参考：
Android OnLowMemory和OnTrimMemory
OnLowMemory
OnLowMemory是Android提供的API，在系统内存不足，所有后台程序（优先级为background的进程，不是指后台运行的进程）都被杀死时，系统会调用OnLowMemory。系统提供的回调有：
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Application.onLowMemory()
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Activity.OnLowMemory()
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Fragement.OnLowMemory()
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Service.OnLowMemory()
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ContentProvider.OnLowMemory()
除了上述系统提供的API，还可以自己实现ComponentCallbacks，通过API注册，这样也能得到OnLowMemory回调。例如：
public static class MyCallback implements ComponentCallbacks { @Override public void onConfigurationChanged(Configuration arg) { } @Override public void onLowMemory() { //do release operation } }
然后，通过Context.registerComponentCallbacks ()在合适的时候注册回调就可以了。通过这种自定义的方法，可以在很多地方注册回调，而不需要局限于系统提供的组件。
onLowMemory 当后台程序已经终止资源还匮乏时会调用这个方法。好的应用程序一般会在这个方法里面释放一些不必要的资源来应付当后台程序已经终止，前台应用程序内存还不够时的情况。
OnTrimMemory
OnTrimMemory是Android 4.0之后提供的API，系统会根据不同的内存状态来回调。系统提供的回调有：
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Application.onTrimMemory()
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Activity.onTrimMemory()
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Fragement.OnTrimMemory()
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Service.onTrimMemory()
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ContentProvider.OnTrimMemory()
OnTrimMemory的参数是一个int数值，代表不同的内存状态：
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TRIM_MEMORY_COMPLETE：内存不足，并且该进程在后台进程列表最后一个，马上就要被清理
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TRIM_MEMORY_MODERATE：内存不足，并且该进程在后台进程列表的中部。
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TRIM_MEMORY_BACKGROUND：内存不足，并且该进程是后台进程。
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TRIM_MEMORY_UI_HIDDEN：内存不足，并且该进程的UI已经不可见了。
以上4个是4.0增加
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TRIM_MEMORY_RUNNING_CRITICAL：内存不足(后台进程不足3个)，并且该进程优先级比较高，需要清理内存
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TRIM_MEMORY_RUNNING_LOW：内存不足(后台进程不足5个)，并且该进程优先级比较高，需要清理内存
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TRIM_MEMORY_RUNNING_MODERATE：内存不足(后台进程超过5个)，并且该进程优先级比较高，需要清理内存
以上3个是4.1增加
系统也提供了一个ComponentCallbacks2，通过Context.registerComponentCallbacks()注册后，就会被系统回调到。
以上int值对应关系：
http://developer.android.com/reference/android/content/ComponentCallbacks2.html
Constants
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public static final int TRIM_MEMORY_BACKGROUND Added in API level 14 Level for onTrimMemory(int): the process has gone on to the LRU list. This is a good opportunity to clean up resources that can efficiently and quickly be re-built if the user returns to the app. Constant Value: 40 (0x00000028)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
public static final int TRIM_MEMORY_COMPLETE
Added in API level 14
Level for onTrimMemory(int): the process is nearing the end of the background LRU list, and if more memory isn‘t found soon it will be killed.
Constant Value: 80 (0x00000050)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
public static final int TRIM_MEMORY_MODERATE
Added in API level 14 Level for onTrimMemory(int): the process is around the middle of the background LRU list; freeing memory can help the system keep other processes running later in the list for better overall performance.
Constant Value: 60 (0x0000003c)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
public static final int TRIM_MEMORY_RUNNING_CRITICAL
Added in API level 16
Level for onTrimMemory(int): the process is not an expendable background process, but the device is running extremely low on memory and is about to not be able to keep any background processes running. Your running process should free up as many non-critical resources as it can to allow that memory to be used elsewhere. The next thing that will happen after this is onLowMemory() called to report that nothing at all can be kept in the background, a situation that can start to notably impact the user.
Constant Value: 15 (0x0000000f)
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public static final int TRIM_MEMORY_RUNNING_LOW
Added in API level 16
Level for onTrimMemory(int): the process is not an expendable background process, but the device is running low on memory. Your running process should free up unneeded resources to allow that memory to be used elsewhere.
Constant Value: 10 (0x0000000a)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
public static final int TRIM_MEMORY_RUNNING_MODERATE
Added in API level 16
Level for onTrimMemory(int): the process is not an expendable background process, but the device is running moderately low on memory. Your running process may want to release some unneeded resources for use elsewhere. Constant Value: 5 (0x00000005)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
public static final int TRIM_MEMORY_UI_HIDDEN
Added in API level 14
Level for onTrimMemory(int): the process had been showing a user interface, and is no longer doing so. Large allocations with the UI should be released at this point to allow memory to be better managed. Constant Value: 20 (0x00000014)
OnLowMemory和OnTrimMemory的比较
1，OnLowMemory被回调时，已经没有后台进程；而onTrimMemory被回调时，还有后台进程。
2，OnLowMemory是在最后一个后台进程被杀时调用，一般情况是low
memory killer
杀进程后触发；而OnTrimMemory的触发更频繁，每次计算进程优先级时，只要满足条件，都会触发。
3，通过一键清理后，OnLowMemory不会被触发，而OnTrimMemory会被触发一次。
使用举例：
1 @Override 2 public void onTrimMemory(int level) { 3 Log.e(TAG, " onTrimMemory ... level:" + level); 6 } 7 8 @Override 9 public void onLowMemory() { 11 Log.e(TAG, " onLowMemory ... "); 13 }
通过 ActivityManager查看进程的内存：
1 private void displayBriefMemory() { 2 final ActivityManager activityManager = (ActivityManager) getSystemService(ACTIVITY_SERVICE); 3 ActivityManager.MemoryInfo info = new ActivityManager.MemoryInfo(); 4 activityManager.getMemoryInfo(info); 5 Log.i(TAG, "系统剩余内存:" + (info.availMem >> 10) + "k"); 6 Log.i(TAG, "系统是否处于低内存运行：" + info.lowMemory); 7 Log.i(TAG, "当系统剩余内存低于" + (info.threshold >> 10) + "k" + "时就看成低内存运行"); 8 9 util.SavedToText(TAG, "meminfo 系统剩余内存:" + (info.availMem >> 10) + "k" 10 + " " + "系统是否处于低内存运行：" + info.lowMemory + " " + "当系统剩余内存低于" 11 + (info.threshold >> 10) + "k" + "时就看成低内存运行"); 12 }