POJO compliance

Flinkboot provides test assertions to ensure your data classes strictly comply with Apache Flink's POJO serialization requirements and prevent runtime Kryo fallback.

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1. Flink POJO requirements

Apache Flink uses an optimized serializer (PojoSerializer) for state and stream data exchange. When Flink recognizes a class as a valid POJO, it can perform direct field access and serialize elements with minimal overhead.

If a class does not satisfy Flink POJO criteria:

• Flink falls back to Kryo serialization, which is slower, less space-efficient, and risky for state schema evolution.
• Keyed operations on nested fields (e.g. keyBy("fieldName")) will fail.

A class must satisfy the following criteria:

1. The class must be public and standalone (or a public static nested class).
2. It must have a public zero-argument constructor.
3. All fields must be either public (non-final) or have public getter and setter methods following JavaBean conventions. Both styles are fully supported.
4. No Kryo fallback fields: No field or nested field may resolve to GenericTypeInfo. FlinkbootAssertions.assertThat(MyClass.class).isPojo() inspects fields recursively to guarantee pure native Flink serialization.

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2. Usage in JUnit 5

Use FlinkbootAssertions.assertThat(Class<?> type).isPojo() to verify your model classes:

import org.junit.jupiter.api.Test;

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

class ModelComplianceTest {

    @Test
    void shouldComplyWithPojoRules() {
        assertThat(MyModel.class).isPojo();
    }
}

If the class violates any of Flink's requirements or contains fields falling back to Kryo serialization, the assertion fails immediately with a descriptive error message indicating the exact path of the invalid field.

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3. Asserting generic types

A Class literal erases generic parameters, so assertThat(Map.class) cannot tell Flink which key and value types are involved. Use a TypeHint to retain them:

import org.apache.flink.api.common.typeinfo.TypeHint;
import java.util.List;
import java.util.Map;

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

class ModelComplianceTest {

    @Test
    void shouldComplyWithGenericTypes() {
        assertThat(new TypeHint<Map<String, List<MyModel>>>() {}).isPojo();
    }
}

Every nested type is validated recursively: the assertion fails if the key type, the value type, or any nested field falls back to Kryo.

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4. Asserting existing TypeInformation

When a type description already exists (produced by a custom TypeInfoFactory, by TypeInformation.of(...), or returned by an operator), pass it directly:

import org.apache.flink.api.common.typeinfo.TypeInformation;

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

class ModelComplianceTest {

    @Test
    void shouldComplyWithTypeInformation() {
        TypeInformation<MyModel> typeInfo = TypeInformation.of(MyModel.class);
        assertThat(typeInfo).isPojo();
    }
}

This verifies directly that a custom factory produces a native Flink serializer instead of a Kryo fallback.