Serialization compliance

In Apache Flink streaming applications, user-defined functions and helper objects are distributed across remote TaskManagers using standard Java serialization.

If any captured object or nested field does not implement java.io.Serializable, Flink fails at submission or initialization with a fatal java.io.NotSerializableException.

Flinkboot provides testing assertions in flinkboot-test to verify that your classes and object graphs comply with standard Java serialization requirements.

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1. How isSerializable() works

The assertion FlinkbootAssertions.assertThat(actual).isSerializable() performs a complete in-memory serialization round-trip:

1. Serialization: Writes the target object into a byte buffer using java.io.ObjectOutputStream.
2. Deserialization: Reconstructs the object from the byte buffer using java.io.ObjectInputStream.

This verifies that:

• The target class implements java.io.Serializable.
• Every nested object, collection element, and referenced type in the object graph is serializable.
• Custom writeObject or readObject hooks execute without errors.

If any element fails serialization, the assertion fails immediately with a descriptive error detailing the exact offending field.

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2. Usage in JUnit 5

Pass any object instance directly to assertThat(actual).isSerializable():

import org.junit.jupiter.api.Test;

import static io.github.sekelenao.flinkboot.test.api.assertion.FlinkbootAssertions.assertThat;

class SerializationComplianceTest {

    @Test
    void shouldBeSerializable() {
        assertThat(payload).isSerializable();
    }
}