java Memory leak Basics

1.Java Memory allocation strategy
（1） Static storage area （ Method area ）： Store static data 、 Global variables, etc , Allocated at compile time , Exist during operation .
（2） The stack area ： Method execution , Local variables are created here , Automatically release after the method ends . High efficiency, limited capacity .
（3） Heap area ： Dynamic memory allocation . Deposit new Out of the objects and arrays , Recycle by garbage collector .

2.java How to manage memory
We allocate memory ,gc Reclaiming memory .
Reachable with vertex 、 Unreachable .jpg

3.java Memory leak in
Useless object （ No longer used ） The memory of continuous comrades in arms or useless objects cannot be released in time , The resulting waste of memory space becomes a memory leak .

Android Memory leak

1. Single case
The singleton static feature makes its life cycle and app As long as , If an object is no longer used , The singleton holds its reference .
e.g. Do not pass in a singleton Activity Of Context, Instead, use context.getApplicationContext
2. Anonymous inner class
Instances created by non static inner classes are not recycled , Hold the reference of this class to the external class .
3.Handler
Change to static inner class , And let it hold weak references to external classes .
4. Avoid using static Member variables
Consider using life cycle management
5. Memory leak due to resource not closed
Activity Close or unregister resources on destruction
6.AsyncTask Memory leak

FAQ

Memory leak reference chain

The main thread / Sub thread -> ThreadLocal -> Looper -> MessageQueue -> Message -> Handler

LeakCanary principle

monitor Activity Life cycle of ,（ adopt Application.registerActivityLifecycleCallbacks() ）
stay onDestroy() When , Wait a few seconds , Then determine whether the object is recycled （ adopt ReferenceQueue+WeakReference）. If not recycled , register IdleHandler, At leisure GC, And then judge whether the object is recycled .
After determining that there is a memory leak , Generate .hprof file （ call heapDumper.dumpHeap()）.
stay IntentService Dealing with subsequent logic 、 Sending notice .

q： Why not use virtual references + Reference queue
a： Objects stored by virtual references are almost directly recycled , As a result, a certain amount of time cannot be stored ,LeakCanary For monitoring and recording , The required data and records should be stored for a certain period of time at least , Because in the actual use scenario, it is generally used if the reference is enough

Good writing