簡(jiǎn)單來(lái)說(shuō)序列化就是一種用來(lái)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(shuō)(就是I/O),我們可以對(duì)流化后的對(duì)象進(jìn)行讀寫(xiě)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(xiě)操作時(shí)會(huì)引發(fā)一些問(wèn)題,而序列化機(jī)制正是用來(lái)解決這些問(wèn)題的!

問(wèn)題的引出:

如上所述,讀寫(xiě)對(duì)象會(huì)有什么問(wèn)題呢?比如:我要將對(duì)象寫(xiě)入一個(gè)磁盤(pán)文件而后再將其讀出來(lái)會(huì)有什么問(wèn)題嗎?別急,其中一個(gè)最大的問(wèn)題就是對(duì)象引用!舉個(gè)例子來(lái)說(shuō):假如我有兩個(gè)類(lèi),分別是A和B,B類(lèi)中含有一個(gè)指向A類(lèi)對(duì)象的引用,現(xiàn)在我們對(duì)兩個(gè)類(lèi)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(lái)我們想將它們寫(xiě)入到磁盤(pán)的一個(gè)文件中去,就在寫(xiě)入文件時(shí)出現(xiàn)了問(wèn)題!因?yàn)閷?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(huà)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(huà),那不是還要搜索它的每一份拷貝來(lái)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(pán)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(guò),只需寫(xiě)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(guò)以上的步驟序列化機(jī)制解決了對(duì)象引用的問(wèn)題!

序列化的實(shí)現(xiàn)

將需要被序列化的類(lèi)實(shí)現(xiàn)Serializable接口,該接口沒(méi)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(lái)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(xiě)出(即保存其狀態(tài)),要恢復(fù)的話(huà)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(guò)harry對(duì)象來(lái)加薪 將在secretary上反映出來(lái)
                         */
                         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(shū)
         *
         */
         public void setSecretary(Employee s)
         {
           secretary = s;
         }
        
         public String toString()
         {
                 return super.toString()+ "[secretary = " + secretary+ "]";
         }

         // secretary代表秘書(shū)

         private Employee secretary;

}

修改默認(rèn)的序列化機(jī)制

在序列化的過(guò)程中,有些數(shù)據(jù)字段我們不想將其序列化,對(duì)于此類(lèi)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(guò)不會(huì)將其寫(xiě)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(lèi)型,這樣的話(huà)我們也必須把他標(biāo)注為transient,可是不能寫(xiě)入又怎么恢復(fù)呢?好在序列化機(jī)制為包含這種特殊問(wèn)題的類(lèi)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(xiě)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(lèi)就是不可序列化的,因?yàn)樵擃?lèi)沒(méi)有實(shí)現(xiàn)Serializable接口,在我的例子中將把它當(dāng)作LabeledPoint類(lèi)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(xiě)入前
                         ObjectOutputStream out = new ObjectOutputStream(new
                         FileOutputStream("Label.txt"));
                         out.writeObject(label);
                         out.close();
                         System.out.println(label);// 寫(xiě)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(guò)調(diào)用defaultWriteObject()方法來(lái)寫(xiě)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;
}


開(kāi)心過(guò)好每一天。。。。。