Java序列化之Externalizable

1.需求

        使用Externalizable实现序列化

2.Externalizable源码

public interface Externalizable extends java.io.Serializable {
    /**
     * The object implements the writeExternal method to save its contents
     * by calling the methods of DataOutput for its primitive values or
     * calling the writeObject method of ObjectOutput for objects, strings,
     * and arrays.
     *
     * @serialData Overriding methods should use this tag to describe
     *             the data layout of this Externalizable object.
     *             List the sequence of element types and, if possible,
     *             relate the element to a public/protected field and/or
     *             method of this Externalizable class.
     *
     * @param out the stream to write the object to
     * @exception IOException Includes any I/O exceptions that may occur
     */
    void writeExternal(ObjectOutput out) throws IOException;

    /**
     * The object implements the readExternal method to restore its
     * contents by calling the methods of DataInput for primitive
     * types and readObject for objects, strings and arrays.  The
     * readExternal method must read the values in the same sequence
     * and with the same types as were written by writeExternal.
     *
     * @param in the stream to read data from in order to restore the object
     * @exception IOException if I/O errors occur
     * @exception ClassNotFoundException If the class for an object being
     *              restored cannot be found.
     */
    void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;
}

首先，我们在序列化对象的时候，由于这个类实现了Externalizable 接口，在writeExternal()方法里定义了哪些属性可以序列化，

哪些不可以序列化，所以，对象在经过这里就把规定能被序列化的序列化保存文件，不能序列化的不处理，然后在反序列的时候自动调用readExternal()方法，根据序列顺序挨个读取进行反序列，并自动封装成对象返回，然后在测试类接收，就完成了反序列

所以说Exterinable的是Serializable的一个扩展

3.举个栗子

需要序列化的类Person

package com.serializable;

import java.io.Externalizable;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectOutput;
import java.text.SimpleDateFormat;
import java.util.Date;

public class Person implements Externalizable {

    private static final long serialVersionUID = 2534579427612174172L;


    private String name;


    private Integer age;

    private transient String gender;

    public Person() {
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    @Override
    public void writeExternal(ObjectOutput out) throws IOException {
         Date date=new Date();
        out.writeObject(name);
        out.writeObject(age);
        out.writeObject(date);
    }
    @Override
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        name=(String) in.readObject();
        age=(Integer) in.readObject();
        SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
        Date date=(Date)in.readObject();
        System.out.println("反序列化后的日期为:"+sdf.format(date));
    }

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

序列化类

public class Client {
    /**
     * 序列化方法
     * @throws IOException
     * @throws FileNotFoundException
     */
    public void serializable(Person person) throws FileNotFoundException, IOException {
        ObjectOutputStream outputStream=new ObjectOutputStream(new FileOutputStream("temp2"));
        outputStream.writeObject(person);
    }

    /**
     * 反序列化的方法
     * @throws IOException
     * @throws FileNotFoundException
     * @throws ClassNotFoundException
     */
    public Person deSerializable() throws FileNotFoundException, IOException, ClassNotFoundException{
        ObjectInputStream ois=new ObjectInputStream(new FileInputStream("temp2"));
        return (Person) ois.readObject();
    }

    @Test
    public  void test() throws Exception {
        Client client=new Client();
        Person person=new Person("村长",20,"man");
        System.out.println("为序列化之前的相关数据如下:\n"+person.toString());
        client.serializable(person);
        Person newPerson=client.deSerializable();
        System.out.println("-------------------------------------------------------");
        System.out.println("序列化之后的相关数据如下:\n"+newPerson.toString());
    }

}

4.注意

1.java中的序列化时transient变量(这个关键字的作用就是告知JAVA我不可以被序列化)和静态变量不会被序列化

2.也是最应该注意的，如果你先序列化对象A后序列化B，那么在反序列化的时候一定记着JAVA规定先读到的对象 是先被序列化的对象，不要先接收对象B，那样会报错.尤其在使用上面的Externalizable的时候一定要注意读取 的先后顺序

3.实现序列化接口的对象并不强制声明唯一的serialVersionUID，是否声明serialVersionUID对于对象序列化的向 上向下的兼容性有很大的影响