JUC collection class is unsafe

Unsafe under multithreading

  List unsafe

java.util.ConcurrentModificationException Concurrent modification exception

CopyOnWrite Copy on write COW An optimization strategy in the field of computer programming ;

When multiple threads call ,list, When reading , fixed , write in （ Cover ）

Avoid overwriting when writing , Cause data problems ！

Read / write separation

Why thread safe Copy no need Vector 

Vector Every method of is locked , and CopyOnWriteArrayList The read operation of is not locked , therefore CopyOnWriteArrayList The read performance of is much higher than Vector, Vector The size of each expansion is the same as that of the original array 2 times , and CopyOnWriteArrayList There's no need to expand , adopt COW The idea can make the array capacity meet the requirements . Both sets are implemented RandomAccess Interface , Supports random reading , Therefore, it is more recommended to use for Loop through . In development , The read operation will be much more than the other operations , Therefore use CopyOnWriteArrayList Aggregation is more efficient

Set unsafe

HashSet What's at the bottom ？

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