执行该方法，如果成功会返回一个对象，否则抛出异常。

The method executes and either an object is returned, if successful, or an exception is thrown.

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如果在处理jsp页面时有一个异常抛出的话，exception对象就会立即通过隐式声明的exception变量的方式抛给指定的错误页面。

Whenever an exception is thrown while processing your JSP page, the exception object will immediately be thrown to the designated error page by means of the implicitly declared exception variable.

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如果还没有创建 XMLHttpRequest 对象，或者已经发送了请求，那么 checkRequest() 函数将抛出一个异常。

The checkRequest() function will throw an exception if the XMLHttpRequest object has not been created or if the request has already been sent.

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