Interview question: what are the functions of fail safe mechanism and fail fast mechanism

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This column is the common interview questions of big factories that I sorted out , I will give answers and explanations , I hope I can help you
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link : Explore the principles from the source code

link : Java Concurrent programming

link : SQL– Knowledge and principle

Some time ago, a classmate went for an interview , Meet such a problem .

fail-safe Mechanism and fail-fast What is the role of the mechanism ？

He said when he heard the question , My mind is full of question marks .

forehead …. Um. …（ After a few seconds , Post a funny picture , such as two years latter And so on , Then match A face blinded ..）

fail-safe Mechanism and fail-fast What are the mechanisms Modus effect

fail-safe and fail-fast, It is a failure handling mechanism for multithreaded concurrent operation collection .

	Fail-fast： Express a quick failure , During collection traversal , Once the data in the container is found to have been modified ,  It will be thrown out immediately  ConcurrentModificationException  abnormal , This leads to traversal failure , In a situation like this .

Usually used in conjunction with iterators , For example, define a Map aggregate , Use Iterator Iterator for data traversal , In the process of traversing , When making changes to set data , It will happen fail-fast.

java.util The collection classes under the package are all of fast failure mechanism .

Common use fail-fast Way to traverse the content Device has HashMap and ArrayList etc. .

	Fail-safe, Indicates failed security , That is, under this mechanism , The modification of set elements appears , I can't throw it   Out  ConcurrentModificationException.

The reason is the collection container with security failure mechanism , Not directly accessed on the collection content when traversing , Instead, copy the contents of the original collection first , Traversing over the set of copies .

Because the iteration is to traverse the copy of the original set , Therefore, changes made to the original set during traversal cannot be detected by the iterator , such as , Defined a CopyOnWriteArrayList, In the process of traversing this collection , After modifying the set elements , Does not throw an exception , But at the same time, the added elements will not be printed .

java.util.concurrent The containers under the package are all security failed , It can be used concurrently in multithreading , Concurrent modification .

often see Of Of send use fail-safe Fang type All over calendar Of Rong device Yes ConcerrentHashMap and CopyOnWriteArrayList etc. .