Classes that implement the
Externalizable interface must provide the
writeExternal() methods. These methods have package-private or public access, and so they can be called by trusted and untrusted code alike. Consequently, programs must ensure that these methods execute only when intended and that they cannot overwrite the internal state of objects at arbitrary points during program execution.
Noncompliant Code Example
This noncompliant code example allows any caller to reset the value of the object at any time because the
readExternal() method is necessarily declared to be public and lacks protection against hostile callers:
This compliant solution protects against multiple initialization through the use of a Boolean flag that is set after the instance fields have been populated. It also protects against race conditions by synchronizing on a private lock object (see LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code).
Note that this compliant solution is inadequate to protect sensitive data.
Failure to prevent the overwriting of an externalizable object can corrupt the state of the object.
Serialization Specification, A.7, Preventing Overwriting of Externalizable Objects