如何在运行时从类序列化中排除字段？ [英] How to exclude field from class serialization in runtime?

问题描述

如何在运行时从序列化过程中排除类字段？
编译时有 transient 修饰符，但运行时呢？
我的意思是使用ObjectOutputStream进行常见的java序列化，而不是gson或者什么。

How to exclude class field from serialization process in runtime ? There is transient modifier for compilation time but what about runtime ? I mean common java serialization with ObjectOutputStream, not gson or something.

抱歉，我认为我没有正确解释。这不完全是关于序列化，而是关于 de - 序列化。我有一批遗留文件并像这样处理它们：

Sorry I think I didn't explain right. This is not exactly about serialization , but about de-serialization. I have batch of legacy files and handle them like this:

public class Deserialize {

/**
 * @param args
 * @throws IOException 
 * @throws ClassNotFoundException 
 */
public static void main(String[] args) throws ClassNotFoundException, IOException {
    File file = new File("/home/developer/workspace/DDFS/some.ddf");
    HackedObjectInputStream in = new HackedObjectInputStream(new GZIPInputStream(new FileInputStream(file)));

    System.out.println("Attempt to open " + file.getAbsolutePath());
    Object obj = in.readObject();
    in.close();


}

 static class HackedObjectInputStream extends ObjectInputStream
    {

        /**
         * Migration table. Holds old to new classes representation.
         */
        private static final Map<String, Class<?>> MIGRATION_MAP = new HashMap<String, Class<?>>();

        static
        {
            MIGRATION_MAP.put("DBOBExit", Exit.class);
        }

        /**
         * Constructor.
         * @param stream input stream
         * @throws IOException if io error
         */
        public HackedObjectInputStream(final InputStream stream) throws IOException
        {
            super(stream);
        }

        @Override
        protected ObjectStreamClass readClassDescriptor() throws IOException, ClassNotFoundException
        {
            ObjectStreamClass resultClassDescriptor = super.readClassDescriptor();

            for (final String oldName : MIGRATION_MAP.keySet())
            {
                if (resultClassDescriptor.getName().equals(oldName))
                {
                    resultClassDescriptor = ObjectStreamClass.lookup(MIGRATION_MAP.get(oldName));   
                }
            }

            return resultClassDescriptor;
        }

    }

}

此代码适用于大多数文件，但有些文件抛出

This code works fine for most of files , but some files throws

Exception in thread "main" java.lang.ClassCastException: cannot assign instance of java.awt.Polygon to field Exit.msgbackPt of type java.awt.Point in instance of Exit
at java.io.ObjectStreamClass$FieldReflector.setObjFieldValues(ObjectStreamClass.java:2053)

因为Exit类的版本不同。新版本有新字段。
当我向新字段添加瞬态时，错误消失，但是另一个文件开始抛出异常（最新文件）。

because of different versions of Exit class . New version has new fields. Error disappearing when I add transient to new fields, but another files starts to throwing an exception (latest files).

如果我检测到遗留的加强文件，我可以在运行时向这些新文件添加瞬态吗？
也许反思或什么？

So can I add transient to these new fileds in runtime if I detect legacy serilized file ? Maybe reflection or something ?

推荐答案

ObjectOutputStream文档说：

默认值对象的序列化机制会写入对象的类，类签名以及所有非瞬态和非静态字段的值。对其他对象的引用（瞬态或静态字段除外）也会导致写入这些对象。

The default serialization mechanism for an object writes the class of the object, the class signature, and the values of all non-transient and non-static fields. References to other objects (except in transient or static fields) cause those objects to be written also.

因此，当您将变量声明为瞬态，ObjectOutputStream应该忽略它。确保使用 transient 关键字而不是 @Transient 注释。某些ORM框架使用此类注释来标记不应保存在数据库中的字段。它们对于buildin序列化框架毫无意义。

So when you declare a variable as transient, it should be ignored by ObjectOutputStream. Make sure that you use the transient keyword and not a @Transient annotation. Such annotations are used by some ORM frameworks to mark fields which are not supposed to be saved in databases. They are meaningless for the buildin serialization framework.

private transient String foo; // Field gets ignored by ObjectOutputStream
@Transient private String bar; // Treated normally by ObjectOutputStream (might mean something for some other framework)

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