（转）为什么第三方应用能早于System的app启动

Android应用的启动顺序网上有一大堆资料可以查阅了，这里就不细述了，这里不阐述ROM启动还有bootloader，软件启动的大致流程应该是启动kernel -> 运行servicemanager 把一些native的服务用命令启动起来（包括wifi, power, rild, surfaceflinger, mediaserver等等）-> 启动Dalivk中的第一个进程Zygote -> 启动java 层的系统服务system_server（包括PowerManagerService, ActivityManagerService , telephony.registry, DisplayManagerService等等）该服务中的这些部件跟native的那些服务关联在一起 -> 启动Luncher和Persistent的App，该程序是系统级的在AndroidManifest.xml里声明android:persistent="true" -> 发出ACTION_BOOT_COMPLETED广播给其他应用。
在这里需要注意的是声明android:persistent属性为true的APP被kill掉后还是会自动重启的。
系统中我们已知android:persistent属性为true的APP肯定有Phone App，也就是说第三方应用应当至少晚于Phone APP启动，如何判断呢？最简单的办法看其PID的大小，PID值越小越先启动。有其第三方应用可以先于Phone APP启动。我们探其应用的AndroidManifest.xml (PS：如何看APK的代码，网上有你懂的apktool等)，发现其在AndroidManifest里定义的静态Receiver的intent-filter的属性如下：

       <receiver android:name="com.anguanjia.safe.AAAReceiver">
            <intent-filter android:priority="2147483647"></intent-filter></span>
                
                
                
                
                
            
            <intent-filter android:priority="2147483647">
                
                
                
                
                
                <data android:scheme="file">
            </data></action></action></action></action></action></intent-filter>
</action></action></action></action></action></receiver>

2147483647 这个值是什么？好大，哦，原来是int的最大值！我们来看下google 文档

android:priority
The priority that should be given to the parent component with regard to handling intents of the type described by the filter. This attribute has meaning for both activities and broadcast receivers:

It provides information about how able an activity is to respond to an intent that matches the filter, relative to other activities that could also respond to the intent. When an intent could be handled by multiple activities with different priorities, Android will consider only those with higher priority values as potential targets for the intent.

It controls the order in which broadcast receivers are executed to receive broadcast messages. Those with higher priority values are called before those with lower values. (The order applies only to synchronous messages; it's ignored for asynchronous messages.)

Use this attribute only if you really need to impose a specific order in which the broadcasts are received, or want to force Android to prefer one activity over others.

The value must be an integer, such as "100". Higher numbers have a higher priority. The default value is 0. The value must be greater than -1000 and less than 1000.


这个值是receiver的优先级，值越大优先级越高，按优先顺序执行，但是文档介绍优先级值大小是-1000~1000. 该应用的是int的最大值， 但android平台没有对android:priority值进行检查。在开机后该应用Receiver的intent-filter的优先级最高，在该filter里的intent被系统发送出来(android.intent.action.MEDIA_MOUNTE, android.net.wifi.WIFI_STATE_CHANGED等等），这个时候App会根据这个intent而被启动起来。
这里需要注意的是该Receiver是个静态的，一定是要注册在AndroidManifest里。当Wifi成功注册后会发出WIFI_STATE_CHANGED的消息， 或者其他的部件完成一些事件后也会发出类似的消息，而这些消息的发出又早于属性为persistent的系统级APP的启动， 由此就会发生第三方应用早于系统级APP的启动的情况。