java 实体类中属性增删查改的泛型写法

Java 实体类中属性增删查改的泛型写法可以使用 Java 的反射机制来实现。具体实现步骤如下：

1. 定义一个通用的泛型 DAO 接口，包含增删查改的方法：

public interface BaseDao<T> {
    boolean insert(T entity);
    boolean delete(T entity);
    boolean update(T entity);
    T findById(Serializable id);
    List<T> findAll();
}

2. 实现这个接口，并使用反射机制实现增删查改的方法：

public class BaseDaoImpl<T> implements BaseDao<T> {
    private Class<T> entityClass;
    private String tableName;

    public BaseDaoImpl(Class<T> entityClass) {
        this.entityClass = entityClass;
        this.tableName = entityClass.getSimpleName();
    }

    @Override
    public boolean insert(T entity) {
        // 使用反射机制获取实体类的属性列表
        Field[] fields = entityClass.getDeclaredFields();
        StringBuilder sb = new StringBuilder();
        sb.append("INSERT INTO ").append(tableName).append("(");

        // 构建 SQL 语句
        for (Field field : fields) {
            sb.append(field.getName()).append(",");
        }
        sb.deleteCharAt(sb.length() - 1);
        sb.append(") VALUES (");
        for (int i = 0; i < fields.length; i++) {
            sb.append("?,");
        }
        sb.deleteCharAt(sb.length() - 1);
        sb.append(")");

        // 执行 SQL 语句
        try (Connection conn = getConnection();
             PreparedStatement ps = conn.prepareStatement(sb.toString())) {
            for (int i = 0; i < fields.length; i++) {
                fields[i].setAccessible(true);
                ps.setObject(i + 1, fields[i].get(entity));
            }
            return ps.executeUpdate() > 0;
        } catch (Exception e) {
            e.printStackTrace();
            return false;
        }
    }

    @Override
    public boolean delete(T entity) {
        // 使用反射机制获取实体类的主键属性
        Field idField = getIdField();
        StringBuilder sb = new StringBuilder();
        sb.append("DELETE FROM ").append(tableName).append(" WHERE ")
                .append(idField.getName()).append(" = ?");

        // 执行 SQL 语句
        try (Connection conn = getConnection();
             PreparedStatement ps = conn.prepareStatement(sb.toString())) {
            idField.setAccessible(true);
            ps.setObject(1, idField.get(entity));
            return ps.executeUpdate() > 0;
        } catch (Exception e) {
            e.printStackTrace();
            return false;
        }
    }

    @Override
    public boolean update(T entity) {
        // 使用反射机制获取实体类的属性列表和主键属性
        Field[] fields = entityClass.getDeclaredFields();
        Field idField = getIdField();
        StringBuilder sb = new StringBuilder();
        sb.append("UPDATE ").append(tableName).append(" SET ");

        // 构建 SQL 语句
        for (Field field : fields) {
            if (!field.equals(idField)) {
                sb.append(field.getName()).append(" = ?,");
            }
        }
        sb.deleteCharAt(sb.length() - 1);
        sb.append(" WHERE ").append(idField.getName()).append(" = ?");

        // 执行 SQL 语句
        try (Connection conn = getConnection();
             PreparedStatement ps = conn.prepareStatement(sb.toString())) {
            int paramIndex = 1;
            for (Field field : fields) {
                if (!field.equals(idField)) {
                    field.setAccessible(true);
                    ps.setObject(paramIndex++, field.get(entity));
                }
            }
            idField.setAccessible(true);
            ps.setObject(paramIndex, idField.get(entity));
            return ps.executeUpdate() > 0;
        } catch (Exception e) {
            e.printStackTrace();
            return false;
        }
    }

    @Override
    public T findById(Serializable id) {
        // 使用反射机制获取实体类的主键属性
        Field idField = getIdField();
        StringBuilder sb = new StringBuilder();
        sb.append("SELECT * FROM ").append(tableName)
                .append(" WHERE ").append(idField.getName()).append(" = ?");

        // 执行 SQL 语句
        try (Connection conn = getConnection();
             PreparedStatement ps = conn.prepareStatement(sb.toString())) {
            ps.setObject(1, id);
            try (ResultSet rs = ps.executeQuery()) {
                if (rs.next()) {
                    // 创建实体类对象
                    T entity = entityClass.newInstance();
                    // 给实体类对象的属性赋值
                    for (Field field : entityClass.getDeclaredFields()) {
                        field.setAccessible(true);
                        field.set(entity, rs.getObject(field.getName()));
                    }
                    return entity;
                } else {
                    return null;
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

    @Override
    public List<T> findAll() {
        StringBuilder sb = new StringBuilder();
        sb.append("SELECT * FROM ").append(tableName);

        // 执行 SQL 语句
        try (Connection conn = getConnection();
             PreparedStatement ps = conn.prepareStatement(sb.toString())) {
            try (ResultSet rs = ps.executeQuery()) {
                List<T> list = new ArrayList<>();
                while (rs.next()) {
                    // 创建实体类对象
                    T entity = entityClass.newInstance();
                    // 给实体类对象的属性赋值
                    for (Field field : entityClass.getDeclaredFields()) {
                        field.setAccessible(true);
                        field.set(entity, rs.getObject(field.getName()));
                    }
                    list.add(entity);
                }
                return list;
            }
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

    /**
     * 获取实体类的主键属性
     */
    private Field getIdField() {
        for (Field field : entityClass.getDeclaredFields()) {
            if (field.isAnnotationPresent(Id.class)) {
                return field;
            }
        }
        throw new RuntimeException("No @Id field found in " + entityClass.getName());
    }

    /**
     * 获取数据库连接
     */
    private Connection getConnection() throws SQLException {
        return DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "123456");
    }
}

3. 在实体类中使用注解来标识主键属性：

public class User {
    @Id
    private Long id;
    private String name;
    private Integer age;
    // 省略 getter 和 setter 方法
}

这样，我们就可以通过一个通用的 DAO 接口来实现实体类属性的增删查改，而不需要针对每个实体类都实现一遍 CRUD 操作。