Why prependId="false" in a jsf form?

A situation where prependId=false is useful is in the login form, if you are using Spring Security, because the ids of the inputtexts have to be exactly "j_username" and "j_password". So you shouldn't put the form id before them, and using prependId=false is a good choice to acheive this.

I prefer to add prependId occasionally to make styling elements via their ID classes easier. For example, a form:

<h:form id="myform" ... >
    <h:inputText id="mytext" ... />
</h:form>

Would give you an ID of myform:mytext. As the colon is reserved in CSS, you have to escape the CSS to read something like #myform\:mytext { ... } which I prefer not to do. With prependId="false" I get to use just #mytext { ... } which is much simpler & nicer to read. It also plays nicer with CSS preprocessors like LESS or Sass.

In my experience, I never use this attribute. However, in some cases it can be useful.

When you use Facelets, you can create templates or include pages inside another page. So you can imagine that a page could be included in several different pages. Take the example where the parent pages contain a form, with different id:

Page 1:

<h:form id="form1">
    <ui:include src="pages/my-page.xhtml"/>
    ...
</h:form>

Page 2:

<h:form id="form2">
    <ui:include src="pages/my-page.xhtml"/>
    ...
</h:form>

Now, in the my-page.xhtml, you have a <h:inputText id="foo"/>. In the first case, the real ID of the input will be form1:foo, while in the second case, it will be form2:foo. This could create complex situations if you need a direct access to this component in Javascript or in Java (using findComponent("...") method).

If you use prependId="false" (or on some components forceId="true"), the real ID will be simply foo, and then your code will be simpler as you will not have to care about the container of the input field.

However, you will have to use this attribute carefully, as you may get a duplicate ID error if you use this prepend attribute too often...

In modern jsf versions it might also break ajax, see UIForm with prependId="false" breaks <f:ajax render>

One scenario where we have to set this flag is in case of Autocomplete control of primefaces library.

I had to set this flag to false when I was trying AutoComplete control of primefaces library. I was not able to get autocomplete working but after setting this flag it worked fine. You can see this link to my question regarding this problem

WARN [Parameters] Parameters: Invalid chunk ignored. warning coming in primefaces application