用一個簡單的例子來看MIDlet 的生命周期
想來估計也沒有比網(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,更詳細的資料可以到www.j2medev.com上查看。我將會以一個例子跟查看官方的源代碼來分析它們。
理解J2ME 的體系結構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)也許會對你理解J2ME 平臺體系結構這個重要的概念有所幫助。在MIDP
中定義了一種新的應用程序模型MIDlet,它是被Application Management Software(AMS)管理
的。AMS 負責MIDlet 的安裝、下載、運行和刪除等操作。在被AMS 管理的同時,MIDlet 可
以和應用管理軟件通信通知應用管理軟件自己狀態(tài)的變化,通常是通過方法notifyDestroyed()
和notifyPaused()實現(xiàn)的
MIDlet 有三個狀態(tài),分別是pause、active 和destroyed。在啟動一個MIDlet 的時候,應用
管理軟件會首先創(chuàng)建一個MIDlet 實例并使得他處于pause 狀態(tài),當startApp()方法被調(diào)用的時候
MIDlet 進入active 狀態(tài),也就是所說的運行狀態(tài)。在active 狀態(tài)調(diào)用destroyApp(boolean
第1 章 J2ME 技術概述
4
unconditional)或者pauseApp()方法可以使得MIDlet 進入destroyed 或者pause 狀態(tài)。值得一提的
是destroyApp(boolean unconditional)方法,事實上,當destroyApp()方法被調(diào)用的時候,AMS 通
知MIDlet 進入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 的時候, MIDlet 可以在接到通知后拋出
MIDletStateChangeException 而保持在當前狀態(tài),如果設置為true 的話,則必須立即進入destroyed
狀態(tài)。下圖說明了MIDlet 狀態(tài)改變情況:
<!--[if !vml]-->
<!--[endif]-->

看看我那個簡單的例子
public class HelloWorld extends MIDlet ......{
public HelloWorld() ......{
System.out.println("這個是程序的構造函數(shù),程序運行的時候首先調(diào)用這個");
}
protected void destroyApp(boolean unconditional)
throws MIDletStateChangeException ......{
System.out.println("這個是程序的destroyed事件,當您按下退出時調(diào)用");
}
protected void pauseApp() ......{
System.out.println("這個是程序的pause事件,當您按下暫停的時調(diào)用");
}
protected void startApp() throws MIDletStateChangeException ......{
System.out.println("這個是程序的active事件,程序啟動時候調(diào)用");
}
}
大家可以運行程序中看到這個程序的運行先后順些。基本上就明白了程序的調(diào)用機制了。
現(xiàn)在大家思考下,j2me的MIDlet是怎么樣運行的呢?sun在里面進行了什么樣子的限制與手腳呢?
一般的應用程序都有個main入門。這里沒有,為什么呢?
我想這個就是ASM的作用了,sun在后臺做了很多處理,比如包括,啟動容器,啟動MIDlet相關的資源等等。
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安徽新華電腦學校專業(yè)職業(yè)規(guī)劃師為你提供更多幫助【在線咨詢】

熱門精品專業(yè)
public class HelloWorld extends MIDlet ......{
public HelloWorld() ......{

