Struct tokio::runtime::Handle

pub struct Handle { /* fields omitted */ }

Handle to the runtime.

The handle is internally reference-counted and can be freely cloned. A handle can be obtained using the Runtime::handle method.

Implementations

impl Handle

pub fn enter<F, R>(&self, f: F) -> R where
    F: FnOnce() -> R, 

Enter the runtime context. This allows you to construct types that must have an executor available on creation such as Delay or TcpStream. It will also allow you to call methods such as tokio::spawn.

This function is also available as Runtime::enter.

Example

use tokio::runtime::Runtime;

fn function_that_spawns(msg: String) {
    // Had we not used `handle.enter` below, this would panic.
    tokio::spawn(async move {
        println!("{}", msg);
    });
}

fn main() {
    let rt = Runtime::new().unwrap();
    let handle = rt.handle().clone();

    let s = "Hello World!".to_string();

    // By entering the context, we tie `tokio::spawn` to this executor.
    handle.enter(|| function_that_spawns(s));
}

pub fn current() -> Self

Returns a Handle view over the currently running Runtime

Panic

This will panic if called outside the context of a Tokio runtime. That means that you must call this on one of the threads being run by the runtime. Calling this from within a thread created by std::thread::spawn (for example) will cause a panic.

Examples

This can be used to obtain the handle of the surrounding runtime from an async block or function running on that runtime.

use tokio::runtime::Handle;

// Inside an async block or function.
let handle = Handle::current();
handle.spawn(async {
    println!("now running in the existing Runtime");
});

thread::spawn(move || {
    // Notice that the handle is created outside of this thread and then moved in
    handle.block_on(async { /* ... */ })
    // This next line would cause a panic
    // let handle2 = Handle::current();
});

pub fn try_current() -> Result<Self, TryCurrentError>

Returns a Handle view over the currently running Runtime

Returns an error if no Runtime has been started

Contrary to current, this never panics

impl Handle

pub fn spawn<F>(&self, future: F) -> JoinHandle<F::Output> where
    F: Future + Send + 'static,
    F::Output: Send + 'static, 

Spawns a future onto the Tokio runtime.

This spawns the given future onto the runtime’s executor, usually a thread pool. The thread pool is then responsible for polling the future until it completes.

See module level documentation for more details.

Examples

use tokio::runtime::Runtime;

// Create the runtime
let rt = Runtime::new().unwrap();
let handle = rt.handle();

// Spawn a future onto the runtime
handle.spawn(async {
    println!("now running on a worker thread");
});

Panics

This function will not panic unless task execution is disabled on the executor. This can only happen if the runtime was built using Builder without picking either basic_scheduler or threaded_scheduler.

pub fn block_on<F: Future>(&self, future: F) -> F::Output

Run a future to completion on the Tokio runtime from a synchronous context.

This runs the given future on the runtime, blocking until it is complete, and yielding its resolved result. Any tasks or timers which the future spawns internally will be executed on the runtime.

If the provided executor currently has no active core thread, this function might hang until a core thread is added. This is not a concern when using the threaded scheduler, as it always has active core threads, but if you use the basic scheduler, some other thread must currently be inside a call to Runtime::block_on. See also the module level documentation, which has a section on scheduler types.

This method may not be called from an asynchronous context.

Panics

This function panics if the provided future panics, or if called within an asynchronous execution context.

Examples

Using block_on with the threaded scheduler.

use tokio::runtime::Runtime;
use std::thread;

// Create the runtime.
//
// If the rt-threaded feature is enabled, this creates a threaded
// scheduler by default.
let rt = Runtime::new().unwrap();
let handle = rt.handle().clone();

// Use the runtime from another thread.
let th = thread::spawn(move || {
    // Execute the future, blocking the current thread until completion.
    //
    // This example uses the threaded scheduler, so no concurrent call to
    // `rt.block_on` is required.
    handle.block_on(async {
        println!("hello");
    });
});

th.join().unwrap();

Using the basic scheduler requires a concurrent call to Runtime::block_on:

use tokio::runtime::Builder;
use tokio::sync::oneshot;
use std::thread;

// Create the runtime.
let mut rt = Builder::new()
    .enable_all()
    .basic_scheduler()
    .build()
    .unwrap();

let handle = rt.handle().clone();

// Signal main thread when task has finished.
let (send, recv) = oneshot::channel();

// Use the runtime from another thread.
let th = thread::spawn(move || {
    // Execute the future, blocking the current thread until completion.
    handle.block_on(async {
        send.send("done").unwrap();
    });
});

// The basic scheduler is used, so the thread above might hang if we
// didn't call block_on on the rt too.
rt.block_on(async {
    assert_eq!(recv.await.unwrap(), "done");
});

impl Handle

pub fn spawn_blocking<F, R>(&self, f: F) -> JoinHandle<R> where
    F: FnOnce() -> R + Send + 'static,
    R: Send + 'static, 

Runs the provided closure on a thread where blocking is acceptable.

In general, issuing a blocking call or performing a lot of compute in a future without yielding is not okay, as it may prevent the executor from driving other futures forward. This function runs the provided closure on a thread dedicated to blocking operations. See the CPU-bound tasks and blocking code section for more information.

Tokio will spawn more blocking threads when they are requested through this function until the upper limit configured on the Builder is reached. This limit is very large by default, because spawn_blocking is often used for various kinds of IO operations that cannot be performed asynchronously. When you run CPU-bound code using spawn_blocking, you should keep this large upper limit in mind; to run your CPU-bound computations on only a few threads, you should use a separate thread pool such as rayon rather than configuring the number of blocking threads.

This function is intended for non-async operations that eventually finish on their own. If you want to spawn an ordinary thread, you should use thread::spawn instead.

Closures spawned using spawn_blocking cannot be cancelled. When you shut down the executor, it will wait indefinitely for all blocking operations to finish. You can use shutdown_timeout to stop waiting for them after a certain timeout. Be aware that this will still not cancel the tasks — they are simply allowed to keep running after the method returns.

Note that if you are using the basic scheduler, this function will still spawn additional threads for blocking operations. The basic scheduler’s single thread is only used for asynchronous code.

Examples

use tokio::runtime::Runtime;

// Create the runtime
let rt = Runtime::new().unwrap();
let handle = rt.handle();

let res = handle.spawn_blocking(move || {
    // do some compute-heavy work or call synchronous code
    "done computing"
}).await?;

assert_eq!(res, "done computing");

Trait Implementations

impl Clone for Handle

fn clone(&self) -> Handle

Returns a copy of the value.

pub fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Handle

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

Formats the value using the given formatter.