Android深入四大元件（二）Service的啟動過程

劉望舒發表於2017-04-24
Android元件

相關文章
Android深入理解四大元件系列

前言

此前我用較長的篇幅來介紹Android應用程式的啟動過程（根Activity的啟動過程），這一篇我們接著來分析Service的啟動過程。建議閱讀此篇文章前，請先閱讀Android深入四大元件（一）應用程式啟動過程（前篇）和Android深入四大元件（一）應用程式啟動過程（後篇）這兩篇文章。

1.ContextImpl到ActivityManageService的呼叫過程

要啟動Service，我們會呼叫startService方法，它的實現在ContextWrapper中，程式碼如下所示。
frameworks/base/core/java/android/content/ContextWrapper.java

public class ContextWrapper extends Context {
    Context mBase;
...
  @Override
    public ComponentName startService(Intent service) {
        return mBase.startService(service);
    }
...    
}

在startService方法中會呼叫mBase的startService方法，Context型別的mBase物件具體指的是什麼呢？在Android深入四大元件（一）應用程式啟動過程（後篇）這篇文章中我們講過ActivityThread啟動Activity時會呼叫如下程式碼建立Activity的上下文環境。
frameworks/base/core/java/android/app/ActivityThread.java

 private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
  ...
            if (activity != null) {
                Context appContext = createBaseContextForActivity(r, activity);//1
         ...
                }
                activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config,
                        r.referrer, r.voiceInteractor, window);
                ...
        }
        return activity;
}

在註釋1處建立上下文物件appContext ，並傳入Activity的attach方法中，將Activity與上下文物件appContext 關聯起來，這個上下文物件appContext 的具體型別是什麼，我們接著檢視createBaseContextForActivity方法，程式碼如下所示。
frameworks/base/core/java/android/app/ActivityThread.java

private Context createBaseContextForActivity(ActivityClientRecord r, final Activity activity) {
...

    ContextImpl appContext = ContextImpl.createActivityContext(
            this, r.packageInfo, r.token, displayId, r.overrideConfig);
    appContext.setOuterContext(activity);
    Context baseContext = appContext;
    ...
    return baseContext;
}

這裡可以得出結論，上下文物件appContext 的具體型別就是ContextImpl 。Activity的attach方法中將ContextImpl賦值給ContextWrapper的成員變數mBase中，因此，mBase具體指向就是ContextImpl 。
那麼，我們緊接著來檢視ContextImpl的startService方法，程式碼如下所示。
frameworks/base/core/java/android/app/ContextImpl.java

Override
public ComponentName startService(Intent service) {
    warnIfCallingFromSystemProcess();
    return startServiceCommon(service, mUser);
}
 private ComponentName startServiceCommon(Intent service, UserHandle user) {
        try {
            validateServiceIntent(service);
            service.prepareToLeaveProcess(this);
            /**
            * 1
            */
            ComponentName cn = ActivityManagerNative.getDefault().startService(
                mMainThread.getApplicationThread(), service, service.resolveTypeIfNeeded(
                            getContentResolver()), getOpPackageName(), user.getIdentifier());
      ...
            return cn;
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

startService方法中會return startServiceCommon方法，在startServiceCommon方法中會在註釋1處呼叫ActivityManageService（AMS）的代理物件ActivityManagerProxy（AMP）的startService方法，最終會呼叫AMS的startService方法。至於註釋1處的程式碼為何會呼叫AMS的startService方法，在Android深入四大元件（一）應用程式啟動過程（前篇）這篇文章中已經講過，這裡不再贅述。
ContextImpl到ActivityManageService的呼叫過程如下面的時序圖所示。

2.ActivityThread啟動Service

我們接著來檢視AMS的startService方法。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

Override
public ComponentName startService(IApplicationThread caller, Intent service,
        String resolvedType, String callingPackage, int userId)
        throws TransactionTooLargeException {
 ...
    synchronized(this) {
 ...
        ComponentName res = mServices.startServiceLocked(caller, service,
                resolvedType, callingPid, callingUid, callingPackage, userId);//1
        Binder.restoreCallingIdentity(origId);
        return res;
    }
}

註釋1處呼叫mServices的startServiceLocked方法，mServices的型別是ActiveServices，ActiveServices的startServiceLocked方法程式碼如下所示。
frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
            int callingPid, int callingUid, String callingPackage, final int userId)
            throws TransactionTooLargeException {
      ...
        return startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
    }

   ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
            boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {

     ...
        String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false, false);
     ...
        return r.name;
    }

startServiceLocked方法的末尾return了startServiceInnerLocked方法，而startServiceInnerLocked方法中又呼叫了bringUpServiceLocked方法：
frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

  private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
            boolean whileRestarting, boolean permissionsReviewRequired)
            throws TransactionTooLargeException {
...
  final String procName = r.processName;//1
  ProcessRecord app;
  if (!isolated) {
            app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);//2
            if (DEBUG_MU) Slog.v(TAG_MU, "bringUpServiceLocked: appInfo.uid=" + r.appInfo.uid
                        + " app=" + app);
            if (app != null && app.thread != null) {//3
                try {
                    app.addPackage(r.appInfo.packageName, r.appInfo.versionCode, mAm.mProcessStats);
                    realStartServiceLocked(r, app, execInFg);//4
                    return null;
                } catch (TransactionTooLargeException e) {
                    throw e;
                } catch (RemoteException e) {
                    Slog.w(TAG, "Exception when starting service " + r.shortName, e);
                }
            }
        } else {
            app = r.isolatedProc;
        }

 if (app == null && !permissionsReviewRequired) {//5
            if ((app=mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
                    "service", r.name, false, isolated, false)) == null) {//6
              ...
            }
            if (isolated) {
                r.isolatedProc = app;
            }
        }

 ...     
}

在註釋1處得到ServiceRecord的processName的值賦值給procName ，其中ServiceRecord用來描述Service的android:process屬性。註釋2處將procName和Service的uid傳入到AMS的getProcessRecordLocked方法中，來查詢是否存在一個與Service對應的ProcessRecord型別的物件app，ProcessRecord主要用來記錄執行的應用程式程式的資訊。註釋5處判斷Service對應的app為null則說明用來執行Service的應用程式程式不存在，則呼叫註釋5處的AMS的startProcessLocked方法來建立對應的應用程式程式。關於建立應用程式程式請檢視Android應用程式程式啟動過程（前篇） 和Android應用程式程式啟動過程（後篇）這兩篇文章。註釋3處判斷如果用來執行Service的應用程式程式存在，則呼叫註釋4處的realStartServiceLocked方法：

frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

private final void realStartServiceLocked(ServiceRecord r,
        ProcessRecord app, boolean execInFg) throws RemoteException {
   ...
    try {
       ...
        app.thread.scheduleCreateService(r, r.serviceInfo,
                mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
                app.repProcState);
        r.postNotification();
        created = true;
    } catch (DeadObjectException e) {
      ...
    } 
    ...
}

在realStartServiceLocked方法中呼叫了app.thread的scheduleCreateService方法。其中app.thread是IApplicationThread型別的，它的實現是ActivityThread的內部類ApplicationThread，其中ApplicationThread繼承了ApplicationThreadNative，而ApplicationThreadNative繼承了Binder並實現了IApplicationThread介面。ApplicationThread的scheduleCreateService方法如下所示。
frameworks/base/core/java/android/app/ActivityThread.java

   public final void scheduleCreateService(IBinder token,
            ServiceInfo info, CompatibilityInfo compatInfo, int processState) {
        updateProcessState(processState, false);
        CreateServiceData s = new CreateServiceData();
        s.token = token;
        s.info = info;
        s.compatInfo = compatInfo;
        sendMessage(H.CREATE_SERVICE, s);
    }

首先將要啟動的資訊封裝成CreateServiceData 物件並傳給sendMessage方法，sendMessage方法向H傳送CREATE_SERVICE訊息，H是ActivityThread的內部類並繼承Handler。這個過程和應用程式的啟動過程（根Activity啟動過程）是類似的。我們接著檢視H的handleMessage方法。
frameworks/base/core/java/android/app/ActivityThread.java

  public void handleMessage(Message msg) {
            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
            switch (msg.what) {
            ...
               case CREATE_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
                    handleCreateService((CreateServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
            ...
             }
          ...
          }
       ...   
  }

handleMessage方法根據訊息型別，呼叫了handleCreateService方法：

frameworks/base/core/java/android/app/ActivityThread.java

 private void handleCreateService(CreateServiceData data) {
        unscheduleGcIdler();
        LoadedApk packageInfo = getPackageInfoNoCheck(
                data.info.applicationInfo, data.compatInfo);//1
        Service service = null;
        try {
            java.lang.ClassLoader cl = packageInfo.getClassLoader();//2
            service = (Service) cl.loadClass(data.info.name).newInstance();//3
        } catch (Exception e) {
           ...
            }
        }
        try {
            if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);

            ContextImpl context = ContextImpl.createAppContext(this, packageInfo);//4
            context.setOuterContext(service);

            Application app = packageInfo.makeApplication(false, mInstrumentation);
            service.attach(context, this, data.info.name, data.token, app,
                    ActivityManagerNative.getDefault());//5
            service.onCreate();//6
            mServices.put(data.token, service);//7
         ...
        } catch (Exception e) {
            ...
        }
    }

註釋1處獲取要啟動Service的應用程式的LoadedApk，LoadedApk是一個APK檔案的描述類。註釋2處通過呼叫LoadedApk的getClassLoader方法來獲取類載入器。接著在註釋3處根據CreateServiceData物件中儲存的Service資訊，將Service載入到記憶體中。註釋4處建立Service的上下文環境ContextImpl物件。註釋5處通過Service的attach方法來初始化Service。註釋6處呼叫Service的onCreate方法，這樣Service就啟動了。在註釋7處將啟動的Service加入到ActivityThread的成員變數mServices中，其中mServices是ArrayMap型別。
最後給出這一節的時序圖。

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