transient 필드와 serialVersionUID

transient를 사용하는 이유

public class Session implements Serializable {
    private String username;
    private transient String password;   // NOT serialized
    private transient Connection dbConn; // NOT serialized
}

역직렬화 후 transient 필드 값

Session s = (Session) ois.readObject();
System.out.println(s.username);  // "alice"
System.out.println(s.password);  // null  (transient)

serialVersionUID란 무엇입니까

public class User implements Serializable {
    private static final long serialVersionUID = 1L;
    private String name;
    private int age;
}

자동 생성 UID와 명시적 UID

import java.io.ObjectStreamClass;
long uid = ObjectStreamClass.lookup(User.class).getSerialVersionUID();
System.out.println(uid); // e.g. 3526749255685047947

버전이 일치하지 않을 때 발생하는 InvalidClassException

안전한 클래스 발전

public class User implements Serializable {
    private static final long serialVersionUID = 1L;
    private String name;
    private int age;
    private String email; // Added in v2 — safe if UID unchanged
}

지연 초기화와 transient

public class Document implements Serializable {
    private static final long serialVersionUID = 1L;
    private String content;
    private transient int wordCount = -1;
    public int getWordCount() {
        if (wordCount == -1) wordCount = content.split("\\s+").length;
        return wordCount;
    }
}

정적 필드는 절대 직렬화되지 않음

public class Counter implements Serializable {
    private static final long serialVersionUID = 1L;
    private static int instanceCount = 0; // NOT serialized
    private int value;
}

serialver 도구 사용

외부 직렬화 방식을 대안으로 사용하기

public class Config implements Externalizable {
    private String host;
    private transient String secret;
    public void writeExternal(ObjectOutput out) throws IOException {
        out.writeUTF(host);
    }
    public void readExternal(ObjectInput in) throws IOException {
        host = in.readUTF();
    }
}

모범 사례

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