用內(nèi)部成員實例做lock，而不用實例自身this做同步有何好處？

譬如Writer定義就用一個lock,

public abstract class Writer implements Appendable, Closeable, Flushable {

  /**
   * The object used to synchronize operations on this stream.  For
   * efficiency, a character-stream object may use an object other than
   * itself to protect critical sections.  A subclass should therefore use
   * the object in this field rather than <tt>this</tt> or a synchronized
   * method.
   */
  protected Object lock;
  。。。
  }

看了英文解釋，說是有利于子類，但是具體好處沒說清楚。