熱門精品專業(yè)用一個(gè)簡單的例子來看MIDlet 的生命周期
想來估計(jì)也沒有比網(wǎng)上教程說的更清楚了，我這里摘錄的只是文字，從www.j2medev.com來獲取，更詳細(xì)的資料可以到www.j2medev.com上查看。我將會(huì)以一個(gè)例子跟查看官方的源代碼來分析它們。
理解J2ME 的體系結(jié)構(gòu)并不像想象的那么容易，我們覺得讀更多的資料幫助也不大，我們
直接邁向J2ME 開發(fā)也許會(huì)對(duì)你理解J2ME 平臺(tái)體系結(jié)構(gòu)這個(gè)重要的概念有所幫助。在MIDP
中定義了一種新的應(yīng)用程序模型MIDlet，它是被Application Management Software（AMS）管理
的。AMS 負(fù)責(zé)MIDlet 的安裝、下載、運(yùn)行和刪除等操作。在被AMS 管理的同時(shí)，MIDlet 可
以和應(yīng)用管理軟件通信通知應(yīng)用管理軟件自己狀態(tài)的變化，通常是通過方法notifyDestroyed()
和notifyPaused()實(shí)現(xiàn)的
MIDlet 有三個(gè)狀態(tài)，分別是pause、active 和destroyed。在啟動(dòng)一個(gè)MIDlet 的時(shí)候，應(yīng)用
管理軟件會(huì)首先創(chuàng)建一個(gè)MIDlet 實(shí)例并使得他處于pause 狀態(tài)，當(dāng)startApp()方法被調(diào)用的時(shí)候
MIDlet 進(jìn)入active 狀態(tài)，也就是所說的運(yùn)行狀態(tài)。在active 狀態(tài)調(diào)用destroyApp(boolean
第1 章 J2ME 技術(shù)概述
4
unconditional)或者pauseApp()方法可以使得MIDlet 進(jìn)入destroyed 或者pause 狀態(tài)。值得一提的
是destroyApp(boolean unconditional)方法，事實(shí)上，當(dāng)destroyApp()方法被調(diào)用的時(shí)候，AMS 通
知MIDlet 進(jìn)入destroyed 狀態(tài)。在destroyed 狀態(tài)的MIDlet 必須釋放了所有的資源，并且保存了
數(shù)據(jù)。如果unconditional 為false 的時(shí)候， MIDlet 可以在接到通知后拋出
MIDletStateChangeException 而保持在當(dāng)前狀態(tài)，如果設(shè)置為true 的話，則必須立即進(jìn)入destroyed
狀態(tài)。下圖說明了MIDlet 狀態(tài)改變情況：
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看看我那個(gè)簡單的例子
public class HelloWorld extends MIDlet ......{
public class HelloWorld extends MIDlet ......{
public HelloWorld() ......{ 
public HelloWorld() ......{ System.out.println("這個(gè)是程序的構(gòu)造函數(shù)，程序運(yùn)行的時(shí)候首先調(diào)用這個(gè)");
} protected void destroyApp(boolean unconditional) throws MIDletStateChangeException ......{ System.out.println("這個(gè)是程序的destroyed事件，當(dāng)您按下退出時(shí)調(diào)用"); } protected void pauseApp() ......{ System.out.println("這個(gè)是程序的pause事件，當(dāng)您按下暫停的時(shí)調(diào)用"); } protected void startApp() throws MIDletStateChangeException ......{ System.out.println("這個(gè)是程序的active事件，程序啟動(dòng)時(shí)候調(diào)用"); }
}
大家可以運(yùn)行程序中看到這個(gè)程序的運(yùn)行先后順些。基本上就明白了程序的調(diào)用機(jī)制了。
現(xiàn)在大家思考下，j2me的MIDlet是怎么樣運(yùn)行的呢？sun在里面進(jìn)行了什么樣子的限制與手腳呢？
一般的應(yīng)用程序都有個(gè)main入門。這里沒有，為什么呢？
我想這個(gè)就是ASM的作用了，sun在后臺(tái)做了很多處理，比如包括，啟動(dòng)容器，啟動(dòng)MIDlet相關(guān)的資源等等。
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安徽新華電腦學(xué)校專業(yè)職業(yè)規(guī)劃師為你提供更多幫助【在線咨詢】
