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[操作系统]【原创】Android内存管理


Application中有两个与内存管理相关的方法:onLowMemory()和 onTrimMemory(int level),源码如下

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

从源码我们可以看到Application收到这两个回调时会通知它的监听者,而Activity和Service都注册了监听,

因此我们可以Application中重写这两个方法,也可以在组件中重写这两个方法。

 

先重点介绍一下onTrimMemory

为了更好的管理内存,OnTrimMemory 方法在 API-14 被引入。这个回调可以在所有组件中获取到(ActivityServiceContentProvider, and Application)。

你应该根据当前设备的限制复写 onTrimMemory(int) 来逐步的释放内存。通过复写这个方法释放资源可以帮助你的app更好的响应系统整体,同时通过让你的app

在系统中存活更久来提高用户体验。如果在系统内存很低时,你仍旧不释放内存,系统将会优先杀死你在的进程。这样当用户返回app时需要重启影响用户体验

 

onTrimMemory(int)的level值并不成线性关系,它只是提供了内存不同状态的线索。

1. 回调时机

 /**   * 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);

当操作系统认为这是一个进程释放无用内存的好时机时,会调用此方法。比如说当已经没有足够的内存来维持目前所有的后台进程,而此进程正好处于后台。
非常不推荐用一个精确的值来与level作比较,因为可能会增加新的差值,推荐的做法是判断一个值是否大于或者等于你感兴趣的level.

为了获取所有进程目前的level,你可以调用{@link android.app.ActivityManager#getMyMemoryState* ActivityManager.getMyMemoryState(RunningAppProcessInfo)}

 

2.level值的具体含义

 

 /**   * 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;

 

当你的app在后台时:

TRIM_MEMORY_COMPLETE :当前进程在LRU列表的尾部,如果没有足够的内存,它将很快被杀死。这时候你应该释放任何不影响app运行的资源。

TRIM_MEMORY_MODERATE :当前进程在LRU列表的中部,如果系统进一步需要内存,你的进程可能会被杀死。

TRIM_MEMORY_BACKGROUND:当前进程在LRU列表的头部,虽然你的进程不会被高优杀死,但是系统已经开始准备杀死LRU列表中的其他进程了,

                   因此你应该尽量的释放能够快速回复的资源,以保证当用户返回你的app时可以快速恢复。                   。

 

当你的app的可见性改变时:

TRIM_MEMORY_UI_HIDDEN:当前进程的界面已经不可见,这时是释放UI相关的资源的好时机。

 

当你的app正在运行时:

TRIM_MEMORY_RUNNING_CRITICAL:虽然你的进程不会被杀死,但是系统已经开始准备杀死其他的后台进程了,这时候你应该释放无用资源以防止性能下降。

                 下一个阶段就是调用"onLowMemory()"来报告开始杀死后台进程了,特别是状况已经开始影响到用户。

TRIM_MEMORY_RUNNING_LOW:虽然你的进程不会被杀死,但是系统已经开始准备杀死其他的后台进程了,你应该释放不必要的资源来提供系统性能,否则会

                 影响用户体验。

TRIM_MEMORY_RUNNING_MODERATE:系统已经进入了低内存的状态,你的进程正在运行但是不会被杀死。

 

再来说一下onLowMemory

在引入OnTrimMemory之前都是使用OnLowMemory方法。如果你的app运行在API-14+的机器上,应该使用 OnTrimMemory(int),OnLowMemory的调用时机大概等同于TRIM_MEMORY_COMPLETE.