从磁盘读取序列化的Java对象，使用较新的类？

Question

Just a simple question:

If I saved a java object on disk, but after that I added some more data member (String type), can new class read those old java object back with those new String member as null?

Answer 1

This is possible, but for that you need to manage compatibility actively. You need to ensure that the new class has the same serialVersionUID value as the old class. This private static final long serialVersionUID field can be auto-generated - or manually assigned. See http://docs.oracle.com/javase/8/docs/api/java/io/Serializable.html

If you already have a serialVersionUID , the new field will be null.

If you don't, you want to create one and use the same value as the auto-generated value of the old class. That's what the tool serialver is for.

Answer 2

The new String member will be whatever the default value is in the class definition I believe (or set in a default constructor).

Do you have to use ObjectOutputStream / ObjectInputStream to serialize your Java objects? Those are 'old-school' and write binary (not human readable format). This format retains actual structural (field) knowledge of the class.

If you used JSON for serialization (or technically XML but it will be larger and harder to deal with), the serialized content will be more 'future-proof'. When reading an old JSON String into Java, if it has missing fields for a newer version of the class, the fields will retain their default values. Most importantly, the serialized data will be human-readable, and easily edited with any modern IDE or a website like: http://www.jsoneditoronline.org/

If you can, try a Java to JSON (and JSON to Java) serialization library like json-io ( https://github.com/jdereg/json-io ). With just one line of code, you can convert your Java object graph into a JSON String:

String json = JsonWriter.objectToJson(root);

Similarly, with one line of code, you can read this JSON back into Java objects:

Object root = JsonReader.jsonToJava(json);

Finally, using JSON format works great with Javascript. If a webserver returns an HTTP response with JSON content, this content will make it's way to the Javascript developer as objects, ready-to-go (the Javascript developer will not have to write parsing code). The objects will be straight-up readable as if they were defined in the Javascript code.