簡(jiǎn)單來說序列化就是一種用來處理對(duì)象流的機(jī)制，所謂對(duì)象流也就是將對(duì)象的內(nèi)容進(jìn)行流化，流的概念這里不用多說(就是I/O)，我們可以對(duì)流化后的對(duì)象進(jìn)行 讀寫操作，也可將流化后的對(duì)象傳輸于網(wǎng)絡(luò)之間(注：要想將對(duì)象傳輸于網(wǎng)絡(luò)必須進(jìn)行流化)！在對(duì)對(duì)象流進(jìn)行讀寫操作時(shí)會(huì)引發(fā)一些問題，而序列化機(jī)制正是用來 解決這些問題的！

問題的引出：

如上所述，讀寫對(duì)象會(huì)有什么問題呢？比如：我要將對(duì)象寫入一個(gè)磁盤文件而后再將其讀出來會(huì) 有什么問題嗎？別急，其中一個(gè)最大的問題就是對(duì)象引用！舉個(gè)例子來說：假如我有兩個(gè)類，分別是A和B，B類中含有一個(gè)指向A類對(duì)象的引用，現(xiàn)在我們對(duì)兩個(gè) 類進(jìn)行實(shí)例化{ A a = new A(); B b = new B(); }，這時(shí)在內(nèi)存中實(shí)際上分配了兩個(gè)空間，一個(gè)存儲(chǔ)對(duì)象a，一個(gè)存儲(chǔ)對(duì)象b，接下來我們想將它們寫入到磁盤的一個(gè)文件中去，就在寫入文件時(shí)出現(xiàn)了問題！因?yàn)? 對(duì)象b包含對(duì)對(duì)象a的引用，所以系統(tǒng)會(huì)自動(dòng)的將a的數(shù)據(jù)復(fù)制一份到b中，這樣的話當(dāng)我們從文件中恢復(fù)對(duì)象時(shí)(也就是重新加載到內(nèi)存中)時(shí)，內(nèi)存分配了三個(gè) 空間，而對(duì)象a同時(shí)在內(nèi)存中存在兩份，想一想后果吧，如果我想修改對(duì)象a的數(shù)據(jù)的話，那不是還要搜索它的每一份拷貝來達(dá)到對(duì)象數(shù)據(jù)的一致性，這不是我們所 希望的！

以下序列化機(jī)制的解決方案：

1.保存到磁盤的所有對(duì)象都獲得一個(gè)序列號(hào)(1, 2, 3等等)

2.當(dāng)要保存一個(gè)對(duì)象時(shí)，先檢查該對(duì)象是否被保存了。

3.如果以前保存過，只需寫入"與已經(jīng)保存的具有序列號(hào)x的對(duì)象相同"的標(biāo)記，否則，保存該對(duì)象

通過以上的步驟序列化機(jī)制解決了對(duì)象引用的問題！

序列化的實(shí)現(xiàn)

將 需要被序列化的類實(shí)現(xiàn)Serializable接口，該接口沒有需要實(shí)現(xiàn)的方法，implements Serializable只是為了標(biāo)注該對(duì)象是可被序列化的，然后使用一個(gè)輸出流(如：FileOutputStream)來構(gòu)造一個(gè) ObjectOutputStream(對(duì)象流)對(duì)象，接著，使用ObjectOutputStream對(duì)象的writeObject(Object obj)方法就可以將參數(shù)為obj的對(duì)象寫出(即保存其狀態(tài))，要恢復(fù)的話則用輸入流。

例子：

import java.io.*;

public class Test {
????????public static void main(String[] args) {
????????????????Employee harry = new Employee("Harry Hacker", 50000);
????????????????Manager manager1 = new Manager("Tony Tester", 80000);
????????????????manager1.setSecretary(harry);
????????????????Employee[] staff = new Employee[2];
????????????????staff[0] = harry;
????????????????staff[1] = manager1;
????????????????try {
????????????????????????ObjectOutputStream out = new ObjectOutputStream(
????????????????????????????????????????new FileOutputStream("employee.dat"));
????????????????????????out.writeObject(staff);
????????????????????????out.close();
????????????????????????ObjectInputStream in = new ObjectInputStream(new FileInputStream(
????????????????????????????????????????"employee.dat"));
????????????????????????Employee[] newStaff = (Employee[]) in.readObject();
????????????????????????in.close();

????????????????????????/**
???????????????????????? * 通過harry對(duì)象來加薪 將在secretary上反映出來
???????????????????????? */
????????????????????????newStaff[0].raiseSalary(10);
????????????????????????for (int i = 0; i < newStaff.length; i++)
????????????????????????????????System.out.println(newStaff[i]);
????????????????} catch (Exception e) {
????????????????????????e.printStackTrace();

????????????????}
????????}
}

class Employee implements Serializable {
????????public Employee(String n, double s) {
????????????????name = n;
????????????????salary = s;
????????}
????????/**
???????? *
???????? * 加薪水
???????? *
???????? */
????????public void raiseSalary(double byPercent) {
????????????????double raise = salary * byPercent / 100;
????????????????salary += raise;
????????}

????????public String toString() {
????????????????return getClass().getName() + "[name = " + name+ ",salary = " + salary+ "]";
????????}
????????private String name;
????????private double salary;
}

class Manager extends Employee

{
????????public Manager(String n, double s)
????????{
????????????????super(n, s);
????????????????secretary = null;
????????}
????????/**
???????? *
???????? * 設(shè)置秘書
???????? *
???????? */
????????public void setSecretary(Employee s)
????????{
?????????? secretary = s;
????????}
????????
????????public String toString()
????????{
????????????????return super.toString()+ "[secretary = " + secretary+ "]";
????????}

????????// secretary代表秘書

????????private Employee secretary;

}

修改默認(rèn)的序列化機(jī)制

在 序列化的過程中，有些數(shù)據(jù)字段我們不想將其序列化，對(duì)于此類字段我們只需要在定義時(shí)給它加上transient關(guān)鍵字即可，對(duì)于transient字段序 列化機(jī)制會(huì)跳過不會(huì)將其寫入文件，當(dāng)然也不可被恢復(fù)。但有時(shí)我們想將某一字段序列化，但它在SDK中的定義卻是不可序列化的類型，這樣的話我們也必須把他 標(biāo)注為transient，可是不能寫入又怎么恢復(fù)呢？好在序列化機(jī)制為包含這種特殊問題的類提供了如下的方法定義：

private void readObject(ObjectInputStream in) throws

IOException, ClassNotFoundException;

private void writeObject(ObjectOutputStream out) throws

IOException;

(注：這些方法定義時(shí)必須是私有的，因?yàn)椴恍枰泔@示調(diào)用，序列化機(jī)制會(huì)自動(dòng)調(diào)用的)

使用以上方法我們可以手動(dòng)對(duì)那些你又想序列化又不可以被序列化的數(shù)據(jù)字段進(jìn)行寫出和讀入操作。

下面是一個(gè)典型的例子，java.awt.geom包中的Point2D.Double類就是不可序列化的，因?yàn)樵擃悰]有實(shí)現(xiàn)Serializable接口，在我的例子中將把它當(dāng)作LabeledPoint類中的一個(gè)數(shù)據(jù)字段，并演示如何將其序列化！

import java.io.*;

import java.awt.geom.*;

public class TransientTest

{

????????public static void main(String[] args)
????????{
????????????????LabeledPoint label = new LabeledPoint("Book", 5.00, 5.00);
????????????????try
????????????????{
????????????????????????System.out.println(label);// 寫入前
????????????????????????ObjectOutputStream out = new ObjectOutputStream(new
????????????????????????FileOutputStream("Label.txt"));
????????????????????????out.writeObject(label);
????????????????????????out.close();
????????????????????????System.out.println(label);// 寫入后
????????????????????????ObjectInputStream in = new ObjectInputStream(new
????????????????????????FileInputStream("Label.txt"));
????????????????????????LabeledPoint label1 = (LabeledPoint) in.readObject();
????????????????????????in.close();
????????????????????????System.out.println(label1);// 讀出并加1.0后
????????????????}
????????????????catch (Exception e)
????????????????{
????????????????????????e.printStackTrace();
????????????????}
????????}
}

class LabeledPoint implements Serializable
{
????????public LabeledPoint(String str, double x, double y)
????????{
????????????????label = str;
????????????????point = new Point2D.Double(x, y);
????????}

????????private void writeObject(ObjectOutputStream out) throws IOException
????????{
????????????????/**
???????????????? *
???????????????? * 必須通過調(diào)用defaultWriteObject()方法來寫入
???????????????? *
???????????????? * 對(duì)象的描述以及那些可以被序列化的字段
???????????????? *
???????????????? */

????????????????out.defaultWriteObject();
????????????????out.writeDouble(point.getX());
????????????????out.writeDouble(point.getY());
????????}

????????private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException
????????{
????????????????/**
???????????????? *
???????????????? * 必須調(diào)用defaultReadObject()方法
???????????????? *
???????????????? */
????????????????in.defaultReadObject();
????????????????double x = in.readDouble() + 1.0;
????????????????double y = in.readDouble() + 1.0;
????????????????point = new Point2D.Double(x, y);

????????}

????????public String toString()
????????{
????????????????return getClass().getName()+ "[label = " + label+ ", point.getX() = " + point.getX()+ ", point.getY() = " + point.getY()+ "]";
????????}
????????private String label;
????????transient private Point2D.Double point;
}