Create Application 的理解.md

blogs/Android/Framework/Interview/应用进程相关/Application 的理解.md

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+---
+谈谈你对 Application 的理解
+---
+
+1. 了解 Application 的作用
+2. 熟悉 Application 的类继承关系以及生命周期
+3. 深入理解 Application 的初始化原理
+
+#### Application 有什么用？
+
+1. 应用进程初始化操作
+2. 提供上下文
+
+#### Application 怎么初始化？
+
+```java
+// ActivityThread#attach
+private void attach() {
+    final IActivityManager mgr = ActivityManagerNative.getDefault();
+    mgr.attachAppliation(mAppThread);
+}
+```
+
+```java
+// AMS
+public final void attachApplication(IApplicationThread thread) {
+    synchronized(this) {
+        attachApplicationLocked(thread, callingPid);
+    }
+}
+```
+
+```java
+boolean attachApplicationLocked(IApplicationThread thread, ...) {
+	//...
+    // IPC 调用，回到应用进程
+	thread.bindApplication(...);
+}
+```
+
+```java
+public final void bindApplication(...) {
+	AppBindData data = new AppBindData();
+	//...
+	sendMessage(H.BIND_APPLICATION, data);
+}
+```
+
+```java
+private void handleBindApplication(AppBindData data) {
+    // LoadedApk
+	data.info = getPackageInfoNoCheck(data.appInfo, data.compatInfo);
+    // LoadedApk#makeApplication
+	Application app = data.info.makeApplication(...);
+    // Application#onCreate
+	mInstrumentation.callApplicationOnCreate(app);
+}
+```
+
+```java
+public Application makeApplication(...) {
+	if(mApplication != null) {
+		return mApplication;
+	}
+	ContextImpl appContext = ContextImpl.createAppContext(...);
+	app = mActivityThread.mInstrumentation.newApplication(...);
+	return app;
+}
+```
+
+```java
+Application new Application(ClassLoader cl, String className, Context context) {
+	return newApplication(cl.loadClass(className), context);
+}
+```
+
+```java
+static Application newApplication(Class<?> clazz, Context context) {
+	Application app = (Application)clazz.newInstance();
+	app.attach(context);
+	return app;
+}
+```
+
+```java
+final void attach(Context context) {
+	attachBaseContext(context);
+}
+```
+
+总结一下 Application 的初始化：
+
+1. new Application()
+2. Application.attachBaseContext()
+3. Application.onCreate()
+
+**不要执行耗时操作**
+
+```java
+boolean attachApplicationLocked(IApplicationThread thread, ...) {
+	//...
+    // IPC 调用，回到应用进程
+	thread.bindApplication(...);
+    //...
+    // 启动待启动的组件
+    mStackSupervisor.attachApplicationLocked(...);
+    mServices.attachApplicationLocked(app, processName);
+    sendPendingBroadcastsLocked(app);
+}
+```
+