MAMP中如何处理'CommunicationsException: Communications link failure'？

Question

我已经在我的机器上安装了 MAMP，上次我在我的旧计算机和另一个运行良好的数据库上使用该代码。现在我不明白我哪里错了。

public class JDBC {
private static String DBURL = "jdbc:mysql://localhost:3306/BookReservationDatabase";
private static String DBLOGIN = "root";
private static String DBPASSWORD = "root";

public static void main(String[] args) {
    // Step1: register JDBC driver
    try {
        Class.forName("com.mysql.jdbc.Driver");
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
    // Step 2: Establish the connection to the database
    Connection conn = null;
    Statement stmt = null;
    ResultSet rset = null;
    try {
        conn = DriverManager.getConnection(DBURL, DBLOGIN, DBPASSWORD);
        // Step 3: Create statements (SQL queries)
        stmt = conn.createStatement();
        // Step 4: Execute statements and retrieve results
        rset = stmt.executeQuery("SELECT * FROM Users");
        // Step 5: Analyze results
        while (rset.next()) {

        }
    } catch (SQLException e) {
        e.printStackTrace();
    } finally {
        // Step 6: Release resources
        try {
            if (rset != null)
                rset.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        try {
            if (stmt != null)
                stmt.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        try {
            if (conn != null)
                conn.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
}

我的数据库名称是 BookReservationDatabase，我在 PhpMyAdmin 上使用 root 作为登录名和密码。

我遇到以下错误：

    com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: 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(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:990)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:342)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2188)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2221)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2016)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:776)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)
    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:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:386)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:330)
    at java.sql.DriverManager.getConnection(DriverManager.java:664)
    at java.sql.DriverManager.getConnection(DriverManager.java:247)
    at features.JDBC.main(JDBC.java:26)
Caused by: java.net.ConnectException: Connection refused (Connection refused)
    at java.net.PlainSocketImpl.socketConnect(Native Method)
    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
    at java.net.Socket.connect(Socket.java:589)
    at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:211)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:301)
    ... 15 more

错误必须在DBURL、DBLOGIN 和DBPASSWORD 之间。因为代码在连接处产生错误

conn = DriverManager.getConnection(DBURL, DBLOGIN, DBPASSWORD);

Answer 1

java.net.ConnectException: Connection refused (Connection refused)

清楚地表明您运行此代码所在的机器上存在通信错误。这可能是由多种原因造成的，因此我只列出最有可能的原因：

MySQL 根本不是运行ning。
- 你开始了吗？通过检查运行ning 进程列表进行验证（取决于 OS）。
MySQL 没有监听默认端口 3306。
- 你改了吗？如果是这样，请相应地更改您的 code/config。
如果防火墙处于活动状态，请检查可能阻止端口 3306 连接的规则。

希望对您有所帮助。

MAMP中如何处理'CommunicationsException: Communications link failure'？

How to deal with 'CommunicationsException: Communications link failure' in MAMP?

java

mysql

database-connection

jdbc