Section 6 of the Java Object Serialization Specification does not explicitly
specify the format of field type strings. The only information given is the
grammar production:

    className1:
      (String) object // String containing the field's type

Instead, it should specify that the field's type string must be in JVM "field
descriptor" format (e.g., "Ljava/lang/String;") as described in section 4.3.2
of the JVM spec. This has been the behavior of serialization since it was
introduced in JDK 1.1.