用内部成员实例做lock，而不用实例自身this做同步有何好处？

Question

譬如Writer定义就用一个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;
  。。。
  }

看了英文解释，说是有利于子类，但是具体好处没说清楚。

Answer 1

哪里提到内部类了？？？
无参构造器不是赋值this吗？

    protected Writer() {
        this.lock = this;
    }

Answer 2

解耦吧。用户可以传自己的lock 进去