构造后可见的 Java 非最终整数

Question

我有一个带有非最终 int 变量的 java 类，我在构造函数中将其显式初始化为 0。对该变量的所有其他访问都由 ReentrantLock 管理。我是否需要担心线程不会看到初始值 0，因为我没有在构造函数中使用锁？

Answer 1

是的，你必须担心。为避免在这种情况下出现问题，您需要安全发布对象引用。

来自 Java Concurrency in Practice :

To publish an object safely, both the reference to the object and the object's state must be made visible to other threads at the same time. A properly constructed object can be safely published by:

• Initializing an object reference from a static initializer;
• Storing a reference to it into a volatile field or AtomicReference;
• Storing a reference to it into a final field of a properly constructed object; or
• Storing a reference to it into a field that is properly guarded by a lock.

在其他情况下，您可以(理论上)面对 new 的结果在构造函数调用完成之前对其他线程可用的情况(由于可能的操作重新排序)。

但是请注意，如果 0 是默认值而不是构造函数中写入的值，则它保证可见(JLS §17.4.4):

• The write of the default value (zero, false or null) to each variable synchronizes- with the first action in every thread. Although it may seem a little strange to write a default value to a variable before the object containing the variable is allocated, conceptually every object is created at the start of the program with its default initialized values