在代码中s.defaultWriteObject();中size应该也被序列化了,为什么下边还要再单独序列化一次呢?
这样写是出于兼容性考虑。
旧版本的JDK中,ArrayList的实现有所不同,会对length字段进行序列化。
而新版的JDK中,对优化了ArrayList的实现,不再序列化length字段。
这个时候,如果去掉s.writeInt(size),那么新版本JDK序列化的对象,在旧版本中就无法正确读取,
因为缺少了length字段。
因此这种写法看起来多此一举,实际上却保证了兼容性。
附上官方解释:defaultReadObject() and defaultWriteObject() should be the first method call inside readObject(ObjectInputStream o) and writeObject(ObjectOutputStream o). It reads and writes all the non-transient fields of the class respectively.
These methods also helps in backward and future compatibility. If in future you add some non-transient field to the class and you are trying to deserialize it by the older version of class then the defaultReadObject() method will neglect the newly added field, similarly if you deserialize the old serialized object by the new version then the new non transient field will take default value from JVM i.e. if its object then null else if primitive then boolean to false, int to 0 etc….