Android 进程回收

1.Android 进程回收策略

众所周知，Android是基于Linux系统的。在Android进程回收策略中，Android进程与Linux进程根据OOM_ADJ阈值进行区分：

• OOM_ADJ >= 4：比较容易被杀死的进程
• OOM_ADJ 0 ~ 3：不容易被杀死的进程
• OOM_ADJ < 0 ：纯Linux进程，非Android进程

当Android系统察觉设备内存不足时，会按照阈值从大到小杀死进程。

 

具体的oom_adj值的意义我们可以查看AOSP中的com.android.server.am.ProcessList 文件（其中本人添加了一些中文注释）：

/** 
 * Activity manager code dealing with processes. 
 */  
final class ProcessList {  
    ...  
  
    // OOM adjustments for processes in various states:  
  
    // Adjustment used in certain places where we don't know it yet.  
    // (Generally this is something that is going to be cached, but we  
    // don't know the exact value in the cached range to assign yet.)  
    // 未知进程，通常是用作缓存  
    static final int UNKNOWN_ADJ = 16;  
  
    // This is a process only hosting activities that are not visible,  
    // so it can be killed without any disruption.  
    // 拥有不可视的Activity的进程，可以不影响影响用户的情况下杀掉  
    static final int CACHED_APP_MAX_ADJ = 15;  
    static final int CACHED_APP_MIN_ADJ = 9;  
  
    // The B list of SERVICE_ADJ -- these are the old and decrepit  
    // services that aren't as shiny and interesting as the ones in the A list.  
    // 一些旧的服务进程  
    static final int SERVICE_B_ADJ = 8;  
  
    // This is the process of the previous application that the user was in.  
    // This process is kept above other things, because it is very common to  
    // switch back to the previous app.  This is important both for recent  
    // task switch (toggling between the two top recent apps) as well as normal  
    // UI flow such as clicking on a URI in the e-mail app to view in the browser,  
    // and then pressing back to return to e-mail.  
    // 用户使用的前一个进程  
    static final int PREVIOUS_APP_ADJ = 7;  
  
    // This is a process holding the home application -- we want to try  
    // avoiding killing it, even if it would normally be in the background,  
    // because the user interacts with it so much.  
    // 主界面进程  
    static final int HOME_APP_ADJ = 6;  
  
    // This is a process holding an application service -- killing it will not  
    // have much of an impact as far as the user is concerned.  
    // 持有应用服务的进程  
    static final int SERVICE_ADJ = 5;  
  
    // This is a process with a heavy-weight application.  It is in the  
    // background, but we want to try to avoid killing it.  Value set in  
    // system/rootdir/init.rc on startup.  
    // 重量级应用进程  
    static final int HEAVY_WEIGHT_APP_ADJ = 4;  
  
    // This is a process currently hosting a backup operation.  Killing it  
    // is not entirely fatal but is generally a bad idea.  
    // 执行备份操作的进程  
    static final int BACKUP_APP_ADJ = 3;  
  
    // This is a process only hosting components that are perceptible to the  
    // user, and we really want to avoid killing them, but they are not  
    // immediately visible. An example is background music playback.  
    // 拥有用户可感知组件的进程  
    static final int PERCEPTIBLE_APP_ADJ = 2;  
  
    // This is a process only hosting activities that are visible to the  
    // user, so we'd prefer they don't disappear.  
    // 拥有用户仅可见、不可交互的Activity的进程  
    static final int VISIBLE_APP_ADJ = 1;  
  
    // This is the process running the current foreground app.  We'd really  
    // rather not kill it!  
    // 前台运行的进程  
    static final int FOREGROUND_APP_ADJ = 0;  
  
    // This is a system persistent process, such as telephony.  Definitely  
    // don't want to kill it, but doing so is not completely fatal.  
    // 系统常驻进程  
    static final int PERSISTENT_PROC_ADJ = -12;  
  
    // The system process runs at the default adjustment.  
    // 系统进程  
    static final int SYSTEM_ADJ = -16;  
  
    // Special code for native processes that are not being managed by the system (so  
    // don't have an oom adj assigned by the system).  
    // 为native进程保留，他们不被系统管理  
    static final int NATIVE_ADJ = -17;  
  
    ...  
}  

2.Android 进程被杀死情况

一般来说，Android进程被杀死有以下几种情况：

1. 触发系统进程管理机制回收（Lowmemorykiller）：这种方法会按照阈值从大到小进行清理
2. 被没有进行Root的第三方应用杀死（使用killBackgroundProcess方法）：这种方法只能杀死OOM_ADJ为4以上的进程
3. 被进行Root的第三方应用杀死（使用force-stop或者kill）：理论上来说可以杀死所有进程，但一般只会清理非系统关键进程和非前台可见进程
4. 厂商的杀进程功能（force-stop或者kill）：理论上来说可以杀死所有进程，包括Linux原生进程
5. 用户主动“强行停止”进程（force-stop）：只能停用第三方和非system/phone进程应用（停用system进程应用会造成Android系统重启）