Why @Transient can't be used with val fields?

Question

I wrote data class

data class FileHeader(
    val relativePath: String,
    val orderNumber: Long,
    val bodySize: Int
) : Serializable {
@Transient
var headerSize: Int = 0
    get() = relativePath.length + 8
}

It works as i expect.

But why i can't use @Transient with val field?

The error is:

This annotation is not applicable to target member property without backing field or delegate

Are there any reasons why it implemented in this way?

Answer 1

The annotation

Marks the JVM backing field of the annotated property as transient, meaning that it is not part of the default serialized form of the object.

The default serialization works on fields and doesn't care about getter methods. So if there's no backing field, there's nothing to serialize (and nothing to mark as transient in bytecode). The annotation would be useless in this case, so the designers chose to make it an error.

If you don't see why there's no backing field:

A backing field will be generated for a property if it uses the default implementation of at least one of the accessors, or if a custom accessor references it through the field identifier.

With your var, the backing field is needed by the default setter; when you change it to val, it isn't.