使类可序列化的用途是什么？

Question

1. I have noticed many of the library classes "ArrayList", "String" even the exceptions are having a serialVersionUID. Why they have made it like this. Whats the practical use of doing that.FYI I am familiar with the concept of Serialization. Please point out the practical purpose of it.

For your reference find the serialversionUid for ClassCastException

public class ClassCastException extends RuntimeException {
private static final long serialVersionUID = -9223365651070458532L;

Where these object's state going to persist? And where will these objects state going to be retrieved ?

2. I am currently working in a project where we are making REST controllers whose input and output parameters will be JSON.We are creating simple POJOs for i/p and o/p parameters.I have seen people making those POJOs serializable .Whats the point in doing that ?

But I havent seen **out.readObject** or out.writeobject which is used to write and read the state of object.Will the POJO's state persist just making it serializable? If yes where it will be stored?

Answer 1

If you want the full story, read the spec: Java Object Serialization Specification .

[...] many of the library classes "ArrayList", "String" even the exceptions are having a serialVersionUID. Why they have made it like this.

To support backwards compatibility when reading objects that were written in an older version of the class. See Stream Unique Identifiers .

Where these object's state going to persist?

Wherever you decide. See Writing to an Object Stream .

And where will these objects state going to be retrieved ?

Wherever you put it. See Reading from an Object Stream .

[...] input and output parameters will be JSON. [...] I have seen people making those POJOs serializable. Whats the point in doing that ?

None. JSON is not using Java serialization. Java serialization creates a binary stream. JSON creates text.

Will the POJO's state persist just making it serializable? If yes where it will be stored?

No, see above.