一 . android进程的优先级

1. Foreground process

2. Visible process

3. Service process

4. Background process

5. Empty process

   系统会主动收回4,5号进程.

二 .android 进程的回收策略

1. Low memory killer: 通过一些比较复杂的评分机制，对进程进行打分，然后将分数高的进程判定为bad进程，杀死并释放内存.

2. OOM_ODJ:判别进程的优先级.

   Low memory killer会根据OOM_ODJ这个值来判定kill那些进程.

三 .进程保活方案.

1. 利用系统广播拉活.

2. 利用系统Service机制拉活: 利用onStartCommand方法返回START_STICKY,此拉活是针对Low memory kill的情况下，如果开始service就被杀死会在5s内系统重启service,如果第二次被杀死那么系统将在10s重启service, 如果第三次被杀死那么系统将在20s重启service. 三次后就无法了哦。

   public int onStartCommand(@Nullable Intent intent, int flags, int startId) {

    onStart\(intent, startId\);
   
    return START_STICKY;
   

   }

   /**

   * Constant to return from {@link #onStartCommand}: if this service's

   * process is killed while it is started (after returning from

   * {@link #onStartCommand}), then leave it in the started state but

   * don't retain this delivered intent. Later the system will try to

   * re-create the service. Because it is in the started state, it will

   * guarantee to call {@link #onStartCommand} after creating the new

   * service instance; if there are not any pending start commands to be

   * delivered to the service, it will be called with a null intent

   * object, so you must take care to check for this.

   *

   * <p>This mode makes sense for things that will be explicitly started

   * and stopped to run for arbitrary periods of time, such as a service

   * performing background music playback.

   */

   public static final int START_STICKY = 1;

3. 利用Native进程拉活：android5.0后就无法使用了，原来是用native service来监控AMS.

4. 利用JobScheduler机制拉活:监听主进程的存活，来决定是否重启service.

5. 利用账号同步机制拉活