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

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:

    public class FooImpl 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 {
                  FooImpl fooimpl = new FooImpl();
                  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:

   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();		
	}

	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.