JAVA序列化基础知识Serializable与Externalizable的区别

转载：http://dovecat.iteye.com/blog/66044

大家都知道Serializable是一个mark interface,告诉JVM这个对象可以被转换成二进制流来传输. 

但是Serializable与Externalizable的转换二进制流的过程是不一样的. 
Serializable 在我们实现这个接口的时候,我们可以使用4个私有方法来控制序列化的过程: 

  我们来看一个例子: 

package test1;import java.io.ByteArrayInputStream;import java.io.ByteArrayOutputStream;import java.io.IOException;import java.io.ObjectInputStream;import java.io.ObjectOutputStream;import java.io.ObjectStreamException;public class FooImpl0 implements java.io.Serializable {private String message;public String getFoo() {return message;}public void setMessage(String message) {this.message = message;}private void writeObject(java.io.ObjectOutputStream out) throws IOException {System.out.println("writeObject invoked");out.writeObject(this.message == null ? "hohohahaha" : this.message);}private void readObject(java.io.ObjectInputStream in) throws IOException,ClassNotFoundException {System.out.println("readObject invoked");this.message = (String) in.readObject();System.out.println("got message:" + message);}private Object writeReplace() throws ObjectStreamException {System.out.println("writeReplace invoked");return this;}private Object readResolve() throws ObjectStreamException {System.out.println("readResolve invoked");return this;}public Object serialize() throws IOException, ClassNotFoundException {ByteArrayOutputStream baos = new ByteArrayOutputStream();ObjectOutputStream oos = new ObjectOutputStream(baos);oos.writeObject(this);ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());ObjectInputStream ois = new ObjectInputStream(bais);return ois.readObject();}public static void main(String[] args) throws IOException,ClassNotFoundException {FooImpl0 fooimpl = new FooImpl0();fooimpl.serialize();}}

我们运行这段代码看到的debug信息: 

writeReplace invoked 
writeObject invoked 
readObject invoked 
readResolve invoked 


当进行序列化的时候: 
首先JVM会先调用writeReplace方法,在这个阶段,我们可以进行张冠李戴,将需要进行序列化的对象换成我们指定的对象. 
跟着JVM将调用writeObject方法,来将对象中的属性一个个进行序列化,我们可以在这个方法中控制住哪些属性需要序列化. 

当反序列化的时候: 
JVM会调用readObject方法,将我们刚刚在writeObject方法序列化好的属性,反序列化回来. 
然后在readResolve方法中,我们也可以指定JVM返回我们特定的对象(不是刚刚序列化回来的对象). 

注意到在writeReplace和readResolve,我们可以严格控制singleton的对象,在同一个JVM中完完全全只有唯一的对象,控制不让singleton对象产生副本. 

Externalizable 是一个有实际方法需要实现的interface,包括writeExternal和readExternal: 

package test1;import java.io.ByteArrayInputStream;import java.io.ByteArrayOutputStream;import java.io.IOException;import java.io.ObjectInput;import java.io.ObjectInputStream;import java.io.ObjectOutput;import java.io.ObjectOutputStream;import java.io.ObjectStreamException;public class FooImpl implements java.io.Externalizable {private String message;public String getFoo() {return message;}public void setMessage(String message) {this.message = message;}private Object writeReplace() throws ObjectStreamException {System.out.println("writeReplace invoked");return this;}private Object readResolve() throws ObjectStreamException {System.out.println("readResolve invoked");return this;}public Object serialize() throws IOException, ClassNotFoundException {ByteArrayOutputStream baos = new ByteArrayOutputStream();ObjectOutputStream oos = new ObjectOutputStream(baos);oos.writeObject(this);ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());ObjectInputStream ois = new ObjectInputStream(bais);return ois.readObject();}public void readExternal(ObjectInput arg0) throws IOException,ClassNotFoundException {System.out.println("readExternal invoked");Object obj = arg0.readObject();System.out.println(obj.toString());}public void writeExternal(ObjectOutput arg0) throws IOException {System.out.println("writeExternal invoked");arg0.writeObject("Hello world");}public static void main(String[] args) throws IOException,ClassNotFoundException {FooImpl fooimpl = new FooImpl();fooimpl.serialize();}}

我们运行这段代码看到的debug信息: 

writeReplace invoked 
writeExternal invoked 
readExternal invoked 
readResolve invoked 
在此writeExternal 和readExternal 的作用与writeObject和readObject 一样. 

最后,当我们同时实现了两个interface的时候,JVM只运行Externalizable 接口里面的writeExternal 和readExternal 方法对序列化内容进行处理. 
需要注意的是:Serializable是一个真正的mark interface, 
writeObject,readObject, writeReplace,readResolve是直接与JVM通信,告诉JVM序列化的内容.