【正文】 all others are ponents perform background processing. When an activity needs to perform some operation that must continue after the user interface disappears (such as download a fle or play music), it monly starts a service specifcally designed for that action. The developer can also use services as applicationspecifc daemons, possibly starting on boot. 26 Services often define an interface for Remote Procedure Call (RPC) that other system ponents can use to send mands and retrieve data, as well as register callbacks. Content provider ? ponents store and share data using a relational database interface. Each content provider has an associated “authority” describing the content it contains. Other ponents use the authority name as a handle to perform SQL queries (such as SELECT, INSERT, or DELETE) to read and write content. Although content providers typically store values in database records, data retrieval is implementationspecifc—for example, fles are also shared through content provider receiver? ponents act as mailboxes for messages from other applications. Commonly, application code broadcasts messages to an implicit destination. Broadcast receivers thus subscribe to such destinations to receive the messages sent to it. Application code can also address a broadcast receiver explicitly by including the namespace assigned to its containing application. Figure 1 shows the FriendTracker and FriendViewer applications containing the diferent ponent types. The developer specifes ponents using a manifest fle (also used to defne policy as described later). There are no restrictions on the number of ponents an application defnes for each type, but as a convention, one ponent has the same name as the application. Frequently, this is an activity, as in the FriendViewer application. This activity usually indicates the primary activity that the system application launcher uses to start the user interface。 although doing so on a mobile platform such as Android remains nontrivial, many researchers hope it provides a clean slate devoid of the plications that legacy software can cause. Android doesn’t ofcially support applications eloped for other platforms: applications execute on top of a Java middleware layer running on an embedded Linux kernel, so developers wishing to port their application to Android must use its custom user interface environment. Additionally, Android restricts application interaction to its special APIs by running each application as its own user identity. Although this controlled interaction has several benefcial 25 security features, our experiences developing Android applications have revealed that designing secure forward. Android uses a simple permission label assignment model to restrict access to resources and other applications, but for reasons of necessity and convenience, its designers have added several potentially confusing refnements as the system has article attempts to unmask the plexity of Android security and note some possible development pitfalls that occur when defning an application’s security. We conclude by attempting to draw some lessons and identify opportunities for future enhancements that should aid in clarity and Applications The Android application framework forces a structure on developers. It doesn’t have a main() function or single entry point for execution—instead, developers must design applications in terms of ponents. Example Application. We developed a pair of applications to help describe how Android applications operate. Interested readers can download the source code from our web Let’s consider a locationsensitive social working application for mobile phones in which users can discover their friends’locations. We split the functionality into two applications: one for tracking friends and one for viewing them. As Figure 1 shows, the FriendTracker application consists of ponents specifc to tracking friend locations (for example, via a Web service), storing geographic coordinates, and sharing those coordinates with other applications. The user then uses the FriendViewer application to retrieve the stored geographic coordinates and view friends on a applications contain multiple ponents for performing their respective tasks。在我們的例子， FriendTracker 結(jié)合到系統(tǒng)中的服務(wù)器位置的管理。服務(wù)組件支持啟動，停止，并結(jié)合行動，所以 FriendTrackerControl 活動，例如，可以啟動和停止 FriendTracker服務(wù)在后臺運行。 可用的 ICC 的動作取決于目標(biāo)的組成部分。在許多方面， ICC是類似于進程間通信（ IPC）在基于 Unix的系統(tǒng)中。在每一種情況下，發(fā)起一個組件與其他的溝通。 Android 系包括附加目標(biāo)的決議規(guī)則，但可選的數(shù)據(jù)操作字符串類型是最常見的。例如一個 view 動作字符串，在一個 intent 中和數(shù)據(jù)域指向一個圖像文件，系統(tǒng)將會直接指首選圖像瀏覽器、 開發(fā)者也能使用動作字符串進行大量廣播發(fā)送和接收。在后一種情況下，系統(tǒng)決定了一個 action 的最佳組件，通過考慮安裝的應(yīng)用程序和用戶的選擇 。 Android 的一個最強大的特點是允許的多種 intent 尋址機制。這個過程中內(nèi)部組件通信稱為一個動作。 Android 的 API 定義了他的方法中傳入 intent ，并使用該信息來啟動一個 activity 例如開始一個 activity(startActivity(intent))，啟動服務(wù)（ startService（ intent））和廣播信息（ sendBroadcast（ intent））。此外，通過分離程序功能和接口，我們可以創(chuàng)建不同的顯示和功能，可選用戶界面是，許多應(yīng)用程序可以重用這些來自 FriendTracker 的功能。FriendReceiver 將會等待接收附近的手機發(fā)送的消息這個消息來一個所指定的朋友。 該 FriendViewer 應(yīng)用主要是顯示有關(guān)好友的位置的信息。在我們的示例代碼中，位置是我們隨機生成的，但直接通過網(wǎng)絡(luò)連接組件接口的服 務(wù)。 在 FriendTracker 應(yīng)用包含四種類型的組件。在在 FriendTracker 應(yīng)用，例如， FriendTrackerControl 活動被標(biāo)記為主用戶界面的啟動點。這一動作通常指示主 activity 作為該系統(tǒng)應(yīng)用程序啟動器用于啟動用戶界 面 。上有一個應(yīng)用程序的組件的數(shù)量沒有限制定義每種類型，但作為習(xí)慣，一組件應(yīng)具有相同的名稱該應(yīng)用程序。 圖 1 顯示了 FriendTracker 和 FriendViewer 應(yīng)用所包含的不同的組件類型。因此，廣播接收器訂閱這些目的地接收發(fā)送給它的消息。 Broadcast receiver 該組件作為為從郵件信箱發(fā)送信息給他應(yīng)用程序。其他組件使用時作為一個 handle 執(zhí)行 SQL 查詢（如 的 SELECT， INSERT 或 DELETE 內(nèi)容。 22 Content 組件存儲和共享數(shù)據(jù) 用關(guān)系數(shù)據(jù)庫接口。當(dāng)一個活動需要進行一些操作，在用戶界面消失以后（如下載一個文件或播放音樂），它通常采取此種動作特殊設(shè)計的服務(wù)。在一時間只有一個鍵盤的系統(tǒng) Activity 可以進行處理，在這個時候所有其他的 Activity 都會被暫停。通常，應(yīng)用程序開發(fā)者定義每一個活動“畫面。 Android 系統(tǒng)定義了 4 種組件類型。 FriendTracker 和 FriendViewer 應(yīng)用由多個不同類型的組件，每個提供一個不同的組功能。 圖 1。 這兩個應(yīng)用程序包含的多個組件包括展示自己任務(wù)，他們組件是由他們組件類型所決定的。如圖