如果我们要把一个对象序列化，必须要实现Serializable接口
这个Serializable接口相当于一个标识，表示实现了该接口的对象允许序列化。
但是我们实现了Serializable接口后，还要在该对象中加一个字段serialVersionUID
如：

public class User implements Serializable {

    private final static Long serialVersionUID = 1235L;

    private String name;
    // 其他字段
    ......

}

很多人在实现Serializable 不加serialVersionUID字段，但是这可能会导致反序列化失败！
我们可以从源码来看

 /**
 * ......
 * The serialization runtime associates with each serializable class a version
 * number, called a serialVersionUID, which is used during deserialization to
 * verify that the sender and receiver of a serialized object have loaded
 * classes for that object that are compatible with respect to serialization.
 * If the receiver has loaded a class for the object that has a different
 * serialVersionUID than that of the corresponding sender's class, then
 * deserialization will result in an {@link InvalidClassException}.  A
 * serializable class can declare its own serialVersionUID explicitly by
 * declaring a field named <code>"serialVersionUID"</code> that must be static,
 * final, and of type <code>long</code>:
 *
 * <PRE>
 * ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;
 * </PRE>
 *
 * If a serializable class does not explicitly declare a serialVersionUID, then
 * the serialization runtime will calculate a default serialVersionUID value
 * for that class based on various aspects of the class, as described in the
 * Java(TM) Object Serialization Specification.  However, it is <em>strongly
 * recommended</em> that all serializable classes explicitly declare
 * serialVersionUID values, since the default serialVersionUID computation is
 * highly sensitive to class details that may vary depending on compiler
 * implementations, and can thus result in unexpected
 * <code>InvalidClassException</code>s during deserialization.  Therefore, to
 * guarantee a consistent serialVersionUID value across different java compiler
 * implementations, a serializable class must declare an explicit
 * serialVersionUID value.  It is also strongly advised that explicit
 * serialVersionUID declarations use the <code>private</code> modifier where
 * possible, since such declarations apply only to the immediately declaring
 * class--serialVersionUID fields are not useful as inherited members. Array
 * classes cannot declare an explicit serialVersionUID, so they always have
 * the default computed value, but the requirement for matching
 * serialVersionUID values is waived for array classes.
 *
 * @author  unascribed
 * @see java.io.ObjectOutputStream
 * @see java.io.ObjectInputStream
 * @see java.io.ObjectOutput
 * @see java.io.ObjectInput
 * @see java.io.Externalizable
 * @since   JDK1.1
 */
public interface Serializable {
}

如果可序列化类未显式声明 serialVersionUID，则序列化运行时将根据该类的各个方面计算该类的默认 serialVersionUID 值，但是默认对 serialVersionUID的 计算对可能因编译器实现而不同。

反序列化时，如果serialVersionUID不相同，会抛出InvalidClassException导致反序列化失败。

所以，我们在实现Serializable接口后，都要在可序列化类显示声明serialVersionUID。
并且serialVersionUID的定义必须是 long final static。 建议使用 private修饰。