用一个简略的例子来看MIDlet的生命周期原文

用一个简单的例子来看MIDlet的生命周期原文
原文
用一个简单的例子来看MIDlet的生命周期

用一个简单的例子来看MIDlet 的生命周期

想来估计也没有比网上教程说的更清楚了，我这里摘录的只是文字，从www.j2medev.com来获取，更详细的资料可以到www.j2medev.com上查看。我将会以一个例子跟查看官方的源代码来分析它们。

理解J2ME 的体系结构并不像想象的那么容易，我们觉得读更多的资料帮助也不大，我们

直接迈向J2ME 开发也许会对你理解J2ME 平台体系结构这个重要的概念有所帮助。在MIDP

中定义了一种新的应用程序模型MIDlet，它是被Application Management Software（AMS）管理

的。AMS 负责MIDlet 的安装、下载、运行和删除等操作。在被AMS 管理的同时，MIDlet 可

以和应用管理软件通信通知应用管理软件自己状态的变化，通常是通过方法notifyDestroyed()

和notifyPaused()实现的

MIDlet 有三个状态，分别是pause、active 和destroyed。在启动一个MIDlet 的时候，应用

管理软件会首先创建一个MIDlet 实例并使得他处于pause 状态，当startApp()方法被调用的时候

MIDlet 进入active 状态，也就是所说的运行状态。在active 状态调用destroyApp(boolean

第1 章 J2ME 技术概述

4

unconditional)或者pauseApp()方法可以使得MIDlet 进入destroyed 或者pause 状态。值得一提的

是destroyApp(boolean unconditional)方法，事实上，当destroyApp()方法被调用的时候，AMS 通

知MIDlet 进入destroyed 状态。在destroyed 状态的MIDlet 必须释放了所有的资源，并且保存了

数据。如果unconditional 为false 的时候， MIDlet 可以在接到通知后抛出

MIDletStateChangeException 而保持在当前状态，如果设置为true 的话，则必须立即进入destroyed

状态。下图说明了MIDlet 状态改变情况：

<!----><!----><!---->

?


看看我那个简单的例子
public?class?HelloWorld?extends?MIDlet?......{

????public?HelloWorld()?......{?
????????System.out.println("这个是程序的构造函数，程序运行的时候首先调用这个");
????}

????protected?void?destroyApp(boolean?unconditional)
????????????throws?MIDletStateChangeException?......{
????????System.out.println("这个是程序的destroyed事件，当您按下退出时调用");
????}

????protected?void?pauseApp()?......{
????????System.out.println("这个是程序的pause事件，当您按下暂停的时调用");

????}

????protected?void?startApp()?throws?MIDletStateChangeException?......{
????????System.out.println("这个是程序的active事件，程序启动时候调用");

????}

}

大家可以运行程序中看到这个程序的运行先后顺些。基本上就明白了程序的调用机制了。
现在大家思考下，j2me的MIDlet是怎么样运行的呢？sun在里面进行了什么样子的限制与手脚呢？
一般的应用程序都有个main入门。这里没有，为什么呢？
我想这个就是ASM的作用了，sun在后台做了很多处理，比如包括，启动容器，启动MIDlet相关的资源等等。

public?static?void?main(String?args[])?...{
????????CommandState?state?=?new?CommandState();

????/**//*
?????*?pass?resource?strings?down?to?the?native?system?menu?and
?????*?popup?choice?group?methods...
?????*/
????initSystemLabels();

????????/**//*
?????????*?We?will?try?to?handle?any?printing?at?this?level,?because
?????????*?displaying?JAM?command?line?errors?is?device?specific.
?????????*/
????????try?...{
????????????initializeInternalSecurity();

????????/**//*?Start?a?inbound?connection?watcher?thread.?*/
????????new?Thread(new?PushRegistryImpl()).start();

????????????restoreCommandState(state);

????????????//?handle?any?development?machine?only?functions?at?this?level
????????????switch?(state.nextCommand)?...{
????????????case?CommandProcessor.RUN_CLASS:
????????????????runLocalClass(state);
????????????????state.nextCommand?=?CommandProcessor.EXIT;
????????????????break;

????????????case?CommandProcessor.MANAGE:
????????????????manage(state);
????????????????break;

????????????case?CommandProcessor.LIST:
????????????case?CommandProcessor.STORAGE_NAMES:
????????????????list(state);
????????????????state.nextCommand?=?CommandProcessor.EXIT;
????????????????break;

????????????case?CommandProcessor.REMOVE:
????????????????if?(DEV_STORAGE_NAME.equals(state.suiteStorageName))?...{
????????????????????removeDevStorage(state);
????????????????????state.nextCommand?=?CommandProcessor.EXIT;
????????????????????break;
????????????????}

????????????????//?fall?through
????????????default:
????????????????CommandProcessor.perform(state);
????????????????if?(state.status?==?CommandProcessor.MIDLET_SUITE_NOT_FOUND)?...{
????????????????????System.out.println("The?MIDlet?suite?was?not?found.");
????????????????}?else?if?(state.initialCommand?==?CommandProcessor.INSTALL?&&
????????????????????????state.status?==?CommandProcessor.OK)?...{
????????????????????System.out.println("Storage?name:?"?+
???????????????????????????????????????state.suiteStorageName);
????????????????}
????????????}
????????}?catch?(InvalidJadException?ije)?...{
????????????System.out.println("**?Error?installing?suite?("?+
???????????????????????????????ije.getReason()?+?"):?"?+?
???????????????????????????????messageForInvalidJadException(ije));
????????}?catch?(IOException?ioe)?...{
????????????System.out.println("**?Error?installing?suite:?"?+
???????????????????????????????ioe.getMessage());
????????}?catch?(ClassNotFoundException?ex)?...{
????????????if?(state.initialCommand?==?CommandProcessor.MANAGE)?...{

??????????????state.runExceptionMessage?=
????????????????????Resource.getString("The?application?cannot?be?launched.?"?+
????????????????????"One?of?the?application?classes?appears?to?be?missing.?"?+
????????????????????"This?could?be?due?to?a?mis-named?class.?Contact?the?"?+
????????????????????"application?provider?to?resolve?the?issue.");
????????????}?else?...{
????????????????System.out.println("MIDlet?class(s)?not?found:?"?+?
???????????????????????????????????ex.getMessage());
????????????}
????????}?catch?(InstantiationException?ex)?...{
????????????if?(state.initialCommand?==?CommandProcessor.MANAGE)?...{
???????????????state.runExceptionMessage?=?Resource.getString(
???????????????????"The?application?cannot?be?launched.?The?application?"?+
???????????????????"may?have?done?an?illegal?operation.?Contact?the?"?+
???????????????????"application?provider?to?resolve?the?issue.")?+?" "?+
???????????????????ex.getMessage();
????????????}?else?...{
????????????????System.out.println(
????????????????????"MIDlet?instance(s)?could?not?be?created:?"?+?
?????????????????????????????????ex.getMessage());
????????????}
????????}?catch?(IllegalAccessException?ex)?...{
????????????if?(state.initialCommand?==?CommandProcessor.MANAGE)?...{
????????????????state.runExceptionMessage?=?Resource.getString(
???????????????????"The?application?cannot?be?launched.?The?application?"?+
???????????????????"may?have?done?an?illegal?operation.?Contact?the?"?+
???????????????????"application?provider?to?resolve?the?issue.")?+?" "?+
???????????????????ex.getMessage();
????????????}?else?...{
????????????????System.out.println(
????????????????????"MIDlet?class(s)?could?not?be?accessed:?"?+?
????????????????????ex.getMessage());
????????????}
????????}?catch?(OutOfMemoryError?ex)?...{
????????????if?(state.initialCommand?==?CommandProcessor.MANAGE)?...{
????????????????state.runExceptionMessage?=?Resource.getString(
????????????????????"The?application?has?unexpectedly?quit?because?it?ran?"?+
????????????????????"out?of?memory.");
????????????}?else?...{
????????????????System.out.println("The?MIDlet?has?run?out?of?memory");
???&nb

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