Java MySQL JDBC驱动程序异常[重复]

Question

The last packet sent successfully to the server was 0 milliseconds ago. The 
driver has not received any packets from the server.
    at com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:172)
    at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64)
    at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:862)
    at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:444)
    at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:230)
    at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:226)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at teabot.database.DatabaseConnection.<init>(DatabaseConnection.java:26)
    at teabot.database.DatabaseHelper.setupDB(DatabaseHelper.java:16)
    at teabot.main.Main.init(Main.java:39)
    at teabot.main.Main.main(Main.java:69)
Caused by: com.mysql.cj.exceptions.CJCommunicationsException: Communications link failure

The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:59)
    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:103)
    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:149)
    at com.mysql.cj.exceptions.ExceptionFactory.createCommunicationsException(ExceptionFactory.java:165)
    at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:92)
    at com.mysql.cj.NativeSession.connect(NativeSession.java:152)
    at com.mysql.cj.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:982)
    at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:852)
    ... 9 more
Caused by: java.net.ConnectException: Connection refused: connect
    at java.net.DualStackPlainSocketImpl.connect0(Native Method)
    at java.net.DualStackPlainSocketImpl.socketConnect(Unknown Source)
    at java.net.AbstractPlainSocketImpl.doConnect(Unknown Source)
    at java.net.AbstractPlainSocketImpl.connectToAddress(Unknown Source)
    at java.net.AbstractPlainSocketImpl.connect(Unknown Source)
    at java.net.PlainSocketImpl.connect(Unknown Source)
    at java.net.SocksSocketImpl.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at com.mysql.cj.protocol.StandardSocketFactory.connect(StandardSocketFactory.java:173)
    at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:66)

源代码：

protected DatabaseConnection(String host, int port, String databaseName, String username, String password) {                
    boolean error = false;
    try {
        String url = "jdbc:mysql://" + host + ":" + port + "/" + databaseName;
        this.connection = DriverManager.getConnection(url, username, password);
    } catch (SQLException e) {
        e.printStackTrace();
        Helper.log(2, e.getMessage());
        error = true;
    }
    if (!error) {
        Helper.log(0, "Database is ready.");
    }
}

编辑：我的问题是我的MySQL服务器没有监听那个端口，因为当我用phpMyAdmin测试它时，我忘记它运行在同一台机器上。

Answer 1

与其说是一个答案，不如说是一种诊断问题的方法：

我通常尝试使用telnet或putty这样的工具在相关端口连接到MySQL数据库。如果你不能连接，你知道问题不是你的代码，而是你的网络连接。这就是堆栈跟踪在我看来的样子--一个网络连接问题，而不是其他任何问题。也许防火墙阻塞了端口？

然后，我将尝试使用带有用户名和密码的MySQL连接工具。