Add ContextPropagatingTaskDecorator

Prior to this commit, `@Async` and `@EventListener` annotated methods
would lose the the logging and observation contexts whenever their
execution was scheduled on a different Thread.

The Context Propagation library supports this use case and can propagate
context values in ThreadLocals, Reactor Context and more.

This commit introduces a new `TaskDecorator` implementation that
leverages the Context Propagation library. When configured on a
`TaskExecutor`, this allows to properly propagate context value through
the execution of the task.

This implementation is completely optional and requires the
"io.micrometer:context-propagation" library on the classpath. Enabling
this feature must be done consciously and sometimes selectively, as
context propagation introduces some overhead.

Closes gh-31130

framework-docs/modules/ROOT/pages/core/beans/context-introduction.adoc

@@ -478,20 +478,21 @@ Kotlin::
 ----
 ======
 
-Notice that `ApplicationListener` is generically parameterized with the type of your
-custom event (`BlockedListEvent` in the preceding example). This means that the
-`onApplicationEvent()` method can remain type-safe, avoiding any need for downcasting.
-You can register as many event listeners as you wish, but note that, by default, event
-listeners receive events synchronously. This means that the `publishEvent()` method
-blocks until all listeners have finished processing the event. One advantage of this
-synchronous and single-threaded approach is that, when a listener receives an event,
-it operates inside the transaction context of the publisher if a transaction context
-is available. If another strategy for event publication becomes necessary, e.g.
-asynchronous event processing by default, see the javadoc for Spring's
-{api-spring-framework}/context/event/ApplicationEventMulticaster.html[`ApplicationEventMulticaster`] interface
-and {api-spring-framework}/context/event/SimpleApplicationEventMulticaster.html[`SimpleApplicationEventMulticaster`]
-implementation for configuration options which can be applied to a custom
-"applicationEventMulticaster" bean definition.
+Notice that `ApplicationListener` is generically parameterized with the type of your custom event (`BlockedListEvent` in the preceding example).
+This means that the `onApplicationEvent()` method can remain type-safe, avoiding any need for downcasting.
+You can register as many event listeners as you wish, but note that, by default, event listeners receive events synchronously.
+This means that the `publishEvent()` method blocks until all listeners have finished processing the event.
+One advantage of this synchronous and single-threaded approach is that, when a listener receives an event,
+it operates inside the transaction context of the publisher if a transaction context is available.
+If another strategy for event publication becomes necessary, e.g. asynchronous event processing by default,
+see the javadoc for Spring's {api-spring-framework}/context/event/ApplicationEventMulticaster.html[`ApplicationEventMulticaster`] interface
+and {api-spring-framework}/context/event/SimpleApplicationEventMulticaster.html[`SimpleApplicationEventMulticaster`] implementation
+for configuration options which can be applied to a custom "applicationEventMulticaster" bean definition.
+In these cases, ThreadLocals and logging context are not propagated for the event processing.
+See xref:integration/observability.adoc#observability.application-events[the `@EventListener` Observability section]
+for more information on Observability concerns.
+
+
 
 The following example shows the bean definitions used to register and configure each of
 the classes above:
@@ -747,6 +748,9 @@ Be aware of the following limitations when using asynchronous events:
   value. If you need to publish another event as the result of the processing, inject an
   {api-spring-framework}/context/ApplicationEventPublisher.html[`ApplicationEventPublisher`]
   to publish the event manually.
+* ThreadLocals and logging context are not propagated by default for the event processing.
+  See xref:integration/observability.adoc#observability.application-events[the `@EventListener` Observability section]
+  for more information on Observability concerns.
 
 
 [[context-functionality-events-order]]

framework-docs/modules/ROOT/pages/integration/observability.adoc

@@ -349,3 +349,28 @@ Instrumentation uses the `org.springframework.web.reactive.function.client.Clien
 |===
 
 
+[[observability.application-events]]
+== Application Events and `@EventListener`
+
+Spring Framework does not contribute Observations for xref:core/beans/context-introduction.adoc#context-functionality-events-annotation[`@EventListener` calls],
+as they don't have the right semantics for such instrumentation.
+By default, event publication and processing is done synchronously and on the same Thread.
+This means that during the execution of that task, the ThreadLocals and logging context will be the same as the event publisher.
+
+If the application configures globally a custom `ApplicationEventMulticaster` with a strategy that schedules event processing on different threads, this is no longer true.
+All `@EventListener` methods will be processed on a different thread, outstide of the main event publication thread.
+In these cases, the https://micrometer.io/docs/contextPropagation[Micrometer Context Propagation library] can help propagating such values and better correlate the processing of the events.
+The application can configure the chosen `TaskExecutor` to use a `ContextPropagatingTaskDecorator` that decorates tasks and propagates context.
+For this to work, the `io.micrometer:context-propagation` library must be present on the classpath:
+
+include-code::./ApplicationEventsConfiguration[]
+
+Similarly, if that asynchronous choice is made locally for each `@EventListener` annotated method, by adding an `@Async` method to it,
+you can choose a `TaskExecutor` that propagates context by referring to it by its qualifier.
+Given the following `TaskExecutor` bean definition, configured with the dedicated task decorator:
+
+include-code::./EventAsyncExecutionConfiguration[]
+
+Annotating event listeners with `@Async` and the relevant qualifier will achieve similar context propagation results:
+
+include-code::./EmailNotificationListener[]

framework-docs/src/main/java/org/springframework/docs/integration/observability/applicationevents/ApplicationEventsConfiguration.java

+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.integration.observability.applicationevents;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.event.SimpleApplicationEventMulticaster;
+import org.springframework.core.task.SimpleAsyncTaskExecutor;
+import org.springframework.core.task.support.ContextPropagatingTaskDecorator;
+
+@Configuration
+public class ApplicationEventsConfiguration {
+
+	@Bean(name = "applicationEventMulticaster")
+	public SimpleApplicationEventMulticaster simpleApplicationEventMulticaster() {
+		SimpleApplicationEventMulticaster eventMulticaster = new SimpleApplicationEventMulticaster();
+		SimpleAsyncTaskExecutor taskExecutor = new SimpleAsyncTaskExecutor();
+		// decorate task execution with a decorator that supports context propagation
+		taskExecutor.setTaskDecorator(new ContextPropagatingTaskDecorator());
+		eventMulticaster.setTaskExecutor(taskExecutor);
+		return eventMulticaster;
+	}
+
+}

framework-docs/src/main/java/org/springframework/docs/integration/observability/applicationevents/EmailNotificationListener.java

+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.integration.observability.applicationevents;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+
+import org.springframework.context.event.EventListener;
+import org.springframework.scheduling.annotation.Async;
+import org.springframework.stereotype.Component;
+
+@Component
+public class EmailNotificationListener {
+
+	private final Log logger = LogFactory.getLog(EmailNotificationListener.class);
+
+	@EventListener(EmailReceivedEvent.class)
+	@Async("propagatingContextExecutor")
+	public void emailReceived(EmailReceivedEvent event) {
+		// asynchronously process the received event
+		// this logging statement will contain the expected MDC entries from the propagated context
+		logger.info("email has been received");
+	}
+
+}

framework-docs/src/main/java/org/springframework/docs/integration/observability/applicationevents/EmailReceivedEvent.java

+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.integration.observability.applicationevents;
+
+import org.springframework.context.ApplicationEvent;
+
+@SuppressWarnings("serial")
+public class EmailReceivedEvent extends ApplicationEvent {
+
+	public EmailReceivedEvent(Object source) {
+		super(source);
+	}
+
+}

framework-docs/src/main/java/org/springframework/docs/integration/observability/applicationevents/EventAsyncExecutionConfiguration.java

+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.integration.observability.applicationevents;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.task.SimpleAsyncTaskExecutor;
+import org.springframework.core.task.TaskExecutor;
+import org.springframework.core.task.support.ContextPropagatingTaskDecorator;
+
+@Configuration
+public class EventAsyncExecutionConfiguration {
+
+	@Bean(name = "propagatingContextExecutor")
+	public TaskExecutor propagatingContextExecutor() {
+		SimpleAsyncTaskExecutor taskExecutor = new SimpleAsyncTaskExecutor();
+		// decorate task execution with a decorator that supports context propagation
+		taskExecutor.setTaskDecorator(new ContextPropagatingTaskDecorator());
+		return taskExecutor;
+	}
+
+}

spring-core/spring-core.gradle

@@ -73,6 +73,7 @@ dependencies {
 	api(project(":spring-jcl"))
 	compileOnly("io.projectreactor.tools:blockhound")
 	compileOnly("org.graalvm.sdk:graal-sdk")
+	optional("io.micrometer:context-propagation")
 	optional("io.netty:netty-buffer")
 	optional("io.netty:netty5-buffer")
 	optional("io.projectreactor:reactor-core")

spring-core/src/main/java/org/springframework/core/task/support/ContextPropagatingTaskDecorator.java

+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.core.task.support;
+
+import io.micrometer.context.ContextSnapshot;
+import io.micrometer.context.ContextSnapshotFactory;
+
+import org.springframework.core.task.TaskDecorator;
+
+/**
+ * {@link TaskDecorator} that {@link ContextSnapshot#wrap(Runnable) wrap the execution} of
+ * tasks, assisting with context propagation.
+ * <p>This operation is only useful when the task execution is scheduled on a different
+ * thread than the original call stack; this depends on the choice of
+ * {@link org.springframework.core.task.TaskExecutor}. This is particularly useful for
+ * restoring a logging context or an observation context for the task execution. Note that
+ * this decorator will cause some overhead for task execution and is not recommended for
+ * applications that run lots of very small tasks.
+ *
+ * @author Brian Clozel
+ * @since 6.1
+ * @see CompositeTaskDecorator
+ */
+public class ContextPropagatingTaskDecorator implements TaskDecorator {
+
+	private final ContextSnapshotFactory factory;
+
+	/**
+	 * Create a new decorator that uses a default instance of the {@link ContextSnapshotFactory}.
+	 */
+	public ContextPropagatingTaskDecorator() {
+		this(ContextSnapshotFactory.builder().build());
+	}
+
+	/**
+	 * Create a new decorator using the given {@link ContextSnapshotFactory}.
+	 * @param factory the context snapshot factory to use.
+	 */
+	public ContextPropagatingTaskDecorator(ContextSnapshotFactory factory) {
+		this.factory = factory;
+	}
+
+	@Override
+	public Runnable decorate(Runnable runnable) {
+		return this.factory.captureAll().wrap(runnable);
+	}
+
+}

spring-core/src/test/java/org/springframework/core/task/support/ContextPropagatingTaskDecoratorTests.java

+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.core.task.support;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import io.micrometer.context.ContextRegistry;
+import io.micrometer.context.ContextSnapshotFactory;
+import io.micrometer.context.ThreadLocalAccessor;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ContextPropagatingTaskDecorator}.
+ * @author Brian Clozel
+ */
+class ContextPropagatingTaskDecoratorTests {
+
+	@Test
+	void shouldPropagateContextInTaskExecution() throws Exception {
+		AtomicReference<String> actual = new AtomicReference<>("");
+		ContextRegistry registry = new ContextRegistry();
+		registry.registerThreadLocalAccessor(new TestThreadLocalAccessor());
+		ContextSnapshotFactory snapshotFactory = ContextSnapshotFactory.builder().contextRegistry(registry).build();
+
+		Runnable task = () -> actual.set(TestThreadLocalHolder.getValue());
+		TestThreadLocalHolder.setValue("expected");
+
+		Thread execution = new Thread(new ContextPropagatingTaskDecorator(snapshotFactory).decorate(task));
+		execution.start();
+		execution.join();
+		assertThat(actual.get()).isEqualTo("expected");
+		TestThreadLocalHolder.reset();
+	}
+
+	static class TestThreadLocalHolder {
+
+		private static final ThreadLocal<String> holder = new ThreadLocal<>();
+
+		public static void setValue(String value) {
+			holder.set(value);
+		}
+
+		public static String getValue() {
+			return holder.get();
+		}
+
+		public static void reset() {
+			holder.remove();
+		}
+
+	}
+
+	static class TestThreadLocalAccessor implements ThreadLocalAccessor<String> {
+
+		public static final String KEY = "test.threadlocal";
+
+		@Override
+		public Object key() {
+			return KEY;
+		}
+
+		@Override
+		public String getValue() {
+			return TestThreadLocalHolder.getValue();
+		}
+
+		@Override
+		public void setValue(String value) {
+			TestThreadLocalHolder.setValue(value);
+		}
+
+		@Override
+		public void setValue() {
+			TestThreadLocalHolder.reset();
+		}
+
+		@Override
+		public void restore(String previousValue) {
+			setValue(previousValue);
+		}
+
+	}
+
+}