Java中transient关键字的作用及使用场景

import java.io.Serializable;

public class User implements Serializable {
    private String username;
    private transient String password;
    private int age;

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

    public String getUsername() {
        return username;
    }

    public String getPassword() {
        return password;
    }

    public int getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "User{" +
                "username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", age=" + age +
                '}';
    }

    public static void main(String[] args) {
        User user = new User("testuser", "secretpassword", 30);

        // 序列化
        try (java.io.FileOutputStream fileOut = new java.io.FileOutputStream("user.ser");
             java.io.ObjectOutputStream out = new java.io.ObjectOutputStream(fileOut)) {
            out.writeObject(user);
            System.out.println("Serialized data is saved in user.ser");
        } catch (java.io.IOException i) {
            i.printStackTrace();
        }

        // 反序列化
        User deserializedUser = null;
        try (java.io.FileInputStream fileIn = new java.io.FileInputStream("user.ser");
             java.io.ObjectInputStream in = new java.io.ObjectInputStream(fileIn)) {
            deserializedUser = (User) in.readObject();
            System.out.println("Deserialized User...");
            System.out.println(deserializedUser);
        } catch (java.io.IOException i) {
            i.printStackTrace();
        } catch (ClassNotFoundException c) {
            System.out.println("User class not found");
            c.printStackTrace();
        }

        // 输出反序列化后的用户信息
        if (deserializedUser != null) {
            System.out.println("Username: " + deserializedUser.getUsername());
            System.out.println("Password: " + deserializedUser.getPassword()); // 输出null
            System.out.println("Age: " + deserializedUser.getAge());
        }
    }
}

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;

public class MyClass implements Serializable {
    private String name;
    private transient int cachedValue;

    public MyClass(String name) {
        this.name = name;
        this.cachedValue = calculateValue(); // 初始计算缓存值
    }

    private int calculateValue() {
        // 模拟耗时计算
        System.out.println("Calculating cached value...");
        return name.length() * 2;
    }

    public String getName() {
        return name;
    }

    public int getCachedValue() {
        return cachedValue;
    }

    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject(); // 先执行默认的反序列化

        // 反序列化后重新计算 cachedValue
        cachedValue = calculateValue();
        System.out.println("cachedValue re-initialized after deserialization.");
    }

    public static void main(String[] args) {
        MyClass obj = new MyClass("example");

        // 序列化
        try (java.io.FileOutputStream fileOut = new java.io.FileOutputStream("myclass.ser");
             java.io.ObjectOutputStream out = new java.io.ObjectOutputStream(fileOut)) {
            out.writeObject(obj);
            System.out.println("Serialized data is saved in myclass.ser");
        } catch (java.io.IOException i) {
            i.printStackTrace();
        }

        // 反序列化
        MyClass deserializedObj = null;
        try (java.io.FileInputStream fileIn = new java.io.FileInputStream("myclass.ser");
             java.io.ObjectInputStream in = new java.io.ObjectInputStream(fileIn)) {
            deserializedObj = (MyClass) in.readObject();
            System.out.println("Deserialized MyClass...");
            System.out.println("Name: " + deserializedObj.getName());
            System.out.println("Cached Value: " + deserializedObj.getCachedValue());
        } catch (java.io.IOException i) {
            i.printStackTrace();
        } catch (ClassNotFoundException c) {
            System.out.println("MyClass class not found");
            c.printStackTrace();
        }
    }
}