java需要关注的知识点---I0之对象序列化

1。对象序列化：
序列化是什么：
　　序列化就是将一个对象的状态（各个属性量）保存起来，然后在适当的时候再获得。
　　序列化分为两大部分：序列化和反序列化。序列化是这个过程的第一部分，将数据分解成字节流，以便存储在文件中或在网络上传输。反序列化就是打开字节流并重构对象。对象序列化不仅要将基本数据类型转换成字节表示，有时还要恢复数据。恢复数据要求有恢复数据的对象实例
　　序列化的什么特点：
　　如果某个类能够被序列化，其子类也可以被序列化。声明为static和transient类型的成员数据不能被序列化。因为static代表类的状态， transient代表对象的临时数据。
　　什么时候使用序列化：
　　一：对象序列化可以实现分布式对象。主要应用例如：RMI要利用对象序列化运行远程主机上的服务，就像在本地机上运行对象时一样。
　　二：java对象序列化不仅保留一个对象的数据，而且递归保存对象引用的每个对象的数据。可以将整个对象层次写入字节流中，可以保存在文件中 或在网络连接上传递。利用对象序列化可以进行对象的"深复制"，即复制对象本身及引用的对象本身。序列化一个对象可能得到整个对象序列

序列化我是实现轻量级持久性。

package com.io;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Random;

public class ObjectSer implements Serializable{
	private static Random random = new Random(47);
	private Data[] d = {
			new Data(random.nextInt(10)),
			new Data(random.nextInt(10)),
			new Data(random.nextInt(10))
	};
	private ObjectSer next;
	private char c;
	
	public ObjectSer(int i, char x) {
		super();
		System.out.println("ObjectSer constructor:" + i);
		c = x;
		if(--i > 0) {
			next = new ObjectSer(i,(char)(x+1));
		}
	}
	
	public String toString() {
		StringBuilder result = new StringBuilder(":");
		result.append(c);
		result.append("(");
		for(Data dat:d)
			result.append(dat);
		result.append(")");
		if(next != null)
		result.append(next);
		return result.toString();
	}

	public ObjectSer() {
		super();
		System.out.println("Default constructor");
	}

	/**
	 * @param args
	 * @throws IOException 
	 * @throws FileNotFoundException 
	 * @throws ClassNotFoundException 
	 */
	public static void main(String[] args) throws FileNotFoundException, IOException, ClassNotFoundException {
		ObjectSer os = new ObjectSer(6,'a');
		System.out.println("os =:" +os);
		ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("worm.out"));
		out.writeObject("Object storage\n");
		out.writeObject(os);
		out.close();
		ObjectInputStream in = new ObjectInputStream(new FileInputStream("worm.out"));
		String s = (String)in.readObject();
		ObjectSer objects = (ObjectSer)in.readObject();
		System.out.println(s+ " objects: " + objects);
		ByteArrayOutputStream bout = new ByteArrayOutputStream();
		ObjectOutputStream out2 = new ObjectOutputStream(bout);
		out2.writeObject("Objectser storage\n");
		out2.writeObject(os);
		out2.flush();
		ObjectInputStream in2 = new ObjectInputStream(new ByteArrayInputStream(bout.toByteArray()));
		s = (String)in2.readObject();
		ObjectSer os2 = (ObjectSer)in2.readObject();
		System.out.println(s + " os2 = "+os2);
	}

}

class Data implements Serializable {
	private int n;
	public Data(int n) {this.n = n;}
	public String toString() {return Integer.toString(n);}
}

结果：

ObjectSer constructor:6
ObjectSer constructor:5
ObjectSer constructor:4
ObjectSer constructor:3
ObjectSer constructor:2
ObjectSer constructor:1
os =::a(853):b(119):c(802):d(788):e(199):f(881)
Object storage
 objects: :a(853):b(119):c(802):d(788):e(199):f(881)
Objectser storage
 os2 = :a(853):b(119):c(802):d(788):e(199):f(881)

序列化控制，Externalizable.

package com.io;

import java.io.Externalizable;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;

public class BlipsExternalizable {
	public static void main(String[] args) throws IOException, IOException, ClassNotFoundException {
		Blip1 b1 = new Blip1();
		Blip2 b2 = new Blip2();
		ObjectOutputStream dos = new ObjectOutputStream(new FileOutputStream("Blp.obj"));
		dos.writeObject(b1);
		dos.writeObject(b2);
		ObjectInputStream ois = new ObjectInputStream(new FileInputStream("Blp.obj"));
		b1 = (Blip1)ois.readObject();
		b2 = (Blip2)ois.readObject();
	}
	
}
class Blip1 implements Externalizable {
	public Blip1(){
		System.out.println("Blip1 constructor");
	}

	@Override
	public void readExternal(ObjectInput in) throws IOException,
			ClassNotFoundException {
		System.out.println("Blip1 read");
	}

	@Override
	public void writeExternal(ObjectOutput out) throws IOException {
		System.out.println("Blip1 write");
	}
}
class Blip2 implements Externalizable {
	Blip2() {
		System.out.println("Blip2 constructor");
	}
	@Override
	public void readExternal(ObjectInput in) throws IOException,
			ClassNotFoundException {
		System.out.println("Blip2 read");
	}

	@Override
	public void writeExternal(ObjectOutput out) throws IOException {
		System.out.println("Blip2 write");
	}
}
结果：
Blip1 constructor
Blip2 constructor
Blip1 write
Blip2 write
Blip1 constructor
Blip1 read
Exception in thread "main" java.io.InvalidClassException: com.io.Blip2; com.io.Blip2; no valid constructor
	at java.io.ObjectStreamClass.checkDeserialize(Unknown Source)
	at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
	at java.io.ObjectInputStream.readObject0(Unknown Source)
	at java.io.ObjectInputStream.readObject(Unknown Source)
	at com.io.BlipsExternalizable.main(BlipsExternalizable.java:21)
Caused by: java.io.InvalidClassException: com.io.Blip2; no valid constructor
	at java.io.ObjectStreamClass.<init>(Unknown Source)
	at java.io.ObjectStreamClass.lookup(Unknown Source)
	at java.io.ObjectOutputStream.writeObject0(Unknown Source)
	at java.io.ObjectOutputStream.writeObject(Unknown Source)
	at com.io.BlipsExternalizable.main(BlipsExternalizable.java:18)

public class Blip3Externalizable implements Externalizable {
	private String s;
	private int i;
	public Blip3Externalizable() {
		super();
		System.out.println("Blip constructor");
	}
	
	public Blip3Externalizable(String s, int i) {
		super();
		System.out.println("Blip constructor(s,i)");
		this.s = s;
		this.i = i;
	}

	@Override
	public void readExternal(ObjectInput in) throws IOException,
			ClassNotFoundException {
		System.out.println("Blip read");
		//reset the attribute value.
		s = (String)in.readObject();
		i = in.readInt();
		
	}

	@Override
	public void writeExternal(ObjectOutput out) throws IOException {
		System.out.println("Blip write");
		out.writeObject(s);
		out.writeInt(i);
	}
	public static void main(String[] args) throws IOException, ClassNotFoundException {
		Blip3Externalizable blip = new Blip3Externalizable("String", 41);
		ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("blip.obj"));
		oos.writeObject(blip);
		ObjectInputStream ois = new ObjectInputStream(new FileInputStream("blip.obj"));
		blip = (Blip3Externalizable) ois.readObject();
		System.out.println(blip.i);
	}

}

使用transient可以关闭序列化，用以保护数据。