通知模板里的单引号，为什么让 {0} 不再替换？

import java.text.MessageFormat;
import java.util.Locale;
public class MessagePatternDemo {
    static int checks;
    static void check(boolean ok) { if(!ok) throw new AssertionError(); checks++; }
    static String render(String pattern, Object... values) {
        return new MessageFormat(pattern, Locale.US).format(values);
    }
    public static void main(String[] args) {
        String broken=render("User's {0}","Ada");
        String fixed=render("User''s {0}","Ada");
        check(broken.equals("Users {0}")); check(fixed.equals("User's Ada"));
        System.out.println("single_quote="+broken+"; doubled_quote="+fixed);
        String literal=render("'{0}' / {0}","Ada");
        check(literal.equals("{0} / Ada"));
        System.out.println("quoted_placeholder="+literal);
        String data=render("user={0}","{1}'$1");
        check(data.equals("user={1}'$1"));
        System.out.println("argument_data="+data);
        String missing=render("first={0}; second={1}","Ada");
        check(missing.equals("first=Ada; second={1}"));
        System.out.println("missing_argument="+missing);
        boolean invalid=false;
        try { new MessageFormat("value={0",Locale.US); } catch(IllegalArgumentException e) { invalid=true; }
        check(invalid); System.out.println("unbalanced_brace=IllegalArgumentException");
        String pattern="{0,number,#,##0.00}";
        MessageFormat format=new MessageFormat(pattern,Locale.US);
        String before=format.format(new Object[]{1234.5});
        format.setLocale(Locale.GERMANY);
        String afterSet=format.format(new Object[]{1234.5});
        format.applyPattern(pattern);
        String afterApply=format.format(new Object[]{1234.5});
        check(before.equals("1,234.50")); check(afterSet.equals(before)); check(afterApply.equals("1.234,50"));
        System.out.println("number: US="+before+"; setLocale_only="+afterSet+"; reapply_pattern="+afterApply);
        System.out.println("PASS checks="+checks);
    }
}