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用XMLEncode输出时候，如果有BigDecimal有时候不好使。
原因是：如果类的变量在定义时候有初始值，而不是null，就必须要重载DefaultPersistenceDelegate的mutatesTo方法。
关于这个说明，在网上这里可以看到：
http://forum.java.sun.com/thread.jspa?threadID=631299&messageID=3642493

有兴趣的可以看看：
//setup big decimal delegate.
          DefaultPersistenceDelegate bigDecimalDelegate = new DefaultPersistenceDelegate() {
              protected Expression instantiate(Object oldInstance, Encoder out) {
                  BigDecimal decimal = (BigDecimal) oldInstance;
                  return new Expression(oldInstance, oldInstance.getClass(), "new", new Object[] {decimal.toString()});
              }
              //must override this method.
              // see http://forum.java.sun.com/thread.jspa?threadID=631299&messageID=3642493
              protected boolean mutatesTo(Object oldInstance, Object newInstance) {
                  return oldInstance.equals(newInstance);
              }
              //--- Joshua
          };

网上的牛人说：

This works for BigDecimal properties that aren't initialized, i.e. null. But if you initialize the property, then this won't work unless you override mutatesTo in addition to instantiate mentioned above:

protected boolean mutatesTo(Object oldInstance, Object newInstance) {
return oldInstance.equals(newInstance);
}