Java静态 block 线程安全

Question

我正在研究 Java 11 并发模型，我发现可以通过 LazyHolder pattern 获得真正的单例。 .特别是，该页面说:

Since the class initialization phase is guaranteed by the JLS to be sequential, i.e., non-concurrent, no further synchronization is required...

好的，所以我明白静态字段只按顺序初始化一次(所以

public class DatabaseQueries {

  private static Map<DatabaseActions, String> database = new HashMap<>();

  public static Map<DatabaseActions, String> getDatabase() {
    return Collections.unmodifiableMap(database);
  }

  static {
    database.put(
      DatabaseActions.LIST_CHIHUAHUA,
      "SELECT id, nome, immagine FROM chihuahua ORDER BY data_nascita DESC"
    );

    // more...

  }

}

上面的代码线程安全吗？即使我有一个静态 block 和一个单独的 map 静态初始化!

---

我在 this 找到了回答:

Static class initialization is guaranteed to be thread-safe by Java.

上面我有一个静态 block 和一个静态变量，这有什么区别吗？是否都在类启动时初始化？

所以我的问题是静态 block 和静态变量(在静态 block 之外)是否都保证不会导致多线程问题

Answer 1

你写的

Even if I have a static block AND a separate static initialization of a map!

这表明错误的心态。您没有单独的初始化。

代码

public class DatabaseQueries {
  private static Map<DatabaseActions, String> database = new HashMap<>();
  static {
    database.put(
      DatabaseActions.LIST_CHIHUAHUA,
      "SELECT id, nome, immagine FROM chihuahua ORDER BY data_nascita DESC"
    );
  }
}

等同于

public class DatabaseQueries {
  private static Map<DatabaseActions, String> database;
  static {
    database = new HashMap<>();
    database.put(
      DatabaseActions.LIST_CHIHUAHUA,
      "SELECT id, nome, immagine FROM chihuahua ORDER BY data_nascita DESC"
    );
  }
}

static 字段的所有初始化器(编译时常量除外)与所有 static {} block 合并为一个初始化器，按照它们在类声明。

线程安全适用于单个结果初始化器的完成和初始化数据的后续读取。

所以如果映射在类初始化后从未被修改过，它就是线程安全的。没有必要，但强烈建议强制执行“初始化后不修改”规则:

public class DatabaseQueries {

  private static final Map<DatabaseActions, String> database;
  static { // keep variable declaration and initializer close for readability
    Map<DatabaseActions, String> map = new HashMap<>();// mutable ref during initialization
    map.put(
      DatabaseActions.LIST_CHIHUAHUA,
      "SELECT id, nome, immagine FROM chihuahua ORDER BY data_nascita DESC"
    );
    // more...
    database = Collections.unmodifiableMap(map);// enforce read-only
  }

  public static Map<DatabaseActions, String> getDatabase() {
    return database;// no wrapping necessary, it’s already wrapped
  }
}