Serializzazione personalizzata: writeObject e readObject

Quando la serializzazione predefinita non basta

Dichiarazione di writeObject

private void writeObject(ObjectOutputStream oos) throws IOException {
    oos.defaultWriteObject(); // write non-transient fields normally
    oos.writeUTF(encrypt(this.secret)); // write encrypted secret
}

Dichiarazione di readObject

private void readObject(ObjectInputStream ois)
        throws IOException, ClassNotFoundException {
    ois.defaultReadObject(); // restore non-transient fields
    this.secret = decrypt(ois.readUTF()); // decrypt and restore
}

L'ordine è importante

private void writeObject(ObjectOutputStream oos) throws IOException {
    oos.writeInt(version);
    oos.writeUTF(data);
}
private void readObject(ObjectInputStream ois)
        throws IOException, ClassNotFoundException {
    this.version = ois.readInt();  // same order
    this.data    = ois.readUTF();
}

Compressione dei dati durante la serializzazione

private void writeObject(ObjectOutputStream oos) throws IOException {
    oos.defaultWriteObject();
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    try (GZIPOutputStream gz = new GZIPOutputStream(bos)) {
        gz.write(largePayload.getBytes());
    }
    oos.writeObject(bos.toByteArray());
}

L'hook readResolve

public class AppConfig implements Serializable {
    private static final AppConfig INSTANCE = new AppConfig();
    private AppConfig() {}
    public static AppConfig getInstance() { return INSTANCE; }
    private Object readResolve() { return INSTANCE; } // return singleton
}

L'hook writeReplace

private Object writeReplace() {
    return new UserProxy(this.id); // serialize a lightweight proxy
}

Convalida dei campi in readObject

private void readObject(ObjectInputStream ois)
        throws IOException, ClassNotFoundException {
    ois.defaultReadObject();
    if (age < 0 || age > 150) {
        throw new InvalidObjectException("Invalid age: " + age);
    }
}

Gestione delle modifiche di versione

private static final int SERIAL_VERSION = 2;
private void writeObject(ObjectOutputStream oos) throws IOException {
    oos.defaultWriteObject();
    oos.writeInt(SERIAL_VERSION);
    if (SERIAL_VERSION >= 2) oos.writeUTF(email); // new field
}
private void readObject(ObjectInputStream ois)
        throws IOException, ClassNotFoundException {
    ois.defaultReadObject();
    int v = ois.readInt();
    if (v >= 2) this.email = ois.readUTF();
}

Pattern del proxy di serializzazione

Considerazioni sulle prestazioni

Verifica rapida

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