重写Freemarker的模板加载器即可:

package cn.com.aweb.vote.test.other;

import java.io.IOException;
import java.io.Reader;
import java.io.StringReader;
import java.util.HashMap;
import java.util.Map;

import freemarker.cache.TemplateLoader;

public class StringTemplateLoader implements TemplateLoader {

		private static final String DEFAULT_TEMPLATE_KEY = "_default_template_key";
		private Map templates = new HashMap();

		public StringTemplateLoader(String defaultTemplate) {
			if (defaultTemplate != null && !defaultTemplate.equals("")) {
				templates.put(DEFAULT_TEMPLATE_KEY, defaultTemplate);
			}
		}

		public void AddTemplate(String name, String template) {
			if (name == null || template == null || name.equals("")
					|| template.equals("")) {
				return;
			}
			if (!templates.containsKey(name)) {
				templates.put(name, template);
			}
		}

		public void closeTemplateSource(Object templateSource)
				throws IOException {

		}

		public Object findTemplateSource(String name) throws IOException {
			if (name == null || name.equals("")) {
				name = DEFAULT_TEMPLATE_KEY;
			}
			return templates.get(name);
		}

		public long getLastModified(Object templateSource) {
			return 0;
		}

		public Reader getReader(Object templateSource, String encoding)
				throws IOException {
			return new StringReader((String) templateSource);
		}

	}

测试类：
package cn.com.aweb.vote.test.other;

import java.io.StringWriter;
import java.util.HashMap;
import java.util.Map;

import freemarker.template.Configuration;
import freemarker.template.Template;

public class Test{
	
	public static void main(String[] args) throws Exception {
		Configuration cfg = new Configuration();    
        cfg.setTemplateLoader(new StringTemplateLoader("hello：${user}"));    
        cfg.setDefaultEncoding("UTF-8");    
   
        Template template = cfg.getTemplate("");    
            
        Map root = new HashMap();    
        root.put("user", "lunzi");    
            
        StringWriter writer = new StringWriter();    
        template.process(root, writer);    
        System.out.println(writer.toString());          
	}
}

附件是 freemarker 学习 pdf