hibernate/hibernate-orm · error · IllegalArgumentException

Could not serialize object of java type: {}

Error message

Could not serialize object of java type: {}

What it means

JacksonXmlFormatMapper.toString serializes a SqlTypes.SQLXML attribute value to XML via the Jackson XmlMapper (JacksonXmlFormatMapper.java:235-240). This IllegalArgumentException wraps a JsonProcessingException thrown while writerFor(type).writeValueAsString(value) runs, meaning the value cannot be rendered as XML by Jackson-dataformat-xml. The cause carries the precise reason (no serializer, non-String map keys, self-reference).

Source

Thrown at hibernate-core/src/main/java/org/hibernate/type/format/jackson/JacksonXmlFormatMapper.java:235

					collectionWrapper = new CollectionWrapper<>( list );
				}
				return writeValueAsString(
						collectionWrapper,
						javaType,
						new ParameterizedTypeImpl( CollectionWrapper.class,
								new Type[] {javaType.getJavaTypeClass().getComponentType()}, null )
				);
			}
		}
		return writeValueAsString( value, javaType, javaType.getJavaType() );
	}

	private <T> String writeValueAsString(Object value, JavaType<T> javaType, Type type) {
		try {
			return objectMapper.writerFor( objectMapper.constructType( type ) ).writeValueAsString( value );
		}
		catch (JsonProcessingException e) {
			throw new IllegalArgumentException( "Could not serialize object of java type: " + javaType, e );
		}
	}

	@JacksonXmlRootElement(localName = "Collection")
	public static class CollectionWrapper<E> {
		@JacksonXmlElementWrapper(useWrapping = false)
		@JacksonXmlProperty(localName = "e")
		Collection<E> value;

		public CollectionWrapper() {
			this.value = new ArrayList<>();
		}

		public CollectionWrapper(Collection<E> value) {
			this.value = value;
		}
	}

View on GitHub (pinned to fad1729dce)

Solutions

1. Read the cause message to find the offending field of the value being flushed.
2. Use String keys in Map attributes (or a List of key/value entries annotated with @JacksonXmlElementWrapper/@JacksonXmlProperty).
3. Annotate the type with Jackson-XML annotations (@JacksonXmlRootElement, @JacksonXmlProperty) so the value renders as valid XML.
4. Register required Jackson modules on the XmlMapper you pass via JacksonXmlFormatMapper(ObjectMapper, legacyFormat).
5. Keep the XML attribute type simple (DTO graph without cycles).

Example fix

// before - non-String map keys cannot be serialized to XML
@JdbcTypeCode(SqlTypes.SQLXML)
Map<UUID, String> items; // IllegalArgumentException on flush

// after - String keys render fine
@JdbcTypeCode(SqlTypes.SQLXML)
Map<String, String> items;

// Verify the attribute value renders as XML before it reaches Hibernate
static void assertXmlSerializable(Object value) {
    try {
        XmlMapper xml = XmlMapper.builder().findAndAddModules().build();
        xml.writeValueAsString( value );
    } catch ( JsonProcessingException e ) {
        throw new IllegalStateException( "Value will fail on flush: " + e.getOriginalMessage(), e );
    }
}

try {
    tx.commit();
} catch ( IllegalArgumentException e ) {
    if ( e.getCause() instanceof com.fasterxml.jackson.core.JsonProcessingException jpe ) {
        // e.g. non-String map keys - fix the attribute structure
    } else throw e;
}

When it happens

Trigger: Flushing an entity whose XML attribute value contains a Map with non-String keys (a known Jackson-XML limitation), a directly self-referencing object, java.time values without the corresponding module, or a root type Jackson-XML cannot serialize (missing properties/annotations for unwrapped collections).

Common situations: Switching a JSON attribute to @JdbcTypeCode(SqlTypes.SQLXML) and hitting XML-specific constraints that JSON tolerated (map keys, arrays without wrapper elements); adding java.time or polymorphic fields to an existing XML-mapped DTO; custom XmlMapper built without findModules.

Related errors

• Could not deserialize string to java type: {}
• Could not serialize object of java type: {}
• Could not serialize object of java type: {}
• Could not serialize object of java type: {}
• Could not deserialize string to java type: {}

AI-assisted analysis of hibernate/hibernate-orm@fad1729dce (2026-08-22). Data as JSON: /api/errors/5c4931571e953cd5. Report an issue: GitHub.