java之手搓简单ORM框架--SQL的SELECT

9--1 / 2023-08-14 / 原文

1.手搓简单SQL增删改查框架-查询(id)

1.1创建简单类，并使用泛型类，这里可能使用到之间写的三篇知识的内容，如果不了解的小伙伴可以去

java高级之泛型

java高级之映射

java高级之反射

当然，前提是必须要把数据库相关连接弄好，这里会专门出一篇 java之jdbc

现在咱们继续手搓框架开始叭！

1.2 前期的准备工作

这里已经在【手搓简单SQL增删改查框架-插入】的时候准备完成，如果没有准备的小伙伴可以去瞅瞅

并且这次依然延续用前两篇文章的模板，及dao、userdao、自定义接口、测试类文件

1.3 手搓！！！

这里依然首先要获取通过子类获取父类的泛型反射类

public class CurrencyTest<T> {
    private Class CurrencyTest;
    public CurrencyTest(){
        Class<? extends com.xw.exercise.CurrencyTest> aClass = this.getClass();
        ParameterizedType genericSuperclass = (ParameterizedType)aClass.getGenericSuperclass();
        Type[] actualTypeArguments = genericSuperclass.getActualTypeArguments();
        CurrencyTest = (Class) actualTypeArguments[0];
    }
}

有了反射类剩下一切都好说

public T seleID(Object id) throws Exception{
    StringBuffer sele = new StringBuffer("select * from ");
    String simpleName = CurrencyTest.getSimpleName();
    TableName annotation = (TableName)CurrencyTest.getAnnotation(TableName.class);
    if(annotation != null){
        simpleName = annotation.value();
    }
    sele.append(simpleName + " where ");
    Field[] declaredFields = CurrencyTest.getDeclaredFields();
    for (Field declaredField : declaredFields) {
        declaredField.setAccessible(true);
        String name = declaredField.getName();
        TableId annotation1 = declaredField.getAnnotation(TableId.class);
        if(annotation1 != null){
            name = annotation1.value();
            sele.append(name + " = " + "'" + id + "'");
        }
        break;
    }
    Connection connection = DButils.getConnection();
    PreparedStatement preparedStatement = connection.prepareStatement(sele.toString());
    ResultSet resultSet = preparedStatement.executeQuery();
    //循环读取每一行结果
    while (resultSet.next()){
        //通过反射new类对象
        T t = (T)CurrencyTest.newInstance();
        //获取所有属性对象并循环
        for (Field declaredField : declaredFields) {
            //暴力反射，授权访问私有属性对象
            declaredField.setAccessible(true);
            //获取属性对象名称
            String name = declaredField.getName();
            //获取自定义注解
            TableId annotation1 = declaredField.getAnnotation(TableId.class);
            //判断属性对象名称是否与表名一致，判断id
            if(annotation1 != null){
                name = annotation1.value();
            }
            TableFiele annotation2 = declaredField.getAnnotation(TableFiele.class);
            //判断属性对象名称是否与表名一致，判断属性对像
            if(annotation2 != null){
                name = annotation2.value();
            }
            //获取最终的属性对象
            Object v = resultSet.getObject(name);
            //将结果装入
            declaredField.set(t,v);
        }
        return t;
    }
    return null;
}

2.0 测试

public static void main(String[] args) throws Exception {
    studentdao studentdao = new studentdao();
        TblStudent tblStudent = studentdao.seleID(5);
        System.out.println(tblStudent);
}

2.1 测试结果：

TblStudent(id=5, sname=赵文帆, sex=男, cid=1)

当然，依次也可以延伸出查询所有的通用语句

3.0 手搓简单SQL增删改查框架-查询(All)

//通用的查询方法（查询所有）
public List<T> seleAll() throws Exception{
    StringBuffer seleall = new StringBuffer("select * from ");
    String simpleName = currency.getSimpleName();
    TableName annotation = (TableName) currency.getAnnotation(TableName.class);
    if(annotation != null){
        simpleName = annotation.value();
    }
    seleall.append(simpleName);
    Connection connection = DButils.getConnection();
    PreparedStatement preparedStatement = connection.prepareStatement(seleall.toString());
    ResultSet resultSet = preparedStatement.executeQuery();
    ArrayList<T> ts = new ArrayList<>();
    Field[] declaredFields = currency.getDeclaredFields();
    while (resultSet.next()){
        T K = (T) currency.newInstance();
        for (Field declaredField : declaredFields) {
            declaredField.setAccessible(true);
            String name = declaredField.getName();
            TableId annotation1 = declaredField.getAnnotation(TableId.class);
            if(annotation1 != null){
                name = annotation1.value();
            }
            TableFiele annotation2 = declaredField.getAnnotation(TableFiele.class);
            if(annotation2 != null){
                name = annotation2.value();
            }
            Object v = resultSet.getObject(name);
            declaredField.set(K,v);
        }
        ts.add(K);
    }
    return ts;
}

这里无非是返回类型变成了List，加了一个ArrayList，其他基本没有变

3.1 测试

public static void main(String[] args) throws Exception {
     studentdao studentdao = new studentdao();
        List<TblStudent> tblStudents = studentdao.seleAll();
        System.out.println(tblStudents);
}

3.1.1 测试结果：

[TblStudent(id=1, sname=王轩, sex=男, cid=2), TblStudent(id=2, sname=张家豪, sex=男, cid=3), TblStudent(id=3, sname=吴军燕, sex=女, cid=1), TblStudent(id=4, sname=罗丽莉, sex=女, cid=4), TblStudent(id=5, sname=赵文帆, sex=男, cid=1)]

数据库：