import java.io.IOException;
import java.io.Writer;
import java.util.Map;

public interface Template {
    public void write(Writer out, Map<String, String> params) throws IOException;
}

import java.io.*;
import java.lang.reflect.Constructor;
import java.util.HashMap;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import javax.tools.*;
import org.mdkt.compiler.InMemoryJavaCompiler;

public class TemplateCompiler {
    private static final Pattern SUBST_PAT = Pattern.compile(
        "(?<LITERAL>.*?)(?:\{\{(?<SUBST>[^}]*)\}\})"
    );

    /**
     * Instantiates a <code>Template</code> that performs simple
     * text substitutions for <code>{{PLACEHOLDERS}}</code>.
     */
    public static Template compile(String templateText) {
        int rest = 0;
        StringBuilder script = new StringBuilder(
            "import java.io.IOException;n" +
            "import java.io.Writer;n" +
            "import java.util.Map;n" +
            "public class C implements Template {n" +
            "    public void write(Writer out, Map<String, String> params) throws IOException {n"
        );
        for (Matcher m = SUBST_PAT.matcher(templateText); m.find(); rest = m.end()) {
            script.append("out.write(")
                  .append(stringLiteral(m.group("LITERAL")))
                  .append(");nout.write(params.get(")
                  .append(stringLiteral(m.group("SUBST")))
                  .append("));n");
        }
        script.append("out.write(")
              .append(stringLiteral(templateText.substring(rest)))
              .append(");n");

        script.append("out.flush();n")
              .append("}}");

        try {
            @SuppressWarnings("unchecked")
            Class <? extends Template> c = (Class <? extends Template>)InMemoryJavaCompiler.compile("C", script.toString());
            Constructor<? extends Template> ctr = c.getConstructor();
            return ctr.newInstance();
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

    private static final Pattern UNSAFE_CHARS = Pattern.compile("[^A-Za-z0-9 ]");

    private static String stringLiteral(String s) {
        StringBuffer result = new StringBuffer(""");
        Matcher matcher = UNSAFE_CHARS.matcher(s);
        while (matcher.find()) {
            char c = matcher.group().charAt(0);
            switch (c) {
              // JLS SE7 3.10.5:
              // It is a compile-time error for a line terminator to appear
              case 'r':
                matcher.appendReplacement(result, "\r");
                break;
              case 'n':
                matcher.appendReplacement(result, "\n");
                break;
              default:
                String.format("\\u%04x", (int)c);
            }
        }
        matcher.appendTail(result);
        result.append(""");
        return result.toString();
    }
}

Template t = TemplateCompiler.compile(
    "Dear {{USER_NAME}},nn" +
    "According to our records, your phone number is {{USER_PHONE}} and " +
    "your e-mail address is {{USER_EMAIL}}.  If this is incorrect, please " +
    "go to {{LOGIN_URL}} and update your contact information."
);
for (Contact c : contacts) {
    Map<String, String> params = new HashMap<>();
    params.put("USER_NAME", c.getUserName());
    params.put("USER_EMAIL", c.getEmail());
    params.put("USER_PHONE", c.getPhone());
    params.put("LOGIN_URL", c.getLoginUrl());
    StringWriter sw = new StringWriter();
    t.write(sw, params);
    sw.toString();
}