java哈希映射可序列化性

Question

HashMap实现了可序列化接口；所以它可以被序列化。我已经研究了HashMap的实现，条目[]表被标记为transient。由于条目[]表是存储映射的全部内容的表，如果无法序列化，那么在反序列化期间映射是如何构造的

Answer 1

哈希映射在序列化期间不会序列化其条目对象。看看它的writeObject方法

javadocs解释道：

The capacity of the HashMap (the length of the bucket array) is emitted (int), followed by the size (an int, the number of key-value mappings), followed by the key (Object) and value (Object) for each key-value mapping. The key-value mappings are emitted in no particular order.

如果查看readObject方法，您将看到如何使用大小、键和值重建条目表

Answer 2

如果查看the source，您将看到它不依赖默认的序列化机制，而是手动写出所有条目（作为键和值的交替流）：

/**
  * Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,
  * serialize it)
  *
  * @serialData The <i>capacity</i> of the HashMap (the length of the
  *             bucket array) is emitted (int), followed by the
  *             <i>size</i> (an int, the number of key-value
  *             mappings), followed by the key (Object) and value (Object)
  *             for each key-value mapping.  The key-value mappings are
  *             emitted in no particular order.
  */
      private void writeObject(java.io.ObjectOutputStream s)
             throws IOException
         {
             Iterator<Map.Entry<K,V>> i =
                 (size > 0) ? entrySet0().iterator() : null;

            // Write out the threshold, loadfactor, and any hidden stuff
            s.defaultWriteObject();

            // Write out number of buckets
            s.writeInt(table.length);

            // Write out size (number of Mappings)
            s.writeInt(size);

            // Write out keys and values (alternating)
            if (i != null) {
                while (i.hasNext()) {
                    Map.Entry<K,V> e = i.next();
                    s.writeObject(e.getKey());
                    s.writeObject(e.getValue());
                }
            }
        }

这比数组更紧凑，因为数组可以包含许多空条目和链接链，以及Map$Entry包装器的开销

注意，它仍然为“easy”字段调用defaultWriteObject。为了让它工作，它必须将所有其他内容标记为transient

Answer 3

HashMap通过使用writeObject和readObject方法来处理自己的序列化