code: public Introspector getIntrospector() {
          Introspector in = introspector.get();
          if (in == null) {
              introspector = new SoftReference<Introspector>(new Introspector());// 重新创建一个
              return getIntrospector();
          } else {
              return in;
          }
      }
docs: ================= setter / getter =================