The difference between JVM memory overflow and memory leak

1、 Distinction of concepts

1.1、 Memory leak （memory leak）

After program running , There is no release Memory space occupied .

A memory leak It doesn't seem to have a big impact , But memory leaks Keep accumulating , Eventually, the available memory will become less and less .

for instance , The total memory size is 100 MB, Yes 40MB Of memory has been unable to recycle , Then only 60MB . this 40MB Memory leak .

Memory leak , After the program runs , No free memory .

1.2、 out of memory （out of memory）

Program runtime , When applying for memory space , There is not enough memory space for its normal use , The program stops , And throw out of memory .

For example, when the program is running, it applies for a 10MB Space , But the current available memory is only 5MB, The program doesn't work properly , This is memory overflow .

 

  out of memory , It can be understood that the memory required for program operation is greater than the currently available memory .

1.3 give an example

（1） In singleton mode , The lifecycle of the singleton is as long as the application , So if a singleton program holds a reference to an external object , Then this external object cannot be recycled , Will lead to   Memory leak   The birth of .

（2） Some offer close Due to the unclosed resources of   Memory leak  . Database connection （dataSource.getConnection() ）, network connections （socket） and IO The connection of the stream must be in finally in close, Otherwise it can't be recycled .

（3） Read large files , The file read at one time is larger than the available memory , It can lead to   out of memory  . The available memory is 1G, How to read 2G What about the documents ？ To build a 100MB Byte array of , read 10 Time .

2、 The difference and connection between the two

2.1 difference

Memory leak ： After program running , The occupied memory is not fully released .

out of memory ： Program runtime , The memory required is larger than the currently available memory , Out of memory , Program cannot continue , Throw out “ out of memory ”, The program is interrupted , end .

2.2 contact

once   Memory leak   It may have no obvious impact on the operation of the program , many times   Memory leak   Will eventually lead to   out of memory  .

For example, the total memory size is 100MB, There are , Yes 10MB There is no release , At present Available memory also 90MB, The program can also run . But after many runs ,  Available memory   Ultimately for 0, When there is no memory available or memory is insufficient , The next time the program runs , Because of insufficient memory , And it appears   out of memory  .

3、 The causes and solutions of memory overflow

3.1 Cause of memory overflow

There are many reasons for memory overflow , Xiaobian lists the following common ：

1. The amount of data loaded in memory is too large , Such as fetching too much data from the database at one time ;
2. There are references to objects in collection classes , Not empty after use , bring JVM Can't recycle ;
3. There are dead loops in the code or loops that produce too many duplicate object entities ;
4. In the third-party software used BUG;
5. The memory value of startup parameter is set too small .

3.2 Memory overflow solution

First step , modify JVM Launch parameters , Add memory directly .(-Xms、-Xmx Parameters must not forget to add )

The second step , Check the error log , see “OutOfMemory” Whether there are other anomalies or errors before the error .

The third step , Walk through and analyze the code , Find out where a memory overflow might occur .

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 . Therefore, the database query should be paged as much as possible .
 
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 .

Original author ：Sharley

Original address ：https://www.cnblogs.com/Sharley/p/5285045.html