Context Propagation Library

Overview

A library that assists with context propagation across different types of context mechanisms such as ThreadLocal, Reactor Context, and others.

Abstractions:

ThreadLocalAccessor - contract to assist with access to a ThreadLocal value.
ContextAccessor - contract to assist with access to a Map-like context.
ContextRegistry - registry for instances of ThreadLocalAccessor and ContextAccessor.
ContextSnapshot - holder of contextual values, that provides methods to capture and to propagate.

You can read the full reference documentation here.

Example Scenarios:

In imperative code, e.g. Spring MVC controller, capture ThreadLocal values into a ContextSnapshot. After that use the snapshot to populate a Reactor Context with the captured values, or to wrap a task (e.g. Runnable, Callable, etc) or an Executor with a decorator that restores ThreadLocal values when the task executes.
In reactive code, e.g. Spring WebFlux controller, create a ContextSnapshot from Reactor Context values. After that use the snapshot to restore ThreadLocal values within a specific stage (operator) of the reactive chain.

Context values can originate from any context mechanism and propagate to any other, any number of times. For example, a value in a Reactor context may originate as a ThreadLocal, and may yet become a ThreadLocal again, and so on.

Generally, imperative code should interact with ThreadLocal values as usual, and likewise Reactor code should interact with the Reactor Context as usual. The Context Propagation library is not intended to replace those, but to assist with propagation when crossing from one type of context to another, e.g. when imperative code invokes a Reactor chain, or when a Reactor chain invokes an imperative component that expects ThreadLocal values.

The library is not limited to context propagation from imperative to reactive. It can assist in asynchronous scenarios to propagate ThreadLocal values from one thread to another. It can also propagate to any other type of context for which there is a registered ContextAccesor instance.

Artifacts

Snapshots are published to repo.spring.io for every successful build on the main branch and maintenance branches.

repositories {
    maven { url 'https://repo.spring.io/snapshot' }
}

dependencies {
    implementation 'io.micrometer:context-propagation:latest.integration'
}

Milestone releases are published to https://repo.spring.io/milestone. Include that as a maven repository in your build configuration to use milestone releases. Note that milestone releases are for testing purposes and are not intended for production use.

These artifacts work with Java 8 or later.

Contributing

See our Contributing Guide for information about contributing to Micrometer Context Propagation.

Code formatting

The spring-javaformat plugin is configured to check and apply consistent formatting in the codebase through the build. The checkFormat task checks the formatting as part of the check task. Apply formatting with the format task. You should rely on the formatting the format task applies instead of your IDE's configured formatting.