Struct tokio::runtime::Handle

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pub struct Handle { /* private fields */ }

Expand description

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§

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impl Handle

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pub fn enter(&self) -> EnterGuard<'_>

Enters the runtime context. This allows you to construct types that must have an executor available on creation such as Sleep or TcpStream. It will also allow you to call methods such as tokio::spawn and Handle::current without panicking.

§Panics

When calling Handle::enter multiple times, the returned guards must be dropped in the reverse order that they were acquired. Failure to do so will result in a panic and possible memory leaks.

§Examples

use tokio::runtime::Runtime;

let rt = Runtime::new().unwrap();

let _guard = rt.enter();
tokio::spawn(async {
    println!("Hello world!");
});

Do not do the following, this shows a scenario that will result in a panic and possible memory leak.

use tokio::runtime::Runtime;

let rt1 = Runtime::new().unwrap();
let rt2 = Runtime::new().unwrap();

let enter1 = rt1.enter();
let enter2 = rt2.enter();

drop(enter1);
drop(enter2);

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pub fn current() -> Self

Returns a Handle view over the currently running Runtime.

§Panics

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, or from a thread with an active EnterGuard. Calling this from within a thread created by std::thread::spawn (for example) will cause a panic unless that thread has an active EnterGuard.

§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.spawn(async { /* ... */ });
    // This next line would cause a panic because we haven't entered the runtime
    // and created an EnterGuard
    // let handle2 = Handle::current(); // panic
    // So we create a guard here with Handle::enter();
    let _guard = handle.enter();
    // Now we can call Handle::current();
    let handle2 = Handle::current();
});

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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

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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.

The provided future will start running in the background immediately when spawn is called, even if you don’t await the returned JoinHandle.

See module level documentation for more details.

§Examples

use tokio::runtime::Runtime;

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

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

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

Runs the provided function on an executor dedicated to blocking operations.

§Examples

use tokio::runtime::Runtime;

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

// Spawn a blocking function onto the runtime using the handle
handle.spawn_blocking(|| {
    println!("now running on a worker thread");
});

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

Runs a future to completion on this Handle’s associated Runtime.

This runs the given future on the current thread, 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.

When this is used on a current_thread runtime, only the Runtime::block_on method can drive the IO and timer drivers, but the Handle::block_on method cannot drive them. This means that, when using this method on a current_thread runtime, anything that relies on IO or timers will not work unless there is another thread currently calling Runtime::block_on on the same runtime.

§If the runtime has been shut down

If the Handle’s associated Runtime has been shut down (through Runtime::shutdown_background, Runtime::shutdown_timeout, or by dropping it) and Handle::block_on is used it might return an error or panic. Specifically IO resources will return an error and timers will panic. Runtime independent futures will run as normal.

§Panics

This function panics if the provided future panics, if called within an asynchronous execution context, or if a timer future is executed on a runtime that has been shut down.

§Examples

use tokio::runtime::Runtime;

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

// Get a handle from this runtime
let handle = rt.handle();

// Execute the future, blocking the current thread until completion
handle.block_on(async {
    println!("hello");
});

Or using Handle::current:

use tokio::runtime::Handle;

#[tokio::main]
async fn main () {
    let handle = Handle::current();
    std::thread::spawn(move || {
        // Using Handle::block_on to run async code in the new thread.
        handle.block_on(async {
            println!("hello");
        });
    });
}

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pub fn runtime_flavor(&self) -> RuntimeFlavor

Returns the flavor of the current Runtime.

§Examples

use tokio::runtime::{Handle, RuntimeFlavor};

#[tokio::main(flavor = "current_thread")]
async fn main() {
  assert_eq!(RuntimeFlavor::CurrentThread, Handle::current().runtime_flavor());
}

use tokio::runtime::{Handle, RuntimeFlavor};

#[tokio::main(flavor = "multi_thread", worker_threads = 4)]
async fn main() {
  assert_eq!(RuntimeFlavor::MultiThread, Handle::current().runtime_flavor());
}