Airflow Java SDK contributor guide

The Java SDK lets Airflow tasks execute JVM code (Java, Kotlin, or any JVM language). You are helping a contributor work in one or both of these locations:

• java-sdk/ — the JVM-side library (Kotlin source, published to Maven)
• task-sdk/src/airflow/sdk/coordinators/java/ — the Python coordinator that launches the JVM subprocess

Read these two documents early in every session — they contain the authoritative reference material:

• airflow-core/docs/authoring-and-scheduling/language-sdks/java.rst — user-facing guide: annotation vs. interface API, XCom type mapping, Gradle/Maven steps, coordinator config.
• java-sdk/README.md — contributor guide: repository layout, detailed execution walkthrough, Gradle + Breeze test commands, coding conventions, common tasks, and PR checklist.

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SDK package architecture

The JVM-side library is split into two packages with distinct visibility rules:

• org.apache.airflow.sdk — public, user-facing API. Classes here (e.g. Client, Bundle, BundleBuilder, Server) are stable contracts that DAG authors and task implementers import directly. Changes to this package are breaking changes.
• org.apache.airflow.sdk.execution — internal implementation detail. Everything in this package (CoordinatorComm, LogSender, Log, Client in execution/, generated schema models, etc.) is not intended to be imported by users. It may change between releases without notice.

When reviewing or writing code, enforce this boundary: user task code and BundleBuilder subclasses must only import from org.apache.airflow.sdk; any import of org.apache.airflow.sdk.execution.* in user-facing API surface is a red flag.

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Bundle composition and coordinator discovery

A bundle is a directory of JAR files (typically build/bundle/) placed on the coordinator's jars_root. The coordinator scans the directory at task-dispatch time to find:

1. Main-Class (standard JAR manifest attribute) — the fully-qualified class name of the entry point that the coordinator invokes with java -classpath … <Main-Class> --comm … --logs …. This must be a class with a public static void main(String[] args) method; the Gradle plugin org.apache.airflow.sdk writes it automatically from airflowBundle { mainClass = "…" } and validates that the class exists and has the right signature at build time.

2. Airflow-Supervisor-Schema-Version (Airflow-specific manifest attribute) — the wire protocol version the JVM side expects when talking to the Python supervisor. In fat-JAR mode (the default), the Gradle plugin reads this value from the airflow-sdk JAR in runtimeClasspath and copies it into the shadow JAR manifest. In thin-JAR mode (fatJar = false), the value stays in the airflow-sdk JAR deployed alongside the bundle JAR.

The Python coordinator (JavaCoordinator) scans every JAR under jars_root with _JarInfo.find(), reads META-INF/MANIFEST.MF out of each ZIP, and collects Main-Class and Airflow-Supervisor-Schema-Version from whichever JARs carry them. The resolved schema version is then passed as the schema_version return value from _build_execute_task_command, which the base SubprocessCoordinator uses to negotiate the supervisor wire protocol.

If main_class is set explicitly on the JavaCoordinator instance (via [sdk] coordinators kwargs), the scan uses it as a filter; otherwise the first JAR with a Main-Class attribute wins. Either way, Airflow-Supervisor-Schema-Version must be present in at least one JAR in jars_root or startup fails.

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Key files to know

File	Purpose
java-sdk/sdk/.../Client.kt	Public API (Variables, Connections, XCom)
java-sdk/sdk/.../execution/Client.kt	Supervisor wire calls
java-sdk/sdk/.../execution/Comm.kt	4-byte-prefix MessagePack framing
java-sdk/sdk/.../Server.kt	Entry-point; drives the execution loop
java-sdk/processor/.../BuilderProcessor.kt	Kapt annotation processor
java-sdk/plugin/.../AirflowSdkPlugin.kt	Gradle bundle plugin
task-sdk/.../coordinators/java/coordinator.py	Python side — spawns the JVM
task-sdk/.../schema/schema.json	Wire protocol definition (both sides)

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Running tests

Always use ./gradlew from inside java-sdk/; never run Gradle via apt's gradle. See java-sdk/README.md#testing for the full list of Gradle commands.

For the Python coordinator, use Breeze (never pytest directly on the host):

breeze testing task-sdk-tests -- task_sdk/coordinators/java

End-to-end test suite:

E2E_TEST_MODE=java_sdk uv run --project airflow-e2e-tests pytest \
    tests/airflow_e2e_tests/java_sdk_tests/ -xvs

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Updating the Python coordinator

coordinator.py extends SubprocessCoordinator. The only method subclasses must implement is _build_execute_task_command, which returns (argv, schema_version). Look at the existing implementation for how jars_root, java_executable, jvm_args, and main_class are assembled into the command. Do not reach into the JVM process from Python beyond what this method provides.

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Upgrading Supervisor Schema client

When upgrading to a newer Supervisor Schema version:

• Regenerate models with ./gradlew generateJsonSchema2Pojo
• Modify execution/Client.kt to handle changes

The java-sdk/README.md#contributing section walks through the full "adding a new Client method" sequence step by step.