无法从 Java 连接到 MySQL:MySQL 驱动程序连接逻辑中的 NullPointerException [英] Can't connect to MySQL from Java: NullPointerException inside MySQL driver connection logic

问题描述

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.

I'm trying to connect to a database I created with MySQL in my Java program, but it always fails.

举个例子，这是我的代码:

For the sake of example, here is my code:

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，并已将 mysql-connector-java-5.1.40.jar 添加到项目中，但每次运行程序时 DriverManager.getConnection() 抛出这个:

I am able to connect to the database from within IntelliJ and have added the mysql-connector-java-5.1.40.jar added to the project, but each time I run the program DriverManager.getConnection() throws this:

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

推荐答案

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.

It might be because you're using an older version of the MySQL driver. You should try using the newest version.

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

To get the newest version, you can check https://mvnrepository.com/artifact/mysql/mysql-connector-java

截至目前，最新版本是 8.0.11.你可以下载here 或将其添加到您的 pom.xml 中:

As of right now, the newest version is 8.0.11. You can download it here or add this to your 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MySQL 8.0.1 中引入的更改:

您报告的问题与 MySQL 中引入的更改有关8.0.1 支持字符集和排序规则，现在添加了默认字符集 'utf8mb4'.这样的改变打破了Connector/J 初始化连接的方式.

The issue you reported is related to the changes introduced in MySQL 8.0.1 wrt the character sets and collations support, with the addition of now being 'utf8mb4' the default char set. Such changes broke the way Connector/J initializes connections.

如您所知，此问题已在 Connector/J 5.1.41 中修复，我相信您已经更新了您的图书馆.

As you know this was fixed in Connector/J 5.1.41 and I'm sure you already updated your library.

参考

如上所述，解决您的问题的另一种方法是使用 5.1.41 而不是 5.1.40.

Like mentionned above, an alternative fix to your problem would have been to use the 5.1.41 instead of 5.1.40.

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