Question

我正在编写与数据库一起工作的应用程序。我有连接到mysql数据库的类：

public class DataBaseConnector {
    private static final String URL = "jdbc:mysql://localhost:3306/dataBase";
    private static final String USER = "user";
    private static final String PASSWORD = "password";

    private Connection connection;

    private static DataBaseConnector instance;

    public synchronized static DataBaseConnector getInstance() {
        if (instance == null) {
            instance = new DataBaseConnector();
        }
        return instance;
    }

    private DataBaseConnector() {
    }   
    public Connection openConnection() {
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }
        try (Connection connection = DriverManager.getConnection(URL, USER, PASSWORD)) {
            System.out.println("Database connected!");
            this.connection = connection;
            return connection;
        } catch (SQLException e) {
            throw new IllegalStateException("Cannot connect the database!", e);
        }
    }
}

执行SQL查询的测试方法：

   public static void main(String[] args) {
        try {
            Connection connection = DataBaseConnector.getInstance().openConnection();
            PreparedStatement stmt = connection.prepareStatement("SELECT * FROM hotel_administrators WHERE login=?");
            stmt.setString(1, "lll");
            ResultSet rs = stmt.executeQuery();
            while (rs.next()) {
                System.out.println(rs.getString("name"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

我不明白为什么出现这样的例外：

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: No operations allowed after connection closed.
Database connected!
    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.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.throwConnectionClosedException(ConnectionImpl.java:1187)
    at com.mysql.jdbc.ConnectionImpl.checkClosed(ConnectionImpl.java:1182)
    at com.mysql.jdbc.ConnectionImpl.prepareStatement(ConnectionImpl.java:4035)
    at com.mysql.jdbc.ConnectionImpl.prepareStatement(ConnectionImpl.java:4004)
    at model.DataBaseManager.main(DataBaseManager.java:81)

怎么了？

Answer 1

创建连接时，使用try-with-resources语句自动关闭新连接。

try (Connection connection = DriverManager.getConnection(URL, USER, PASSWORD)) {
    return connection; 
}
/* this is automatically executed:
finally { 
   connection.close(); 
}
*/

现在，当您执行查询时，MySQL只会抱怨连接已经关闭。为了避免错误，你可以写：

try {
     this.connection = DriverManager.getConnection(URL, USER, PASSWORD));
     System.out.println("Database connected!");
     return this.connection;
} catch (SQLException e) {
     throw new IllegalStateException("Cannot connect the database!", e);
}

Java为什么会出现MySQLNonTransientConnectionException：不允许操作......？

1 个答案: