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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.:

public void readExternal(ObjectInput in) 
                         throws IOException, ClassNotFoundException {
   // Read instance fields
   this.name = (String) in.readObject();
   this.UID = in.readInt();
   // ...
}

Compliant Solution

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 rule LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code).

private final Object lock = new Object();
private boolean initialized = false;

public void readExternal(ObjectInput in)
                         throws IOException, ClassNotFoundException {
  synchronized (lock) {
    if (!initialized) {
      // Read instance fields
      this.name = (String) in.readObject();
      this.UID = in.readInt();
      // ...  
      initialized = true;
    } else {
      throw new IllegalStateException();
    }
  }
}

Note that this compliant solution is inadequate to protect sensitive data.

Risk Assessment

Failure to prevent the overwriting of an externalizable object can corrupt the state of the object.

Bibliography

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