Java:how to pass value from class/bean to servlet

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栀梦 2020-12-21 09:59

i am new to java, i\'m having problem passing value from a class/bean (which are stored in arraylist) to servlet. any idea how can i achieve that? below is my code.

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难免孤独 (楼主)

2020-12-21 10:25

You don't pass something into a servlet. You just let the servlet access something.

You should get rid of that main() method and move the database interaction code into a DAO class. I'd also give the model class (with timezone and location) a more sensitive name starting with an uppercase. So all with all, you should update the code so that it look something like the following:

Model class, the Area (name it whatever you want, as long as it makes sense), it should just represent a single entity:

public class Area {
    private String location;
    private String timezone;

    public String getLocation() { return location; }
    public String getTimezone() { return timezone; }

    public void setLocation(String location) { this.location = location; }
    public void setTimezone(String timezone) { this.timezone = timezone; }
}

Basic connection manager class, the Database, here you load the driver just once and provide a getter for the connection:

public class Database {
    private String url;
    private String username;
    private String password;

    public Database(String driver, String url, String username, String password) {
        try { 
            Class.forName(driver);
        } catch (ClassNotFoundException e) {
            throw new RuntimeException("Driver class is missing in classpath", e);
        }
        this.url = url;
        this.username = username;
        this.password = password;
    }

    public Connection getConnection() {
        return DriverManager.getConnection(url, username, password);
    }
}

DAO class, the AreaDAO, here you put all DB interaction methods:

public class AreaDAO {
    private Database database;

    public AreaDAO(Database database) {
        this.database = database;
    }

    public List list() throws SQLException {
        Connection connection = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        List areas = new ArrayList();

        try {
            connection = database.getConnection();
            statement = connection.prepareStatement("SELECT location, timezone FROM userclient");
            resultSet = statement.executeQuery();
            while (resultSet.next()) {
                Area area = new Area();
                area.setLocation(resultSet.getString("location"));
                area.setTimezone(resultSet.getString("timezone"));
                areas.add(area);
            }
        } finally {
            if (resultSet != null) try { resultSet.close(); } catch (SQLException logOrIgnore) {}
            if (statement != null) try { statement.close(); } catch (SQLException logOrIgnore) {}
            if (connection != null) try { connection.close(); } catch (SQLException logOrIgnore) {}
        }

        return areas;
    }
}

Finally, in the servlet initialize the DAO once and obtain the list in the HTTP method:

public class AreaServlet extends HttpServlet {
    private AreaDAO areaDAO;

    public void init() throws ServletException {
        String driver = "com.mysql.jdbc.Driver";
        String url = "jdbc:mysql://localhost:3306/dbname";
        String username = "user";
        String password = "pass";
        Database database = new Database(driver, url, username, password);
        this.areaDAO = new AreaDAO(database);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response)  throws ServletException, IOException {
        try {
            List areas = areaDAO.list();
            request.setAttribute("areas", areas);
            request.getRequestDispatcher("/WEB-INF/areas.jsp").forward(request, response);
        } catch (SQLException e) {
            throw new ServletException("Cannot retrieve areas", e);
        }
    }
}

Map this servlet on an url-pattern of /areas in web.xml so that you can invoke it by http://example.com/contextname/areas

The /WEB-INF/areas.jsp can look like something this, assuming that you want to display the areas in a table:


        
    
            ${area.location}
            ${area.timezone}

Java:how to pass value from class/bean to servlet

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