进程终止分析之二

1, 概述

在系统内存较低时就会杀死一些进程,最明显的在卸载一个apk之前会杀死该apk所在的进程。在PMS的deletePackageLI方法中会调用killApplication方法杀死进程。因此,本文以killApplication方法为例来论述杀死进程的过程。当然在android系统中,还有很多情况下会杀死进程,例如在很多异常的时候会直接杀死进程。Java层基本都是在AMS中通过Process的方法杀死进程的。

 

2, kill进程

ProcessRecord的kill方法如下,

void kill(String reason, boolean noisy) {        if (!killedByAm) {            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "kill");            if (noisy) {                Slog.i(TAG, "Killing " + toShortString() + " (adj " + setAdj + "): " + reason);            }          EventLog.writeEvent(EventLogTags.AM_KILL, userId, pid, processName, setAdj, reason);            Process.killProcessQuiet(pid);            Process.killProcessGroup(uid, pid);            if (!persistent) {                killed = true;                killedByAm = true;            }            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);        }    }

直接调用Process的静态方法killProcessQuiet/ killProcessGroup

其实,在Process中,还有一个killProcess方法也可以杀死进程。

 

killProcess方法如下,

public static final void killProcess(int pid) {        sendSignal(pid, SIGNAL_KILL);    }

public static final native void sendSignal(int pid, int signal);

Java层的sendSignal方法对应着android_util_Process.cpp的android_os_Process_sendSignal方法

android_os_Process_sendSignal方法如下,

void android_os_Process_sendSignal(JNIEnv* env, jobject clazz, jint pid, jint sig){    if (pid > 0) {        ALOGI("Sending signal. PID: %" PRId32 " SIG: %" PRId32, pid, sig);        kill(pid, sig);    }}

ProcessRecord的kill调用流程图如下,

android_util_Process.cpp的android_os_Process_sendSignalQuiet方法如下,

void android_os_Process_sendSignalQuiet(JNIEnv* env, jobject clazz, jint pid, jint sig){    if (pid > 0) {        kill(pid, sig);    }}

由此可见, killProcess和killProcessQuiet方法都是通过JNI机制最后调用kill方法来实现的。

 

再来看killProcessGroup方法是如何调用的。

android_os_Process_killProcessGroup方法如下,

jint android_os_Process_killProcessGroup(JNIEnv* env, jobject clazz, jint uid, jint pid){    return killProcessGroup(uid, pid, SIGKILL);}

processgroup.cpp中的killProcessGroup方法如下,

static int killProcessGroupOnce(uid_t uid, int initialPid, int signal){    int processes = 0;    struct ctx ctx;    pid_t pid;    ctx.initialized = false;     // 获取进程,逐个杀死    while ((pid = getOneAppProcess(uid, initialPid, &ctx)) >= 0) {        processes++;        if (pid == 0) {            // Should never happen...  but if it does, trying to kill this            // will boomerang right back and kill us!  Let's not let that happen.            SLOGW("Yikes, we've been told to kill pid 0!  How about we don't do that.");            continue;        }        if (pid != initialPid) {            // We want to be noisy about killing processes so we can understand            // what is going on in the log; however, don't be noisy about the base            // process, since that it something we always kill, and we have already            // logged elsewhere about killing it.            SLOGI("Killing pid %d in uid %d as part of process group %d", pid, uid, initialPid);        }        int ret = kill(pid, signal); // 杀死进程        if (ret == -1) {            SLOGW("failed to kill pid %d: %s", pid, strerror(errno));        }    }    if (ctx.initialized) {        close(ctx.fd);    }    return processes;}

所以,process的三个杀死进程的方法都是调用kill方法完成,该方法位于用户空间的Native层。

更为详细的细节在此就不论述了。