实际上并没有提供Externalizable接口来优化序列化进程的性能!而是提供实现您自己的自定义处理的方法，并为对象及其超类型提供对流的格式和内容的完全控制!

这方面的例子是AMF (ActionScript Message Format)远程处理的实现，通过网络传输本机操作脚本对象。

Serializable和Externalizable之间的主要区别

Marker interface: Serializable is marker interface without any methods. Externalizable interface contains two methods: writeExternal() and readExternal(). Serialization process: Default Serialization process will be kicked-in for classes implementing Serializable interface. Programmer defined Serialization process will be kicked-in for classes implementing Externalizable interface. Maintenance: Incompatible changes may break serialisation. Backward Compatibility and Control: If you have to support multiple versions, you can have full control with Externalizable interface. You can support different versions of your object. If you implement Externalizable, it's your responsibility to serialize super class public No-arg constructor: Serializable uses reflection to construct object and does not require no arg constructor. But Externalizable demands public no-arg constructor.

更多细节请参考Hitesh Garg的博客。

There are so many difference exist between Serializable and Externalizable but when we compare difference between custom Serializable(overrided writeObject() & readObject()) and Externalizable then we find that custom implementation is tightly bind with ObjectOutputStream class where as in Externalizable case , we ourself provide an implementation of ObjectOutput which may be ObjectOutputStream class or it could be some other like org.apache.mina.filter.codec.serialization.ObjectSerializationOutputStream

如果是Externalizable接口

@Override
public void writeExternal(ObjectOutput out) throws IOException {
    out.writeUTF(key);
    out.writeUTF(value);
    out.writeObject(emp);
}

@Override
public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
    this.key = in.readUTF();
    this.value = in.readUTF();
    this.emp = (Employee) in.readObject();
}





**In case of Serializable interface**


        /* 
             We can comment below two method and use default serialization process as well
             Sequence of class attributes in read and write methods MUST BE same.
        // below will not work it will not work . 
        // Exception = java.io.StreamCorruptedException: invalid type code: 00\
              private void writeObject(java.io.ObjectOutput stream) 
              */
            private void writeObject(java.io.ObjectOutputStream Outstream)
                    throws IOException {

                System.out.println("from writeObject()");
                /*     We can define custom validation or business rules inside read/write methods.
 This way our validation methods will be automatically 
    called by JVM, immediately after default serialization 
    and deserialization process 
    happens.
                 checkTestInfo();
                */

                stream.writeUTF(name);
                stream.writeInt(age);
                stream.writeObject(salary);
                stream.writeObject(address);
            }

            private void readObject(java.io.ObjectInputStream Instream)
                    throws IOException, ClassNotFoundException {
                System.out.println("from readObject()");
                name = (String) stream.readUTF();
                age = stream.readInt();
                salary = (BigDecimal) stream.readObject();
                address = (Address) stream.readObject();
                // validateTestInfo();
            }

为了更好地解释，我添加了示例代码。请检查Externalizable的入/出对象情况。它们不直接绑定到任何实现。 其中Outstream/ stream与类紧密绑定。我们可以扩展ObjectOutputStream/ObjectInputStream，但使用起来有点困难。

一些差异:

For Serialization there is no need of default constructor of that class because Object because JVM construct the same with help of Reflection API. In case of Externalization contructor with no arg is required, because the control is in hand of programmar and later on assign the deserialized data to object via setters. In serialization if user want to skip certain properties to be serialized then has to mark that properties as transient, vice versa is not required for Externalization. When backward compatiblity support is expected for any class then it is recommended to go with Externalizable. Serialization supports defaultObject persisting and if object structure is broken then it will cause issue while deserializing.

实际上并没有提供Externalizable接口来优化序列化进程的性能!而是提供实现您自己的自定义处理的方法，并为对象及其超类型提供对流的格式和内容的完全控制!

这方面的例子是AMF (ActionScript Message Format)远程处理的实现，通过网络传输本机操作脚本对象。

在考虑提高性能的选项时，不要忘记自定义序列化。您可以免费让Java做它擅长的事情，或者至少做得足够好，并为它做得不好的事情提供自定义支持。这通常比完全的外部化支持少得多。