What causes "Could not serialize object of java type: {}" in hibernate/hibernate-orm?

Read the cause message to find the offending property/type.

hibernate/hibernate-orm · error · IllegalArgumentException

Could not serialize object of java type: {}

Error message

Could not serialize object of java type: {}

What it means

JsonBJsonFormatMapper.toString serializes a SqlTypes.JSON attribute value to column text using jakarta.json.bind Jsonb.toJson (JsonBJsonFormatMapper.java:44-51). This IllegalArgumentException wraps a JsonbException thrown while serializing the current value, i.e. JSON-B cannot render the attribute's runtime object. The Yasson message in the cause names the property or type it failed on.

Source

Thrown at hibernate-core/src/main/java/org/hibernate/type/format/jakartajson/JsonBJsonFormatMapper.java:50

	}

	@Override
	public <T> T fromString(CharSequence charSequence, Type type) {
		try {
			return jsonb.fromJson( charSequence.toString(), type );
		}
		catch (JsonbException e) {
			throw new IllegalArgumentException( "Could not deserialize string to java type: " + type, e );
		}
	}

	@Override
	public <T> String toString(T value, Type type) {
		try {
			return jsonb.toJson( value, type );
		}
		catch (JsonbException e) {
			throw new IllegalArgumentException( "Could not serialize object of java type: " + type, e );
		}
	}
}

View on GitHub (pinned to fad1729dce)

Solutions

Read the cause message to find the offending property/type.
Annotate the value type for JSON-B (@JsonbProperty, @JsonbDateFormat, @JsonbTransient) or convert java.time fields to strings.
Configure a Jsonb with the needed adapters/serializers and pass it via new JsonBJsonFormatMapper(jsonb) through the hibernate.type.json_format_mapper setting.
Keep the attribute type a plain DTO graph so any JSON-B runtime can serialize it.

Example fix

// before - OffsetDateTime field fails under default Yasson serialization
public class Audit {
    public OffsetDateTime changedAt;
}

// after - explicit format makes it serializable
public class Audit {
    @JsonbDateFormat("yyyy-MM-dd'T'HH:mm:ssXXX")
    public OffsetDateTime changedAt;
}

Defensive patterns

Strategy: validation

Validate before calling

// Confirm JSON-B can serialize a value before depending on the flush
try ( Jsonb jsonb = JsonbBuilder.create() ) {
    jsonb.toJson( value ); // throws JsonbException early if not serializable
}

Try / catch

try {
    tx.commit();
} catch ( IllegalArgumentException e ) {
    if ( e.getCause() instanceof jakarta.json.bind.JsonbException je ) {
        // fix the value type (annotations/adapters) per je.getMessage()
    } else throw e;
}

Prevention

Annotate java.time fields with @JsonbDateFormat; add JsonbAdapter for exotic types.
Keep the attribute type to plain DTOs; avoid polymorphic fields unless annotated for JSON-B.
Prefer configuring a custom Jsonb instance via the mapper setting over fighting defaults.

When it happens

Trigger: Flushing an entity whose JSON attribute holds a value JSON-B cannot serialize (e.g. java.time types without a format JSON-B supports, polymorphic types without @JsonbTypeInfo, custom classes without readable properties); getters of the value throwing exceptions during serialization.

Common situations: Moving an application from a Jackson-based mapper to JSON-B where serialization defaults differ; adding java.time or generic Object fields to a JSON-mapped DTO; custom Jsonb instances configured with strict property visibility.

Related errors

AI-assisted analysis of hibernate/hibernate-orm@fad1729dce (2026-08-22). Data as JSON: /api/errors/4155e31727a4e36e. Report an issue: GitHub.