Ankur Mhatre
9b3fe88813
Every @Async behaviour that surprises people, asserted by a test and captured to docs/output/: the self-invocation trap, what CGLIB cannot override, the IllegalArgumentException a plain return type throws, the unbounded queue that makes max-size decoration, spring.task.execution.propagate-context (new in Boot 4.1.0), the two Executor beans that leave @Async on an unpooled SimpleAsyncTaskExecutor, and JEP 491 measured on JDK 21 against JDK 25.
51 lines
2.6 KiB
Markdown
51 lines
2.6 KiB
Markdown
[README](../README.md) · next: [The self-invocation trap](02-the-self-invocation-trap.md)
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# 1. What `@Async` actually does
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`@Async` is not a keyword and it is not a thread. It is a marker that
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[`AsyncAnnotationBeanPostProcessor`](https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/scheduling/annotation/AsyncAnnotationBeanPostProcessor.html)
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looks for while the context is being built. When it finds one, it wraps the bean in a proxy. The
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proxy's version of the method does not call your code; it wraps your code in a `Callable`, hands
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that to an `AsyncTaskExecutor`, and returns immediately.
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Three consequences follow from that sentence, and between them they explain almost every
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`@Async` question ever asked:
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1. **The proxy is the whole mechanism.** A call that does not go through the proxy is not
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asynchronous. See [chapter 2](02-the-self-invocation-trap.md) and
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[chapter 3](03-what-the-proxy-cannot-see.md).
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2. **The return value has to be produced before your code runs.** So the return type is
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constrained, and exceptions have nowhere obvious to go. See
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[chapter 4](04-return-types-and-exceptions.md).
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3. **Which executor gets the `Callable` is resolved separately, by type and then by name.** It is
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not necessarily the one you configured. See [chapter 7](07-which-executor-runs-it.md).
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## `@EnableAsync` is not automatic
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Spring Boot auto-configures the *executor*. It does not enable the *annotation*. Without
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`@EnableAsync` somewhere in the context, no post-processor is installed, no proxy is created, and
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every `@Async` method in the application runs on its caller's thread. There is no warning at any
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log level, because from Spring's point of view nothing unusual has happened — you have a bean
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with an annotation nobody asked it to process.
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`AsyncDemoApplication` carries the annotation for exactly this reason.
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## The evidence in this module
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Every claim in these chapters is asserted by a test and captured in `docs/output/`. The
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measurement is always the same one: the name of the thread the method body actually ran on,
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returned from the method itself.
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```
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caller thread : main (virtual=false)
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service.annotated() : task-5 (virtual=false)
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service.viaSelfInvocation() : main (virtual=false)
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service.viaSelfReference() : task-6 (virtual=false)
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```
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Timing cannot tell you this. A method that runs synchronously in 3 ms and a method that runs on a
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pool thread in 3 ms look identical from the outside, which is why `@Async` failures survive so
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long in production.
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next: [The self-invocation trap](02-the-self-invocation-trap.md)
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