java编程思想笔记（十六）对象序列化
java对象序列化是将对象转化成一个字节序列，一般是将对象实现Serializable接口来实现对象序列化，序列化后，可以通过网络传输数据，也能存在数据库或硬盘中。
反序列化则是字节序列转为JAVA对象
下面是实现Serializable接口后对象序列化和反序列化的栗子：
import java.io.*;
public class TestObject implements Serializable{}
import java.io.*;
//序列化
public class SerializeObject{
public static void main(String[] args)throws Exception{
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(“x.file”));
TestObject obj = new TestObject();
//将TestObject对象序列化到x.file文件中
out.write(obj);
}
}
import java.io.*;
//反序列化
public class DeserializeObject{
public static void main(String[] args)throw Exception{
ObjectInputStream in = new ObjectInputStream(new FileInputStream(“x.file”));
Object myObj = in.readObject();
System.out.println(myObj.getClass());
}
}
输出结果：
class TestObject
在序列化时，使用ObjectOutputStream将对象序列化写出，反序列化时，使用ObjectInputStream将对象反序列化读入。
注意：反序列化对象使用时，对象的class文件必须在classpath中，否则，JVM找不到对象的class文件会抛出ClassNotFoundException。
2.序列化控制
用Serializable会把整个对象都给序列化，使用Externalizable则把对象部分序列化，Externalizable需要实现2个方法，一个是writeExternal(ObjectOutput out)，另一个是readExternal(ObjectInput in)，把你想序列化的对象写在writeExternal方法中，反序列化的对象写在readExternal方法中，在对象序列化和反序列化过程中，这两个方法会自动调用。
import java.io.*;
class Blip1 implements Externalizable{
public Blip1(){
System.out.println(“Blip1 Constructor”);
}
public void writeExternal(ObjectOutput out)throws IOException{
System.out.println(“Blip1.writeExternal”);
}
public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException{
System.out.println(Blip1.readExternal);
}
}
class Blip2 implements Externalizable{
Blip1(){
System.out.println(“Blip2 Constructor”);
}
public void writeExternal(ObjectOutput out)throws IOException{
System.out.println(“Blip2.writeExternal”);
}
public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException{
System.out.println(Blip2.readExternal);
}
}
public class Blips{
public static void main(String[] args)throws IOException, ClassNotFoundException{
System.out.println(“Constructing objects:”);
Blip1 b1 = new Blip1();
Blip2 b2 = new Blip2();
//序列化
ObjectOutputStream o = new ObjectOutputStream(new FileOutputStream(“Blips.out”));
System.out.println(“Saving objects:”);
o.writeObject(b1);
o.writeObject(b2);
//反序列化
ObjectInputStream in = new ObjectInputStream(new FileInputStream(“Blips.out”));
System.out.println(“Recovering b1:”);
b1 = (Blip1)in.readObject();
//由于Blip2的默认无参数构造方法不是public的，所以会抛异常
//System.out.println(“Recovering b2:”);
//b2 = (Blip2)in.readObject();
}
}
输出结果：
Constructing objects:
Blip1 Constructor
Blip2 Constructor
Saving objects:
Blip1.writeExternal
Blip2.writeExternal
Recovering b1:
Blip1 Constructor
Blip1.readExternal
3.Externalizable指定部分字段初始化
用writeExternal()和readExternal()可指定所需字段去初始化，不需要的则不初始化
import java.io.*;
public class Test implements Externalizable{
private int i;
private String s;
public Test(){
System.out.println(“Test Constructor”);
}
public Test(String x, int a){
System.out.println(“Test(String x, int a)”);
i = a;
s = x;
}
public void toString(){
return s + i;
}
public void writeExternal(ObjectOutput out)throws IOException{
System.out.println(“Test.writeExternal”);
out.writeObject(s);
out.writeInt(i);
}
public void readExternal(ObjectInput in)throws IOException, ClassNotFoundException{
System.out.println(“Test.readExternal”);
s = (String)in.readObject();
i = in.readInt();
}
public static void main()throws IOException, ClassNotFoundException{
System.out.println(“Constructing objects:”);
Test t1 = new Test(“A String”, 47);
Test t2 = new Test();
System.out.println(t1);
System.out.println(t2);
//序列化
ObjectOutputStream o = new ObjectOutputStream(new FileOutputStream(“Test.out”));
System.out.println(“Saving objects:”);
o.writeObject(t1);\
o.writeObject(t2);
o.close();
//反序列化
ObjectInputStream in = new ObjectInputStream(new FileInputStream(“Test.out”));
System.out.println(“Recovering objects:”);
t1 = (Test)in.readObject();
System.out.println(t1);
t2 = (Test)in.readObject();
System.out.println(t2);
}
}
输出结果：
Constructing objects:
Test(String x, int a)
A String 47
Test Constructor
null0
Saving objects:
Test.writeExternal
Test.writeExternal
Recovering objects:
Test Constructor
Test.readExternal
A String 47
Test Constructor
Test.readExternal
null 0
该例子中，当创建对象使用非无参数构造方法时，对象字段被初始化，因此序列化时可以将初始化的值保存。当创建对象使用无参数的构造方法时，对象的字段没有被赋值而使用默认的初始化值。t1使用readExternal()反序列化时因为没有参数，所以会调用默认构造器.
4.transient关键字：
当类中某些字段不想被序列化，就需要使用transient关键字，适用于自动序列化的Serializable , Externalizable可通过writeExternal()方法也可以实现此功能.
例子如下：
import java.io.*;
public class User implements Serializable{
private String username;
private transient String password;
public User(String name, String pwd){
username = name;
password = pwd;
}
public String toString(){
return “username:” + username + “, password:” + password;
}
public static void main(String[] args)throws Exception{
User user = new User(“Test”, “passwd”);
System.out.println(“User=” + user);
ObjectOutputStream o = new ObjectOutputStream(new FileOutputStream(“User.out”));
o.writeObject(user);
o.close();
ObjectInputStream in = new ObjectInputStream(new FileInputStream(“User.out”));
System.out.println(“Recovering object:”);
user = (User)in.readObject();
System.out.println(“User=” + user);
}
}
输出结果：
User=username:Test,password:passwd
Recovering object:
User=username:Test,password:null