我们在程序创建的Java对象都是存在于JVM内存中的，也就是Java对象的生命周期一定不会长于JVM，所以如何以一种持久化的方式保留用户创建的对象呢，这就需要用到对象序列化的知识，将序列化的对象重新还原称为Java对象，即为反序列化。关于Java对象序列化与反序列化，我们需要理解的一点是，序列化只是将Java对象以一种二进制数据流的方式保存到本地文件或其他介质中，它实际上保存的只是实例化对象的一种状态，对于静态成员，是无法序列化保存的。

Java对象要支持序列化，该类必须要具有可序列化的特性，真正提供实现序列化与反序列化方式的ObjectInputStream和ObjectOutputStream类。Java对象序列化的方式可以分为普通方式和定制方式两种，普通方式，借助Serializable接口完成，不需要用户做过多的干涉和修改；定制方式，用户可以使用transient关键字修饰类中某个属性，使该属性屏蔽序列化特性，使用transient关键字屏蔽之后，被transient修饰的字段序列化后取默认值，而非序列化之前的实例化对象值。如果需要序列化该字段，我们可以将transient关键字去掉，也可以在该类中重写readObject和writeObject方法，并在重写方法中继续序列化屏蔽的字段，从而达到反屏蔽的效果。除了使用transient关键字，重写readObject和writeObject方法实现定制序列化的方式，还有一种与Serializable方式完全不同的支持定制序列化的方式，是指定该类继承自Externalizable接口，但是继承自该接口的类必须明确重写readExternal和writeExternal方法才能实现对象序列化与反序列化，在这两个方法里指定需要序列化和反序列化的属性字段，没有指明的属性将不会被序列化。

下面是提供一个程序示例。

import java.io.*;
public class TestObjSer{public static void main(String[] args) throws Exception{String stuFilePath = "./student.out";String codFilePath = "./coder.out";File file = new File(codFilePath);if(!file.exists()){System.out.println("The serialization file does not exist, which will be created right now!");file.createNewFile();}Student stu = new Student("zhangsan", 17, Gender.MALE, "CSU");Coder coder = new Coder("lisi", 18, Gender.FEMALE, "JAVA");FileInputStream fis = new FileInputStream(file);FileOutputStream fos = new FileOutputStream(file);//folowing two stream can not be disordered, I have no idea why is thisObjectOutputStream oos = new ObjectOutputStream(fos);ObjectInputStream ois = new ObjectInputStream(fis);serObj(oos,coder);deSerObj(ois);}public static void serObj(ObjectOutputStream oos,Object obj) throws Exception {oos.writeObject(obj);oos.close();}public static void deSerObj(ObjectInputStream ois) throws Exception {Person per = (Person)ois.readObject();ois.close();System.out.println(per);}
}/*** the enum type is a special type in Java, which is generated from * class Enum by default and class Enum implements the interface Seralizable* so it can be serialized and all member of enum should be capital*/
enum Gender{MALE,FEMALE
}
/*** superclass should implement Serialzable*/
class Person implements Serializable{private String name;private int age;private Gender gender;public Person(){System.out.println("Non-Arg constructor be called!");}public Person(String name, int age, Gender gender){System.out.println("Arg constructor be called!");this.name = name;this.age = age;this.gender = gender;}public String getName() {return this.name;}public int getAge() {return this.age;}public Gender getGender(){return this.gender;}public void setName(String name) {this.name = name;}public void setAge(int age) {this.age = age;}public void setGender(Gender gender){this.gender = gender;}public String toString(){return "Per(name = " + this.getName() + ", age = " + this.getAge() + ", gender = " + this.getGender() +")";}
}
class Student extends Person{//use transient to avoid the field school be serializedprivate transient String school;public Student(){super();}public Student(String name, int age, Gender gender, String school){super(name, age, gender);this.school = school;}public String getSchool() {return this.school;}public void setSchool(String school) {this.school = school;}public String toString(){return "Stu(name = " + this.getName() + ", age = " + this.getAge() + ", gender = " + this.getGender() + ", school = " + this.getSchool() +")";}private void writeObject(ObjectOutputStream oos) throws IOException {oos.defaultWriteObject();//call superoos.writeObject(school);}private void readObject(ObjectInputStream ois) throws Exception{ois.defaultReadObject();//call superthis.school = (String)ois.readObject();}
}
class Coder extends Person implements Externalizable {private String language;public Coder(){super();}public Coder(String name, int age, Gender gender, String language){super(name, age, gender);this.language = language;}public String getLanguage() {return this.language;}public void setLanguage(String language) {this.language = language;}public String toString(){return "Cod(name = " + this.getName() + ", age = " + this.getAge() + ", gender = " + this.getGender() + ", language = " + this.getLanguage() +")";}/*** [readExternal description]serialize by this method should * specify field in readExternal and writeExternal method by user,* it is completely different from Serialzable* @param  in                     [description]* @throws IOException            [description]* @throws ClassNotFoundException [description]*/public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {this.setName((String)in.readObject());this.setAge(in.readInt());this.setGender((Gender)in.readObject());this.language = (String)in.readObject();}public void writeExternal(ObjectOutput out) throws IOException {out.writeObject(this.getName());out.writeInt(this.getAge());out.writeObject(this.getGender());out.writeObject(this.language);}
}

上面的程序示例基本展示了Java对象序列化与反序列化的所有知识，还有一些相关知识，希望在这里啰嗦几句：

1）enum属于Java中的特殊类型，凡是enum类型声明的变量都是默认继承自Enum类，且默认具有序列化特性。

2）Java对象序列化采取级联机制，即如果一个对象引用了其它对象作为其属性，那么在序列化时，引用对象也将被序列化。

3）要想从父类继承的成员也被序列化，则父类必须明确支持序列化特性，即继承Serializable接口。

4）Java序列化的底层原理利用的是反射机制，这个将在后续介绍。

5）从其他博客看到的一点，关于单例模式单例对象的序列化，要想实现反序列化后的对象与单例对象相等，一个解决方法就是替代序列化过程，在该类中添加一个readResolve方法：无论是实现Serializable接口，或是Externalizable接口，当从I/O流中读取对象时，readResolve()方法都会被调用到。实际上就是用readResolve()中返回的对象直接替换在反序列化过程中创建的对象，而被创建的对象则会被垃圾回收掉（这句话是别人的）。