Authoring Java SDK Tasks

Authoring Java SDK Tasks

The Airflow Java SDK implements the language-SDK model for the JVM: your DAG stays in Python, and each task instance runs in a short-lived JVM subprocess. This skill covers the Java-specific native API. The shared model — the Python @task.stub pattern, ID matching, and the XCom-as-JSON contract — lives in authoring-language-sdk-tasks; read that first if you're new to language SDKs.

> Experimental. The Java SDK is in preview. Artifact coordinates and APIs may change.

> Related skills: authoring-language-sdk-tasks (shared Python stub + concepts), configuring-airflow-language-sdks (route the queue to JavaCoordinator), deploying-java-sdk-bundles (compile and ship the JAR).

Recap: the Python side

Java tasks are paired with Python stubs that carry no logic — they declare the task, queue, dependency graph, and retries. IDs must match the Java annotations exactly, and an upstream argument on a stub only declares the dependency (the value is fetched in Java). Full rules are in authoring-language-sdk-tasks; the minimal shape:

from airflow.sdk import dag, task


@dag
def sales_pipeline():                     # dag_id "sales_pipeline" -> @Builder.Dag(id="sales_pipeline")
    @task.stub(queue="java")
    def extract(): ...                    # task_id "extract" -> @Builder.Task(id="extract")

    @task.stub(queue="java")
    def transform(extracted): ...

    transform(extract())


sales_pipeline()

Java side: two APIs

Both APIs produce identical runtime behavior; pick by style, and you can mix them in one bundle.

Annotation-based API (recommended)

Annotate a plain class; an annotation processor generates the wiring (<ClassName>Builder) at compile time.

import static java.lang.System.Logger.Level.INFO;
import org.apache.airflow.sdk.*;

@Builder.Dag(id = "sales_pipeline")          // must match the Python dag_id
public class SalesPipeline {
  private static final System.Logger log = System.getLogger(SalesPipeline.class.getName());

  @Builder.Task(id = "extract")              // must match the Python @task.stub name
  public long extract(Client client) {
    var conn = client.getConnection("sales_db");
    log.log(INFO, "connected to {0}", conn.host);
    return 42L;                              // return value is pushed as the return_value XCom
  }

  @Builder.Task(id = "transform")
  public long transform(
      Client client,
      @Builder.XCom(task = "extract") long recordCount) {  // pulls extract's return_value
    var threshold = (String) client.getVariable("transform_threshold");
    return recordCount * 2;
  }

  @Builder.Task   // id omitted -> the method name "load" is used
  public void load(Context context, @Builder.XCom(task = "transform") long transformed) {
    log.log(INFO, "attempt {0}, value {1}", context.ti.tryNumber, transformed);
  }
}

Annotation reference:

| Annotation | Purpose |

|------------|---------|

| @Builder.Dag(id = "...") | Marks the class as a task container. id must match the Python dag_id; if omitted, the class name is used. Optional to = "..." renames the generated builder (default <ClassName>Builder). |

| @Builder.Task(id = "...") | Marks a method as a task. id must match the Python @task.stub function name; if omitted, the method name is used. |

| @Builder.XCom(task = "...", key = "...") | Injects an upstream task's XCom as a parameter. task defaults to the parameter name; key defaults to the producing task's return_value. The parameter type must be compatible with the stored JSON value. |

A task method's return value is automatically pushed as that task's return_value XCom. A method may declare throws Exception; any uncaught exception fails the task instance (which triggers retries if the stub configured them).

Interface-based API

Implement Task directly when you want full control over registration and XCom handling.

import org.apache.airflow.sdk.*;

public class ExtractTask implements Task {
  @Override
  public void execute(Context context, Client client) throws Exception {
    var conn = client.getConnection("sales_db");
    // ... do work ...
    client.setXCom(42L);   // push return_value explicitly
  }
}

Register tasks manually in a Dag and expose it through a BundleBuilder:

public class MyBundle implements BundleBuilder {
  @Override
  public Iterable<Dag> getDags() {
    var dag = new Dag("sales_pipeline");      // DAG ID matches Python
    dag.addTask("extract", ExtractTask.class);
    dag.addTask("transform", TransformTask.class);
    return java.util.List.of(dag);
  }
}

Each Task class needs a public no-arg constructor. Task IDs must be unique within a DAG, and DAG IDs unique within a bundle.

The entry point

Every bundle has a main that hands your DAGs to the SDK server. The server connects to the coordinator, runs one task instance, and exits.

import java.util.List;
import org.apache.airflow.sdk.*;

public class Main implements BundleBuilder {
  @Override
  public Iterable<Dag> getDags() {
    // With the annotation API, the *Builder classes are generated at compile time.
    return List.of(SalesPipelineBuilder.build());
  }

  public static void main(String[] args) {
    Server.create(args).serve(new Main().build());
  }
}

Server.create(args) parses the connection details Airflow passes on the command line — don't construct them by hand. Record this main class as the bundle's main class when you build it (see deploying-java-sdk-bundles).

Talking to Airflow from a task: Client

A Client is passed into every task and is scoped to the current DAG run and task instance.

| Call | Returns | Notes |

|------|---------|-------|

| client.getConnection(id) | Connection | Fields: id, type, host, schema, login, password, port, extra. Any unset field is null. Throws if the connection doesn't exist. |

| client.getVariable(key) | Object (or null) | Cast to the type you expect, e.g. (String) client.getVariable("threshold"). |

| client.getXCom(taskId) | Object (or null) | Reads another task's return_value by default. Overloads accept key, dagId, runId, mapIndex, and includePriorDates for cross-DAG/run reads and mapped tasks. |

| client.setXCom(value) | — | Pushes the return_value XCom (interface API). Value must be JSON-serializable. With the annotation API, returning a value does this for you. |

Context

The Context parameter exposes run metadata: context.dagRun (dagId, runId) and context.ti (dagId, runId, taskId, mapIndex, tryNumber). tryNumber is useful for retry-aware logic.

XCom: Java types

XComs cross the boundary as JSON (the shared contract is in authoring-language-sdk-tasks). When you read one back in Java you get:

| Python type | JSON | Java type from getXCom |

|-------------|------|--------------------------|

| int | integer | Long (or BigInteger if too large) |

| float | decimal | Double |

| str | string | String |

| bool | boolean | Boolean |

| None | null | null |

| list | array | List<Object> |

| dict | object | Map<String, Object> |

Declare @Builder.XCom parameter types to match. A mismatch (e.g. declaring int when the value is a String) fails the task.

Logging

Declare a logger as a static field named after the class — the conventional pattern regardless of framework:

private static final System.Logger log = System.getLogger(SalesPipeline.class.getName());

For records to reach Airflow's task log store (and show in the UI), the bundle must include one of the SDK logging integration artifacts (airflow-sdk-jpl, airflow-sdk-slf4j, airflow-sdk-log4j2, or airflow-sdk-jul). The dependencies and per-framework setup are in the logging integration section of deploying-java-sdk-bundles. System.Logger (JPL) with airflow-sdk-jpl is the lightest option and needs no configuration.

A complete worked example ships with the SDK

The SDK repository includes a runnable example under java-sdk/example/:

• src/resources/dags/java_examples.py — Python DAGs pairing Python tasks with Java stubs, including a load stub with retries=1.
• src/java/.../AnnotationExample.java — annotation API, including a task that fails on tryNumber == 1 and succeeds on retry.
• src/java/.../InterfaceExampleBuilder.java — the same tasks via the Task interface and Dag.addTask(...).
• src/java/.../ExampleBundleBuilder.java — a BundleBuilder returning both DAGs plus the main entry point.

Point users there for an end-to-end reference.

Java-specific pitfalls

• Cast Object returns deliberately. getVariable and getXCom return Object; match the cast to the JSON type (see the table above).
• @Builder.XCom parameter types must match the stored JSON type, or the task fails at runtime.
• The annotation processor must be on the build for the annotation API (generates <ClassName>Builder); it is not needed for the interface API. See deploying-java-sdk-bundles.
• See authoring-language-sdk-tasks for the language-agnostic pitfalls (ID matching, one JVM per task instance, queue/retries on the stub).

Related Skills

• authoring-language-sdk-tasks: Shared Python-stub pattern and concepts (read first).
• configuring-airflow-language-sdks: Route the java queue to JavaCoordinator and set JRE/coordinator options.
• deploying-java-sdk-bundles: Build the bundle (Gradle/Maven) and place the JAR where Airflow can find it.
• authoring-dags: General Airflow DAG authoring.