当MIDIet程序运行时出现错误的时候,应用程序治理器(application manager)会发出这个异常(exception). 在MIDIet类中也有两个方法可以发出这个异常(exception): destroyApp()和startApp().
下面是一个捕捉和发送例外的例子。这个简单的MIDIet显示一个FORM和一个关闭程序的按钮(显示EXIT).
创建这个例子的时候,我们会有这个问题:当按下关闭按钮或者应用程序治理器(application manager)要关闭这个MIDIet的时候,假如MIDIet正在下载程序,会发生什么情况?我们可以抛出异常(exception)而不是马上关闭程序。也就是说,下载请求不应在此时被关闭。应用程序治理器会在以后重试。
这个例子如何工作:当使用者第一次按下EXIT按钮,我们调用destroyApp(false)方法。这里传入参数false告诉方法这个请求不是无条件的,而且我们将会抛出MIDIetStateException异常.这个异常(exception)会在commandAction()里被捕捉到,同时会设置一个标志,这个标志意味着我们将可以关闭程序. 这样,当我们再一次按下EXIT按钮的时候。MIDIet将要继续运行直到EXIT按钮第二次被按下。
注重:以下例子基于MIDP和CLDC 1.0.3.
1. Java源代码:
/*----------------------------------------------------
* a look inside MIDletStateChangeException and the
* destroyApp() method
*
* www.CoreJ2ME.com
*---------------------------------------------------*/
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
public class ExceptionTest extends MIDlet implements CommandListener
{
private Display display;// Reference to Display object
private Form frmMain; // A Form
private Command cmdExit;// A Command to exit the MIDlet
private boolean safeToExit = false; // Is it safe to exit the MIDlet?
public ExceptionTest()
{
display = Display.getDisplay(this);
cmdExit = new Command("Exit", Command.SCREEN, 1);
frmMain = new Form("Test Exception");
frmMain.addCommand(cmdExit);
frmMain.setCommandListener(this);
}
// Called by application manager to start the MIDlet.
public void startApp()
{
display.setCurrent(frmMain);
}
// We are about to be placed in the Paused state
public void pauseApp()
{
}
// We are about to enter the Destroyed state
public void destroyApp(boolean unconditional) throws MIDletStateChangeException
{
System.out.println("Inside destroyApp()");
// If we do not need to unconditionally exit
if (unconditional == false)
{
System.out.println("Requesting not to be shutdown");
throw new MIDletStateChangeException("Please don't shut me down.");
}
}
// Check to see if the Exit command was selected
public void commandAction(Command c, Displayable s)
{
if (c == cmdExit)
{
try
{
// Is it ok to exit?
if (safeToExit == false)
destroyApp(false);
else
{
destroyApp(true);
notifyDestroyed();
}
}
catch (MIDletStateChangeException excep)
{
safeToExit = true; // Next time, let's exit
System.out.println(excep.getMessage());
System.out.println("Resuming the Active state");
}
}
}
}