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.。