Use lazy initialization.

使用延迟初始化。

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Instead of double-checked locking, use the Initialize-on-demand Holder Class idiom, which provides lazy initialization, is thread-safe, and is faster and less confusing than double-checked locking.

不使用双重检查锁定，而使用Initialize - on - demand Holder Class idiom，它提供了迟缓初始化，是线程安全的，而且比双重检查锁定更快且没那么混乱。

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