Kotlin coroutines provide an efficient way to manage concurrency on the server side. By using coroutines, developers can write code that is more responsive to user input, while still providing a rich user experience.
Coroutines were introduced in Kotlin 1.1 and have been a stable feature since Kotlin 1.3. In the coroutine context, a coroutine is a light-weight thread that can be used to offload work from the main thread. This allows developers to write code that is more responsive to user input, while still providing a rich user experience.
Coroutines can be used with any existing Java library, making them a great tool for asynchronous server-side development. In this article, we'll take a look at how to use coroutines in a Java web application.
We'll start by setting up a simple Maven project. Add the following dependencies to your pom.xml
:
<dependencies>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-stdlib-jdk8</artifactId>
<version>1.3.72</version>
</dependency>
<dependency>
<groupId>org.jetbrains.kotlinx</groupId>
<artifactId>kotlinx-coroutines-core</artifactId>
<version>1.3.7</version>
</dependency>
</dependencies>
We're using Kotlin 1.3.72 and Kotlinx Coroutines 1.3.7. The kotlin-stdlib-jdk8
dependency includes the Kotlin standard library, while the kotlinx-coroutines-core
dependency provides the core coroutine functionality.
Coroutines can be created using the launch
or async
functions from the kotlinx.coroutines
package. The launch
function creates a new coroutine and starts it immediately. The async
function creates a new coroutine, but it does not start it immediately. Instead, it returns an instance of the Deferred
class, which can be used to start the coroutine later.
For example, we can create a coroutine that sleeps for one second using the launch
function:
import kotlinx.coroutines.*
fun main() {
launch {
// Suspend this coroutine for one second
delay(1000)
println("Hello, world!")
}
}
We can also create a coroutine using the async
function:
import kotlinx.coroutines.*
fun main() {
val deferred = async {
// Suspend this coroutine for one second
delay(1000)
println("Hello, world!")
}
// Start the coroutine
deferred.start()
}
The launch
function is a suspending function, which means that it can be used within another coroutine. The async
function is not a suspending function, so it can only be used from within a coroutine.
Coroutines can be cancelled using the cancel
function. This function will cause the coroutine to throw a CancellationException
.
For example, we can cancel a coroutine that is sleeping for one second using the cancel
function:
import kotlinx.coroutines.*
fun main() {
val deferred = launch {
try {
// Suspend this coroutine for one second
delay(1000)
println("Hello, world!")
} catch (e: CancellationException) {
println("Coroutine was cancelled")
}
}
// Cancel the coroutine after half a second
delay(500)
deferred.cancel()
}
Coroutines can be timed out using the withTimeout
function. This function will cause the coroutine to throw a TimeoutCancellationException
if it does not complete within the specified time.
For example, we can time out a coroutine that is sleeping for one second using the withTimeout
function:
import kotlinx.coroutines.*
fun main() {
try {
withTimeout(500) {
// Suspend this coroutine for one second
delay(1000)
println("Hello, world!")
}
} catch (e: TimeoutCancellationException) {
println("Coroutine timed out")
}
}
Coroutines can be supervised using the supervisorScope
function. This function will catch any exceptions that are thrown by the coroutine and will continue to execute the remaining code.
For example, we can supervisor a coroutine that is sleeping for one second using the supervisorScope
function:
import kotlinx.coroutines.*
fun main() {
supervisorScope {
val deferred = launch {
// Suspend this coroutine for one second
delay(1000)
println("Hello, world!")
}
// Cancel the coroutine after half a second
delay(500)
deferred.cancel()
}
}
Coroutines can be used to write asynchronous web applications. In this section, we'll take a look at how to use coroutines in a Java web application.
We'll start by creating a simple servlet that uses a coroutine to sleep for one second:
@WebServlet("/sleep")
public class SleepServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// Create a new coroutine
launch {
// Suspend this coroutine for one second
delay(1000)
// Send a response to the client
resp.getWriter().println("Hello, world!");
}
}
}
In this servlet, we use the launch
function to create a new coroutine. This coroutine sleeps for one second and then sends a response to the client.
We can also use the async
function to create a new coroutine. However, in this case, we need to start the coroutine manually:
@WebServlet("/sleep")
public class SleepServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// Create a new coroutine
val deferred = async {
// Suspend this coroutine for one second
delay(1000)
// Send a response to the client
resp.getWriter().println("Hello, world!");
}
// Start the coroutine
deferred.start()
}
}
In this servlet, we use the async
function to create a new coroutine. This coroutine sleeps for one second and then sends a response to the client. We use the start
function to start the coroutine.
It's also possible to use the withTimeout
function to time out a coroutine:
@WebServlet("/sleep")
public class SleepServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
try {
// Time out the coroutine after 500 milliseconds
withTimeout(500) {
// Create a new coroutine
launch {
// Suspend this coroutine for one second
delay(1000)
// Send a response to the client
resp.getWriter().println("Hello, world!");
}
}
} catch (e: TimeoutCancellationException) {
// Send a timeout response to the client
resp.getWriter().println("Request timed out");
}
}
}
In this servlet, we use the withTimeout
function to time out the coroutine. If the coroutine does not complete within 500 milliseconds, it will be cancelled and a timeout response will be sent to the client.
It's also possible to use the supervisorScope
function to supervisor a coroutine:
@WebServlet("/sleep")
public class SleepServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// Supervisor the coroutine
supervisorScope {
val deferred = launch {
// Suspend this coroutine for one second
delay(1000)
// Send a response to the client
resp.getWriter().println("Hello, world!");
}
// Cancel the coroutine after half a second
delay(500)
deferred.cancel()
}
}
}
In this servlet, we use the supervisorScope
function to supervisor the coroutine. This coroutine is cancelled after half a second, but the remaining code in the servlet is executed.
Kotlin coroutines provide an efficient way to manage concurrency on the server side. They can be used with any existing Java library, making them a great tool for asynchronous server-side development.