send for JoinHandle<T>` [src]

### `impl<T: Send> Sync for JoinHandle<T>` [src]

### `impl<T> Unpin for JoinHandle<T>` [src]

## Auto Trait Implementations

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

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

## Blanket Implementations

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `pub fn type_id(&self) -> TypeId` [src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T` [src]

#### `pub fn from(t: T) -> T` [src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `pub fn into(self) -> U` [src]

Performs the conversion.

### `impl<F> IntoFuture for F where
F: Future,` [src]

#### `type Output = <F as Future>::Output`

🔬 This is a nightly-only experimental API. (`into_future`)

The output that the future will produce on completion.

#### `type Future = F`

🔬 This is a nightly-only experimental API. (`into_future`)

Which kind of future are we turning this into?

#### `pub fn into_future(self) -> <F as IntoFuture>::Future` [src]

🔬 This is a nightly-only experimental API. (`into_future`)

Creates a future from a value.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>` [src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

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

The type returned in the event of a conversion error.

#### `pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>` [src]

Performs the conversion.