今天看郭哥的LitePal框架的源码,刚打开LitePalApplication里面的源码看到了这样一幕

@Override
	public void onLowMemory() {
		super.onLowMemory();
		mContext = getApplicationContext();
	}

首先通过郭哥的那段代码,就看到了,如下部分

public void onLowMemory() {
        Object[] callbacks = collectComponentCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ComponentCallbacks)callbacks[i]).onLowMemory();
            }
        }
    }

/**
     * This is called when the overall system is running low on memory, and
     * actively running processes should trim their memory usage.  While
     * the exact point at which this will be called is not defined, generally
     * it will happen when all background process have been killed.
     * That is, before reaching the point of killing processes hosting
     * service and foreground UI that we would like to avoid killing.
     *
     * <p>You should implement this method to release
     * any caches or other unnecessary resources you may be holding on to.
     * The system will perform a garbage collection for you after returning from this method.
     * <p>Preferably, you should implement {@link ComponentCallbacks2#onTrimMemory} from
     * {@link ComponentCallbacks2} to incrementally unload your resources based on various
     * levels of memory demands.  That API is available for API level 14 and higher, so you should
     * only use this {@link #onLowMemory} method as a fallback for older versions, which can be
     * treated the same as {@link ComponentCallbacks2#onTrimMemory} with the {@link
     * ComponentCallbacks2#TRIM_MEMORY_COMPLETE} level.</p>
     */
    void onLowMemory();

public void onTrimMemory(int level) {
        Object[] callbacks = collectComponentCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                Object c = callbacks[i];
                if (c instanceof ComponentCallbacks2) {
                    ((ComponentCallbacks2)c).onTrimMemory(level);
                }
            }
        }
    }

/**
     * Level for {@link #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.
     */
    static final int TRIM_MEMORY_COMPLETE = 80;
    
    /**
     * Level for {@link #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.
     */
    static final int TRIM_MEMORY_MODERATE = 60;
    
    /**
     * Level for {@link #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.
     */
    static final int TRIM_MEMORY_BACKGROUND = 40;
    
    /**
     * Level for {@link #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.
     */
    static final int TRIM_MEMORY_UI_HIDDEN = 20;

    /**
     * Level for {@link #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 {@link #onLowMemory()} called to report that
     * nothing at all can be kept in the background, a situation that can start
     * to notably impact the user.
     */
    static final int TRIM_MEMORY_RUNNING_CRITICAL = 15;

    /**
     * Level for {@link #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.
     */
    static final int TRIM_MEMORY_RUNNING_LOW = 10;


    /**
     * Level for {@link #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.
     */
    static final int TRIM_MEMORY_RUNNING_MODERATE = 5;

    /**
     * Called when the operating system has determined that it is a good
     * time for a process to trim unneeded memory from its process.  This will
     * happen for example when it goes in the background and there is not enough
     * memory to keep as many background processes running as desired.  You
     * should never compare to exact values of the level, since new intermediate
     * values may be added -- you will typically want to compare if the value
     * is greater or equal to a level you are interested in.
     *
     * <p>To retrieve the processes current trim level at any point, you can
     * use {@link android.app.ActivityManager#getMyMemoryState
     * ActivityManager.getMyMemoryState(RunningAppProcessInfo)}.
     *
     * @param level The context of the trim, giving a hint of the amount of
     * trimming the application may like to perform.  May be
     * {@link #TRIM_MEMORY_COMPLETE}, {@link #TRIM_MEMORY_MODERATE},
     * {@link #TRIM_MEMORY_BACKGROUND}, {@link #TRIM_MEMORY_UI_HIDDEN},
     * {@link #TRIM_MEMORY_RUNNING_CRITICAL}, {@link #TRIM_MEMORY_RUNNING_LOW},
     * or {@link #TRIM_MEMORY_RUNNING_MODERATE}.
     */
    void onTrimMemory(int level);

TRIM_MEMORY_COMPLETE:

系统处于低内存的运行状态中如果系统现在没有内存回收你的应用将会第一个被杀掉. 你必须释放掉所有非关键的资源从而恢复应用的状态.

TRIM_MEMORY_MODERATE

 系统处于低内存的运行状态中并且你的应用处于缓存应用列表的中级阶段. 如果系运行内存收到限制, 你的应用有被杀掉的风险.

TRIM_MEMORY_BACKGROUND:

    系统处于低内存的运行状态中并且你的应用处于缓存应用列表的初级阶段.  虽然你的应用不会处于被杀的高风险中, 但是系统已经开始清除缓存列表中的其它应用, 所以你必须释放资源使你的应用继续存留在列表中以便用户再次回到你的应用时能快速恢复进行使用.

TRIM_MEMORY_UI_HIDDEN

     这个过程显示到用户界面,提示占用内存比较大的应用和ui即将被释放,ui不可见

TRIM_MEMORY_RUNNING_CRITICAL

    应用处于运行状态但是系统已经把大多数缓存应用杀掉了, 你必须释放掉不是非常关键的资源, 如果系统不能回收足够的运行内存, 系统会清除所有缓存应用并且会把正在活动的应用杀掉.

TRIM_MEMORY_RUNNING_LOW

    应用处于运行状态并且不会被杀掉, 设备可以使用的内存非常低, 可以把不用的资源释放一些提高性能(会直接影响程序的性能)

TRIM_MEMORY_RUNNING_MODERATE

   应用处于运行状态并且不会被杀掉, 设备使用的内存比较低, 系统级会杀掉一些其它的缓存应用.

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。

Android乱弹onLowMemory()和onTrimMemory()