手动实现让查询结果返回为一个对象集合
实体类
package com.jdbc;
public class User {
private Integer id;
private String name;
private Integer age;
@Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", age=" + age +
'}';
}
}
反射工具类
package com.jdbc;
import java.lang.reflect.*;
/**
* 反射工具类，访问私有变量, 获取泛型类型 Class, 提取集合中元素属性等 Utils 函数
*/
public class ReflectionUtils {
/**
* 通过反射, 获得定义 Class 时声明的父类的泛型参数的类型
* 如: public EmployeeDao extends BaseDao<Employee, String>
*/
@SuppressWarnings("unchecked")
public static Class getSuperClassGenricType(Class clazz, int index) {
Type genType = clazz.getGenericSuperclass();
if (!(genType instanceof ParameterizedType)) {
return Object.class;
}
Type[] params = ((ParameterizedType) genType).getActualTypeArguments();
if (index >= params.length || index < 0) {
return Object.class;
}
if (!(params[index] instanceof Class)) {
return Object.class;
}
return (Class) params[index];
}
/**
* 通过反射, 获得 Class 定义中声明的父类的泛型参数类型
* 如: public EmployeeDao extends BaseDao<Employee, String>
*/
@SuppressWarnings("unchecked")
public static <T> Class<T> getSuperGenericType(Class clazz) {
return getSuperClassGenricType(clazz, 0);
}
/**
* 循环向上转型, 获取对象的 DeclaredMethod
*/
public static Method getDeclaredMethod(Object object, String methodName, Class<?>[] parameterTypes) {
for (Class<?> superClass = object.getClass(); superClass != Object.class; superClass = superClass.getSuperclass()) {
try {
//superClass.getMethod(methodName, parameterTypes);
return superClass.getDeclaredMethod(methodName, parameterTypes);
} catch (NoSuchMethodException e) {
//Method 不在当前类定义, 继续向上转型
}
}
return null;
}
/**
* 使 filed 变为可访问
*/
public static void makeAccessible(Field field) {
if (!Modifier.isPublic(field.getModifiers())) {
field.setAccessible(true);
}
}
/**
* 循环向上转型, 获取对象的 DeclaredField
*/
public static Field getDeclaredField(Object object, String filedName) {
for (Class<?> superClass = object.getClass(); superClass != Object.class; superClass = superClass.getSuperclass()) {
try {
return superClass.getDeclaredField(filedName);
} catch (NoSuchFieldException e) {
//Field 不在当前类定义, 继续向上转型
}
}
return null;
}
/**
* 直接调用对象方法, 忽略修饰符(private, protected)
*/
public static Object invokeMethod(Object object, String methodName, Class<?>[] parameterTypes, Object[] parameters) throws InvocationTargetException {
Method method = getDeclaredMethod(object, methodName, parameterTypes);
if (method == null) {
throw new IllegalArgumentException("Could not find method [" + methodName + "] on target [" + object + "]");
}
method.setAccessible(true);
try {
return method.invoke(object, parameters);
} catch (IllegalAccessException e) {
e.printStackTrace();
}
return null;
}
/**
* 直接设置对象属性值, 忽略 private/protected 修饰符与 setter
*/
public static void setFieldValue(Object object, String fieldName, Object value) {
Field field = getDeclaredField(object, fieldName);
if (field == null)
throw new IllegalArgumentException("Could not find field [" + fieldName + "] on target [" + object + "]");
makeAccessible(field);
try {
field.set(object, value);
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
/**
* 直接读取对象的属性值, 忽略 private/protected 修饰符与 setter
*/
public static Object getFieldValue(Object object, String fieldName) {
Field field = getDeclaredField(object, fieldName);
if (field == null)
throw new IllegalArgumentException("Could not find field [" + fieldName + "] on target [" + object + "]");
makeAccessible(field);
Object result = null;
try {
result = field.get(object);
} catch (IllegalAccessException e) {
e.printStackTrace();
}
return result;
}
}
测试
package test.jdbc;
import com.jdbc.ReflectionUtils;
import com.jdbc.User;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import java.io.InputStream;
import java.sql.*;
import java.util.*;
public class JDBCResultObject {
private Connection conn;
private ResultSet resultSet;
@BeforeEach
public void start() throws Exception {
Properties properties = new Properties();
InputStream in = this.getClass().getClassLoader().getResourceAsStream("jdbc.properties");
properties.load(in);
String driver = properties.getProperty("driver");
String jdbcUrl = properties.getProperty("jdbcUrl");
String user = properties.getProperty("user");
String password = properties.getProperty("password");
Class.forName(driver);
conn = DriverManager.getConnection(jdbcUrl, user, password);
}
@AfterEach
public void end() throws Exception {
if (resultSet != null) {
resultSet.close();
}
if (conn != null) {
conn.close();
}
}
@Test
public void testResultSetOfObject() {
ResultSetMetaData(User.class, "SELECT * FROM user where id > ?", 0);
}
public <T> void ResultSetMetaData(T t, String sql, Object... args) {
try {
PreparedStatement preparedStatement = conn.prepareStatement(sql);
for (int i = 0; i < args.length; i++) {
preparedStatement.setObject(i + 1, args[i]);
}
resultSet = preparedStatement.executeQuery();
// 得到 ResultSetMetaData 对象
ResultSetMetaData rsmd = resultSet.getMetaData();
// 封装结果集
List<Map<String, Object>> resultList = new ArrayList<>(100);
Map<String, Object> values = null;
while (resultSet.next()) {
values = new HashMap<>(20);
for (int i = 0; i < rsmd.getColumnCount(); i++) {
String columnLabel = rsmd.getColumnLabel(i + 1);
Object columnValue = resultSet.getObject(columnLabel);
values.put(columnLabel, columnValue);
}
resultList.add(values);
}
List<T> list = new ArrayList<>(100);
T object = null;
// 结果集转为对象
for (Map<String, Object> objectMap : resultList) {
object = (T) ((Class) t).newInstance();
for (Map.Entry<String, Object> entry : objectMap.entrySet()) {
String fieldName = entry.getKey();
Object fieldValue = entry.getValue();
ReflectionUtils.setFieldValue(object, fieldName, fieldValue);
}
list.add(object);
}
System.out.println(list);
} catch (Exception e) {
e.printStackTrace();
}
}
}
原文地址：https://www.cnblogs.com/jhxxb/p/10439567.html
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