使用 JDBC 获取到 MySQL 的连接时“连接过多”

Question

Having an odd issue attempting to query a MySQL database; I'm getting a "too many connections" error on my first call to getConnection . Surely I must be doing something idiotic? For reference, I know the basics of closing connection , and not only am I only making one call, its failing on the very first attempt. So I must be doing something extremely stupid?

Two classes, one to call for a query, and the other to execute it: import java.sql.*;

public class Main {

    public static void main(String[] args) {

        CollectorDb.findEvents();
    }
}

public class CollectorDb {
    private static String username = "foo";
    private static String password = "bar";
    private static String connectionString = "jdbc:mysql://awsdb/dbName" +
            "?characterEncoding=UTF-8";

    public static void findEvents() {
        Connection conn = null;
        Statement stmt = null;
        ResultSet rset = null;

        try {
            Class.forName("com.mysql.jdbc.Driver");
        }
        catch (ClassNotFoundException e) {
            e.printStackTrace();
        }


        try {
            // Next line throws Exception
            conn = DriverManager.getConnection(connectionString, username, password);
            // ** NEVER GET HERE **
            // single query, then close all connections in a finally block
            ...
    }

}

First lines of the stacktrace:

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Too many connections
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:408)
...
Exception in thread "main" java.lang.RuntimeException: Too many connections
    at com.lifesize.CollectorDb.findEvents(CollectorDb.java:64)
    at com.lifesize.Main.main(Main.java:13)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
...

Have truncated the details that I think aren't relevant - happy to provide more code or full stacktrace, or any other information of course.

Answer 1

As pointed out by SleimanJneidi and zmf in the comments, the issue was not with my code at all, but the Server itself; the biggest clue being failure to connect on first attempt .

In my case, I don't have direct access to the server, but the following will display current connected clients:

SHOW STATUS WHERE variable_name = 'Threads_connected';

To answer my follow up of "How can I connect to check this if max connections are exceeded" - MySQL reserves a connection for the admin , specifically for this reason. This can be found in their documention :

The number of connections permitted is controlled by the max_connections system variable. The default value is 151 to improve performance when MySQL is used with the Apache Web server. (Previously, the default was 100.) If you need to support more connections, you should set a larger value for this variable.

mysqld actually permits max_connections+1 clients to connect. The extra connection is reserved for use by accounts that have the SUPER privilege. By granting the SUPER privilege to administrators and not to normal users (who should not need it), an administrator can connect to the server and use SHOW PROCESSLIST to diagnose problems even if the maximum number of unprivileged clients are connected. See Section 13.7.5.30, “SHOW PROCESSLIST Syntax”.

Answer 2

u have to put

                if(conn != null) {
                    try {
                    conn.close();   
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }