Java 日看一类（53）之IO包中的SerialCallbackContext类

该类是内部类，无法在包外引用，且该类为final，无法被继承

该类无引入包和继承类

该类的类头注释如下：

/**
 * Context during upcalls from object stream to class-defined
 * readObject/writeObject methods.
 * Holds object currently being deserialized and descriptor for current class.
 *
 * This context keeps track of the thread it was constructed on, and allows
 * only a single call of defaultReadObject, readFields, defaultWriteObject
 * or writeFields which must be invoked on the same thread before the class's
 * readObject/writeObject method has returned.
 * If not set to the current thread, the getObj method throws NotActiveException.
 */

大意如下：

从对象流到类上下文定义的readObject和writeObject方法会调用该类

存储当前被反序列化的类和该类的描述符

这个上下文跟踪它构建的线程，并允许只调用defaultReadObject，readFields，defaultWriteObject或writeFields，它们必须在类之前在同一个线程上调用 readObject / writeObject方法已经返回

如果没设置当前线程，getOj方法会抛出NotActiveException

该类含有如下的成员变量：

被操作的Object：

private final Object obj;

内含的对象流类：

private final ObjectStreamClass desc;

当前操作线程

private Thread thread;

该类含有如下的成员方法：

构造函数（指定操作类和操作流类

public SerialCallbackContext(Object obj, ObjectStreamClass desc) {
    this.obj = obj;
    this.desc = desc;
    this.thread = Thread.currentThread();
}

获取当前操作对象

public Object getObj() throws NotActiveException {
    checkAndSetUsed();
    return obj;
}

获取当前对象流类描述符

public ObjectStreamClass getDesc() {
    return desc;
}

检测当前操作线程可用性

public void check() throws NotActiveException {
    if (thread != null && thread != Thread.currentThread()) {//仅当当前线程不为空切非当前操作线程时抛出异常
        throw new NotActiveException(
            "expected thread: " + thread + ", but got: " + Thread.currentThread());
    }
}

检测当前线程是否为当前操作线程，如果是则置空，否则抛异常

private void checkAndSetUsed() throws NotActiveException {
    if (thread != Thread.currentThread()) {
         throw new NotActiveException(
          "not in readObject invocation or fields already read");
    }
    thread = null;
}

将当前操作线程置空

public void setUsed() {
    thread = null;
}

该类主要用于序列化操作时对对象，对象类的记录，我们编程无法调用该类，看看即可