JSP tag to render Handlebars templates (Java code, Spring config and TLD here: https://gist.github.com/ndemengel/2d0ce37ad326570cecfc2daccaae2267)

ContextProperty.java

import lombok.Value;

@Value
public class ContextProperty {
    String name;
    Object value;
}

ContextPropertyAware.java

import javax.servlet.jsp.JspTagException;

public interface ContextPropertyAware {
    void addContextProperty(ContextProperty property) throws JspTagException;
}

ContextPropertyTag.java

import lombok.Setter;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.BodyTagSupport;

public class ContextPropertyTag extends BodyTagSupport {

    @Setter
    private String name;
    @Setter
    private Object value;

    @Override
    public int doEndTag() throws JspException {
        ContextPropertyAware propertyAwareTag = (ContextPropertyAware) findAncestorWithClass(this, ContextPropertyAware.class);
        if (propertyAwareTag == null) {
            throw new JspException("The contextProperty tag must be a descendant of a tag that supports context properties");
        }
        propertyAwareTag.addContextProperty(new ContextProperty(name, value));

        return EVAL_PAGE;
    }

    @Override
    public void release() {
        super.release();
        this.name = null;
        this.value = null;
    }
}

RenderTag.java

import com.github.jknack.handlebars.Handlebars;
import lombok.Setter;
import org.springframework.web.context.WebApplicationContext;

import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.TagSupport;
import java.io.IOException;
import java.util.*;

import static java.util.Collections.emptyMap;
import static java.util.stream.Collectors.toMap;
import static org.springframework.web.context.support.WebApplicationContextUtils.getWebApplicationContext;

public class RenderTag extends TagSupport implements ContextPropertyAware {

    @Setter
    private Object context;
    @Setter
    private String template;
    private List<ContextProperty> contextProperties = new ArrayList<>();

    @Override
    public int doStartTag() throws JspException {
        contextProperties = new ArrayList<>();
        return EVAL_BODY_INCLUDE;
    }

    @Override
    public int doEndTag() throws JspException {
        WebApplicationContext appContext = getWebApplicationContext(pageContext.getServletContext());
        Handlebars handlebars = appContext.getBean(Handlebars.class);
        try {
            handlebars.compile(template).apply(getOrBuildContext(), pageContext.getOut());
            return EVAL_PAGE;
        } catch (IOException ex) {
            throw new JspException(ex.toString(), ex);
        }
    }

    private Object getOrBuildContext() {
        if (context != null) {
            return context;
        }

        if (contextProperties == null) {
            return emptyMap();
        }

        return contextProperties.stream().collect(toMap(ContextProperty::getName, ContextProperty::getValue));
    }

    @Override
    public void addContextProperty(ContextProperty property) throws JspTagException {
        contextProperties.add(property);
    }

    @Override
    public void release() {
        super.release();
        context = null;
        template = null;
        contextProperties = null;
    }
}