The purpose of synchronization is to ensure that only one thread executes a given block of code at a time. There's no real problem with marking writeObject synchronized, but if it's highly suspicious if this serialization-related method the only synchronized code in a class.

Noncompliant Code Example

public class RubberBall {

  private Color color;
  private int diameter;

  public RubberBall(Color color, int diameter) {
    // ...
  }

  public void bounce(float angle, float velocity) {
    // ...
  }

  private synchronized void writeObject(ObjectOutputStream stream) throws IOException { // Noncompliant
    // ...
  }
}

Compliant Solution

public class RubberBall {

  private Color color;
  private int diameter;

   public RubberBall(Color color, int diameter) {
    // ...
  }

  public void bounce(float angle, float velocity) {
    // ...
  }

  private void writeObject(ObjectOutputStream stream) throws IOException {
    // ...
  }
}