java - 非最终初始化程序是线程安全的吗？

Question

是否保证每个线程都看到非final字段的实例初始化器(字段等号右边的表达式)的值？例如:

class Foo {
  private boolean initialized = false; // Initializer
  private final Lock lock = new ReentrantLock();

  public void initialize() {
    lock.lock()
    try {
      // Is initialized always false for the first call of initialize()?
      if (initialized) {
        throw new IllegalStateException("already initialized");
      }
      // ...
      initialized = true;
    } finally {
      lock.unlock();
    }
  }
}

Answer 1

在那种特定情况下你没问题，因为false 也是boolean 字段 的默认值。如果您的实例变量初始化是:

private boolean initialized = true;

那么您将无法保证线程会读取 true。

请注意，如果该字段是静态的，则由于类加载语义，您将获得这样的保证。

引用:JLS 17.4.4 (强调我的)

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.