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:

Java代码







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();

}

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:

Java代码







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();

}

}

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.