凌亂.png

System_Server是系統(tǒng)進(jìn)程击罪，而Zygote是Android java世界的基石。這兩位都在Android中扮演著十分重要的角色。有個(gè)很形象的比喻疏日，Zygote是爸爸盈滴，System_Server是大兒子涯肩，應(yīng)用是一堆小兒子就不說了。今天來說說這兩位仁兄的兩個(gè)特性：
1.System_Server使用捕捉異常的方式啟動(dòng)巢钓。
2.Zygote與System_Server共存亡病苗。

System_Server使用捕捉異常的方式啟動(dòng)

源文件:/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
入口是ZygoteInit的startSystemServer函數(shù),利用forkSystemServer函數(shù)產(chǎn)生了子進(jìn)程SystemServer，并且設(shè)置了SystemServer的pid症汹，uid硫朦，uidgroud等。但這時(shí)并沒有進(jìn)入SystemServer的main函數(shù)入口背镇，只是一個(gè)進(jìn)程跑起來了咬展，懂嗎。

    private static boolean startSystemServer(String abiList, String socketName)
            throws MethodAndArgsCaller, RuntimeException {
        String args[] = {
            "--setuid=1000",   //SystemServer pid=1000,gid=1000,還設(shè)置了權(quán)限組
            "--setgid=1000",
            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007",
            "--capabilities=" + capabilities + "," + capabilities,
            "--nice-name=system_server",
            "--runtime-args",
            "com.android.server.SystemServer",    //SystemServer的類名
        };
         ...................
         ...................
         //會(huì)利用linux的fork函數(shù)產(chǎn)生SystemServer子進(jìn)程
            pid = Zygote.forkSystemServer(
                    parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids,
                    parsedArgs.debugFlags,
                    null,
                    parsedArgs.permittedCapabilities,
                    parsedArgs.effectiveCapabilities);
        } catch (IllegalArgumentException ex) {
            throw new RuntimeException(ex);
        }

        /* For child process */
        if (pid == 0) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }
            //子進(jìn)程 SystemServer
            handleSystemServerProcess(parsedArgs);
        }

        return true;
    }

進(jìn)入handleSystemServerProcess函數(shù),這里的parsedArgs.remainingArgs就是類名com.android.server.SystemServer

    private static void handleSystemServerProcess(
            ZygoteConnection.Arguments parsedArgs)
            throws ZygoteInit.MethodAndArgsCaller {
            ......
            RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
           ..................
        }
    }

/frameworks/base/core/java/com/android/internal/os/RuntimeInit.java
再進(jìn)入RuntimeInit類的zygoteInit函數(shù)

    public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
            throws ZygoteInit.MethodAndArgsCaller {
        commonInit();    //初始化時(shí)區(qū)啥的
        nativeZygoteInit();  //native的初始化啥的
        applicationInit(targetSdkVersion, argv, classLoader);  //參數(shù)argv是類名com.android.server.SystemServer
    }

再進(jìn)入applicationInit函數(shù)

    private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
            throws ZygoteInit.MethodAndArgsCaller {
        invokeStaticMain(args.startClass, args.startArgs, classLoader);
    }

再進(jìn)入invokeStaticMain函數(shù)瞒斩，在這里反射調(diào)用了SystemServer類破婆，并且獲取到main方法，但并沒有執(zhí)行胸囱，而是直接拋出一個(gè)異常throw new ZygoteInit.MethodAndArgsCaller荠割。

    private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader)
            throws ZygoteInit.MethodAndArgsCaller {
        Class<?> cl;
       cl = Class.forName(className, true, classLoader);//反射獲取SystemServer類
       Method m;
        m = cl.getMethod("main", new Class[] { String[].class });  //獲取main方法
        throw new ZygoteInit.MethodAndArgsCaller(m, argv);   //拋出異常，argv為空
    }

那這個(gè)異常是由誰來捕捉呢旺矾？其實(shí)是在ZygoteInit的main函數(shù)捕捉的蔑鹦，如下圖

異常捕捉.png

異常捕捉后，調(diào)用 caller.run();
這里caller是MethodAndArgsCaller類箕宙，是ZygoteInit的內(nèi)部類嚎朽。run方法會(huì)執(zhí)行SystemServer的main方法。從而完成SystemServer的啟動(dòng)柬帕。

    public static class MethodAndArgsCaller extends Exception
            implements Runnable {
        public void run() {
                mMethod.invoke(null, new Object[] { mArgs });
    }
}

為什么SystemServer要采用捕捉異常的方式啟動(dòng)呢哟忍？源碼里面其實(shí)有注釋，應(yīng)該是啟動(dòng)一個(gè)新進(jìn)程時(shí)陷寝，用來清除之前的堆棧信息的锅很。

Paste_Image.png

Zygote與System_Server共存亡

System_Server是一個(gè)很重要的進(jìn)程，如果掛了凤跑，Zygote必須重啟爆安，如果不重啟，手機(jī)就會(huì)進(jìn)入假死的狀態(tài),手機(jī)黑屏仔引，無聲扔仓，無法跟用戶交互等褐奥。
Zygote與System_Server共存亡是如何實(shí)現(xiàn)的呢？
入口:/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
Zygote調(diào)用forkSystemServer產(chǎn)生子進(jìn)程SystemServer翘簇。

    private static boolean startSystemServer(String abiList, String socketName)
            throws MethodAndArgsCaller, RuntimeException {
            pid = Zygote.forkSystemServer(
                    parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids,
                    parsedArgs.debugFlags,
                    null,
                    parsedArgs.permittedCapabilities,
                    parsedArgs.effectiveCapabilities);
        } catch (IllegalArgumentException ex) {
            throw new RuntimeException(ex);
        }
    }

/frameworks/base/core/java/com/android/internal/os/Zygote.java
進(jìn)入forkSystemServer函數(shù)看看

    public static int forkSystemServer(int uid, int gid, int[] gids, int debugFlags,
            int[][] rlimits, long permittedCapabilities, long effectiveCapabilities) {
        int pid = nativeForkSystemServer(
                uid, gid, gids, debugFlags, rlimits, permittedCapabilities, effectiveCapabilities);
        return pid;
    }

會(huì)jni進(jìn)入native函數(shù)
/frameworks/base/core/jni/com_android_internal_os_Zygote.cpp

static jint com_android_internal_os_Zygote_nativeForkSystemServer(
        JNIEnv* env, jclass, uid_t uid, gid_t gid, jintArray gids,
        jint debug_flags, jobjectArray rlimits, jlong permittedCapabilities,
        jlong effectiveCapabilities) {
 ............................
  pid_t pid = ForkAndSpecializeCommon(env, uid, gid, gids,
                                      debug_flags, rlimits,
                                      permittedCapabilities, effectiveCapabilities,
                                      MOUNT_EXTERNAL_DEFAULT, NULL, NULL, true, NULL,
                                      NULL, NULL);
 ................................

}

繼續(xù)進(jìn)入ForkAndSpecializeCommon函數(shù)

static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArray javaGids,
                                     jint debug_flags, jobjectArray javaRlimits,
                                     jlong permittedCapabilities, jlong effectiveCapabilities,
                                     jint mount_external,
                                     jstring java_se_info, jstring java_se_name,
                                     bool is_system_server, jintArray fdsToClose,
                                     jstring instructionSet, jstring dataDir) {
  ...............
...............
  SetSigChldHandler();   //注冊(cè)信號(hào)撬码，這里將關(guān)系共存亡

  pid_t pid = fork();//fork產(chǎn)生子進(jìn)程systemserver

  if (pid == 0) {
    .........                //子進(jìn)程的一些操作
  } else if (pid > 0) {
    // the parent process
  }
  return pid;
}

SetSigChldHandler用于注冊(cè)信號(hào)，這里就是關(guān)系到Zygote與System_Server的共存亡版保。

static void SetSigChldHandler() {
  struct sigaction sa;
  memset(&sa, 0, sizeof(sa));
  sa.sa_handler = SigChldHandler;  //信號(hào)處理函數(shù)

  int err = sigaction(SIGCHLD, &sa, NULL);   
  if (err < 0) {
    ALOGW("Error setting SIGCHLD handler: %s", strerror(errno));
  }
}

SetSigChldHandler定義了信號(hào)處理函數(shù)SigChldHandler呜笑，當(dāng)信號(hào)SIGCHLD到來的時(shí)候，會(huì)進(jìn)入信號(hào)處理函數(shù)彻犁。
看看信號(hào)處理函數(shù)SigChldHandler叫胁，可以看到如果子進(jìn)程SystemServer掛了，Zygote就會(huì)自殺袖裕。從而完成了共存亡的光輝使命

static void SigChldHandler(int /*signal_number*/) {
  pid_t pid;
  int status;
 //Zygote監(jiān)聽所有子進(jìn)程的存亡
  while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
 //某一個(gè)子進(jìn)程掛了
 if (WIFSIGNALED(status)) {
      if (WTERMSIG(status) != SIGKILL) {
        ALOGI("Process %d exited due to signal (%d)", pid, WTERMSIG(status));
      }
    }
   //如果掛掉的是SystemServer
    if (pid == gSystemServerPid) {
      ALOGE("Exit zygote because system server (%d) has terminated", pid);
      kill(getpid(), SIGKILL);   //Zygote自殺
    }
  }

}

總結(jié)

1.System_Server采用拋出異常和捕捉異常的方式啟動(dòng)，可以清除進(jìn)程的堆棧溉瓶，減少內(nèi)存占用急鳄。
2.System_Server與Zygote共存亡是通過父進(jìn)程調(diào)用waitpid的方式監(jiān)聽子進(jìn)程System_Server的死亡，當(dāng)子進(jìn)程死亡的時(shí)候堰酿，父進(jìn)程就自殺