J2SE中的序列化之继承

J2SE中的序列化是将对象转换成字节流，用于对象的存储和传输。而在序列化对象时，如果该对象实现了Serializable接口，那么子类也会自动实现序列化，这就是所谓的“继承序列化”。

下面通过示例说明继承序列化的几个要点:

1.子类序列化时父类属性的序列化与反序列化：

public class Parent implements Serializable{
    private static final long serialVersionUID = 1L;
    private int parentId;
    public Parent(int parentId) {
        this.parentId = parentId;
    }
    public int getParentId() {
        return parentId;
    }
    public void setParentId(int parentId) {
        this.parentId = parentId;
    }
}

public class Child extends Parent {
    private static final long serialVersionUID = 1L;
    private int childId;
    public Child(int parentId, int childId) {
        super(parentId);
        this.childId = childId;
    }
    public int getChildId() {
        return childId;
    }
    public void setChildId(int childId) {
        this.childId = childId;
    }
}

public static void main(String[] args) throws Exception {
    FileOutputStream fos = new FileOutputStream("serial.txt");
    ObjectOutputStream oos = new ObjectOutputStream(fos);
    Child child = new Child(100, 200);
    oos.writeObject(child);
    oos.close();
    fos.close();

    FileInputStream fis = new FileInputStream("serial.txt");
    ObjectInputStream ois = new ObjectInputStream(fis);
    Child newChild = (Child) ois.readObject();
    System.out.println(newChild.getParentId());
    System.out.println(newChild.getChildId());
    ois.close();
    fis.close();
}

在父类属性没有实现Serializable接口时，子类序列化时，父类属性不会被序列化和反序列化，因此在输出结果中只能得到子类的属性值。

2.子类实现自己的readObject与writeObject：

public class Child extends Parent {
    private static final long serialVersionUID = 1L;
    private int childId;
    public Child(int parentId, int childId) {
        super(parentId);
        this.childId = childId;
    }
    public int getChildId() {
        return childId;
    }
    public void setChildId(int childId) {
        this.childId = childId;
    }
    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        out.writeInt(getParentId());
    }
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        setParentId(in.readInt());
    }
}

public static void main(String[] args) throws Exception {
    FileOutputStream fos = new FileOutputStream("serial.txt");
    ObjectOutputStream oos = new ObjectOutputStream(fos);
    Child child = new Child(100, 200);
    oos.writeObject(child);
    oos.close();
    fos.close();

    FileInputStream fis = new FileInputStream("serial.txt");
    ObjectInputStream ois = new ObjectInputStream(fis);
    Child newChild = (Child) ois.readObject();
    System.out.println(newChild.getParentId());
    System.out.println(newChild.getChildId());
    ois.close();
    fis.close();
}

我们在Child类中添加了自己的writeObject和readObject方法，序列化时只需要序列化子类的属性，而在反序列化时，我们显式地从输入流中读取并设置父类的属性值，这样就能够正确地反序列化子类。

综上，了解继承序列化可以让我们更好地掌握Java的序列化机制，避免一些问题的出现。