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;}