Java.Serialization.How can I exclude fields from serialization?

🚫 Use the transient Keyword

When a field is marked as transient, Java skips it during serialization — it will not be written to the stream, and when deserialized, it will be set to its default value.

import java.io.*;

public class User implements Serializable {
    private static final long serialVersionUID = 1L;

    String username;
    transient String password; // ❌ Do not serialize this

    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }
}

🔄 When deserialized:

User u = new User("Stanley", "mySecret123");
serializeToFile(u); // Save to file

User restored = deserializeFromFile(); // Load from file
System.out.println(restored.username); // "Stanley"
System.out.println(restored.password); // null ✅ because it was transient

🧠 Why Use transient?

Field Type	Reason to Exclude
Passwords / API Keys	🛡 Security
Cache / derived values	♻ Can be rebuilt, don’t waste space
Database connections / threads	🔌 Can’t be serialized
Large fields not needed after restart	💾 Save space

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⚠️ What Happens to Transient Fields?

Field Type	Deserialized Value
String	null
int, long, double, etc.	0
boolean	false
Object	null

You can rebuild or initialize them in:

• readObject() method
• A custom constructor or init method

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🧵 TL;DR

Feature	Usage
Exclude field from serialization	transient keyword
Default value after deserialization	Yes
Used for	Sensitive, non-serializable, or unnecessary data