MySQL 出现too many connections错误
出现这个错误的原因是mysql服务器配置了指定数量的连接个数(mysql安装目录下my.ini【测试环境Windows XP】, max_connections=10【默认为100，测试修改后的值】)。


通过DriverManager.getConnection(url, username, pwd)获取连接数大于mysql中配置的max_connections数就会报【mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Data source rejected establishment of connection, message from server: "Too many connections"】

报错的测试代码：

 for (int i = 1; i <= 20; i++) {      new Thread(new Runnable() {public void run() {Connection conn = null;try {conn = DBUtils.getConnection();synchronized (counter) {System.out.print(Thread.currentThread().getName());System.out.print("    counter = " + counter+++ "  conn = " + conn);System.out.println();conn.close();}} catch (SQLException e) {e.printStackTrace();}}}).start();}

解决方案：修改my.ini配置文件中的连接数max_connections 或 通过连接池。
【个人推荐】：连接池。