前面寫了一個(gè)序列化的文章，這次重新整理一下。
1.Serialize概念
Java的對(duì)象序列化就是將那些實(shí)現(xiàn)了Serializable接口的對(duì)象轉(zhuǎn)換成一個(gè)字節(jié)序列。并能夠
在以后將這個(gè)字節(jié)完全恢復(fù)為原來(lái)的對(duì)象。
對(duì)象序列化的概念加入到語(yǔ)言當(dāng)中就是為了支持兩種主要特性
一是Java的遠(yuǎn)程方法調(diào)用（Remote Method Invocation, RMI）。當(dāng)向遠(yuǎn)程對(duì)象發(fā)送消息時(shí)，
需要通過(guò)序列化來(lái)傳輸參數(shù)和返回值。
二是Javabean對(duì)象序列化。有些服務(wù)器通過(guò)將所有的SESSION 數(shù)據(jù)（包括BEAN）寫入磁盤來(lái)支持任意長(zhǎng)的SESSION生命期，即使服務(wù)器停機(jī)也不會(huì)丟失。當(dāng)服務(wù)器重新啟動(dòng)后，串行化的數(shù)據(jù)被恢復(fù)。同樣的理由，在重負(fù)載的站點(diǎn)上支持服務(wù)器分簇的環(huán)境中，許多服務(wù)器通過(guò)串行化來(lái)復(fù)制SESSION。如果你的BEAN不支持串行化，服務(wù)器就不能正確地保存和傳輸類。
只要對(duì)象實(shí)現(xiàn)了Serializable接口，對(duì)象的序列化處理就會(huì)非常簡(jiǎn)單。
2.Serialize方法
要序列化一個(gè)對(duì)象，首先要?jiǎng)?chuàng)建某些OutputStream對(duì)象，然后將其封裝在一個(gè)ObjectOutputStream對(duì)象內(nèi)。
這時(shí)，只需要調(diào)用writeObject()即可將對(duì)象序列化，并將其發(fā)送給OutputStream。反之，將一個(gè)序列化對(duì)象還原為一個(gè)對(duì)象，需要將一個(gè)InputStream封裝在ObjectInputStream內(nèi)，然后調(diào)用readObject()。
OutputStream和InputStream常用的是ByteArrayOutputStream/FileOutputStream和ByteArrayInputStream/FileInputStream。
下面兩個(gè)類以ByteArray方式實(shí)現(xiàn)序列化：
public final class Serialization
{
/**
* Serialize the object into a byte array.
*/
public static byte[] serialize( Object obj )
throws IOException
{
ByteArrayOutputStream baos;
ObjectOutputStream oos;
baos = new ByteArrayOutputStream();
oos = new ObjectOutputStream( baos );
oos.writeObject( obj );
oos.close();
return baos.toByteArray();
}
/**
* Deserialize an object from a byte array
*/
public static Object deserialize( byte[] buf )
throws ClassNotFoundException, IOException
{
ByteArrayInputStream bais;
ObjectInputStream ois;
bais = new ByteArrayInputStream( buf );
ois = new ObjectInputStream( bais );
return ois.readObject();
}
}
public class DefaultSerializer
implements Serializer
{
public static final DefaultSerializer INSTANCE = new DefaultSerializer();
/**
* Construct a DefaultSerializer.
*/
public DefaultSerializer()
{
// no op
}
/**
* Serialize the content of an object into a byte array.
*
* @param obj Object to serialize
* @return a byte array representing the object's state
*/
public byte[] serialize( Object obj )
throws IOException
{
return Serialization.serialize( obj );
}
/**
* Deserialize the content of an object from a byte array.
*
* @param serialized Byte array representation of the object
* @return deserialized object
*/
public Object deserialize( byte[] serialized )
throws IOException
{
try {
return Serialization.deserialize( serialized );
} catch ( ClassNotFoundException except ) {
throw new WrappedRuntimeException( except );
}
}
}
File方式的序列化大致如下：
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(out.txt));
out.writeObject(obj);
out.close();
ObjectInputStream in = new ObjectInputStream(new FileInputStream(out.txt));
Object obj = (Object)in.readObject();
in.close();
3.Serialize定制
缺省的序列化機(jī)制并不難操縱，如果有特殊的需求那又該怎么辦？例如，也許要考慮特殊的安全問(wèn)題，而且
你不希望對(duì)象的某一部分被序列化；或者一個(gè)對(duì)象被還原后，某子對(duì)象需要重新創(chuàng)建，從而不需要將該子對(duì)象
序列化。在這樣的特殊情況下，可通過(guò)實(shí)現(xiàn)Externalizable接口來(lái)對(duì)序列化過(guò)程進(jìn)行控制。這個(gè)Externalizable
接口繼承了Serializable接口，同時(shí)增加了兩個(gè)方法：writeExternal(ObjectOutput out)和readExternal(ObjectInput in)
Serializable對(duì)象完全以它存儲(chǔ)的二進(jìn)制位為基礎(chǔ)來(lái)構(gòu)造，而不用調(diào)用構(gòu)造器。而對(duì)于一個(gè)Externalizable對(duì)象
缺省構(gòu)造器都會(huì)被調(diào)用，然后調(diào)用readExternal()。下面列子中，如果Blip3()構(gòu)造器注釋掉，程序會(huì)報(bào)錯(cuò)：no valid constructor
// Reconstructing an externalizable object.
// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// www.BruceEckel.com. See copyright notice in CopyRight.txt.
import com.bruceeckel.simpletest.*;
import java.io.*;
import java.util.*;
public class Blip3 implements Externalizable {
private static Test monitor = new Test();
private int i;
private String s; // No initialization
public Blip3() {
System.out.println("Blip3 Constructor");
// s, i not initialized
}
public Blip3(String x, int a) {
System.out.println("Blip3(String x, int a)");
s = x;
i = a;
// s & i initialized only in nondefault constructor.
}
public String toString() { return s + i; }
public void writeExternal(ObjectOutput out)
throws IOException {
System.out.println("Blip3.writeExternal");
// You must do this:
out.writeObject(s);
out.writeInt(i);
}
public void readExternal(ObjectInput in)
throws IOException, ClassNotFoundException {
System.out.println("Blip3.readExternal");
// You must do this:
s = (String)in.readObject();
i = in.readInt();
}
public static void main(String[] args)
throws IOException, ClassNotFoundException {
System.out.println("Constructing objects:");
Blip3 b3 = new Blip3("A String ", 47);
System.out.println(b3);
ObjectOutputStream o = new ObjectOutputStream(
new FileOutputStream("Blip3.out"));
System.out.println("Saving object:");
o.writeObject(b3);
o.close();
// Now get it back:
ObjectInputStream in = new ObjectInputStream(
new FileInputStream("Blip3.out"));
System.out.println("Recovering b3:");
b3 = (Blip3)in.readObject();
System.out.println(b3);
monitor.expect(new String[] {
"Constructing objects:",
"Blip3(String x, int a)",
"A String 47",
"Saving object:",
"Blip3.writeExternal",
"Recovering b3:",
"Blip3 Constructor",
"Blip3.readExternal",
"A String 47"
});
}
}