java - writeObject() 如何访问 serialPersistantFields

Question

需要序列化，我理解这一点，但是 stream.defaultWriteObject() 如何实际访问它们，因为它们是私有(private)的、静态的和最终的。

我的 CS 讲师无法解释这一点(或者他只是不想解释)，而且(据我所知)也没有任何文档可以解释这种机制。

即使是抽象的解释也是非常受欢迎的!

Answer 1

我们通常采用的获取私有(private)字段的方式:反射。具体通过Field.setAccessible(true)。这是他们执行此操作的代码，来自 java.io.ObjectStreamClass，Java 8:

/**
 * Returns serializable fields of given class as defined explicitly by a
 * "serialPersistentFields" field, or null if no appropriate
 * "serialPersistentFields" field is defined.  Serializable fields backed
 * by an actual field of the class are represented by ObjectStreamFields
 * with corresponding non-null Field objects.  For compatibility with past
 * releases, a "serialPersistentFields" field with a null value is
 * considered equivalent to not declaring "serialPersistentFields".  Throws
 * InvalidClassException if the declared serializable fields are
 * invalid--e.g., if multiple fields share the same name.
 */
private static ObjectStreamField[] getDeclaredSerialFields(Class<?> cl)
    throws InvalidClassException
{
    ObjectStreamField[] serialPersistentFields = null;
    try {
        Field f = cl.getDeclaredField("serialPersistentFields");
        int mask = Modifier.PRIVATE | Modifier.STATIC | Modifier.FINAL;
        if ((f.getModifiers() & mask) == mask) {
            f.setAccessible(true);
            serialPersistentFields = (ObjectStreamField[]) f.get(null);
        }
    } catch (Exception ex) {
    }