why does my JSP code generate blank lines?

The problem you have is because after the directives you are using they have a return of the car after them.

Look at the following example:

<%@include file="..." %>(retorno del carro)
HOLA

It will generate the following output:

(lo que genere el archivo ...)(retorno del carro)
HOLA

However the following example:

<%@include file="..." %>HOLA

It will generate:

(lo que genere el archivo ...)HOLA

To change this behavior in a particular file you can use the trimDirectiveWhitespaces directive as follows:

<%@ page trimDirectiveWhitespaces="true" %>
<%@include file="..." %>(retorno del carro)
HOLA

Generating directly the following output:

(lo que genere el archivo ...)HOLA

You can check at runtime whether this policy is enabled or not by using JspPropertyGroupDescriptor.getTrimDirectiveWhitespaces() .

You can also set it globally in web.xml with a set of group properties (in this case all files *.jsp ) of the following way:

<jsp-config>
  <jsp-property-group>
    <url-pattern>*.jsp</url-pattern>
    <trim-directive-whitespaces>true</trim-directive-whitespaces>
  </jsp-property-group>
</jsp-config>