Is it ok to use same BasicDataSource, Connection, Statement and ResultSet Object in multiple class methods.?

问题

I have below code which uses static objects of BasicDataSource, Sql Connection, Statement and ResultSet. The code below is working fine, but i just want to know about the safety of using these kinds of coding practices. or how can i optimize the below code so that it can become more stable and can reliable.

public class Testing {
     static BasicDataSource bds = DBConnection.getInstance().getBds();
     static Connection con = null;
     static PreparedStatement stmt = null;
     static ResultSet rs = null;

    private void show() {
        try {
            con = bds.getConnection();
            stmt = con.prepareStatement("SELECT * FROM users");
            rs = stmt.executeQuery();
            if(rs.next()) {
                System.out.println(rs.getString("firstname") + " " + rs.getString("lastname"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            try {
                con.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
    private void display() {
        try {
            con = bds.getConnection();
            stmt = con.prepareStatement("SELECT * FROM agent_cities");
            rs = stmt.executeQuery();
            while(rs.next()) {
                System.out.println(rs.getString("city_name"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            try {
                con.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
    private void add() {
        try {
            con = bds.getConnection();
            stmt = con.prepareStatement("UPDATE users SET firstname = 'shsh' WHERE id = 2");
            stmt.executeUpdate();
            System.out.println("updated successfully");
        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            try {
                con.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args) {
        Testing t = new Testing();
        t.show();
        t.display();
        t.add();
    }
}

Thanks in advance. Do share your cases on which you can break above code and question about its safety.

Update : Updating only to ensure that no one should use static fields as i have used in above program because above program contains bug when deployed on dev server.

After using above code on large systems i found the bug. one month ago i had no problem with the above code and it was working fine but today i found the bug.

Bug:

After hitting my APIs 6-7 times it stopped giving response at 8th hit. i really don't know why and have no idea about loop holes present in program. But now as i have accepted the answer i changed my source code and started using try-with resources in my code and removed static fields.

But i am still curious to know about the bug that i found in the above code. that doesn't gives response and hangs after 7-8 API hits. Please share your thoughts on this. i'm using apache tomcat 8.5.32 server. Thanks in advance.

回答1:

Better use try-with-resources. This automatically closes Connection, Statement and ResultSet, even when an exception was raised, or on an inner return.

    String sql = "UPDATE users SET firstname = ? WHERE id = ?";
    try (Connection con = bds.getConnection();
            PreparedStatement stmt = con.prepareStatement()) {
        stmt.setString(1, "shsh");
        stmt.setLong(2, 2);
        stmt.executeUpdate();
        System.out.println("updated successfully");
    }

    String sql = "SELECT city_name FROM agent_cities";
    try (Connection con = bds.getConnection();
            PreparedStatement stmt = con.prepareStatement()) {
        try (ResultSet rs = stmt.executeQuery()) {
            while(rs.next()) {
                System.out.println(rs.getString("city_name"));
            }
        }
    }

This is better for garbage collection. Prevents unnice rs2, rs3. Allows multi-user concurrency, like in a server application. Calls that query themselves. And static is even more in the style of global variables.

回答2:

It is more or less ok if we are talking about such a small program. But there is no need to keep con, stmt and rs as a static variables, they can be declared inside a method. Also, you need to rewrite try catch finally blocks and close resources properly:

Connection con = null;
PreparedStatement stmt = null;
ResultSet rs = null;
try {
  // your code
} catch (SQLException e) {
  e.printStackTrace();
} finally {
  try { if (rs != null) rs.close(); } catch (Exception e) {e.printStackTrace();}
  try { if (stmt != null) stmt.close(); } catch (Exception e) {e.printStackTrace();}
  try { if (conn != null) conn.close(); } catch (Exception e) {e.printStackTrace();}
}

As a next step you can check try-with-resources construction to clean up this code.

回答3:

You should use try-with-resources to avoid any kind of connection leaks.

The following example reads the first line from a file. It uses an instance of BufferedReader to read data from the file. BufferedReader is a resource that must be closed after the program is finished with it:

static String readFirstLineFromFile(String path) throws IOException {
    try (BufferedReader br =
                   new BufferedReader(new FileReader(path))) {
        return br.readLine();
    }
}

来源：https://stackoverflow.com/questions/55023282/is-it-ok-to-use-same-basicdatasource-connection-statement-and-resultset-object