将数据库中的数据以Java上的形式输出给用户

本文介绍了将数据库中的数据以Java上的形式输出给用户的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！问题描述 29岁程序员，3月因学历无情被辞！我最近开始学习 Java.我需要用 Java 编写一个 Web 应用程序，用户可以在其中从下拉列表的主页 html 页面上的表单中选择他需要的产品.产品列表存储在数据库的表中(使用 MySQL).然后应将所选产品写入订单历史"表中.如何从数据库中输出数据库到下拉列表?如何实现用户对所需产品的选择?我该如何开始?有没有人举个小例子? 解决方案 Vaadin这是一个完整的工作示例，使用 Java 和用户首先从下拉列表中选择一个星球产品，然后点击订购"按钮.在此数据输入区域下方，您会看到一对 Grid 小部件，作为数据库的后门窥视.左边是产品列表，没有变化.右侧是所有订单的列表，每次用户使用订单"按钮下订单时都会更新.请注意，数据库是在内存中的，不是持久的，因为这只是一个小演示.因此，每次启动应用程序时，都会从头开始重建数据库.order_ 表在每次运行时都是空的.最后，这绝不是可用于生产的代码，只是一个展示可能性的示例.刚才在拍摄该屏幕截图时，我发现了一个与根本不选择任何产品有关的错误.C'est la vie.有关详细信息，请参阅 Vaadin 手册的以下部分:按钮 –按钮小部件概览.NativeSelect —此下拉列表小部件的快速演示选择组件 — 讨论诸如 NativeSelect 之类的小部件如何在 Vaadin 中工作网格 —如何使用这个强大的数据网格小部件.主应用类基本上是样板文件，让 Vaadin 运行.重要的是中间的那对线，用于 ProductPickerLayout.package com.basilbourque.example;导入 javax.servlet.annotation.WebServlet;导入 com.vaadin.annotations.Theme;导入 com.vaadin.annotations.VaadinServletConfiguration;导入 com.vaadin.server.VaadinRequest;导入 com.vaadin.server.VaadinServlet;导入 com.vaadin.ui.*;/*** 此 UI 是应用程序入口点.UI 可以代表一个浏览器窗口*(或选项卡)或嵌入 Vaadin 应用程序的 HTML 页面的某些部分.* <p>* UI 使用 {@link #init(VaadinRequest)} 初始化.这种方法旨在* 重写以将组件添加到用户界面并初始化非组件功能.*/@Theme ( "mytheme" )公共类 MyUI 扩展了 UI {@覆盖受保护的无效初始化(VaadinRequest vaadinRequest){最终布局布局 = 新的 ProductPickerLayout();this.setContent( 布局 );}@WebServlet ( urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true )@VaadinServletConfiguration ( ui = MyUI.class, productionMode = false )公共静态类 MyUIServlet 扩展了 VaadinServlet {}}Servlet 上下文监听器对网络应用启动和退出做出反应.package com.basilbourque.example;导入 javax.servlet.ServletContextEvent;导入 javax.servlet.ServletContextListener;导入 javax.servlet.annotation.WebListener;导入 java.time.Instant;@WebListener公共类 MyServletContextListener 实现 ServletContextListener {MyDatabaseService 数据库服务；@覆盖public void contextInitialized ( ServletContextEvent servletContextEvent ) {System.out.println( "TRACE - contextInitialized " + Instant.now());//数据库.MyDatabaseService db = new MyDatabaseService();db.建立数据库()；}@覆盖public void contextDestroyed ( ServletContextEvent servletContextEvent ) {//此方法有意留空.}}数据库服务类定义并预加载数据库.提供插入和查询，将数据移入和移出数据库.package com.basilbourque.example;导入 java.sql.*;导入 java.time.Instant;导入 java.util.ArrayList;导入 java.util.List;导入 java.util.Locale;公共类 MyDatabaseService {//---------|会员|------------------------------------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";//设置延迟以保持内存数据库，即使在上次连接关闭后也是如此.static final private String productTableName = "product_";static final private String orderTableName = "order_";公共无效建立数据库(){//验证 JDBC 驱动程序.尝试 {Class.forName( MyDatabaseService.driverName );} catch ( ClassNotFoundException e ) {e.printStackTrace();}//连接，并创建数据库.尝试(连接连接 = DriverManager.getConnection(MyDatabaseService.jdbcPath)；){字符串 sql = null;//创建 product_ 表.//列:pkey_name_尝试(语句 stmt = conn.createStatement() ; ) {sql = "CREATE TABLE " + productTableName + " (" +" pkey_ IDENTITY PRIMARY KEY ," +" name_ VARCHAR ( 80 ) NOT NULL" +");";System.out.println( "TRACE - SQL:" + sql );stmt.execute(sql);}System.out.println( "TRACE - 创建的表 product_." );//创建 order_ 表.//列: pkey_ fkey_product_ when_ordered_尝试(语句 stmt = conn.createStatement() ; ) {sql = "CREATE TABLE " + orderTableName + " (" +" pkey_ IDENTITY PRIMARY KEY ," +" fkey_product_ LONG NOT NULL ," +" when_ordered_ TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP" +") ;" +"ALTER TABLE" + orderTableName + " ADD FOREIGN KEY ( fkey_product_ ) REFERENCES product_ ( pkey_ ) ; ";System.out.println( "TRACE - SQL:" + sql );stmt.execute(sql);}//列出表格DatabaseMetaData md = conn.getMetaData();尝试(结果集 rs = md.getTables(空，空，空，空)){而(rs.next()){System.out.println( rs.getString( 3 ) );}}//列出 `product_` 表的列.尝试 ( ResultSet rs = md.getColumns( null , null , productTableName.toUpperCase( Locale.US ) , null ) ) {System.out.println("表的列数:" + productTableName );而(rs.next()){System.out.println( rs.getString( 4 ) + " | " + rs.getString( 5 ) + " | " + rs.getString( 6 ) );//列名称、数据类型、类型名称.}}//列出 `order_` 表的列.尝试 ( ResultSet rs = md.getColumns( null , null , orderTableName.toUpperCase( Locale.US ) , null ) ) {System.out.println("表的列数:" + orderTableName );而(rs.next()){System.out.println( rs.getString( 4 ) + " | " + rs.getString( 5 ) + " | " + rs.getString( 6 ) );//列名称、数据类型、类型名称.}}//添加行sql =插入产品_(名称_)"+值(?)" +"; ";列表fetchAllProducts () {System.out.println( "TRACE MyDatabaseService::fetchAllOrders at " + Instant.now() );列表产品 = 新的 ArrayList();//询问.循环 ResultSet，实例化对象，并收集.尝试(连接连接 = DriverManager.getConnection(MyDatabaseService.jdbcPath)；){System.out.println( "TRACE - fetchAllProducts at " + Instant.now() );String sql = "SELECT * FROM " + productTableName + ";";尝试 (语句 stmt = conn.createStatement() ;结果集 rs = stmt.executeQuery(sql);){int rowCount = 0;而(rs.next()){Long pkey = rs.getLong("pkey_");String name = rs.getString("name_");//从列值创建对象.Product p = new Product( pkey , name );产品添加( p );//收集从数据库中检索到的每个 `Order` 对象.}}} catch ( SQLException e ) {e.printStackTrace();}退货产品；}公共列表();//询问.循环 ResultSet，实例化对象，并收集.尝试(连接连接 = DriverManager.getConnection(MyDatabaseService.jdbcPath)；){String sql = "SELECT * FROM " + orderTableName + " ORDER BY pkey_ DESC ;";尝试 (语句 stmt = conn.createStatement() ;结果集 rs = stmt.executeQuery(sql);){int rowCount = 0;而(rs.next()){Long pkey = rs.getLong("pkey_");Long fkey_product = rs.getLong( "fkey_product_" );即时 when_ordered = rs.getObject( "when_ordered_" , Instant.class );//从列值创建对象.Order o = new Order( pkey , fkey_product , when_ordered );订单.add(o);//收集从数据库中检索到的每个 `Order` 对象.}}} catch ( SQLException e ) {e.printStackTrace();}退货单；}公共无效插入订单(长 fkeyProduct){System.out.println( "TRACE - MyDatabaseService::insertOrder at " + Instant.now() );尝试(连接连接 = DriverManager.getConnection(MyDatabaseService.jdbcPath)；){String sql = "INSERT INTO " + orderTableName + "( fkey_product_ )" + " VALUES ( ? ) ;";PreparedStatement ps = conn.prepareStatement(sql);ps.setLong( 1 , fkeyProduct );ps.executeUpdate();} catch ( SQLException e ) {e.printStackTrace();}}}内容布局显示我们的小部件:NativeSelect 产品下拉列表、订购按钮和一对 Grid 数据网格.用作视图和控制器，用于与我们的用户进行交互.package com.basilbourque.example;导入 com.vaadin.data.HasValue;导入 com.vaadin.data.HasValue.ValueChangeEvent;导入 com.vaadin.ui.*;导入 java.time.Instant;导入 java.util.List;公共类 ProductPickerLayout 扩展 VerticalLayout {标签 prodPopLabel;本机选择产品选择；按钮订单按钮；网格产品网格；网格();//添加一些项目列表products = new MyDatabaseService().fetchAllProducts();this.productSelect.setItems(产品);this.productSelect.setItemCaptionGenerator(Product::getName);//如果有更多，则显示 5 个项目和一个滚动条//select.setRows( 3 );productSelect.addValueChangeListener(new HasValue.ValueChangeListener(){@覆盖public void valueChange (ValueChangeEvent< Product > valueChangeEvent ) {产品 p = valueChangeEvent.getValue();orderButton.setEnabled( null != p );Notification.show("选择:" + p.name );}});this.orderButton = new Button("订单");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(产品);this.productsGrid.setCaption("产品");this.ordersGrid = new Grid(Order.class);列表模型类 - Product &订单Vaadin 会自动检测并使用 getter/setter 访问器方法来在 Grid 数据网格小部件中显示数据.当然，您也可以改为手动配置.我们的 Product 类.package com.basilbourque.example;公共类产品{长密钥；字符串名称；公共产品(长 pkey ，字符串名称){this.pkey = pkey;this.name = 名称；}@覆盖公共字符串 toString() {返回产品{" +"pkey=" + pkey +|姓名="+姓名+" }";}//-----------|配件|-----------------公共长getPkey(){返回密钥；}公共无效setPkey(长pkey){this.pkey = pkey;}公共字符串getName(){返回名称；}public void setName(字符串名称){this.name = 名称；}}我们的Order类.package com.basilbourque.example;导入 java.time.Instant;公共类订单{长密钥；//此订单的标识符.长 fkeyProduct;//所订购产品的标识符.立即订购；//下单的那一刻.公共订单(Long pkey，Long fkeyProduct，Instant whenOrdered){this.pkey = pkey;this.fkeyProduct = fkeyProduct;this.whenOrdered = whenOrdered;}@覆盖公共字符串 toString() {返回订单{" +"pkey=" + pkey +"| fkeyProduct=" + fkeyProduct +"| whenOrdered=" + whenOrdered +" }";}//-----------|配件|-----------------公共长getPkey(){返回密钥；}公共无效setPkey(长pkey){this.pkey = pkey;}公共长getFkeyProduct(){返回 fkeyProduct;}public void setFkeyProduct (Long fkeyProduct) {this.fkeyProduct = fkeyProduct;}公共即时getWhenOrdered(){下单时退货；}public void setWhenOrdered(即时whenOrdered){this.whenOrdered = whenOrdered;}}Maven POMpom.<目标>编译主题</目标></目标></执行></执行></插件><groupId>org.apache.maven.plugins</groupId><artifactId>maven-clean-plugin</artifactId><version>3.1.0</version><配置><文件集><文件集><目录>src/main/webapp/VAADIN/themes</directory><包括><include>**/styles.css</include><include>**/styles.scss.cache</include></包括></文件集></文件集></配置></插件><groupId>org.eclipse.jetty</groupId><artifactId>jetty-maven-plugin</artifactId><version>${jetty.plugin.version}</version><配置><scanIntervalSeconds>2</scanIntervalSeconds></配置></插件></插件></build><个人资料><个人资料><id>vaadin-prerelease</id><激活><activeByDefault>false</activeByDefault></激活><存储库><存储库><id>vaadin-prereleases</id><url>http://maven.vaadin.com/vaadin-prereleases</url></repository><存储库><id>vaadin-snapshots</id><url>https://oss.sonatype.org/content/repositories/vaadin-snapshots/</url><发布><启用>假</启用></发布><快照><启用>真</启用></快照></repository></repositories><pluginRepositories><插件库><id>vaadin-prereleases</id><url>http://maven.vaadin.com/vaadin-prereleases</url></pluginRepository><插件库><id>vaadin-snapshots</id><url>https://oss.sonatype.org/content/repositories/vaadin-snapshots/</url><发布><启用>假</启用></发布><快照><启用>真</启用></快照></pluginRepository></pluginRepositories></个人资料></个人资料></项目>顺便说一下，在 macOS 世界中，下拉列表"被称为弹出菜单"或弹出菜单".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 the drop-down list. The list of products is stored in a table in the database (using MySQL). Then the selected product should be written in the "order history" table.How to output the database from the database to the drop-down list?How to realize the choice of the necessary product for user?How can I start? Have anyone got a small example? 解决方案 VaadinHere 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 widgetSelection Components — Discussion of how widgets such as NativeSelect work in VaadinGrid — How to use this powerful data-grid widget.Main app classBasically 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. * <p> * 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 listenerReacts 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;@WebListenerpublic 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 classDefines 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 + " (" + " pkey_ IDENTITY PRIMARY KEY ," + " name_ VARCHAR ( 80 ) NOT NULL" + ") ;"; System.out.println( "TRACE - SQL:" + 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 + " (" + " pkey_ IDENTITY PRIMARY KEY ," + " fkey_product_ LONG NOT NULL ," + " when_ordered_ TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP" + ") ;" + "ALTER TABLE " + orderTableName + " ADD FOREIGN KEY ( fkey_product_ ) REFERENCES product_ ( pkey_ ) ; " ; System.out.println( "TRACE - SQL:" + 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_ )" + "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 + " ORDER BY pkey_ DESC ;"; 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_ )" + " VALUES ( ? ) ;"; PreparedStatement ps = conn.prepareStatement( sql ); ps.setLong( 1 , fkeyProduct ); ps.executeUpdate(); } catch ( SQLException e ) { e.printStackTrace(); } }}Content layoutDisplays 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 & OrderThe 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 POMThe pom.xml file, controlling Maven for loading dependencies (libraries) and for building/running the web app.<?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>com.basilbourque.example</groupId> <artifactId>prodpop</artifactId> <packaging>war</packaging> <version>1.0-SNAPSHOT</version> <name>prodpop</name> <prerequisites> <maven>3</maven> </prerequisites> <properties> <vaadin.version>8.5.2</vaadin.version> <vaadin.plugin.version>8.5.2</vaadin.plugin.version> <jetty.plugin.version>9.4.12.v20180830</jetty.plugin.version> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> <maven.compiler.source>10</maven.compiler.source> <maven.compiler.target>10</maven.compiler.target>  <vaadin.widgetset.mode>local</vaadin.widgetset.mode> </properties> <repositories> <repository> <id>vaadin-addons</id> <url>http://maven.vaadin.com/vaadin-addons</url> </repository> </repositories> <dependencyManagement> <dependencies> <dependency> <groupId>com.vaadin</groupId> <artifactId>vaadin-bom</artifactId> <version>${vaadin.version}</version> <type>pom</type> <scope>import</scope> </dependency> </dependencies> </dependencyManagement> <dependencies> <dependency> <groupId>javax.servlet</groupId> <artifactId>javax.servlet-api</artifactId> <version>3.1.0</version> <scope>provided</scope> </dependency> <dependency> <groupId>com.vaadin</groupId> <artifactId>vaadin-server</artifactId> </dependency> <dependency> <groupId>com.vaadin</groupId> <artifactId>vaadin-push</artifactId> </dependency> <dependency> <groupId>com.vaadin</groupId> <artifactId>vaadin-client-compiled</artifactId> </dependency> <dependency> <groupId>com.vaadin</groupId> <artifactId>vaadin-themes</artifactId> </dependency>  <dependency> <groupId>com.h2database</groupId> <artifactId>h2</artifactId> <version>1.4.197</version> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-war-plugin</artifactId> <version>3.2.2</version> <configuration> <failOnMissingWebXml>false</failOnMissingWebXml>  <packagingExcludes>WEB-INF/classes/VAADIN/widgetsets/WEB-INF/**</packagingExcludes> </configuration> </plugin> <plugin> <groupId>com.vaadin</groupId> <artifactId>vaadin-maven-plugin</artifactId> <version>${vaadin.plugin.version}</version> <executions> <execution> <goals> <goal>update-theme</goal> <goal>update-widgetset</goal> <goal>compile</goal>  <goal>compile-theme</goal> </goals> </execution> </executions> </plugin> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-clean-plugin</artifactId> <version>3.1.0</version>  <configuration> <filesets> <fileset> <directory>src/main/webapp/VAADIN/themes</directory> <includes> <include>**/styles.css</include> <include>**/styles.scss.cache</include> </includes> </fileset> </filesets> </configuration> </plugin>  <plugin> <groupId>org.eclipse.jetty</groupId> <artifactId>jetty-maven-plugin</artifactId> <version>${jetty.plugin.version}</version> <configuration> <scanIntervalSeconds>2</scanIntervalSeconds> </configuration> </plugin> </plugins> </build> <profiles> <profile>  <id>vaadin-prerelease</id> <activation> <activeByDefault>false</activeByDefault> </activation> <repositories> <repository> <id>vaadin-prereleases</id> <url>http://maven.vaadin.com/vaadin-prereleases</url> </repository> <repository> <id>vaadin-snapshots</id> <url>https://oss.sonatype.org/content/repositories/vaadin-snapshots/</url> <releases> <enabled>false</enabled> </releases> <snapshots> <enabled>true</enabled> </snapshots> </repository> </repositories> <pluginRepositories> <pluginRepository> <id>vaadin-prereleases</id> <url>http://maven.vaadin.com/vaadin-prereleases</url> </pluginRepository> <pluginRepository> <id>vaadin-snapshots</id> <url>https://oss.sonatype.org/content/repositories/vaadin-snapshots/</url> <releases> <enabled>false</enabled> </releases> <snapshots> <enabled>true</enabled> </snapshots> </pluginRepository> </pluginRepositories> </profile> </profiles></project>By the way, in the macOS world, a "drop-down list" is known as a "pop-up menu" or a "popup". 这篇关于将数据库中的数据以Java上的形式输出给用户的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！上岸，阿里云！

VaAdin

将数据库中的数据以Java上的形式输出给用户