声音和图象同步；摄影机前同时举起的挡板。
Synchronizes sound and picture; boards held in front of a movie camera are banged together.
仅当所有线程在访问该集合前都锁定该对象时，同步才有效。
Synchronization is effective only if all threads lock this object before accessing the collection.
仅当所有线程在访问该集合前都锁定该对象时，同步才有效。
Synchronization is effective only if all threads lock this object before accessing the collection.
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