prev: [Pool sizing](05-pool-sizing.md) · [README](../README.md) · next: [Which executor runs it](07-which-executor-runs-it.md)

# 6. `spring.task.execution.propagate-context` — new in Boot 4.1

Handing a `Callable` to another thread leaves every `ThreadLocal` behind. That is why the async
thread in a stock application sees no MDC entries, no `RequestAttributes`, and no
`SecurityContext` — and why "the trace id disappears in the async part" is such a common report.

Spring Boot 4.1.0 added a property for it:

```yaml
spring:
  task:
    execution:
      propagate-context: true
```

It decorates the auto-configured executor with `ContextPropagatingTaskDecorator`, which takes a
snapshot of everything registered with micrometer's `ContextRegistry` at submission time and
restores it around the task on the executing thread.

## How new is it, exactly

New in **4.1.0**. It is absent from `TaskExecutionProperties` in 4.0.8 and in 3.5.9 — checked by
extracting `TaskExecutionProperties.class` from each `spring-boot-autoconfigure` jar and running
`javap` over it, rather than by reading release notes. The accessor pair
`getPropagateContext()`/`setPropagateContext(boolean)` appears first in the 4.1.0 jar.

Note also what it is *not*: there is no matching `spring.task.scheduling.propagate-context`. The
scheduler is not covered.

## Measured

`RequestId` in this module is a `ThreadLocal<String>` with a `ThreadLocalAccessor` registered
against `ContextRegistry`. Two tests differ only in the property:

- `ContextNotPropagatedTest` — the async thread reads `null`.
- `ContextPropagatedTest` — the async thread reads `req-4711`
  ([output](output/context-propagation.txt)).

## The three things that make it silently do nothing

1. **micrometer's `context-propagation` is not on the classpath.** The property is still bound
   and still accepted; nothing decorates the executor. This module declares
   `io.micrometer:context-propagation` explicitly for that reason.
2. **Nothing registered an accessor.** The snapshot only carries what `ContextRegistry` knows
   about. Libraries that ship accessors (Micrometer tracing, Reactor) register their own; a
   `ThreadLocal` of your own does not register itself.
3. **The executor is not the auto-configured one.** The decorator is applied by Boot's
   auto-configuration. Declare your own `Executor` bean (chapter 7) and you have opted out of
   the property along with everything else Boot was doing.

`SecurityContext` propagation is a related but separate mechanism, with its own set of ways to
get it wrong; that is covered in
[Virtual Threads and SecurityContext Propagation](https://ankurm.com/spring-security-virtual-threads-context-propagation/).

next: [Which executor runs it](07-which-executor-runs-it.md)