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Java编程中的序列化 深入剖析Java编程中的序列化

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 Java提供一种机制叫做序列化，通过有序的格式或者字节序列持久化java对象，其中包含对象的数据，还有对象的类型，和保存在对象中的数据类型。

所以，如果我们已经序列化了一个对象，那么它可以被读取并通过对象的类型和其他信息进行反序列化，并最终获取对象的原型。

ObjectInputStream 和 ObjectOutputStream对象是高级别的流对象，包含序列化和反序列化的方法。

ObjectOutputStream 拥有很多序列化对象的方法，最常用的是：
 

private void writeObject(ObjectOutputStream os) throws IOException
 {
   
 }

类似的 ObjectInputStream 提供如下方法：
 

 private void readObject(ObjectInputStream is) throws IOException, ClassNotFoundException
 {
   
 }

那么哪里会需要序列化呢？序列化通常在需要通过网络传输数据，或者保存对象到文件的场合使用。这里说的数据是对象而不是文本。

现在的问题是，我们的网络架构和硬盘都只能识别二进制和字节，而不能识别Java对象。

序列化就是把Java对象中的value/states翻译为字节，以便通过网络传输或者保存。另外，反序列化就是通过读取字节码，并把它翻译回java对象。

serialVersionUID概念

serialVersionUID 是用于保证同一个对象（在序列化中会被用到）可以在Deserialization过程中被载入。serialVersionUID 是用于对象的版本控制。你可以参考serialVersionUID in java serialization获取更多信息。

对于序列化：

步骤如下：

让我们看一个列子：

在 src->org.arpit.javapostsforlearning 创建Employee.java

1.Employee.java 
 

package org.arpit.javapostsforlearning;
import java.io.Serializable;
public class Employee implements Serializable{
 
 int employeeId;
 String employeeName;
 String department;
  
 public int getEmployeeId() {
  return employeeId;
 }
 public void setEmployeeId(int employeeId) {
  this.employeeId = employeeId;
 }
 public String getEmployeeName() {
  return employeeName;
 }
 public void setEmployeeName(String employeeName) {
  this.employeeName = employeeName;
 }
 public String getDepartment() {
  return department;
 }
 public void setDepartment(String department) {
  this.department = department;
 }
}

就如你所见的，如果你需要序列化任何类，那么你 必须实现 Serializable 接口 ，这个接口是标记接口（marker interface）。


Java中的标记接口（marker interface）就是一个没有任何字段或者方法的接口，简单的来说，java中把空接口叫做标记接口（marker interface）

2.SerializeMain.java
 

package org.arpit.javapostsforlearning;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
 public class SerializeMain {
 
 /**
 * @author Arpit Mandliya
 */
 public static void main(String[] args) {
 
 Employee emp = new Employee();
 emp.setEmployeeId(101);
 emp.setEmployeeName("Arpit");
 emp.setDepartment("CS");
 try
 {
 FileOutputStream fileOut = new FileOutputStream("employee.ser");
 ObjectOutputStream outStream = new ObjectOutputStream(fileOut);
 outStream.writeObject(emp);
 outStream.close();
 fileOut.close();
 }catch(IOException i)
 {
 i.printStackTrace();
 }
 }
}

对于反序列化:
步骤是

在包src->org.arpit.javapostsforlearning中，创建 DeserializeMain.java
3.DeserializeMain.java
 

package org.arpit.javapostsforlearning;
import java.io.IOException;
import java.io.ObjectInputStream;
 
public class DeserializeMain {
 /**
 * @author Arpit Mandliya
 */
 public static void main(String[] args) {
 
 Employee emp = null;
  try
  {
   FileInputStream fileIn =new FileInputStream("employee.ser");
   ObjectInputStream in = new ObjectInputStream(fileIn);
   emp = (Employee) in.readObject();
   in.close();
   fileIn.close();
  }catch(IOException i)
  {
   i.printStackTrace();
   return;
  }catch(ClassNotFoundException c)
  {
   System.out.println("Employee class not found");
   c.printStackTrace();
   return;
  }
  System.out.println("Deserialized Employee...");
  System.out.println("Emp id: " + emp.getEmployeeId());
  System.out.println("Name: " + emp.getEmployeeName());
  System.out.println("Department: " + emp.getDepartment());
 }
}


4.运行:
首先运行SerializeMain.java，然后运行 DeserializeMain.java，你会得到如下的结果:
 

Deserialized Employee...
Emp id: 101
Name: Arpit
Department: CS

就这样，我们序列化了一个employee对象，并对它进行反序列化。这看起来和简单，但是如果其中包含对象引用，继承，那么情况就会变得复杂。接下来让我们一个接一个的看一下例子，看看如何在各种场合中实现序列化。

案例1 - 如果对象引用了其他对象，那该如何

我们已经看过最简单的序列化例子，现在看看，如何处理对象中引用了其他对象的场合。我们该如何序列化？引用对象也会被序列化吗？对的，你不需要显式的序列化引用对象。当你序列化任何对象，如果它包含引用对象，那么Java序列化会自动序列化该对象的整个对象图。例如，Employee现在引用了一个address对象，并且Address也引用了其他对象（例如，Home），那么当你序列化Employee对象的时候，所有其他引用对象，例如address和home将会被自动地被序列化。让我们来创建Address类，并它Address的对象作为引用，添加到employee类中。

Employee.java: 
 

package org.arpit.javapostsforlearning;
import java.io.Serializable;
 
public class Employee implements Serializable{
 
 int employeeId;
 String employeeName;
 String department;
 Address address;
 
 public int getEmployeeId() {
 return employeeId;
 }
 public void setEmployeeId(int employeeId) {
 this.employeeId = employeeId;
 }
 public String getEmployeeName() {
 return employeeName;
 }
 public void setEmployeeName(String employeeName) {
 this.employeeName = employeeName;
 }
 public String getDepartment() {
 return department;
 }
 public void setDepartment(String department) {
 this.department = department;
 }
 public Address getAddress() {
 return address;
 }
 public void setAddress(Address address) {
 this.address = address;
 }
}

在 org.arpit.javapostsforlearning 包中，创建Address.java
Address.java: 
 

package org.arpit.javapostsforlearning;
public class Address {
 
 int homeNo;
 String street;
 String city;
 public Address(int homeNo, String street, String city) {
 super();
 this.homeNo = homeNo;
 this.street = street;
 this.city = city;
 }
 public int getHomeNo() {
 return homeNo;
 }
 public void setHomeNo(int homeNo) {
 this.homeNo = homeNo;
 }
 public String getStreet() {
 return street;
 }
 public void setStreet(String street) {
 this.street = street;
 }
 public String getCity() {
 return city;
 }
 public void setCity(String city) {
 this.city = city;
 }
}

在包 org.arpit.javapostsforlearning中，创建SerializeDeserializeMain.java
SerializeDeserializeMain.java:
 

package org.arpit.javapostsforlearning;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
 
public class SerializeDeserializeMain {
 /**
 * @author Arpit Mandliya
 */
 public static void main(String[] args) {
 
 Employee emp = new Employee();
 emp.setEmployeeId(101);
 emp.setEmployeeName("Arpit");
 emp.setDepartment("CS");
 Address address=new Address(88,"MG road","Pune");
 emp.setAddress(address);
 //Serialize
 try
 {
 FileOutputStream fileOut = new FileOutputStream("employee.ser");
 ObjectOutputStream outStream = new ObjectOutputStream(fileOut);
 outStream.writeObject(emp);
 outStream.close();
 fileOut.close();
 }catch(IOException i)
 {
 i.printStackTrace();
 }
 
 //Deserialize
 emp = null;
 try
 {
 FileInputStream fileIn =new FileInputStream("employee.ser");
 ObjectInputStream in = new ObjectInputStream(fileIn);
 emp = (Employee) in.readObject();
 in.close();
 fileIn.close();
 }catch(IOException i)
 {
 i.printStackTrace();
 return;
 }catch(ClassNotFoundException c)
 {
 System.out.println("Employee class not found");
 c.printStackTrace();
 return;
 }
 System.out.println("Deserialized Employee...");
 System.out.println("Emp id: " + emp.getEmployeeId());
 System.out.println("Name: " + emp.getEmployeeName());
 System.out.println("Department: " + emp.getDepartment());
 address=emp.getAddress();
 System.out.println("City :"+address.getCity());
 }
}

运行它:
当你运行SerializeDeserializeMain.java。你会得到这样的结果：
 

java.io.NotSerializableException: org.arpit.javapostsforlearning.Address
 at java.io.ObjectOutputStream.writeObject0(Unknown Source)
 at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
 at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
 at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
 at java.io.ObjectOutputStream.writeObject0(Unknown Source)
 at java.io.ObjectOutputStream.writeObject(Unknown Source)

我们将解释哪里出错了。我忘记了说，Address 类也必须是serializable。那么Address类必须继承serialzable接口。

Address.java:
 

import java.io.Serializable;
 
public class Address implements Serializable{
 
 int homeNo;
 String street;
 String city;
 public Address(int homeNo, String street, String city) {
 super();
 this.homeNo = homeNo;
 this.street = street;
 this.city = city;
 }
 public int getHomeNo() {
 return homeNo;
 }
 public void setHomeNo(int homeNo) {
 this.homeNo = homeNo;
 }
 public String getStreet() {
 return street;
 }
 public void setStreet(String street) {
 this.street = street;
 }
 public String getCity() {
 return city;
 }
 public void setCity(String city) {
 this.city = city;
 }
}

再次运行:
当你再次运行SerializeDeserializeMain.java。你可以得到如下的结果
 

Deserialized Employee...
Emp id: 101
Name: Arpit
Department: CS
City :Pune

案例2：如果我们不能访问引用对象的源代码（例如，你不能访问上面的Address类的源码）

如果我们不能访问到address类，那么我们该如何在Address类中实现serializable 接口？是否有另外的途径来实现呢？对的，你可以创建另外一个类，并继承Address，然后让它继承serializable 接口，但是对于下面的情况，这个方案会失败：

    如果引用类被定义为final
    如果引用类引用了另外一个非可序列化的对象

那么，我们该如何序列化Employee对象？解决的办法是，标记transient。如果你不需要序列化任何字段，只需把它标记为transient。
 
transient Address address
在Employee类中，标记了address为transient之后，运行程序。你会得到nullPointerException，因为在反序列化过程中，Address引用将会是null。


案例3 - 如果我仍然需要保存引用对象的状态呢？（例如address对象）

如果你在反序列化过程中，标记了address为transient，它将会返回null结果。但是如果你仍然需要保存它的状态，你就需要序列化address对象。 Java序列化提供一个机制，如果你有特定签名的private方法，那么它们就会在序列化和反序列化过程中被调用，所以我们将重写Employee类的writeObject和readObject方法，然后它们就会在Employee对象序列化/反序列化过程中被调用。

Employee.java:

package org.arpit.javapostsforlearning;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
 
public class Employee implements Serializable{
 
 int employeeId;
 String employeeName;
 String department;
 transient Address address;
 
 public int getEmployeeId() {
 return employeeId;
 }
 public void setEmployeeId(int employeeId) {
 this.employeeId = employeeId;
 }
 public String getEmployeeName() {
 return employeeName;
 }
 public void setEmployeeName(String employeeName) {
 this.employeeName = employeeName;
 }
 public String getDepartment() {
 return department;
 }
 public void setDepartment(String department) {
 this.department = department;
 }
 public Address getAddress() {
 return address;
 }
 public void setAddress(Address address) {
 this.address = address;
 }
 
 private void writeObject(ObjectOutputStream os) throws IOException, ClassNotFoundException
 {
 try {
 os.defaultWriteObject();
 os.writeInt(address.getHomeNo());
 os.writeObject(address.getStreet());
 os.writeObject(address.getCity());
 }
 catch (Exception e)
 { e.printStackTrace(); }
 }
 
 private void readObject(ObjectInputStream is) throws IOException, ClassNotFoundException
 {
 try {
 is.defaultReadObject();
 int homeNo=is.readInt();
 String street=(String) is.readObject();
 String city=(String) is.readObject();
 address=new Address(homeNo,street,city);
 
 } catch (Exception e) { e.printStackTrace(); }
 }
}

另外有一点需要牢记的，ObjectInputStream读取数据的顺序和ObjectOutputStream写入数据的顺序是一致的.

在包org.arpit.javapostsforlearning 中创建Address.java
Address.java: 
 

package org.arpit.javapostsforlearning;
import java.io.Serializable;
 
public class Address {
 
 int homeNo;
 String street;
 String city;
 
 
 public Address(int homeNo, String street, String city) {
 super();
 this.homeNo = homeNo;
 this.street = street;
 this.city = city;
 }
 public int getHomeNo() {
 return homeNo;
 }
 public void setHomeNo(int homeNo) {
 this.homeNo = homeNo;
 }
 public String getStreet() {
 return street;
 }
 public void setStreet(String street) {
 this.street = street;
 }
 public String getCity() {
 return city;
 }
 public void setCity(String city) {
 this.city = city;
 }
}

在包org.arpit.javapostsforlearning中创建SerializeDeserializeMain.java
SerializeDeserializeMain.java:
 

package org.arpit.javapostsforlearning;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
 
public class SerializeDeserializeMain {
 /**
 * @author Arpit Mandliya
 */
 public static void main(String[] args) {
 
 Employee emp = new Employee();
 emp.setEmployeeId(101);
 emp.setEmployeeName("Arpit");
 emp.setDepartment("CS");
 Address address=new Address(88,"MG road","Pune");
 emp.setAddress(address);
 //Serialize
 try
 {
 FileOutputStream fileOut = new FileOutputStream("employee.ser");
 ObjectOutputStream outStream = new ObjectOutputStream(fileOut);
 outStream.writeObject(emp);
 outStream.close();
 fileOut.close();
 }catch(IOException i)
 {
 i.printStackTrace();
 }
 
 //Deserialize
 emp = null;
 try
 {
 FileInputStream fileIn =new FileInputStream("employee.ser");
 ObjectInputStream in = new ObjectInputStream(fileIn);
 emp = (Employee) in.readObject();
 in.close();
 fileIn.close();
 }catch(IOException i)
 {
 i.printStackTrace();
 return;
 }catch(ClassNotFoundException c)
 {
 System.out.println("Employee class not found");
 c.printStackTrace();
 return;
 }
 System.out.println("Deserialized Employee...");
 System.out.println("Emp id: " + emp.getEmployeeId());
 System.out.println("Name: " + emp.getEmployeeName());
 System.out.println("Department: " + emp.getDepartment());
 address=emp.getAddress();
 System.out.println("City :"+address.getCity());
 }
}

运行 :
当你运行SerializeDeserializeMain.java.你会得到如下的结果： 
 

Deserialized Employee...
Emp id: 101
Name: Arpit
Department: CS
City :Pune

现在我们就得到了address对象的状态，就如它被序列化前的一样。

序列化中的继承：

现在我们看看继承是如何影响序列化的。不管父类是不是可序列化，这将引出很多个例子。如果父类是非可序列化的，我们将如何处理，并且它是如何工作的。让我们看看例子。

我们将创建一个Person.java，作为 Employee的父类。

案例4： 如果父类是可序列化的

如果父类可序列化，那么所有的继承类将是可序列化的。

案例5： 如果父类为非可序列化呢？

如果父类为非可序列化的 ，那么我们的处理办法会很不一样。

    如果父类为非可序列化的，那么它必然不会有参数构造函数。

Person.java 
 

package org.arpit.javapostsforlearning;
public class Person {
 
 String name="default";
 String nationality;
 
 public Person()
 {
 System.out.println("Person:Constructor");
 }
 
 public Person(String name, String nationality) {
 super();
 this.name = name;
 this.nationality = nationality;
 }
 
 public String getName() {
 return name;
 }
 
 public void setName(String name) {
 this.name = name;
 }
 
 public String getNationality() {
 return nationality;
 }
 
 public void setNationality(String nationality) {
 this.nationality = nationality;
 }
 
}

在包org.arpit.javapostsforlearning 中创建Employee.java
Employee.java:

 

package org.arpit.javapostsforlearning;
import java.io.Serializable;
 
public class Employee extends Person implements Serializable{
 
 int employeeId;
 String department;
 
 public Employee(int employeeId,String name,String department,String nationality)
 {
 super(name,nationality);
 this.employeeId=employeeId;
 this.department=department;
 System.out.println("Employee:Constructor");
 }
 
 public int getEmployeeId() {
 return employeeId;
 }
 public void setEmployeeId(int employeeId) {
 this.employeeId = employeeId;
 }
 
 public String getDepartment() {
 return department;
 }
 public void setDepartment(String department) {
 this.department = department;
 }
}

在org.arpit.javapostsforlearning包中创建SerializeDeserializeMain.java

SerializeDeserializeMain.java:


package org.arpit.javapostsforlearning;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
 
public class SerializeDeserializeMain {
 
 /**
 * @author Arpit Mandliya
 */
 public static void main(String[] args) {
 
 //Serialize
 Employee emp = new Employee(101,"Arpit","CS","Indian");
 System.out.println("Before serializing");
 System.out.println("Emp id: " + emp.getEmployeeId());
 System.out.println("Name: " + emp.getName());
 System.out.println("Department: " + emp.getDepartment());
 System.out.println("Nationality: " + emp.getNationality());
 System.out.println("************");
 System.out.println("Serializing");
 try
 {
 FileOutputStream fileOut = new FileOutputStream("employee.ser");
 ObjectOutputStream outStream = new ObjectOutputStream(fileOut);
 outStream.writeObject(emp);
 outStream.close();
 fileOut.close();
 }catch(IOException i)
 {
 i.printStackTrace();
 }
 
 //Deserialize
 System.out.println("************");
 System.out.println("Deserializing");
 emp = null;
 try
 {
 FileInputStream fileIn =new FileInputStream("employee.ser");
 ObjectInputStream in = new ObjectInputStream(fileIn);
 emp = (Employee) in.readObject();
 in.close();
 fileIn.close();
 }catch(IOException i)
 {
 i.printStackTrace();
 return;
 }catch(ClassNotFoundException c)
 {
 System.out.println("Employee class not found");
 c.printStackTrace();
 return;
 }
 System.out.println("After serializing");
 System.out.println("Emp id: " + emp.getEmployeeId());
 System.out.println("Name: " + emp.getName());
 System.out.println("Department: " + emp.getDepartment());
 System.out.println("Nationality: " + emp.getNationality());
 }
}

运行：

当你运行SerializeDeserializeMain.java后，你会得到如下的输出，如果父类是非可序列化的，那么在反序列化过程中，所有继承于父类的实例变量值，将会通过调用非序列化构造函数来初始化。 这里 name继承于person，所以在反序列化过程中，name将会被初始化为默认值。


案例6 - 如果父类是可序列化，但你不需要继承类为可序列化

如果你不希望继承类为可序列化，那么你需要实现 writeObject() 和readObject() 方法，并且需要抛出NotSerializableException 异常。

案例7 - 可否序列化静态变量？

不能。因为静态变量是类级别的，不是对象级别的，当你序列化一个对象的时候，是不能序列化静态变量。


总结：

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