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：
- 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
- 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
- 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
- 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
- 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）