The lifecycle of an android application(转载)

In most cases, every Android application runs in its own Linux process. This process is created for the application when some of its code needs to be run, and will remain running until it is no longer needed and the system needs to reclaim its memory for use by other applications.

在多数情况下, 每个Android应用运行在自己的Linux进程中. 当一个应用的某段code需要运行的时候这个进程将会被创建, 直到不再需要该应用或系统要为其他的应用释放内存的时候才停止.

An important and unusual feature of Android is that an application process's lifetime is not directly controlled by the application itself. Instead, it is determined by the system through a combination of the parts of the application that the system knows are running, how important these things are to the user, and how much overall memory is available in the system.

一个非常重要且少有的特性是, 应用进程的存活时间不是由这个应用直接控制的. 而是由系统决定的, 系统会根据每个已知的正在运行的应用情况来定夺, 包括, 该应用对用户的重要性和系统全部可用内存.

It is important that application developers understand how different application components (in particular Activity, Service, and IntentReceiver) impact the lifetime of the application's process. Not using these components correctly can result in the system killing the application's process while it is doing important work.

对于开发人员来讲, 了解每个应用组件(尤其是, Activity, Service, 和IntentReceiver)对于应用进程存活时间的影响是非常重要的. 如果没有正确使用, 可能会导致应用进程在处理重要工作的时候被系统杀掉.

A common example of a process lifecycle bug is an IntentReceiver that starts a thread when it receives an Intent in its onReceiveIntent() method, and then returns from the function. Once it returns, the system considers that IntentReceiver to be no longer active, and thus its hosting process no longer needed (unless other application components are active in it). Thus, it may kill the process at any time to reclaim memory, terminating the spawned thread that is running in it. The solution to this problem is to start a Service from the IntentReceiver, so the system knows that there is still active work being done in the process.

在对应用进程生命周期的理解中, 一个典型的错误就是当一个IntentReceiver 接收到Intent 之后, 会在自己的onReceiveIntent()方法中开起一个线程, 而后return这个方法. 一旦这个方法return, 系统会认为这个IntentReceiver 不在处于活跃状态, 也就认为他的宿主进程不再需要(除非还包有其他活跃的应用组件). 以至于当系统需要回收内存的时候会随时释kill掉这个进程, 中止其中的子线程. 解决这个问题的办法是在IntentReceiver中启动一个Service, 这样系统会知道在这个进程中还有活跃的任务需要完成.

To determine which processes should be killed when low on memory, Android places them into an "importance hierarchy" based on the components running in them and the state of those components. These are, in order of importance:

为了决定在内存较低的时候杀掉哪个进程, Android会根据运行在这些进程内的组件及他们的状态把进程划分成一个"重要程度层次". 其重要的程度按以下规则排序:

       1. A foreground process is one holding an Activity at the top of the screen that the user is interacting with (its onResume() method has been called) or an IntentReceiver that is currently running (its onReceiveIntent() method is executing). There will only ever be a few such processes in the system, and these will only be killed as a last resort if memory is so low that not even these processes can continue to run. Generally at this point the device has reached a memory paging state, so this action is required in order to keep the user interface responsive.

       1. 前端进程可以是一个持有运行在屏幕最前端并与用户交互的Activity的进程 (onResume方法被调用时)，也可以是持有一个正在运行的IntentReceiver(也就是说他正在执行自己的onReceiveIntent方法)的进程. 在系统中, 只会有少数这样的进程, 并且除非内存已经低到不够这些进程运行, 否则系统不会主动杀掉这些进程. 这时, 设备通常已经达到了需要内存整理的状态, 所以杀掉这些进程是为了不让用户界面停止响应.

       1. A visible process is one holding an Activity that is visible to the user on-screen but not in the foreground (its onPause() method has been called). This may occur, for example, if the foreground activity has been displayed with a dialog appearance that allows the previous activity to be seen behind it. Such a process is considered extremely important and will not be killed unless doing so is required to keep all foreground processes running.

       1. 可视进程是持有一个被用户可见, 但没有显示在最前端 (onPause方法被调用时) 的Activity的进程. 举例来说, 这种进程通常出现在一个前端Activity以一个对话框出现并保持前一个Activity可见时. 这种进程被系统认为是极其重要的, 并且通常不会被杀掉, 除非为了保持所有前端进程正常运行不得不杀掉这些可见进程.

       1. A service process is one holding a Service that has been started with the startService() method. Though these processes are not directly visible to the user, they are generally doing things that the user cares about (such as background mp3 playback or background network data upload or download), so the system will always keep such processes running unless there is not enough memory to retain all foreground and visible process.

       1. 服务进程是持有一个Service的进程, 该Service是由startService()方法启动的, 尽管这些进程用户不能直接看到, 但是通常他们做的工作用户是十分关注的(例如, 在后台播放mp3或是在后台下载 上传文件), 所以, 除非为了保持所有的前端进程和可视进程正常运行外, 系统是不会杀掉服务进程的.

       1. A background process is one holding an Activity that is not currently visible to the user (its onStop() method has been called). These processes have no direct impact on the user experience. Provided they implement their activity lifecycle correctly (see Activity for more details), the system can kill such processes at any time to reclaim memory for one of the three previous processes types. Usually there are many of these processes running, so they are kept in an LRU list to ensure the process that was most recently seen by the user is the last to be killed when running low on memory.

       1. 后台进程是持有一个不再被用户可见的Activity(onStop()方法被调用时)的进程. 这些进程不会直接影响用户体验. 加入这些进程已经完整的,正确的完成了自己的生命周期(访问Activity查看更多细节), 系统会在为前三种进程释放内存时随时杀掉这些后台进程. 通常会有很多的后台进程在运行, 所以这些进程被存放在一个LRU列表中, 以保证在低内存的时候, 最近一个被用户看到的进程会被最后杀掉.

       1. An empty process is one that doesn't hold any active application components. The only reason to keep such a process around is as a cache to improve startup time the next time a component of its application needs to run. As such, the system will often kill these processes in order to balance overall system resources between these empty cached processes and the underlying kernel caches.

       1. 空进程是没有持有任何活动应用组件的进程. 保留这种进程的唯一理由是为了提供一种缓存机制, 缩短他的应用下次运行时的启动时间. 就其本身而言, 系统杀掉这些进程的目的是为了在这些空进程和底层的核心缓存之间平衡整个系统的资源.

When deciding how to classify a process, the system picks the most important level of all the components currently active in the process. See the Activity, Service, and IntentReceiver documentation for more detail on how each of these components contribute to the overall lifecycle of a process. The documentation for each of these classes describes in more detail how they impact the overall lifecycle of their application.

当需要给一个进程分类的时候, 系统会在该进程中处于活动状态的所有组件里掉选一个重要等级最高作为分类依据. 查看Activity, Service,和IntentReceiver的文档, 了解每个组件在进程整个生命周期中的贡献. 每一个classes的文档详细描述他们在各自应用的生命周期中所起得作用.