序列化和反序列化

1. Serializable接口
只是一个标记接口，不包含任何的方法的定义

public class Teacher implements Serializable {

    private static final long serialVersionUID = 1915987587027965086L;

    private String name;

    private Integer age;

    public Teacher(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    //getter,setter......

    @Override
    public String toString() {
        return "Teacher{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

作用：
serialVersionUID用于验证反序列化的类和本地的类的版本是否一致，比如从网络获取反序列化的UID为1L,而本地的UID为2L，则会产生InvalidCastException异常，只有版本一致才能转化
同时本地Classpath也必须有这个类的.class文件，否者也没有比较UID这么一说,若不存在该文件会产生ClassNotFoundException

生成规则：
一是默认的1L，比如：private static final long serialVersionUID = 1L;
二是根据类名、接口名、成员方法及属性等来生成一个64位的哈希字段

2. Externalizable接口
继承自Serializable，定义了两个方法，用于序列化的反序列化时自动调用，可以自定义需要序列化的属性，以及属性的初始化

void writeExternal(ObjectOutput out) throws IOException;

void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;

序列化时自动调用writeExternal，在方法中可以自定义那些属性字段需序列化

 @Override
    public void writeExternal(ObjectOutput out) throws IOException {
        out.writeObject(name);
        out.writeObject(age);
        //out.writeObject(teacher); 无需序列化则省略
    }

反序列化自动调用readExternal，在反序列化时可以设置被忽略字段的初始值

 @Override
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        name = (String)in.readObject();
        age = (Integer)in.readObject();
        //teacher = (Teacher)in.readObject();
        teacher = new Teacher("default", 0);  //自定义初始值
    }

序列化和反序列化的字段的顺序需要一致，否则不同类型的数据会产生ClassCastException
会调用Student的默认的无参构造函数，必须提供无参构造函数（没有无参构造函数：InvalidClassException: com.sxd.bean.Student; no valid constructor），对于Serializable对象是完全以它存储的二进制位为基础来构造，而不是通过构造器构造的，对于Enternalizable对象会通过默认无参构造函数初始化构造对象

3. ObjectOutputStream/ObjectInputStream
Student类中的属性必须都为可序列化对象，有一个为不可序列化对象（没有继承Serializable）则会产生java.io.NotSerializableException异常

public class SerializeableTest {

    public static void main(String[] args) throws Exception {
        writeSerializeStudent();
        readSerializeStudent();
    }

    private static void writeSerializeStudent() throws Exception {
        Teacher teacher = new Teacher("haha", 36);
        Student student = new Student("sxd", 22, teacher);
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("F:/student.out"));
        oos.writeObject(student);
        oos.close();
    }

    public static void readSerializeStudent() throws Exception {
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("F:/student.out"));
        Student student = (Student)ois.readObject();
        System.out.println(student);
    }
}

//Student
public class Student implements Externalizable {

    private static final long serialVersionUID = 2662734246959142949L;

    private String name;

    private Integer age;

    private Teacher teacher;

    public Student() {
        System.out.println("constructor");
    }

    public Student(String name, Integer age, Teacher teacher) {
        this.name = name;
        this.age = age;
        this.teacher = teacher;
    }

    //getter,setter......

    @Override
    public void writeExternal(ObjectOutput out) throws IOException {
        out.writeObject(name);
        out.writeObject(age);
        out.writeObject(teacher);
    }

    @Override
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        name = (String)in.readObject();
        age = (Integer)in.readObject();
        //teacher = new Teacher("default", 0);
        teacher = (Teacher)in.readObject();
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", teacher=" + teacher +
                '}';
    }
}

4. transient关键字
处理继承Externalizable实现相关的方法，在writeExternal方法内部决定属性的序列化与否之外，还可以通过transient实现，被该关键字修饰的字段会自动关闭序列化操作，和Serializable一起使用的效果和Externalizabel类似