JDBC-通用查询

手动实现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.ArrayList;
import java.util.List;
import java.util.Properties;

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<T> resultList = new ArrayList<>(100);
            T object = null;
            while (resultSet.next()) {
                object = (T) ((Class) t).newInstance();
                for (int i = 0; i < rsmd.getColumnCount(); i++) {
                    String columnLabel = rsmd.getColumnLabel(i + 1);
                    Object columnValue = resultSet.getObject(columnLabel);
                    ReflectionUtils.setFieldValue(object, columnLabel, columnValue);
                }
                resultList.add(object);
            }

            System.out.println(resultList);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}