连接为空，但为什么

Question

在该方法中readAll()我得到了NullPointerException我Connection，但为什么，我怎么可能改变这种状况？如果我用主类测试它一切正常，但如果我在服务器（网络服务器 - 本地）上做同样的事情，它会抛出一个NullPointerException.这是我的ConnectionManager课：public class ConnectionManager {    private volatile static ConnectionManager connectionManager = null;    private Connection connection = null;    private ConnectionManager() throws SQLException, ClassNotFoundException, InstantiationException, IllegalAccessException {        DBProperties properties = DBProperties.getInstance();        String url = properties.getStringProperty("databaseURL") + properties.getStringProperty("pathToDB") + properties.getStringProperty("urlSettings");        String user = properties.getStringProperty("user");        String passwort = properties.getStringProperty("password");        try { // load driver            Class.forName(properties.getStringProperty("dbDriver")).newInstance();            connection = DriverManager.getConnection(url, user, passwort);        } catch (SQLException ex) {            Logger.getLogger(ConnectionManager.class.getName()).log(Level.SEVERE, null, ex);        }    }    public synchronized static ConnectionManager getInstance() throws SQLException {        if (connectionManager == null) {            try {                connectionManager = new ConnectionManager();            } catch (Exception ex) {                Logger.getLogger(ConnectionManager.class.getName()).log(Level.SEVERE, null, ex);            }        }        return connectionManager;    }    public Connection getConnection() {        return connection;    }    public static void closeConnection() {        //close the connection only if there is a instance        //--> closeConnection() can always be called.         //(e.g. if non JDBC-Dao's are used  --> ConnectionManager is not instantiated)        if (connectionManager != null) {            try {                connectionManager.connection.close();            } catch (SQLExce

Answer 1

很可能你有一个SQLException你只登录的：

try { // load driver

    Class.forName(properties.getStringProperty("dbDriver")).newInstance();

    connection = DriverManager.getConnection(url, user, passwort);

} catch (SQLException ex) {

    Logger.getLogger(ConnectionManager.class.getName()).log(Level.SEVERE, null, ex);

}

没有理由在SQLException这里处理。您没有在您的 中的任何地方实现重新连接逻辑ConnectionManager，如果您没有有效的连接，您的应用程序将无法工作。您应该重新抛出它并让您的应用程序退出：

} catch (SQLException ex) {

    Logger.getLogger(ConnectionManager.class.getName()).log(Level.SEVERE, null, ex);

    throw ex;

}