使用calcite为对象List封装SQL接口

最新推荐文章于 2025-10-13 12:51:20 发布

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本文展示了一个使用Scala编写的示例程序，该程序利用Apache Calcite实现了表连接操作。通过对比Java和Scala两种实现方式，揭示了Scala中字段默认为私有的特性，并探讨了解决方案。

编写示例程序如下，注意这是一个scala程序：

import java.sql.DriverManager
import org.apache.calcite.jdbc.CalciteConnection
import org.apache.calcite.adapter.java.ReflectiveSchema
import java.util.Properties

object CalciteTest {
  def main(args: Array[String]) = {
    Class.forName("org.apache.calcite.jdbc.Driver");
    val info = new Properties();
    info.setProperty("lex", "JAVA");
    val connection = DriverManager.getConnection("jdbc:calcite:", info);
    val calciteConnection =
      connection.asInstanceOf[CalciteConnection];
    val rootSchema = calciteConnection.getRootSchema();

    val hrs = new ReflectiveSchema(new JavaHrSchema())
    rootSchema.add("hr", hrs);
    val statement = calciteConnection.createStatement();

    val resultSet = statement.executeQuery(
      """select * from hr.emps as e 
        join hr.depts as d on e.deptno = d.deptno""");

    while (resultSet.next()) {
      (1 to resultSet.getMetaData.getColumnCount).foreach(x => print(resultSet.getObject(x) + "\t"));
      println("");
    }

    resultSet.close();
    statement.close();
    connection.close();
  }
}

其中的JavaHrSchema如下，它包含2个字段emps和deps，分别模拟2张表：

public class JavaHrSchema
{
	public static class Employee
	{
		public final int empid;
		public final String name;
		public final int deptno;

		public Employee(int empid, String name, int deptno)
		{
			this.empid = empid;
			this.name = name;
			this.deptno = deptno;
		}
	}

	public static class Department
	{
		public final String name;
		public final int deptno;

		public Department(int deptno, String name)
		{
			this.name = name;
			this.deptno = deptno;
		}
	}

	public final Employee[] emps = { new Employee(100, "bluejoe", 1),
			new Employee(200, "wzs", 2), new Employee(150, "wxz", 1) };

	public final Department[] depts = { new Department(1, "dev"),
			new Department(2, "market") };

}

运行结果如下：

100	bluejoe	1	dev	1	
200	wzs	2	market	2	
150	wxz	1	dev	1

这么臃肿的java代码确实有点丑陋，原本想把JavaHrSchema写成如下scala代码：

class HrSchema {
  val emps: Array[Employee] = Array(new Employee(100, "bluejoe", 1),
    new Employee(200, "wzs", 2),
    new Employee(150, "wxz", 1));

  val depts: Array[Department] = Array(new Department(1, "dev"),
    new Department(2, "market"));
}

class Employee(val empid: Int, val name: String, val deptno: Int) {}
class Department(val deptno: Int, val name: String) {}

结果出错了，原因是ReflectiveSchema读取不到HrSchema里面的public字段，因为scala自动把它变成了private。。。