Oom out of memory memory overflow

When app Overhead memory exceeds allocation when memory , There will be a memory overflow

There are several reasons

The one-time memory cost is too large   ： Load macro, etc
Memory persistence overhead ：listview Load picture scroll , Loop to create objects, etc
Memory recycling is not timely ： Memory overhead is too high ,GC The frequency can not keep up with the overhead speed
Memory cannot be recycled ： Memory leak leads to memory overflow, etc

Android Memory allocation and recycling mechanism

Android The system will only judge before the new memory allocation Heap Whether there is enough space left at the end of , If not enough, it will trigger GC operation , So as to free up more free memory space .

Android Of Heap Space is a GenerationalHeapMemory Model of , Recently assigned objects will be stored in YoungGeneration Area , When an object stays in this area for a certain time , It will be moved to OldGeneration, Finally, accumulate a certain time and move to PermanentGeneration Area .

The system will execute different data types according to different memory data types GC operation . for example , Just assigned to YoungGeneration Objects in the area are usually easier to destroy and recycle , At the same time YoungGeneration Regional gc Operating speed ratio OldGeneration Regional gc Faster . every last Generation All memory areas have a fixed size , As new objects are assigned to this area , When the total size of these objects reaches the threshold of this level of memory area , Will trigger GC operation , To make room for other new objects . Usually ,GC When it happened , All threads are suspended . perform GC The time it took and where it happened Generation It also matters , YoungGeneration Every time GC The operation time is the shortest , OldGeneration secondly , PermanentGeneration The longest . The length of execution is also different from the current Generation It depends on the number of objects in .

For the whole Android Memory control needs of the system ,Android The system sets a hard for each application DalvikHeapSize Maximum limit threshold , This threshold value will be changed on different devices because RAM It's different in size . If your application's memory footprint is close to this threshold , If you try to allocate memory again at this time , It's easy to cause OutOfMemoryError Error of .

//  obtain Heap Size threshold 
ActivityManager am =(ActivityManager)context.getSystemService(Context.ACTIVITY_SERVICE);
//  The return value is in Mb In units of 
int memoryClass = am.getMemoryClass();

as long as allocated + Newly allocated memory >= getMemoryClass() When it happens OOM.

avoid OOM Methods

1. Reduce the memory footprint of objects

avoid OOM The first step is to minimize the memory occupied by newly allocated objects , Try to use more lightweight objects .

Use lighter data structures

Use ArrayMap/ SparseArray replace HashMap And so on . ArrayMap yes Android The system is a container specially written for mobile operating system , in the majority of cases , Than HashMap More efficient , Less memory . SparseArray More efficient because they avoid key and value Of autobox Automatic boxing , And avoid unpacking after packing .

To avoid the Android It uses Enum

Android Official statement ”Enums often require more than twice as much memory as static constants. You should strictly avoid using enums on Android.“, Therefore, it should be avoided in Android Enumeration is used inside .

  Reduce Bitmap Object's memory footprint

Bitmap Is a fat man who is very easy to consume memory , Reduce the of creation processing Bitmap Memory usage is very important , Usually there are the following 2 Measures ：

inSampleSize： Zoom ratio , Before loading pictures into memory , We need to work out an appropriate scale first , Avoid unnecessary large image loading .
decode format： Decoding format , choice ARGB_8888/ RGB_565/ ARGB_4444/ ALPHA_8, There is a big difference .
  Use smaller pictures

Corresponding resource picture , Pay special attention to whether there is compressible space in this picture , Can I use a smaller picture . Trying to use smaller pictures can not only reduce memory usage , It can also avoid a large number of InflationException. Suppose a big picture is XML The document refers directly to , It is very likely that when initializing the view, it will occur due to insufficient memory InflationException, The root cause of this problem is actually that OOM.

Reuse of memory objects

Reuse of most objects , The final implementation scheme is to use object pool technology , Or create an object pool in the program displayed when writing code , Then handle the reuse implementation logic , Or use some reuse features of the system framework to reduce the repeated creation of objects , So as to reduce the allocation and recycling of memory . stay Android The most commonly used caching algorithm above is LRU(Least Recently Use), The recommended operating principle is shown in the figure below ：

Reuse the resources of the system

Android The system itself has many built-in resources , Such as a string 、 Color 、 picture 、 Animation 、 Style and simple layout, etc , These resources can be used directly in the application . This not only reduces the application's own load , Reduce APK Size . But be careful Android System version differences .

Pay attention to ListView/GridView Wait for a view with a large number of repeated sub components ConvertView Reuse of

 Bitmap Reuse of objects

stay RecyclerView、ListView、GridView And other controls that display a large number of pictures LRU Mechanism to cache processed Bitmap. utilize inBitmap Advanced features of Android The system is in Bitmap Improve the efficiency of distribution and release execution . Use inBitmap Properties can tell Bitmap Decoder to try to use the existing memory area , New decoded bitmap Will try to use the previous one bitmap stay heap Occupied by pixel data Memory area , Instead of asking the memory to reapply an area to store bitmap. Take advantage of this feature , Even thousands of pictures , It will only take up the size of the number of pictures that can be displayed on the screen .

Use inBitmap Several restrictions that need attention ：

stay SDK 11~18 Between , Reusable bitmap The size must be consistent , For example, to inBitmap The assigned image size is 100x100, So the new application bitmap Must also be 100x100 Can be reused . from SDK 19 Start , Newly applied bitmap The size must be less than or equal to the assigned bitmap size .
Newly applied bitmap With the old bitmap Must have the same decoding format , For example, the preceding bitmap yes 8888, I can't support it for so long 4444 and 565 Format bitmap 了 . We can create one that contains a variety of typical reusable bitmap Object pool , And so on bitmap You can find the right one ” Templates “ To reuse .

 

  To avoid the onDraw Method to create objects

similar onDraw And other frequently called methods , You must pay attention to avoid creating objects here , Because it will rapidly increase the use of memory , And it's easy to cause frequent GC, Even memory jitter .

 StringBuilder

When a large number of string splicing operations need to be used in the code , It is necessary to consider the use of StringBuilder To replace frequent ”+“.

Avoid memory leaks of objects

Memory object leak , Some objects that are no longer in use cannot be released in time , On the one hand, it takes up valuable memory space , It can easily lead to subsequent memory allocation , There is not enough free space OOM. obviously , It also makes every level Generation The free space of the memory area is reduced ,GC It's easier to trigger , Prone to memory jitter , This causes performance problems .

Be careful Activity Leakage of

Generally speaking ,Activity Memory leak is the most serious problem in memory leak , It takes up the most memory , Wide range of influence . Lead to Activity Two cases of leakage ：

Internal class references cause Activity Leakage of

Activity Context Passed to other instances , This may cause itself to be referenced and leaked .
Consider using Application Context instead of Activity Context

For most non essential applications Activity Context The situation of （Dialog Of Context It has to be Activity Context）, You can consider using Application Context instead of Activity Of Context, This can avoid inadvertent Activity leak .

Pay attention to temporary Bitmap Timely recycling of objects

A relatively large... Created temporarily bitmap object , After conversion, get a new one bitmap After object , The original should be recycled as soon as possible bitmap, This will release the original more quickly bitmap The space occupied .

Be careful Bitmap Class createBitmap() Method ：

This function returns bitmap It's possible and source bitmap Is the same , When it comes to recycling , Special inspection is required source bitmap And return bitmap Is the quotation the same , Only in unequal circumstances , To be able to execute source bitmap Of recycle Method .

Pay attention to the cancellation of the listener

stay Android There are many needs in the program register and unregister The monitor for , Need to ensure timely delivery at the right time unregister Those listeners . Manual add Of listener, Need to remember in time remove This listener.

Note the object leak in the cache container

If the container is static or global , Then the objects stored inside should be timely remove.

Be careful WebView Leakage of

Android in WebView There are big compatibility issues , It needs to be destroyed at an appropriate time .

Be careful Cursor Whether the object is closed in time

For database queries Cursor, If it is not closed in time, it will cause leakage .