读取查询（ Read Query）

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2023-12-01

以下示例将演示如何在DBUtils的帮助下使用Read查询读取记录。我们将从Employees Table中读取记录。

语法 (Syntax)

ResultSetHandler<Employee> resultHandler = new BeanHandler<Employee>(Employee.class);
Employee emp = queryRunner.query(conn, "SELECT * FROM employees WHERE first=?", resultHandler, "Sumit");

Where,

resultHandler - 将结果集映射到Employee对象的ResultSetHandler对象。
queryRunner - 从数据库中读取员工对象的QueryRunner对象。

要理解与DBUtils相关的上述概念，让我们编写一个运行读取查询的示例。要编写我们的示例，让我们创建一个示例应用程序。

步	描述
1	更新在 DBUtils - First Application章节下创建的文件MainApp.java 。
2	编译并运行应用程序，如下所述。

以下是Employee.java的内容。

public class Employee {
   private int id;
   private int age;
   private String first;
   private String last;
   public int getId() {
      return id;
   }
   public void setId(int id) {
      this.id = id;
   }
   public int getAge() {
      return age;
   }
   public void setAge(int age) {
      this.age = age;
   }
   public String getFirst() {
      return first;
   }
   public void setFirst(String first) {
      this.first = first;
   }
   public String getLast() {
      return last;
   }
   public void setLast(String last) {
      this.last = last;
   }
}

以下是MainApp.java文件的内容。

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import org.apache.commons.dbutils.DbUtils;
import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.ResultSetHandler;
import org.apache.commons.dbutils.handlers.BeanHandler;
public class MainApp {
   // JDBC driver name and database URL
   static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
   static final String DB_URL = "jdbc:mysql://localhost:3306/emp";
   //  Database credentials
   static final String USER = "root";
   static final String PASS = "admin";
   public static void main(String[] args) throws SQLException {
      Connection conn = null;
      QueryRunner queryRunner = new QueryRunner();
      //Step 1: Register JDBC driver
      DbUtils.loadDriver(JDBC_DRIVER);
      //Step 2: Open a connection
      System.out.println("Connecting to database...");
      conn = DriverManager.getConnection(DB_URL, USER, PASS);
      //Step 3: Create a ResultSet Handler to handle Employee Beans
      ResultSetHandler<Employee> resultHandler = 
         new BeanHandler<Employee>(Employee.class);
      try {
         Employee emp = queryRunner.query(conn, "SELECT * FROM employees WHERE id=?",
            resultHandler, 104);
         //Display values
         System.out.print("ID: " + emp.getId());
         System.out.print(", Age: " + emp.getAge());
         System.out.print(", First: " + emp.getFirst());
         System.out.println(", Last: " + emp.getLast());
      } finally {
         DbUtils.close(conn);
      }        
   }
}

完成源文件创建后，让我们运行应用程序。如果您的应用程序一切正常，它将打印以下消息。

ID: 104, Age: 30, First: Sohan, Last: Kumar