java中final的sessionID是干嘛用的？

[boooch]

经常见别人写的代码中会有一个final的sessionID，一串特别长的数字，为什么要这么写呢，这是做啥用的？这串数字是怎么生成的？

[Rainness_Zhang]

新手，同求

[shine333]

不是sessionId，是serialVersionUID。
简单说就是为了检查反序列化的时候运行的class版本和序列化的时候是否一致。
假设

public class A implements Serializable {int x;}

你把它序列化到了一个文件里面，若干天后，你想重新读出来，这个时候，你的版本已经改为了

public class A implements Serializable {int x;String y;}

这个时候两个版本就不一致了，当然这个例子，并不会导致很严重的问题，只不过y=null而已。

但是如果你不想要这个效果，或者不允许有版本不一致，那么就：

public class A implements Serializable {
private static final long serialVersionUID = 1L; // 或者其他随机数字
int x;}

public class A implements Serializable {
private static final long serialVersionUID = 555L; // 或者其他不同数字
int x;String y;
}

这样就会检查出版本不一致

/**
 * 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>:<p>
 *
 * <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.
 */

[shine333]

最好定义一个，因为你不定义，他也会帮你根据class文件算一个