JDBC连接太多

Question

这里是连接方法：

private static Connection connection = null;

/**
 * Gets the database connection.
 */
public static Connection getConnection() {
    return Storage.connection;
}

/**
 * Connects to the database.
 */
public static void connectToDatabase() {
    try {
        String userName = "root";
        String password = "mysql";
        String url = "jdbc:mysql://localhost/secondsemesterprojectdanishcrowncaos";
        Class.forName("com.mysql.jdbc.Driver").newInstance();
        Storage.connection = DriverManager.getConnection(url, userName,
                password);
        Storage.connection
                .setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);
        Storage.connection.setAutoCommit(false);
    } catch (Exception e) {
        System.err.println("Cannot connect to database server." + e);
    }
}

/**
 * Terminates the database connection.
 */
public static void disconnectDatabase() {
    if (Storage.connection != null) {
        try {
            Storage.connection.close();
        } catch (Exception e) {
        }
    }
}

这是我的连接和查询：我所有的数据库连接几乎都和这个一样：

public void addLoadingDock(LoadingDock loadingDock) {
    loadingDocks.add(loadingDock);

    Storage.connectToDatabase();

    PreparedStatement s;
    try {
        String query = "INSERT INTO LoadingDocks(dock_no, active, product_type_id) VALUES (?, ?, ?);";
        s = Storage.connection.prepareStatement(query,
                Statement.RETURN_GENERATED_KEYS);
        s.setInt(1, loadingDock.getDockNo());
        s.setBoolean(2, loadingDock.getActive());
        s.setInt(3, loadingDock.getProductType().getId());
        int rows = s.executeUpdate();
        Storage.connection.commit();
        System.out.println(rows + " row(s) created in database");
        ResultSet generatedKeys;
        generatedKeys = s.getGeneratedKeys();
        if (generatedKeys.next()) {
            loadingDock.setId(generatedKeys.getInt(1));
        } else {
            throw new SQLException(
                    "Creating LoadingDock failed, no generated key obtained.");
        }
        generatedKeys.close();
        s.close();
    } catch (Exception e) {
        System.err.println("Error adding LoadingDock. Details: " + e);
        try {
            Storage.connection.rollback();
        } catch (SQLException e1) {
            System.err.println("Error on rollback. Details: " + e1);
        }
    } finally {
        Storage.disconnectDatabase();
    }
}

在某一时刻程序遇到了太多的连接问题。

请告诉我，连接出了什么问题。我没有正确关闭它们的地方？或任何其他问题。

Answer 1

有两件事突然出现在我面前……

1. 您应该避免以这种方式使用static ...
2. 在创建 Connection 之前，您永远不会检查它是否为 null，因此可能存在仍在运行的恶意连接，例如...

如果您有两段代码创建一个连接实例，而第二段是在第一个关闭之前创建的，则只有 Connection 的第二个实例被释放

Storage.connectToDatabase();
//...
Storage.connectToDatabase();
//...
Storage.disconnectDatabase();

您可以首先检查connectToDatabase 中的null，例如

public static void connectToDatabase() {
    if (Storage.connection == null) {
        try {
            String userName = "root";
            String password = "mysql";
            String url = "jdbc:mysql://localhost/secondsemesterprojectdanishcrowncaos";
            Class.forName("com.mysql.jdbc.Driver").newInstance();
            Storage.connection = DriverManager.getConnection(url, userName,
                    password);
            Storage.connection
                    .setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);
            Storage.connection.setAutoCommit(false);
        } catch (Exception e) {
            System.err.println("Cannot connect to database server." + e);
        }
    }
}

要克服static 的使用，您可以使用单例模式，例如...

public enum Storage {

    INSTANCE;

    private static Connection connection = null;

    /**
     * Gets the database connection.
     */
    public Connection getConnection() {
        return connection;
    }

    /**
     * Connects to the database.
     */
    public void connectToDatabase() {
        if (connection == null) {
            try {
                String userName = "root";
                String password = "mysql";
                String url = "jdbc:mysql://localhost/secondsemesterprojectdanishcrowncaos";
                Class.forName("com.mysql.jdbc.Driver").newInstance();
                connection = DriverManager.getConnection(url, userName,
                        password);
                connection
                        .setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);
                connection.setAutoCommit(false);
            } catch (Exception e) {
                System.err.println("Cannot connect to database server." + e);
            }
        }
    }

    /**
     * Terminates the database connection.
     */
    public void disconnectDatabase() {
        if (connection != null) {
            try {
                connection.close();
            } catch (Exception e) {
            }
        }
    }
}

这保证了Storage 的单个实例，并且如果小心操作，可以更好地管理Connection。

另一种选择是使用某种连接池，这将允许它为您管理连接...

Answer 2

使用连接池。有很多可用的，但c3p0 和bonecp 很受欢迎。连接池将允许您更轻松地重用数据库连接，这样您就不会耗尽“打开”资源的最大数量。