Output the data from the database to the user in the form on Java

Question

I am recently start learning Java. I need to write a web-application in Java, where the user can select the product necessary for him from the form on the home html-page from t

Answer 1

Vaadin

Here is a complete working example using Java with the Vaadin Framework version 8.5.2 to create a web app where an database run by H2 Database Engine tracks a list of products (10 planets of the solar system). A NativeSelect widget in Vaadin is populated from a List of Product objects loaded from a product_ table in that database. Each time the user clicks the Order button, an order is recorded as a row in the order_ table.

Here is a simple ERD diagram of the two tables in the database.

The user first selects a planet product from the drop-down list, then clicks the “Order” button.

Below this data-entry area you see pair of Grid widgets as a back-door peek into the database. On the left is the list of products, which does not change. On the right is a list of all orders, which updates every time the user places an order with that “Order” button.

Note that the database is in-memory, not persistent, as this is just a little demo. So every time you launch the app, the database is rebuilt from scratch. The order_ table starts out empty each run.

Lastly, this is by no means production-ready code, just an example to show possibilities. Just now while shooting that screenshot I found a bug related to choosing no product at all. C’est la vie.

For more info, see these sections of the Vaadin manual:

• Button – Overview of the button widget.
• NativeSelect — Quick demo of this drop-down list widget
• Selection Components — Discussion of how widgets such as NativeSelect work in Vaadin
• Grid — How to use this powerful data-grid widget.

Main app class

Basically boilerplate, to make Vaadin run. What matters is the pair of lines in the middle, for ProductPickerLayout.

package com.basilbourque.example;

import javax.servlet.annotation.WebServlet;

import com.vaadin.annotations.Theme;
import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinServlet;
import com.vaadin.ui.*;

/**
 * This UI is the application entry point. A UI may either represent a browser window
 * (or tab) or some part of an HTML page where a Vaadin application is embedded.
 * 

 * The UI is initialized using {@link #init(VaadinRequest)}. This method is intended to be
 * overridden to add component to the user interface and initialize non-component functionality.
 */
@Theme ( "mytheme" )
public class MyUI extends UI {

    @Override
    protected void init ( VaadinRequest vaadinRequest ) {
        final Layout layout = new ProductPickerLayout();
        this.setContent( layout );
    }

    @WebServlet ( urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true )
    @VaadinServletConfiguration ( ui = MyUI.class, productionMode = false )
    public static class MyUIServlet extends VaadinServlet {
    }
}

Servlet context listener

Reacts to web app launching and exiting.

package com.basilbourque.example;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import java.time.Instant;

@WebListener
public class MyServletContextListener implements ServletContextListener {
    MyDatabaseService databaseService;

    @Override
    public void contextInitialized ( ServletContextEvent servletContextEvent ) {
        System.out.println( "TRACE - contextInitialized " + Instant.now() );

        // Database.
        MyDatabaseService db = new MyDatabaseService();
        db.establishDatabase();
    }

    @Override
    public void contextDestroyed ( ServletContextEvent servletContextEvent ) {
        // This method intentionally left blank.
    }
}

Database service class

Defines and preloads the database. Provides inserts and queries, to move data in and out of database.

package com.basilbourque.example;

import java.sql.*;
import java.time.Instant;
import java.util.ArrayList;
import java.util.List;
import java.util.Locale;

public class MyDatabaseService {
    // ---------|  Members  |------------------------------------
    static final private String driverName = "org.h2.Driver";
    static final private String catalogName = "ProdPop";
    static final private String jdbcPath = "jdbc:h2:mem:" + MyDatabaseService.catalogName + ";DB_CLOSE_DELAY=-1";  // "jdbc:h2:mem:autogrid";  // Set delay to keep in-memory database even after last connection closed.
    static final private String productTableName = "product_";
    static final private String orderTableName = "order_";

    public void establishDatabase () {
        // Verify JDBC driver.
        try {
            Class.forName( MyDatabaseService.driverName );
        } catch ( ClassNotFoundException e ) {
            e.printStackTrace();
        }

        // Connect, and create database.
        try ( Connection conn = DriverManager.getConnection( MyDatabaseService.jdbcPath ) ;
        ) {
            String sql = null;

            // Create product_ table.
            // Columns: pkey_  name_
            try ( Statement stmt = conn.createStatement() ; ) {
                sql = "CREATE TABLE " + productTableName + " ( \n" +
                      " pkey_ IDENTITY PRIMARY KEY , \n" +
                      " name_ VARCHAR ( 80 ) NOT NULL \n" +
                      ") ; \n";
                System.out.println( "TRACE - SQL:\n" + sql );
                stmt.execute( sql );
            }
            System.out.println( "TRACE - Created table product_." );

            // Create order_ table.
            // Columns: pkey_  fkey_product_  when_ordered_
            try ( Statement stmt = conn.createStatement() ; ) {
                sql = "CREATE TABLE " + orderTableName + " ( \n" +
                      "  pkey_ IDENTITY PRIMARY KEY  , \n" +
                      "  fkey_product_ LONG NOT NULL , \n" +
                      "  when_ordered_  TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP  \n" +
                      ") ; \n" +
                      "ALTER TABLE " + orderTableName + " ADD FOREIGN KEY ( fkey_product_ ) REFERENCES product_ ( pkey_ ) ; \n "
                ;
                System.out.println( "TRACE - SQL:\n" + sql );
                stmt.execute( sql );
            }

            // List tables
            DatabaseMetaData md = conn.getMetaData();
            try ( ResultSet rs = md.getTables( null , null , null , null ) ) {
                while ( rs.next() ) {
                    System.out.println( rs.getString( 3 ) );
                }
            }

            // List columns of `product_` table.
            try ( ResultSet rs = md.getColumns( null , null , productTableName.toUpperCase( Locale.US ) , null ) ) {
                System.out.println( "Columns of table: " + productTableName );
                while ( rs.next() ) {
                    System.out.println( rs.getString( 4 ) + " | " + rs.getString( 5 ) + " | " + rs.getString( 6 ) ); // COLUMN_NAME, DATA_TYPE , TYPE_NAME.
                }
            }

            // List columns of `order_` table.
            try ( ResultSet rs = md.getColumns( null , null , orderTableName.toUpperCase( Locale.US ) , null ) ) {
                System.out.println( "Columns of table: " + orderTableName );
                while ( rs.next() ) {
                    System.out.println( rs.getString( 4 ) + " | " + rs.getString( 5 ) + " | " + rs.getString( 6 ) ); // COLUMN_NAME, DATA_TYPE , TYPE_NAME.
                }
            }

            // Add rows
            sql = "INSERT INTO product_ ( name_ ) \n" +
                  "VALUES ( ? ) " +
                  "; ";

            List< String > planets = List.of( "Mercury" , "Venus" , "Earth" , "Mars" , "Ceres" , "Jupiter" , "Saturn" , "Uranus" , "Neptune" , "Pluto" );
            try ( PreparedStatement ps = conn.prepareStatement( sql ) ; ) {
                for ( String planet : planets ) {
                    ps.setString( 1 , planet );
                    ps.executeUpdate();
                }
            }

            System.out.println( "TRACE - Dumping tables in their initial state. " + Instant.now() );
            this.dumpTableToConsole( MyDatabaseService.productTableName );
            this.dumpTableToConsole( MyDatabaseService.orderTableName );
        } catch ( SQLException e ) {
            e.printStackTrace();
        }
    }

    public void dumpTableToConsole ( String tableName ) {

        try ( Connection conn = DriverManager.getConnection( MyDatabaseService.jdbcPath ) ;
        ) {
            System.out.println( "TRACE - « " + tableName + " » table dump to console at " + Instant.now() );
            String sql = "SELECT * FROM " + tableName + " ;";
            try ( Statement stmt = conn.createStatement() ;
                  ResultSet rs = stmt.executeQuery( sql ) ; ) {
                ResultSetMetaData meta = rs.getMetaData();
                int colCount = meta.getColumnCount();
                int rowCount = 0;
                while ( rs.next() ) {
                    rowCount++;
                    System.out.print( "Row # " + rowCount + ": " );
                    for ( int col = 1 ; col <= colCount ; col++ ) {
                        System.out.print( meta.getColumnLabel( col ) + "=" );
                        Object o = rs.getObject( col );
                        if ( null != o ) {
                            System.out.print( o.toString() + " " );
                        }
                    }
                    System.out.println( "" ); // Newline.
                }
                System.out.println( "« fin de " + tableName + " »" );
            }
        } catch ( SQLException e ) {
            e.printStackTrace();
        }
    }

    public List< Product > fetchAllProducts () {
        System.out.println( "TRACE MyDatabaseService::fetchAllOrders at " + Instant.now() );

        List< Product > products = new ArrayList<>();

        // Query. Loop ResultSet, instantiating object, and collecting.
        try ( Connection conn = DriverManager.getConnection( MyDatabaseService.jdbcPath ) ;
        ) {
            System.out.println( "TRACE - fetchAllProducts at " + Instant.now() );
            String sql = "SELECT * FROM " + productTableName + " ;";
            try (
            Statement stmt = conn.createStatement() ;
            ResultSet rs = stmt.executeQuery( sql ) ;
            ) {
                int rowCount = 0;
                while ( rs.next() ) {
                    Long pkey = rs.getLong( "pkey_" );
                    String name = rs.getString( "name_" );
                    // Make object from column values.
                    Product p = new Product( pkey , name );
                    products.add( p ); // Collect each `Order` object retrieved from database.
                }
            }
        } catch ( SQLException e ) {
            e.printStackTrace();
        }

        return products;
    }

    public List< Order > fetchAllOrders () {
        System.out.println( "TRACE MyDatabaseService::fetchAllOrders at " + Instant.now() );

        List< Order > orders = new ArrayList<>();

        // Query. Loop ResultSet, instantiating object, and collecting.
        try ( Connection conn = DriverManager.getConnection( MyDatabaseService.jdbcPath ) ;
        ) {
            String sql = "SELECT * FROM " + orderTableName + " \n ORDER BY pkey_ DESC \n ;";
            try (
            Statement stmt = conn.createStatement() ;
            ResultSet rs = stmt.executeQuery( sql ) ;
            ) {
                int rowCount = 0;
                while ( rs.next() ) {
                    Long pkey = rs.getLong( "pkey_" );
                    Long fkey_product = rs.getLong( "fkey_product_" );
                    Instant when_ordered = rs.getObject( "when_ordered_" , Instant.class );
                    // Make object from column values.
                    Order o = new Order( pkey , fkey_product , when_ordered );
                    orders.add( o ); // Collect each `Order` object retrieved from database.
                }
            }
        } catch ( SQLException e ) {
            e.printStackTrace();
        }

        return orders;
    }

    public void insertOrder ( Long fkeyProduct ) {
        System.out.println( "TRACE - MyDatabaseService::insertOrder at " + Instant.now() );
        try ( Connection conn = DriverManager.getConnection( MyDatabaseService.jdbcPath ) ;
        ) {
            String sql = "INSERT INTO " + orderTableName + "( fkey_product_ )\n" + " VALUES ( ? ) ;\n";
            PreparedStatement ps = conn.prepareStatement( sql );
            ps.setLong( 1 , fkeyProduct );
            ps.executeUpdate();
        } catch ( SQLException e ) {
            e.printStackTrace();
        }
    }
}

Content layout

Displays our widgets: the NativeSelect drop-down list of products, order button, and pair of Grid data-grids. Serves as the view and the controller, for interacting with our user.

package com.basilbourque.example;

import com.vaadin.data.HasValue;
import com.vaadin.data.HasValue.ValueChangeEvent;
import com.vaadin.ui.*;

import java.time.Instant;
import java.util.List;

public class ProductPickerLayout extends VerticalLayout {
    Label prodPopLabel;
    NativeSelect< Product > productSelect;
    Button orderButton;

    Grid< Product > productsGrid;
    Grid< Order > ordersGrid;

    // Constructor
    public ProductPickerLayout () {
        this.widgetsMake();
        this.widgetsArrange();
    }

    private void widgetsMake () {

        // Create the selection component
        this.prodPopLabel = new Label( "Products: " );
        this.productSelect = new NativeSelect<>();
        // Add some items
        List< Product > products = new MyDatabaseService().fetchAllProducts();
        this.productSelect.setItems( products );
        this.productSelect.setItemCaptionGenerator( Product :: getName );
        // Show 5 items and a scrollbar if there are more
//        select.setRows( 3 );

        productSelect.addValueChangeListener( new HasValue.ValueChangeListener< Product >() {
            @Override
            public void valueChange ( ValueChangeEvent< Product > valueChangeEvent ) {
                Product p = valueChangeEvent.getValue();
                orderButton.setEnabled( null != p );
                Notification.show( "Selected: " + p.name );
            }
        } );

        this.orderButton = new Button( "Order" );
        this.orderButton.setEnabled( this.productSelect.getValue() != null );
        this.orderButton.addClickListener( ( Button.ClickEvent e ) -> {
            this.placeOrder();
            this.ordersGrid.setItems( new MyDatabaseService().fetchAllOrders() );
        } );

        MyDatabaseService db = new MyDatabaseService();

        this.productsGrid = new Grid<>( Product.class );
        this.productsGrid.setItems( products );
        this.productsGrid.setCaption( "Products" );

        this.ordersGrid = new Grid<>( Order.class );
        List< Order > orders = db.fetchAllOrders();
        this.ordersGrid.setItems( orders );
        this.ordersGrid.setCaption( "Orders" );
    }

    private void widgetsArrange () {
        HorizontalLayout orderBar = new HorizontalLayout();
        orderBar.setSpacing( true );
        orderBar.addComponents( this.prodPopLabel , this.productSelect , this.orderButton );
        orderBar.setComponentAlignment( this.prodPopLabel , Alignment.MIDDLE_CENTER );
        orderBar.setComponentAlignment( this.productSelect , Alignment.MIDDLE_CENTER );
        orderBar.setComponentAlignment( this.orderButton , Alignment.MIDDLE_CENTER );

        HorizontalLayout gridsBar = new HorizontalLayout();
        gridsBar.setSpacing( true );
        gridsBar.addComponents( this.productsGrid , this.ordersGrid );

        this.addComponents( orderBar , gridsBar );
        this.setExpandRatio( gridsBar , 1.0F );
    }

    private void placeOrder () {
        // Get pkey of the currently selected product.

        Product p = this.productSelect.getValue();
        if ( null == p ) {
            throw new IllegalStateException( "The `productSelect` NativeSelect does not have a current value." );
        }
        Long fkeyProduct = p.pkey;

        // Insert row into table.
        new MyDatabaseService().insertOrder( fkeyProduct );

        this.updateOrdersGrid();
    }

    private void updateOrdersGrid () {
    }
}

Model classes – Product & Order

The getter/setter accessor methods are automatically detected and used by Vaadin to show data in the Grid data-grid widgets. Of course, you could instead do manual configuration.

Our Product class.

package com.basilbourque.example;

public class Product {
    Long pkey;
    String name;

    public Product ( Long pkey , String name ) {
        this.pkey = pkey;
        this.name = name;
    }

    @Override
    public String toString () {
        return "Product{ " +
               "pkey=" + pkey +
               "| name='" + name +
               " }";
    }


    // -----------|  Accessors  |-----------------

    public Long getPkey () {
        return pkey;
    }

    public void setPkey ( Long pkey ) {
        this.pkey = pkey;
    }

    public String getName () {
        return name;
    }

    public void setName ( String name ) {
        this.name = name;
    }
}

Our Order class.

package com.basilbourque.example;

import java.time.Instant;

public class Order {
    Long pkey;  // Identifier of this order.
    Long fkeyProduct; // Identifier of the product being ordered.
    Instant whenOrdered; // The moment the order was placed.

    public Order ( Long pkey , Long fkeyProduct , Instant whenOrdered ) {
        this.pkey = pkey;
        this.fkeyProduct = fkeyProduct;
        this.whenOrdered = whenOrdered;
    }

    @Override
    public String toString () {
        return "Order{ " +
               "pkey=" + pkey +
               "| fkeyProduct=" + fkeyProduct +
               "| whenOrdered=" + whenOrdered +
               " }";
    }

    // -----------|  Accessors  |-----------------

    public Long getPkey () {
        return pkey;
    }

    public void setPkey ( Long pkey ) {
        this.pkey = pkey;
    }

    public Long getFkeyProduct () {
        return fkeyProduct;
    }

    public void setFkeyProduct ( Long fkeyProduct ) {
        this.fkeyProduct = fkeyProduct;
    }

    public Instant getWhenOrdered () {
        return whenOrdered;
    }

    public void setWhenOrdered ( Instant whenOrdered ) {
        this.whenOrdered = whenOrdered;
    }
}

Maven POM

The pom.xml file, controlling Maven for loading dependencies (libraries) and for building/running the web app.

    4.0.0

    com.basilbourque.example
    prodpop
    war
    1.0-SNAPSHOT
    prodpop

    
        3
    

    
        8.5.2
        8.5.2
        9.4.12.v20180830
        UTF-8
        10
        10
        
        local
    

    
        
            vaadin-addons
            http://maven.vaadin.com/vaadin-addons
        
    

    
        
            
                com.vaadin
                vaadin-bom
                ${vaadin.version}
                pom
                import
            
        
    

    
        
            javax.servlet
            javax.servlet-api
            3.1.0
            provided
        
        
            com.vaadin
            vaadin-server
        
        
            com.vaadin
            vaadin-push
        
        
            com.vaadin
            vaadin-client-compiled
        
        
            com.vaadin
            vaadin-themes
        

        
        
            com.h2database
            h2
            1.4.197
        

    

    
        
            
                org.apache.maven.plugins
                maven-war-plugin
                3.2.2
                
                    false
                    
                    WEB-INF/classes/VAADIN/widgetsets/WEB-INF/**
                
            
            
                com.vaadin
                vaadin-maven-plugin
                ${vaadin.plugin.version}
                
                    
                        
                            update-theme
                            update-widgetset
                            compile
                            
                            compile-theme
                        
                    
                
            
            
                org.apache.maven.plugins
                maven-clean-plugin
                3.1.0
                
                
                    
                        
                            src/main/webapp/VAADIN/themes
                            
                                **/styles.css
                                **/styles.scss.cache
                            
                        
                    
                
            

            
            
                org.eclipse.jetty
                jetty-maven-plugin
                ${jetty.plugin.version}
                
                    2
                
            
        
    

    
        
            
            vaadin-prerelease
            
                false
            

            
                
                    vaadin-prereleases
                    http://maven.vaadin.com/vaadin-prereleases
                
                
                    vaadin-snapshots
                    https://oss.sonatype.org/content/repositories/vaadin-snapshots/
                    
                        false
                    
                    
                        true
                    
                
            
            
                
                    vaadin-prereleases
                    http://maven.vaadin.com/vaadin-prereleases
                
                
                    vaadin-snapshots
                    https://oss.sonatype.org/content/repositories/vaadin-snapshots/
                    
                        false
                    
                    
                        true
                    
                
            
        
    

---

By the way, in the macOS world, a “drop-down list” is known as a “pop-up menu” or a “popup”.