譬如Writer定義就用一個(gè)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?zhuān)f(shuō)是有利于子類(lèi)，但是具體好處沒(méi)說(shuō)清楚。