java - 在 Java 中序列化静态属性

Question

如果我尝试序列化静态属性会发生什么？

谢谢

Answer 1

从此article :

Tip 1: Handling Static Variables

Java classes often hold some globally relevant value in a static class variable. We won't enter into the long history of the debate over the propriety of global variables - let's just say that programmers continue to find them useful and the alternatives suggested by purists aren't always practical.

For static variables that are initialized when declared, serialization doesn't present any special problems. The first time the class is used, the variable in question will be set to the correct value.

Some statics can't be initialized this way. They may, for instance, be set by a human during the running time of the program. Let's say we have a static variable that turns on debugging output in a class. This variable can be set on a server by sending it some message, perhaps from a monitor program. We'll also imagine that when the server gets this message, the operator wants debugging turned on in all subsequent uses of the class in the clients that are connected to that server.

The programmer is now faced with a difficulty. When the class in question arrives at the client, the static variable's value doesn't come with it. However, it contains the default static state that's set when the class's no-argument constructor is called by writeObject(). How can the client programs receive the new correct value?

The programmer could create another message type and transmit that to the client; however, this requires a proliferation of message types, marring the simplicity that the use of serialization can achieve in messaging. The solution we've come up with is for the class that needs the static transmitted to include a "static transporter" inner class. This class knows about all the static variables in its outer class that must be set. It contains a member variable for each static variable that must be serialized. StaticTransporter copies the statics into its member variables in the writeObject() method of the class. The readObject() method "unwraps" this bundle and transmits the server's settings for the static variables to the client. Since it's an inner class, it'll be able to write to the outer class's static variables, regardless of the level of privacy with which they were declared.

来自另一个article :

Static or transient data

However, this "ease" is not true in all cases. As we shall see, serialization is not so easily applied to classes with static or transient data members. Only data associated with a specific instance of a class is serialized, therefore static data, that is, data associated with a class as opposed to an instance, is not serialized automatically. To serialize data stored in a static variable one must provide class-specific serialization.

Similarly, some classes may define data members to use as scratch variables. Serializing these data members may be unnecessary. Some examples of transient data include runtime statistics or hash table mapping references. These data should be marked with the transient modifier to avoid serialization. Transient, by definition, is used to designate data members that the programmer does not want or need to be serialized. See Java in a Nutshell, page 174: mouse position, preferred size, file handles (machine specific (native code)).

When writing code if something is declared transient, then this triggers (to programmer) necessity of the posibility of special code for serialization later.

To serialize an object, you create some sort of OutputStream object and then wrap it inside an ObjectOutputStream object. At this point you only need to call writeObject() and your object is magically serialized and sent to the OutputStream. To reverse the process, you wrap an InputStream inside an ObjectInputStream and call readObject(). What comes back is, as usual, a handle to an upcast Object, so you must downcast to set things straight. If you need to dynamically query the type of the object, you can use the getClass method. Specifically dk.getClass.getName() returns the name of the class that dk is an instance of. I.e., this asks the object for the name of its corresponding class object. (Hmmm, True, but what about syntax? I still need to know what it is to declare it...too bad) (C++ can do this in one operation (dynamic_cast (gives null if wrong type)), java can use instanceof operator to check if it is what I think (see Core Java, Ch5 Inheritence, Casting section)