How can I retain HTML form field values in JSP after submitting form to Servlet?

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You could access single-value request parameters by ${param}.

<%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %>
...
<input name="foo" value="${fn:escapeXml(param.foo)}">
<textarea name="bar">${fn:escapeXml(param.bar)}</textarea>
...
<input type="radio" name="faz" value="a" ${param.faz == 'a' ? 'checked' : ''} />
<input type="radio" name="faz" value="b" ${param.faz == 'b' ? 'checked' : ''} />
<input type="radio" name="faz" value="c" ${param.faz == 'c' ? 'checked' : ''} />
...
<select name="baz">
    <option value="a" ${param.baz == 'a' ? 'selected' : ''}>label a</option>
    <option value="b" ${param.baz == 'b' ? 'selected' : ''}>label b</option>
    <option value="c" ${param.baz == 'c' ? 'selected' : ''}>label c</option>
</select>

Do note that JSTL’s fn:escapeXml() is necessary in order to prevent XSS attacks. See also XSS prevention in JSP/Servlet web application.

You could access multi-value request parameters by ${paramValues} and EL 3.0 streams. 

<input type="checkbox" name="far" value="a" ${paramValues.far.stream().anyMatch(v->v == 'a').get() ? 'checked' : ''} />
<input type="checkbox" name="far" value="b" ${paramValues.far.stream().anyMatch(v->v == 'b').get() ? 'checked' : ''} />
<input type="checkbox" name="far" value="c" ${paramValues.far.stream().anyMatch(v->v == 'c').get() ? 'checked' : ''} />
...
<select name="boo" multiple>
    <option value="a" ${paramValues.boo.stream().anyMatch(v->v == 'a').get() ? 'selected' : ''}>label a</option>
    <option value="b" ${paramValues.boo.stream().anyMatch(v->v == 'b').get() ? 'selected' : ''}>label b</option>
    <option value="c" ${paramValues.boo.stream().anyMatch(v->v == 'c').get() ? 'selected' : ''}>label c</option>
</select>

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