Activity ，Service与Context

Activity 的创建流程

追溯到 ActivityThread.java 的 performLaunchActivity 方法：

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

        ActivityInfo aInfo = r.activityInfo;
        if (r.packageInfo == null) {
          //1. 获取LoadedApk对象
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

       .......

      //2.创建 ContextImpl 对象 appContext
        ContextImpl appContext = createBaseContextForActivity(r);
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = appContext.getClassLoader();
          //3. 创建 Activity  对象
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            StrictMode.incrementExpectedActivityCount(activity.getClass());
            r.intent.setExtrasClassLoader(cl);
            r.intent.prepareToEnterProcess();
            if (r.state != null) {
                r.state.setClassLoader(cl);
            }
        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to instantiate activity " + component
                    + ": " + e.toString(), e);
            }
        }

        try {
            //4. 创建Application 对象 app
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);
            ........
              //appContext 我们知道是ContextImpl类的对象 ，将activity 设置进去
                appContext.setOuterContext(activity);
              //appContext window相联合绑定
                activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigpurationInstances, config,
                        r.referrer, r.voiceInteractor, window, r.configCallback);

               ......
              
                activity.mCalled = false;
                if (r.isPersistable()) {
                    //5. 调用 activity 的 onCreate 方法
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                ........
        return activity;
    }

1.获取 LoadedApk 对象
2.创建ContextImpl 对象 appContext
3.创建 Activity 对象 activity
4.创建Application 对象app
5.调用activity 的 onCreate 方法启动

Service 创建流程

service 创建和 activity 的创建类似。
ActivityThread.java 的 handleCreateService 方法：

private void handleCreateService(CreateServiceData data) {
      .....
        //1. 获取LoadedApk 对象
        LoadedApk packageInfo = getPackageInfoNoCheck(
                data.info.applicationInfo, data.compatInfo);
      //2.创建 service 对象
        Service service = null;
        try {
            java.lang.ClassLoader cl = packageInfo.getClassLoader();
            service = (Service) cl.loadClass(data.info.name).newInstance();
        } catch (Exception e) {
           ....
        }

        try {
          .....
          // 3. 创建ContextImpl 对象 context
            ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
            context.setOuterContext(service);
          //4. 创建Application对象 app
            Application app = packageInfo.makeApplication(false, mInstrumentation);
          
            service.attach(context, this, data.info.name, data.token, app,
                    ActivityManager.getService());
        //5.调用 service 的 onCreate 方法
            service.onCreate();
            mServices.put(data.token, service);
            .....
        } catch (Exception e) {
            ......
        }
    }

1.获取 LoadedApk 对象
2.创建 service 对象
3.创建 ContextImpl 对象 context
4.创建 Application 对象 app
5.调用 Service 的 onCreate 方法

Application 最终通过 Instrumentation.java 的 newApplication() 方法创建完成：

 static public Application newApplication(Class clazz, Context context)
            throws InstantiationException, IllegalAccessException, 
            ClassNotFoundException {
        Application app = (Application)clazz.newInstance();
        app.attach(context);
        return app;
    }

这里我们发现 ContextImpl 的 setOuterContext 中设置 context 的不同：

• performLaunchActivity------activity

//appContext 就是ContextImpl 实例对象
appContext.setOuterContext(activity);

• makeApplication ----- application

Application app = null;
.....
appContext.setOuterContext(app);

• handleCreateService ---- service

context.setOuterContext(service);

Activity context 的创建

ContextImpl appContext = createBaseContextForActivity(r);
private ContextImpl createBaseContextForActivity(ActivityClientRecord r) {
       ....
        //创建appContext, 将应用的若干信息传进去
        ContextImpl appContext = ContextImpl.createActivityContext(
                this, r.packageInfo, r.activityInfo, r.token, displayId, r.overrideConfig);
     ....
        return appContext;
    }

static ContextImpl createActivityContext(ActivityThread mainThread,
            LoadedApk packageInfo, ActivityInfo activityInfo, IBinder activityToken, int displayId,
            Configuration overrideConfiguration) {
      .....
    //将信息传入 ContextImpl 中
        ContextImpl context = new ContextImpl(null, mainThread, packageInfo, activityInfo.splitName,
                activityToken, null, 0, classLoader);

        // Clamp display ID to DEFAULT_DISPLAY if it is INVALID_DISPLAY.
        displayId = (displayId != Display.INVALID_DISPLAY) ? displayId : Display.DEFAULT_DISPLAY;

        final CompatibilityInfo compatInfo = (displayId == Display.DEFAULT_DISPLAY)
                ? packageInfo.getCompatibilityInfo()
                : CompatibilityInfo.DEFAULT_COMPATIBILITY_INFO;

        final ResourcesManager resourcesManager = ResourcesManager.getInstance();

        // Create the base resources for which all configuration contexts for this Activity
        // will be rebased upon.
        //设置 resources        
        context.setResources(resourcesManager.createBaseActivityResources(activityToken,
                packageInfo.getResDir(),
                splitDirs,
                packageInfo.getOverlayDirs(),
                packageInfo.getApplicationInfo().sharedLibraryFiles,
                displayId,
                overrideConfiguration,
                compatInfo,
                classLoader));
        context.mDisplay = resourcesManager.getAdjustedDisplay(displayId,
                context.getResources());
        return context;
    }

Service context 创建

ContextImpl context = ContextImpl.createAppContext(this, packageInfo);

createAppContext 方法：

static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo) {
        if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
        ContextImpl context = new ContextImpl(null, mainThread, packageInfo, null, null, null, 0,
                null);
        context.setResources(packageInfo.getResources());
        return context;
    }

以上是针对这个 Activity 和 Service 与 Context 之间的联系做了查看。