SystemServer

概述

• SystemServer的进程名实际上叫做“system_server”,通常简称为SS。
• SS是由Zygote通过Zygote.forkSystemServer函数fork诞生出来的
• 与Zygote生死与共.SS诞生后,便和生父Zygote分道扬镳,它有了自己的历史使命

诞生

image.png

• ZygoteInit.java frameworks/base/core/com/android/internal/os/ZygoteInit.java

// frameworks/base/core/com/android/internal/os/Zygote.java
pid = Zygote.forkSystemServer(
                    parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids,
                    parsedArgs.debugFlags,
                    null,
                    parsedArgs.permittedCapabilities,
                    parsedArgs.effectiveCapabilities);
        } catch (IllegalArgumentException ex) {
            throw new RuntimeException(ex);
        }

//
/* dalvik/vm/native/dalvik_system_Zygote
 *
 * native public static int nativeForkSystemServer(int uid, int gid,
 *     int[] gids, int debugFlags, int[][] rlimits,
 *     long permittedCapabilities, long effectiveCapabilities);
 */
static void Dalvik_dalvik_system_Zygote_forkSystemServer(
        const u4* args, JValue* pResult)
{
    pid_t pid;
    // 根据参数fork一个子进程
    pid = forkAndSpecializeCommon(args, true);

    /* The zygote process checks whether the child process has died or not. */
    if (pid > 0) {
        int status;

        ALOGI("System server process %d has been created", pid);
        gDvm.systemServerPid = pid;  //保存system_server的进程id
        /* There is a slight window that the system server process has crashed
         * but it went unnoticed because we haven't published its pid yet. So
         * we recheck here just to make sure that all is well.
         */
         //函数推出前需要却确认刚创建的子进程是否退出
        if (waitpid(pid, &status, WNOHANG) == pid) {
            ALOGE("System server process %d has died. Restarting Zygote!", pid);
            //如果system_server 退出,    zygote 自杀
            kill(getpid(), SIGKILL);
        }
    }
    RETURN_INT(pid);
}

• 由此可见，作为Zygote的嫡长子，竟然到了影响到Zygote

使命

image.png

• SS 调用 handleSystemServerProcess 来承担自己的责任
• SS 调用 zygoteInitNative后，在 onZygote中，执行了 proc->startThreadPool(); ，这一句会启动一个线程，用于Binder线。即，将与Binder通信系统建立联系，这样就能够使用Binder。
• invokeStaticMain 会抛出一个异常，会在ZygoteInit的main方法中被截获。而最后在caller.run() 方法中，会执行 mMethod.invoke(null, new Object[] { mArgs }); ，这个mMethod为 com.android.server.SystemServer 的main函数被调用。

真面目

• ZygoteInit分裂产生的SS,其实就是为了调用com.android.server.SystemServer的main函数。

   public static void main(String[] args) {
  
          /*
           * In case the runtime switched since last boot (such as when
           * the old runtime was removed in an OTA), set the system
           * property so that it is in sync. We can't do this in
           * libnativehelper's JniInvocation::Init code where we already
           * had to fallback to a different runtime because it is
           * running as root and we need to be the system user to set
           * the property. http://b/11463182
           */
          SystemProperties.set("persist.sys.dalvik.vm.lib",
                               VMRuntime.getRuntime().vmLibrary());
  
          if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
              // If a device's clock is before 1970 (before 0), a lot of
              // APIs crash dealing with negative numbers, notably
              // java.io.File#setLastModified, so instead we fake it and
              // hope that time from cell towers or NTP fixes it
              // shortly.
              Slog.w(TAG, "System clock is before 1970; setting to 1970.");
              SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
          }
  
          if (SamplingProfilerIntegration.isEnabled()) {
              SamplingProfilerIntegration.start();
              timer = new Timer();
              timer.schedule(new TimerTask() {
                  @Override
                  public void run() {
                      SamplingProfilerIntegration.writeSnapshot("system_server", null);
                  }
              }, SNAPSHOT_INTERVAL, SNAPSHOT_INTERVAL);
          }
  
          // Mmmmmm... more memory!
          dalvik.system.VMRuntime.getRuntime().clearGrowthLimit();
  
          // The system server has to run all of the time, so it needs to be
          // as efficient as possible with its memory usage.
          VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
  
          Environment.setUserRequired(true);
  
          System.loadLibrary("android_servers");  //加载库
  
          Slog.i(TAG, "Entered the Android system server!");
  
          // Initialize native services.
          nativeInit();  //在 /base/services/jni/com_android_server_SystemServer.cpp
  
          // This used to be its own separate thread, but now it is
          // just the loop we run on the main thread.
          ServerThread thr = new ServerThread();
          thr.initAndLoop();
      }
  

  //  frameworks/base/services/jni/com_android_server_SystemServer.cpp
  static void android_server_SystemServer_nativeInit(JNIEnv* env, jobject clazz) {
      char propBuf[PROPERTY_VALUE_MAX];
      property_get("system_init.startsensorservice", propBuf, "1");
      if (strcmp(propBuf, "1") == 0) {
          // Start the sensor service
          SensorService::instantiate();
      }
  }
  
  

• 没看懂这波操作

zygote 分裂

• zygote 分裂出 system_server 之后，通过 runSelectLoopMode 等待并处理来自客户的消息

ActivityManagerService 发送请求

image.png

• ActivityManagerService.java /framewroks/base/services/java/com.android.services/am/ActivityManangerService.java
• Process /frameworks/base/core/java/android/os/Prcoess.java

• zygoteSendArgsAndGetResult

  • openZygoteSocketIfNeeded
  • 把请求的参数发到 Zygote

• openZygoteSocketIfNeeded

          try {
                  sZygoteSocket = new LocalSocket();
                  // 链接 Zygote
                  sZygoteSocket.connect(new LocalSocketAddress(ZYGOTE_SOCKET, 
                          LocalSocketAddress.Namespace.RESERVED));
                  sZygoteInputStream
                          = new DataInputStream(sZygoteSocket.getInputStream());
                  sZygoteWriter =
                      new BufferedWriter(
                              new OutputStreamWriter(
                                      sZygoteSocket.getOutputStream()),
                              256);
                  Log.i("Zygote", "Process: zygote socket opened");
                  sPreviousZygoteOpenFailed = false;
          ......
          }
  

• ActicityManagerService 终于向 zygote 发送请求了。

  • 请求的参数中有一个字符串，它的值是 "android.app.ActicityThread"

响应请求

• 请求之后，会回到 ZygoteInit 之中

image.png

• ZygoteConnection.java frameworks/base/core/com/android/internal/os/ZygoteConnection.java

  if (parsedArgs.runtimeInit) {
            if (parsedArgs.invokeWith != null) {
                WrapperInit.execApplication(parsedArgs.invokeWith,
                        parsedArgs.niceName, parsedArgs.targetSdkVersion,
                        pipeFd, parsedArgs.remainingArgs);
            } else {
                //此时，会调用这里
                RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion,
                        parsedArgs.remainingArgs);
            }
        } else {
            ......
                try {
                    ZygoteInit.invokeStaticMain(cloader, className, mainArgs);
                } catch (RuntimeException ex) {
                    logAndPrintError(newStderr, "Error starting.", ex);
                }
            }
        }
    }

• 而 RuntimeInit.zygoteInit 方法中，nativeZygoteInit这个jni函数，最终实现的是 proc->startThreadPool() 即：建立了Binder服务。