jsp的ajax代码

How to write Ajax code to retrieve information on to a particular part of webpage when we select option from a dropdown box?

I want information of a particular item that I had selected from a dropdown menu on particular part of my webpage.

Let's say your markup looks something like:

<select id="dropdown">
    <option value="1">Some option</option>
    <option value="2">Other option</option>
    <option value="3">Helpfull option</option>
    <option value="4">Don't pick this option</option>
</select>
<div id="details"></div>

In order to make AJAX calls I would use some sort of library. Using for instance jQuery will be much easier than writing code that handles ajax calls across different browsers. The code could look like this:

$('#dropdown').on('change', function(e) {
    var currentValue = $('#dropdown').val();
    $.get('<someurl>', function(data) {
        $('#details').html(data);
    });
};

Replace '<someurl>' with the url to the resource you want. Doing this without using jQuery or similar library is a bit more involved. For some guidance you can look at this answer: https://stackoverflow.com/a/2557268/355499