所有内容均来源于官方文档 https://developer.android.com/training/articles/memory.html
only way to completely release memory from your app is to release object references you may be holding, making the memory available to the garbage collector.
从应用完全释放内存的唯一方式就是释放你所引用的的对象，使其对GC可见。
Android系统如何管理内存
共享内存
In order to fit everything it needs in RAM, Android tries to share RAM pages across processes. It can do so in the following ways:
为了适应内存所需，Android会跨进程共享内存页面，通过下列方式：
1）Each app process is forked from an existing process called Zygote.
每个应用进程从一个叫做Zygote的已有进程fork来
2）Most static data is mmapped into a process.
大部分静态数据被映射在一个进程
3）In many places, Android shares the same dynamic RAM across processes using explicitly allocated shared memory regions (either with ashmem or gralloc).
很多场合，Android使用指定的分配内存区域来跨进程共享相同的动态内存。
分配和回收内存
Here are some facts about how Android allocates then reclaims memory from your app:
以下是一些系统回收和分配内存的策略：
1）The Dalvik heap for each process is constrained to a single virtual memory range.
每个进程的Dalvik heap被限定在一个内存区间。
2）The logical size of the heap is not the same as the amount of physical memory used by the heap.
堆内存的逻辑大小和物理大小不相同
3）The Dalvik heap does not compact the logical size of the heap, meaning that Android does not defragment the heap to close up space. Android can only shrink the logical heap size when there is unused space at the end of the heap.
Dalvik heap不会为了使内存连续而整理内存碎片
限制应用内存
To maintain a functional multi-tasking environment, Android sets a hard limit on the heap size for each app. The exact heap size limit varies between devices based on how much RAM the device has available overall. If your app has reached the heap capacity and tries to allocate more memory, it will receive an OutOfMemoryError.
为了维持一个多任务环境，系统严格限制每个应用的堆内存大小，并且随不同设备有所不同，超出这个容量就会造成OOM。
应用切换
Instead of using swap space when the user switches between apps, Android keeps processes that are not hosting a foreground (“user visible”) app component in a least-recently used (LRU) cache. For example, when the user first launches an app, a process is created for it, but when the user leaves the app, that process does not quit. The system keeps the process cached, so if the user later returns to the app, the process is reused for faster app switching.
系统以LRU策略保持非前台进程在缓存里面，例如，当你启动APP时，产生一个进程，但是当你离开这个APP时，进程并未结束。当你再次进入应用时，进程会从缓存里面快速恢复。
应用该如何管理内存
You should apply the following techniques while designing and implementing your app to make it more memory efficient.
为了你的APP更加高效使用内存，你应该采用下列技术：
保守地使用service
1）If your app needs a service to perform work in the background, do not keep it running unless it’s actively performing a job. Also be careful to never leak your service by failing to stop it when its work is done.
当service的工作完成后，永远别忘了stop你的service。
2）When you start a service, the system prefers to always keep the process for that service running. This makes the process very expensive because the RAM used by the service can’t be used by anything else or paged out.
当你开始一个服务时，系统将倾向一直保持这个进程，这会使得这个进程非常昂贵，因为被service使用的内存无法再被得到任何使用。
3）The best way to limit the lifespan of your service is to use an IntentService, which finishes itself as soon as it’s done handling the intent that started it.
限制service生命周期的最好方式是使用IntentService，它会在完成任务后自动总结自己。
当应用界面不在时，释放内存。
1） When the user navigates to a different app and your UI is no longer visible, you should release any resources that are used by only your UI.
当用户切换到其他应用你的应用UI不再可视时，应该释放所有仅被你的UI使用的资源
2）To be notified when the user exits your UI, implement the onTrimMemory() callback in your Activity classes. You should use this method to listen for the TRIM_MEMORY_UI_HIDDEN level, which indicates your UI is now hidden from view and you should free resources that only your UI uses.
重写onTrimMemory()方法判断TRIM_MEMORY_UI_HIDDEN来监听用户是否离开UI，一旦离开就要释放仅被UI使用的资源。
3）So although you should implement onStop() to release activity resources such as a network connection or to unregister broadcast receivers, you usually should not release your UI resources until you receive onTrimMemory(TRIM_MEMORY_UI_HIDDEN). This ensures that if the user navigates back from another activity in your app, your UI resources are still available to resume the activity quickly.
我们需要在onStop里面释放一些activity资源，如网络连接，广播注册等，但只有系统回调onTrimMemory(TRIM_MEMORY_UI_HIDDEN)时才释放UI资源。
当内存紧张时释放内存
During any stage of your app’s lifecycle, the onTrimMemory() callback also tells you when the overall device memory is getting low. You should respond by further releasing resources based on the following memory levels delivered by onTrimMemory():
在应用的所有生命周期中，onTrimMemory() 回调可以告诉你设备内存是不是下降着，你应该根据以下反应不同程度内存使用状况的系统回调作出相应的资源释放操作。
1）TRIM_MEMORY_RUNNING_MODERATE
2）TRIM_MEMORY_RUNNING_LOW
3）TRIM_MEMORY_RUNNING_CRITICAL
Also, when your app process is currently cached, you may receive one of the following levels from onTrimMemory():
当你的应用进程当前被缓存时，你也可以接收到下列反应不同程度内存使用状况的系统回调。
1）TRIM_MEMORY_BACKGROUND
2）TRIM_MEMORY_MODERATE
3）TRIM_MEMORY_COMPLETE
When the system begins killing processes in the LRU cache, although it primarily works bottom-up, it does give some consideration to which processes are consuming more memory and will thus provide the system more memory gain if killed
当系统开始杀LRU缓存中的进程时，尽管主要是遵从LRU策略，但是那些占用大量内存的进程也会优先被考虑。
检查你应该使用多少内存
As mentioned earlier, each Android-powered device has a different amount of RAM available to the system and thus provides a different heap limit for each app. You can call getMemoryClass() to get an estimate of your app’s available heap in megabytes. If your app tries to allocate more memory than is available here, it will receive an OutOfMemoryError.
你可以调用getMemoryClass()方法来估计你的应用可用堆内存，如果你的应用使用超过这个量的内存就会造成OOM。
In very special situations, you can request a larger heap size by setting the largeHeap attribute to “true” in the manifest tag. If you do so, you can call getLargeMemoryClass() to get an estimate of the large heap size.
特殊情况下，你可以通过在manifest文件 节点下设置largeHeap属性为true申请更大的堆内存，如果你这样做，可以调用getLargeMemoryClass()来查看可分配到的更大的堆内存。
Additionally, the large heap size is not the same on all devices and, when running on devices that have limited RAM, the large heap size may be exactly the same as the regular heap size.
另外，由分配规则可以知道，即使设了largeHeap，也可能不起作用。
谨慎处理Bitmap，避免浪费内存。参见具体章节。 http://developer.android.com/training/displaying-bitmaps/load-bitmap.html
使用优化的数据容器
Take advantage of optimized containers in the Android framework, such as SparseArray, SparseBooleanArray, and LongSparseArray.
使用Android API中的优化容器，如SparseArray、SparseBooleanArray,、LongSparseArray等来替代HaspMap等容器。
清楚内存开销
1）Enums often require more than twice as much memory as static constants. You should strictly avoid using enums on Android.
Enums和静态常量相比通常需要两倍内存，在Android中应该避免使用enums
2）Every class in Java (including anonymous inner classes) uses about 500 bytes of code.
每个Class使用大概500个字节码
3）Every class instance has 12-16 bytes of RAM overhead.
每个对象大概有12-16个字节内存的开销
4）Putting a single entry into a HashMap requires the allocation of an additional entry object that takes 32 bytes (see the previous section about optimized data containers).
HashMap每个key对象需要额外大约32个字节的内存，这也是为什么推荐使用6中提到的优化容器。
小心抽象编程
序列化数据时使用nano protobufs
避免依赖注入框架
these frameworks tend to perform a lot of process initialization by scanning your code for annotations, which can require significant amounts of your code to be mapped into RAM even though you don’t need it.
这些框架会通过扫描你的代码中的注解来进行大量的进程初始化，这会导致相当分量的代码被映射进内存，而这是不必要的。
谨慎使用外部库
优化整体表现，这在training doc中有一个专门章节。
使用ProGuard来剔除不必要的代码
The ProGuard tool shrinks, optimizes, and obfuscates your code by removing unused code and renaming classes, fields, and methods with semantically obscure names.Using ProGuard can make your code more compact, requiring fewer RAM pages to be mapped.
使用ProGuard工具来对你的代码进行瘦身、优化和混淆，使得你代码更紧凑，减少内存的使用。
使用zipalign工具操作你的最终APK
If you do any post-processing of an APK generated by a build system (including signing it with your final production certificate), then you must run zipalign on it to have it re-aligned. Failing to do so can cause your app to require significantly more RAM, because things like resources can no longer be mmapped from the APK.
你必须用zipalign工具来操作你的最终APK，这会减少你的应用的内存使用，因为一些不必要的资源可以不再需要从APK映射到内存。
分析你的内存使用
Once you achieve a relatively stable build, begin analyzing how much RAM your app is using throughout all stages of its lifecycle. For information about how to analyze your app, read Investigating Your RAM Usage.
详细信息，看Investigating Your RAM Usage https://developer.android.com/tools/debugging/debugging-memory.html 这一章节。里面不仅会介绍常用的MAT，还有一些其他技巧。
使用多进程
If it’s appropriate for your app, an advanced technique that may help you manage your app’s memory is dividing components of your app into multiple processes.This technique must always be used carefully and most apps should not run multiple processes, as it can easily increase—rather than decrease—your RAM footprint if done incorrectly. It is primarily useful to apps that may run significant work in the background as well as the foreground and can manage those
operations separately.
You can specify a separate process for each app component by declaring the android:process attribute for each component in the manifest file.
当应用的前后台工作分工明确时，可以使用此技巧，但一定要谨慎使用，否则会适得其反。
在官方文档中，我们可以得到最准确的一手信息。