如何把ResultSet转换成Java对象

有时候我们不想使用任何框架，但又需要用JDBC实现类似于iBATIS的orm映射功能，把一个ResultSet转换成我们的JavaBeans,我们可以模仿iBATIS的方式自己写一个ResultSetMapper实现类，利用反射原理把ResultSet转换成一个JavaBeans，下面是网上的一个开源实现，主要用到的是注解和反射机制，我们先看一下使用效果：

package com.heaven.mapper;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.List;
public class SampleMain {
		public static void main(String ...args){
		try {
			ResultSetMapper<SamplePojo> resultSetMapper = new ResultSetMapper<SamplePojo>();
			ResultSet resultSet = null;
			// simple JDBC code to run SQL query and populate resultSet - START
			Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
			String database = "jdbc:odbc:AkDb";
			Connection connection = DriverManager.getConnection( database ,"","");
			PreparedStatement statement = connection.prepareStatement("SELECT * FROM UsersSample");
			resultSet = statement.executeQuery();
			/******************************/
			List<SamplePojo> pojoList = resultSetMapper.mapRersultSetToObject(resultSet, SamplePojo.class);
			/******************************/
			if(pojoList != null){
				for(SamplePojo pojo : pojoList){
					System.out.println(pojo);
				}
			}else{
				System.out.println("ResultSet is empty. Please check if database table is empty");
			}
			connection.close();
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		} catch (SQLException e) {
			e.printStackTrace();
		}

		}
}

如上图所示，把ResultSet转换成我们的JavaPojo对象只需要调用ResultSetMapper的一个方法即可完成，使用起来非常方便。

下面是ResultSetMapper源代码：

package com.heaven.mapper;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import javax.persistence.Column;
import javax.persistence.Entity;
import org.apache.commons.beanutils.BeanUtils;

public class ResultSetMapper<T> {
	@SuppressWarnings("unchecked")
	public List<T> mapRersultSetToObject(ResultSet rs, Class outputClass) {
		List<T> outputList = null;
		try {
			// make sure resultset is not null
			if (rs != null) {
				// check if outputClass has 'Entity' annotation
				if (outputClass.isAnnotationPresent(Entity.class)) {
					// get the resultset metadata
					ResultSetMetaData rsmd = rs.getMetaData();
					// get all the attributes of outputClass
					Field[] fields = outputClass.getDeclaredFields();
					while (rs.next()) {
						T bean = (T) outputClass.newInstance();
						for (int _iterator = 0; _iterator < rsmd.getColumnCount(); _iterator++) {
							// getting the SQL column name
							String columnName = rsmd.getColumnName(_iterator + 1);
							// reading the value of the SQL column
							Object columnValue = rs.getObject(_iterator + 1);
							// iterating over outputClass attributes to check if
							// any attribute has 'Column' annotation with
							// matching 'name' value
							for (Field field : fields) {
								if (field.isAnnotationPresent(Column.class)) {
									Column column = field.getAnnotation(Column.class);
									if (column.name().equalsIgnoreCase(columnName) && columnValue != null) {
										BeanUtils.setProperty(bean, field.getName(), columnValue);
										break;
									}
								}
							}
						}
						if (outputList == null) {
							outputList = new ArrayList<T>();
						}
						outputList.add(bean);
					}

				} else {
					// throw some error
				}
			} else {
				return null;
			}
		} catch (IllegalAccessException e) {
			e.printStackTrace();
		} catch (SQLException e) {
			e.printStackTrace();
		} catch (InstantiationException e) {
			e.printStackTrace();
		} catch (InvocationTargetException e) {
			e.printStackTrace();
		}
		return outputList;
	}
}

使用注解的pojo对象：

<pre name="code" class="java">package com.heaven.mapper;
import javax.persistence.Column;
import javax.persistence.Entity;
@Entity
public class SamplePojo {
	@Column(name="User_Id")
	private int id;
	@Column(name="User_Name")
	private String name;
	@Column(name="Address")
	private String address;
	@Column(name="Gender")
	private boolean gender;
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address = address;
	}
	public boolean isGender() {
		return gender;
	}
	public void setGender(boolean gender) {
		this.gender = gender;
	}

	@Override
	public String toString() {
		return 	"id: " + id + "\n" +
				"name: " + name + "\n"+
				"address: " + address + "\n" +
				"gender: " + (gender ? "Male" : "Female") + "\n\n";
	}
}