final transient fields and serialization

Question

Is it possible to have final transient fields that are set to any non-default value after serialization in Java? My usecase is a cache variable — that\'s why i

Answer 1

Yes, this is easily possible by implementing the (apparently little known!) readResolve() method. It lets you replace the object after it is deserialized. You can use that to invoke a constructor that will initialize a replacement object however you want. An example:

import java.io.*;
import java.util.*;

public class test {
    public static void main(String[] args) throws Exception {
        X x = new X();
        x.name = "This data will be serialized";
        x.cache.put("This data", "is transient");
        System.out.println("Before: " + x + " '" + x.name + "' " + x.cache);

        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        new ObjectOutputStream(buffer).writeObject(x);
        x = (X)new ObjectInputStream(new ByteArrayInputStream(buffer.toByteArray())).readObject();
        System.out.println("After: " + x + " '" + x.name + "' " + x.cache);
    }

    public static class X implements Serializable {
        public final transient Map cache = new HashMap<>();
        public String name;

        public X() {} // normal constructor

        private X(X x) { // constructor for deserialization
            // copy the non-transient fields
            this.name = x.name;
        }

        private Object readResolve() {
            // create a new object from the deserialized one
            return new X(this);
        }
    }
}

Output -- the string is preserved but the transient map is reset to an empty (but non-null!) map:

Before: test$X@172e0cc 'This data will be serialized' {This data=is transient}
After: test$X@490662 'This data will be serialized' {}