hibernate/hibernate-orm · error · SerializationException

could not serialize

Error message

could not serialize

What it means

serialize() hands the object to ObjectOutputStream.writeObject; any IOException from the JDK is wrapped into SerializationException with this message. The dominant cause is NotSerializableException for the object itself or something reachable from a non-transient field, though any serialization-time IO failure on the destination stream lands here too. The original IOException is preserved as the cause and names the offending class.

Source

Thrown at hibernate-core/src/main/java/org/hibernate/internal/util/SerializationHelper.java:112

	public static void serialize(Serializable obj, OutputStream outputStream) throws SerializationException {
		if ( outputStream == null ) {
			throw new IllegalArgumentException( "The OutputStream must not be null" );
		}

		if ( CORE_LOGGER.isTraceEnabled() ) {
			if ( Hibernate.isInitialized( obj ) ) {
				CORE_LOGGER.tracev( "Starting serialization of object [{0}]", obj );
			}
			else {
				CORE_LOGGER.trace( "Starting serialization of [uninitialized proxy]" );
			}
		}

		try ( var out = new ObjectOutputStream( outputStream ) ) {
			out.writeObject( obj );
		}
		catch (IOException ex) {
			throw new SerializationException( "could not serialize", ex );
		}
	}

	/**
	 * Serializes an object to a byte array for storage or
	 * externalization.
	 *
	 * @param obj the object to serialize to bytes
	 *
	 * @return a byte[] with the converted Serializable
	 *
	 * @throws SerializationException (runtime) if the serialization fails
	 */
	public static byte[] serialize(Serializable obj) throws SerializationException {
		final var byteArrayOutputStream = new ByteArrayOutputStream( 512 );
		serialize( obj, byteArrayOutputStream );
		return byteArrayOutputStream.toByteArray();
	}

View on GitHub (pinned to fad1729dce)

Solutions

1. Read getCause(): NotSerializableException names the exact offending class — make it implement Serializable or mark the field transient.
2. For third-party types you cannot modify, store a serializable projection (a DTO) or switch the column format to JSON.
3. If the cause is a genuine IO error (disk, channel), fix the destination resource rather than the object graph.

Example fix

// before
public class SessionData implements Serializable {
    private Connection jdbcConnection; // NotSerializableException -> "could not serialize"
}

// after
public class SessionData implements Serializable {
    private transient Connection jdbcConnection;
    private String connectionUrl; // serializable descriptor instead
}

static void requireSerializable(Object obj) {
    if (obj != null && !(obj instanceof java.io.Serializable)) {
        throw new IllegalArgumentException("Not serializable: " + obj.getClass().getName());
    }
}

static boolean isSerializable(Object obj) {
    return obj == null || obj instanceof java.io.Serializable;
}

try {
    byte[] bytes = SerializationHelper.serialize(obj);
} catch (org.hibernate.type.SerializationException e) {
    if (e.getCause() instanceof java.io.NotSerializableException nse) {
        // nse.getMessage() names the offending class; make it Serializable or mark the field transient
    }
}

When it happens

Trigger: Serializing an object whose class (or one of its fields' classes) does not implement java.io.Serializable; graphs containing non-serializable runtime types such as Connection, Socket, or InputStream; IO failures when the destination stream breaks mid-write (disk full, closed channel).

Common situations: Entities or DTOs stored as serialized blobs (SERIALIZED varbinary columns) that gained a non-serializable field; objects wrapping JDBC or service references; third-party value types without Serializable; cache stores that serialize detached state after a refactor.

Related errors

• Unable to deserialize from cached file [%s]
• Cannot serialize Session while connected
• Blobs may not be accessed after serialization
• Clobs may not be accessed after serialization
• Cannot serialize an EntityUniqueKey which represents a non s

AI-assisted analysis of hibernate/hibernate-orm@fad1729dce (2026-08-22). Data as JSON: /api/errors/c866d54fc5692472. Report an issue: GitHub.