The default JSP file encoding is specified by JSR315 as ISO-8859-1. This is the encoding that the JSP engine uses to read the JSP file and it is unrelated to the servlet request or response encoding.
If you have non-latin characters in your JSP files, save the JSP file as UTF-8 with BOM or set pageEncoding in the beginning of the JSP page:
<%@page pageEncoding="UTF-8" %>
However, you might want to change the default to UTF-8 globally for all JSP pages. That can be done via web.xml:
<jsp-config>
    <jsp-property-group>
        <url-pattern>/*</url-pattern>
        <page-encoding>UTF-8</page-encoding>
    </jsp-property-group>
</jsp-config>
Or, when using Spring Boot with an (embedded) Tomcat, via a TomcatContextCustomizer:
@Component
public class JspConfig implements TomcatContextCustomizer {
    @Override
    public void customize(Context context) {
        JspPropertyGroup pg = new JspPropertyGroup();
        pg.addUrlPattern("/*");
        pg.setPageEncoding("UTF-8");
        pg.setTrimWhitespace("true"); // optional, but nice to have
        ArrayList<JspPropertyGroupDescriptor> pgs = new ArrayList<>();
        pgs.add(new JspPropertyGroupDescriptorImpl(pg));
        context.setJspConfigDescriptor(new JspConfigDescriptorImpl(pgs, new ArrayList<TaglibDescriptor>()));
    }
}
For JSP to work with Spring Boot, don't forget to include these dependencies:
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-tomcat</artifactId>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>org.apache.tomcat.embed</groupId>
        <artifactId>tomcat-embed-jasper</artifactId>
        <scope>provided</scope>
    </dependency>
And to make a "runnable" .war file, repackage it:
<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <executions>
                <execution>
                    <phase>package</phase>
                    <goals>
                        <goal>repackage</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
   . . .