无法从Java连接到MySQL:MySQL驱动程序连接逻辑中的NullPointerException

Question

我正在尝试连接到我在Java程序中使用MySQL创建的数据库,但它总是失败.

为了举例,这是我的代码:

import java.sql.*;

public class Squirrel {
    public static void main(String[] args) {
        String user;
        String password;
        Connection connection;
        Statement statement;
        try {
            Class.forName("com.mysql.jdbc.Driver");

            connection = DriverManager.getConnection(
                "jdbc:mysql://localhost:3306", user, password);

            statement = connection.createStatement();

            // Other code
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (statement != null) {
                    statement.close();
                }
                if (connection != null) {
                    connection.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

我能够从IntelliJ中连接到数据库并添加mysql-connector-java-5.1.40.jar添加到项目中,但每次运行程序时都会DriverManager.getConnection()抛出:

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Could not create connection to database server.
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:488)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
    at com.mysql.jdbc.Util.getInstance(Util.java:408)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:918)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:897)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:886)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:860)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2330)
    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 java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:488)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:410)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:328)
    at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:678)
    at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:229)
    at Squirrel.main(Squirrel.java:12)
Caused by: java.lang.NullPointerException
    at com.mysql.jdbc.ConnectionImpl.getServerCharset(ConnectionImpl.java:2997)
    at com.mysql.jdbc.MysqlIO.sendConnectionAttributes(MysqlIO.java:1934)
    at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1863)
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1226)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2253)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2284)
... 13 more

Answer 1

这可能是因为您使用的是旧版本的MySQL驱动程序.您应该尝试使用最新版本.

要获得最新版本,您可以查看https://mvnrepository.com/artifact/mysql/mysql-connector-java

截至目前,最新版本为8.0.11.您可以在此处下载或将其添加到您的pom.xml:

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.11</version>
</dependency>

更新

经过进一步调查,似乎是因为在MySQL 8.0.1以下方面引入了一项变更:

您报告的问题与MySQL 8.0.1中引入的更改有关,即字符集和排序规则支持,现在添加的是"utf8mb4"默认字符集.这些更改打破了Connector/J初始化连接的方式.

如您所知,这已在Connector/J 5.1.41中修复,我确定您已经更新了库.

参考

如上所述,对您的问题的另一种解决方法是使用5.1.41而不是5.1.40.

Answer 2

听起来像是潜在的版本不匹配或过时的客户端。当您在 IDE 之外运行它时，您可能会引入错误的版本。我会确保客户端使用最新版本或类似于服务器使用的版本。