${var}变量替换

${age},${name} 给定一个上下文 Map<String,String> context 使用上下文替换变量
private static final Pattern VAR_PATTERN =
              Pattern.compile("\\$\\{[^\\}\\$\u0020]+\\}");

          private static final int MAX_SUBST = 20;

          private static String substituteVars(String expr,Map<String,String> paramMap) {
            if (expr == null) {
              return null;
            }
            Matcher match = VAR_PATTERN.matcher("");
            String eval = expr;
            for(int s=0; s<MAX_SUBST; s++) {
              match.reset(eval);
              if (!match.find()) {
                return eval;
              }
              String var = match.group();
              var = var.substring(2, var.length()-1); // remove ${ .. }
              String val = null;
              try {
                val = paramMap.get(var);
              } catch(SecurityException se) {
              }
              /*if (val == null) {
                val = "aa";
              }*/
              if (val == null) {
                return eval; // return literal ${var}: var is unbound
              }
              // substitute
              eval = eval.substring(0, match.start())+val+eval.substring(match.end());
            }
            throw new IllegalStateException("Variable substitution depth too large: " 
                                            + MAX_SUBST + " " + expr);
          }