MySQL異常刨析：ata source rejected establishment of connection, message from server: “Too many connection

異常現象

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Data source rejected establishment of connection,  message from server: "Too many connections"
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
    at com.mysql.jdbc.Util.getInstance(Util.java:387)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:917)

    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:896)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:885)
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1042)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2253)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2284)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2083)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:806)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)

    at sun.reflect.GeneratedConstructorAccessor47.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:410)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:328)
    at java.sql.DriverManager.getConnection(DriverManager.java:571)

    at java.sql.DriverManager.getConnection(DriverManager.java:215)
    at com.yuanhe.leasing.util.DBUtil.getConn(DBUtil.java:37)
    at com.yuanhe.leasing.service.StatusListService.showManage(StatusListService.java:221)
    at com.yuanhe.leasing.servlet.ListStatusServlet.doPost(ListStatusServlet.java:56)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:650)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:218)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)

    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:442)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1082)

    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:623)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2517)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2506)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)

    at java.lang.Thread.run(Thread.java:745)

原因分析：

1，未使用連線池，每次建立連結且沒有關閉

解決方法： 

關閉連線，且依次關閉資料庫連線順序關閉！（RS，PS，CONN），確保異常時也要關閉。

finally{
    try {
        DBUtil.close(ps, conn, rs);
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

原因2：連線數設定的太少

1.Mysql資料庫的預設連線數是100。

2.預設值100對於一般的程式都不是很夠用，如果只是在自己的電腦上除錯程式，有可能發現不了這個問題，因為除錯程式時會經常重起Tomcat。

3.如果使用Hibernate框架來進行資料庫操作，這個問題尤其明顯。

4.解決方法：加大Mysql連線數。在Mysql安裝檔案中，找到my.ini檔案，在裡面找到

max_connections=100，將這個100變大一點，直接來個1000