The difference between memory overflow and memory leak

out of memory out of memory, When a program requests memory , Not enough memory for it , appear out of memory; Like applying for a integer, But save it long Number that can be saved , That's memory overflow .

Memory leak memory leak, Refers to the program after applying for memory , Unable to free requested memory space , The harm of a memory leak can be ignored , But the memory leak has serious consequences , No matter how much memory , Sooner or later it will be taken up .

memory leak Will eventually lead to out of memory！

Memory overflow is when you ask for more memory than the system can give you , The system can't meet the demand , So there's overflow .

A memory leak is when you apply to the system to allocate memory for use (new), But they don't return it after using it (delete), As a result, you can't access the memory you applied for （ Maybe you lost its address ）, And the system can't assign it again to the program it needs . A plate can only hold 4 A fruit , You put on 5 individual , As a result, I fell to the ground and couldn't eat . This is overflow ！ Like stacks , When the stack is full, it will cause space overflow , Call for overflow , When the stack is empty, it will cause space overflow , It's called underflow . The memory allocated is not enough to put down the sequence of data items , It's called a memory overflow .

Classify in the way that they happen , Memory leaks can be divided into 4 class ：

1. Frequent memory leaks . Code with memory leaks will be executed multiple times to , Each time it is executed, it will cause a memory leak . 2. Occasional memory leak . Code that has a memory leak only happens under certain circumstances or operating procedures . Recurrent and episodic are relative . For a particular environment , What happens occasionally may become what happens frequently . So test environment and test method are very important to detect memory leak . 3. One time memory leak . Code with a memory leak will only be executed once , Or because of algorithm flaws , There will always be a single memory leak . such as , Allocate memory in the class's constructor , In the destructor, the memory is not freed , So memory leaks only happen once . 4. Implicit memory leaks . Program in the process of running the allocation of memory , But don't release memory until the end . Strictly speaking, there is no memory leak , Because eventually the program frees up all of the requested memory . But for a server program , It takes a few days to run , Weeks or even months , Not releasing memory in time can also lead to the eventual exhaustion of all the system's memory . therefore , We call such memory leaks implicit memory leaks .

From the point of view of the user using the program , Memory leaks themselves don't do any harm , As a general user , There is no memory leak at all . The real danger is the accumulation of memory leaks , This will eventually consume all the memory of the system . From this perspective , One time memory leaks don't hurt , Because it doesn't pile up , The implicit memory leak is very harmful , Because it is more difficult to detect than frequent and accidental memory leaks

Focus on the following points ： 1. Check the database query , Is there a query to get all the data . Generally speaking , If you take 100000 records to memory at a time , May cause memory overflow . This problem is more hidden , Before going online , There is less data in the database , It's not easy to have problems , After the launch , There's a lot of data in the database , A query may cause memory overflow . So for Try to query the database by paging .

2. Check the code for dead loops or recursive calls .

3. Check if there is a large loop that repeatedly generates new object entities .

4. Check the database query , Is there a query to get all the data . Generally speaking , If you take 100000 records to memory at a time , May cause memory overflow . This problem is more hidden , Before going online , There is less data in the database , It's not easy to have problems , After the launch , There's a lot of data in the database , A query may cause memory overflow . Therefore, the database query should be paged as much as possible .

5. Check List、MAP Wait until the collection object is used , Problems not cleared .List、MAP The collection object will always have a reference to the object , So that these objects cannot be GC Recycling .

Step four , Use the memory view tool to dynamically view memory usage

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