Android开发之旅：HelloWorld项目的目录结构

前面Android开发之旅：环境搭建及HelloWorld，我们介绍了如何搭建Android开发环境及简单地建立一个HelloWorld项目，本篇将通过HelloWorld项目来介绍Android项目的目录结构。本文的主要主题如下：

• 1、HelloWorld项目的目录结构
  • 1.1、src文件夹
  • 1.2、gen文件夹
  • 1.3、Android 2.1文件夹
  • 1.4、assets 
  • 1.5、res文件夹
  • 1.6、AndroidManifest.xml
  • 1.7、default.properties

1、HelloWorld项目的目录结构

（这个HelloWorld项目是基于Android 2.1的）在Eclipse的左侧展开HelloWorld项目，可以看到如下图的目录结构：

图1、HelloWorld项目目录结构

下面将分节介绍上面的各级目录结构。

1.1、src文件夹

顾名思义（src, source code）该文件夹是放项目的源代码的。打开HelloWorld.java文件会看到如下代码：

HelloWorld.java

可以知道：我们新建一个简单的HelloWorld项目，系统为我们生成了一个HelloWorld.java文件。他导入了两个类android.app.Activity和android.os.Bundle，HelloWorld类继承自Activity且重写了onCreate方法。

以下说明针对没有学过Java或者Java基础薄弱的人

@Override

在重写父类的onCreate时，在方法前面加上@Override 系统可以帮你检查方法的正确性。例如，public void onCreate(Bundle savedInstanceState){…….}这种写法是正确的，如果你写成public void oncreate(Bundle savedInstanceState){…….}这样编译器回报如下错误——The method oncreate(Bundle) of type HelloWorld must override or implement a supertype method，以确保你正确重写onCreate方法。（因为oncreate应该为onCreate）

而如果你不加@Override，则编译器将不会检测出错误，而是会认为你新定义了一个方法oncreate。

android.app.Activity类：因为几乎所有的活动（activities）都是与用户交互的，所以Activity类关注创建窗口，你可以用方法setContentView(View)将自己的UI放到里面。然而活动通常以全屏的方式展示给用户，也可以以浮动窗口或嵌入在另外一个活动中。有两个方法是几乎所有的Activity子类都实现的：

1. onCreate(Bundle)：初始化你的活动（Activity），比如完成一些图形的绘制。最重要的是，在这个方法里你通常将用布局资源（layout resource）调用setContentView(int)方法定义你的UI，和用findViewById(int)在你的UI中检索你需要编程地交互的小部件（widgets）。setContentView指定由哪个文件指定布局（main.xml），可以将这个界面显示出来，然后我们进行相关操作，我们的操作会被包装成为一个意图，然后这个意图对应有相关的activity进行处理。
2. onPause()：处理当离开你的活动时要做的事情。最重要的是，用户做的所有改变应该在这里提交（通常ContentProvider保存数据）。

更多的关于Activity类的详细信息此系列以后的文章将做介绍，如果你想了解更多请参阅相关文档。

android.os.Bundle类：从字符串值映射各种可打包的（Parcelable）类型（Bundle单词就是捆绑的意思，所有这个类很好理解和记忆）。如该类提供了公有方法——public boolean containKey(String key)，如果给定的key包含在Bundle的映射中返回true，否则返回false。该类实现了Parceable和Cloneable接口，所以它具有这两者的特性。

1.2、gen文件夹

该文件夹下面有个R.java文件，R.java是在建立项目时自动生成的，这个文件是只读模式的，不能更改。R.java文件中定义了一个类——R，R类中包含很多静态类，且静态类的名字都与res中的一个名字对应，即R类定义该项目所有资源的索引。看我们的HelloWorld项目是不是如此，如下图：

图2、R.java对应res

通过R.java我们可以很快地查找我们需要的资源，另外编绎器也会检查R.java列表中的资源是否被使用到，没有被使用到的资源不会编绎进软件中，这样可以减少应用在手机占用的空间。

1.3、Android 2.1文件夹

该文件夹下包含android.jar文件，这是一个Java 归档文件，其中包含构建应用程序所需的所有的Android SDK 库（如Views、Controls）和APIs。通过android.jar将自己的应用程序绑定到Android SDK和Android Emulator，这允许你使用所有Android的库和包，且使你的应用程序在适当的环境中调试。例如上面的HelloWorld.java源文件中的：

import android.app.Activity;
import android.os.Bundle;

这里两行代码就是从android.jar导入包。

1.4、assets

包含应用系统需要使用到的诸如mp3、视频类的文件。

1.5、res文件夹

资源目录，包含你项目中的资源文件并将编译进应用程序。向此目录添加资源时，会被R.java自动记录。新建一个项目，res目录下会有三个子目录：drawabel、layout、values。

• drawabel-?dpi：包含一些你的应用程序可以用的图标文件(*.png、*.jpg)
• layout：界面布局文件(main.xml)与WEB应用中的HTML类同，没修改过的main.xml文件如下（HelloWorld的就没有修改过）：

main.xml

• values：软件上所需要显示的各种文字。可以存放多个*.xml文件，还可以存放不同类型的数据。比如arrays.xml、colors.xml、dimens.xml、styles.xml

1.6、AndroidManifest.xml

项目的总配置文件，记录应用中所使用的各种组件。这个文件列出了应用程序所提供的功能，在这个文件中，你可以指定应用程序使用到的服务(如电话服务、互联网服务、短信服务、GPS服务等等)。另外当你新添加一个Activity的时候，也需要在这个文件中进行相应配置，只有配置好后，才能调用此Activity。AndroidManifest.xml将包含如下设置：application permissions、Activities、intent filters等。

如果你跟我一样是ASP.NET出生或者学过，你会发现AndroidManifest.xml跟web.config文件很像，可以把它类同于web.config文件理解。

如果你不是，你可以这样理解——众所周知xml是一种数据交换格式，AndroidManifest.xml就是用来存储一些数据的，只不过这些数据时关于android项目的配置数据。

HelloWorld项目的AndroidManifest.xml如下所示：

AndroidManifest.xml

关于AndroidManifest.xml现在就讲这么多，此系列后面的文章将单独详细介绍。

1.7、default.properties

记录项目中所需要的环境信息，比如Android的版本等。 HelloWorld的default.properties文件代码如下所示，代码中的注释已经把default.properties解释得很清楚了：

default.properties

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