JPA-Test/src/main/java/com/guwan/backend/builder/JoinBuilder.java

package com.guwan.backend.builder;

import jakarta.persistence.EntityManager;
import jakarta.persistence.criteria.*;

import java.lang.reflect.Constructor;
import java.util.*;
import java.util.stream.Collectors;

/**
 * 自定义连表查询构建器
 * 用于构建复杂的多表连接查询，支持指定返回字段和条件
 */
public class JoinBuilder<T> {
    private final Class<T> rootClass;
    private final Map<Class<?>, JoinInfo> joins = new HashMap<>();
    private final Map<Class<?>, List<Condition>> conditions = new HashMap<>();
    private Map<String, Object> namedSelections = new LinkedHashMap<>();

    private JoinBuilder(Class<T> rootClass) {
        this.rootClass = rootClass;
    }

    public static <T> JoinBuilder<T> from(Class<T> rootClass) {
        return new JoinBuilder<>(rootClass);
    }

    /**
     * 添加连接表
     *
     * @param joinClass   要连接的表实体类
     * @param sourceField 主表的外键字段
     * @param targetField 连接表的目标字段
     */
    public void join(Class<?> joinClass, String sourceField, String targetField) {
        joins.put(joinClass, new JoinInfo(sourceField, targetField));
    }

    /**
     * 添加等值条件
     *
     * @param entityClass 实体类
     * @param fieldName   字段名
     * @param value       字段值
     */
    public <V> void equal(Class<?> entityClass, String fieldName, V value) {
        if (!conditions.containsKey(entityClass)) {
            conditions.put(entityClass, new ArrayList<>());
        }
        conditions.get(entityClass).add(new Condition(fieldName, value, Operator.EQUAL));
    }
    

    /**
     * 添加一个具有别名的字段选择（字符串版本）
     *
     * @param alias       字段别名
     * @param entityClass 实体类
     * @param fieldName   字段名称
     */
    public <E> void selectAs(String alias, Class<E> entityClass, String fieldName) {

        namedSelections.put(alias, new SelectionInfo(entityClass, fieldName));

    }

    /**
     * 执行查询并返回结果
     * @param entityManager EntityManager实例
     * @return 查询结果列表
     */
    public List<Object[]> execute(EntityManager entityManager) {
        // 创建CriteriaBuilder
        CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();

        // 创建CriteriaQuery
        CriteriaQuery<Object[]> criteriaQuery = criteriaBuilder.createQuery(Object[].class);

        // 设置根实体
        Root<T> root = criteriaQuery.from(rootClass);

        // 创建连接关系
        Map<Class<?>, From<?, ?>> fromMap = createJoins(root, criteriaQuery);

        // 应用所有条件，包括表之间的连接条件和额外查询条件
        List<Predicate> predicates = applyAllConditions(root, fromMap, criteriaBuilder);

        // 添加条件到查询
        if (!predicates.isEmpty()) {
            criteriaQuery.where(criteriaBuilder.and(predicates.toArray(new Predicate[0])));
        }

        // 设置查询结果投影
        configureSelections(root, fromMap, criteriaQuery);

        // 执行查询并返回结果
        return entityManager.createQuery(criteriaQuery).getResultList();
    }

    public long count(EntityManager entityManager) {
        CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
        CriteriaQuery<Long> countQuery = criteriaBuilder.createQuery(Long.class);
        Root<T> root = countQuery.from(rootClass);

        // 创建连接关系
        Map<Class<?>, From<?, ?>> fromMap = createJoins(root, countQuery);

        // 应用条件，包括表之间的连接条件和额外查询条件
        List<Predicate> predicates = applyAllConditions(root, fromMap, criteriaBuilder);

        // 添加条件到查询
        if (!predicates.isEmpty()) {
            countQuery.where(criteriaBuilder.and(predicates.toArray(new Predicate[0])));
        }

        // 设置count查询
        countQuery.select(criteriaBuilder.count(root));

        // 执行查询并返回结果
        return entityManager.createQuery(countQuery).getSingleResult();
    }

    public <R> List<R> executeAndMapWithPaging(EntityManager entityManager,
                                               Class<R> dtoClass,
                                               int page,
                                               int size) {
        List<Object[]> results = executeWithPaging(entityManager, page, size);
        return mapToDto(results, dtoClass);
    }


    public List<Object[]> executeWithPaging(EntityManager entityManager, int page, int size) {
        // 创建CriteriaBuilder
        CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();

        // 创建CriteriaQuery
        CriteriaQuery<Object[]> criteriaQuery = criteriaBuilder.createQuery(Object[].class);

        // 设置根实体
        Root<T> root = criteriaQuery.from(rootClass);

        // 创建连接关系
        Map<Class<?>, From<?, ?>> fromMap = createJoins(root, criteriaQuery);

        // 应用所有条件
        List<Predicate> predicates = applyAllConditions(root, fromMap, criteriaBuilder);

        // 添加条件到查询
        if (!predicates.isEmpty()) {
            criteriaQuery.where(criteriaBuilder.and(predicates.toArray(new Predicate[0])));
        }

        // 设置查询结果投影
        configureSelections(root, fromMap, criteriaQuery);

        // 创建查询并设置分页
        jakarta.persistence.Query query = entityManager.createQuery(criteriaQuery);
        query.setFirstResult(page * size);
        query.setMaxResults(size);

        // 执行查询并返回结果
        return query.getResultList();
    }



    
    /**
     * 创建连接 - 修改为使用from而不是join
     */
    private Map<Class<?>, From<?, ?>> createJoins(Root<T> root, CriteriaQuery<?> query) {
        Map<Class<?>, From<?, ?>> fromMap = new HashMap<>();
        fromMap.put(rootClass, root);
        
        // 对于每个需要连接的表，创建一个单独的From
        for (Map.Entry<Class<?>, JoinInfo> entry : joins.entrySet()) {
            Class<?> joinClass = entry.getKey();
            From<?, ?> joinRoot = query.from(joinClass);
            fromMap.put(joinClass, joinRoot);
        }
        
        return fromMap;
    }
    
    /**
     * 应用所有条件，包括表之间的连接条件和额外查询条件
     */
    private List<Predicate> applyAllConditions(Root<T> root, Map<Class<?>, From<?, ?>> fromMap, CriteriaBuilder criteriaBuilder) {
        List<Predicate> predicates = new ArrayList<>();
        
        // 首先添加表连接条件
        for (Map.Entry<Class<?>, JoinInfo> entry : joins.entrySet()) {
            Class<?> joinClass = entry.getKey();
            JoinInfo joinInfo = entry.getValue();
            
            if (fromMap.containsKey(joinClass)) {
                From<?, ?> joinFrom = fromMap.get(joinClass);
                Predicate joinCondition = criteriaBuilder.equal(
                    root.get(joinInfo.sourceField),
                    joinFrom.get(joinInfo.targetField)
                );
                predicates.add(joinCondition);
            }
        }
        
        // 然后添加普通查询条件
        for (Class<?> entityClass : conditions.keySet()) {
            for (Condition condition : conditions.get(entityClass)) {
                if (fromMap.containsKey(entityClass)) {
                    From<?, ?> from = fromMap.get(entityClass);
                    predicates.add(applyCondition(condition, from, criteriaBuilder));
                }
            }
        }
        
        return predicates;
    }

    
    /**
     * 将查询结果映射到DTO对象
     * @param results 查询结果
     * @param dtoClass DTO类
     * @param <R> DTO类型
     * @return DTO对象列表
     */
    public <R> List<R> mapToDto(List<Object[]> results, Class<R> dtoClass) {
        try {
            // 获取所有字段名（顺序与查询结果一致）
            List<String> fieldNames = new ArrayList<>(namedSelections.keySet());
            
            // 获取构造函数
            Class<?>[] paramTypes = new Class[fieldNames.size()];
            Arrays.fill(paramTypes, Object.class);
            Constructor<R> constructor = dtoClass.getDeclaredConstructor(paramTypes);
            
            // 映射结果
            return results.stream()
                    .map(row -> {
                        try {
                            return constructor.newInstance(row);
                        } catch (Exception e) {
                            throw new RuntimeException("Failed to map result to DTO: " + e.getMessage(), e);
                        }
                    })
                    .collect(Collectors.toList());
        } catch (Exception e) {
            throw new RuntimeException("Failed to create DTO mapper: " + e.getMessage(), e);
        }
    }
    
    /**
     * 配置查询结果投影
     */
    private void configureSelections(Root<T> root, Map<Class<?>, From<?, ?>> fromMap, CriteriaQuery<?> query) {
        if (!namedSelections.isEmpty()) {
            // 处理命名选择
            List<Selection<?>> selections = new ArrayList<>();
            
            for (Map.Entry<String, Object> entry : namedSelections.entrySet()) {
                String alias = entry.getKey();
                Object value = entry.getValue();
                
                if (value instanceof SelectionInfo) {
                    SelectionInfo info = (SelectionInfo) value;
                    Path<?> path;
                    
                    if (fromMap.containsKey(info.entityClass)) {
                        path = fromMap.get(info.entityClass).get(info.fieldName);
                        selections.add(path.alias(alias));
                    }
                }
            }
            
            if (!selections.isEmpty()) {
                query.multiselect(selections);
                return;
            }
        }

    }

    private <X> Predicate applyCondition(Condition condition, From<?, X> from, CriteriaBuilder criteriaBuilder) {
        switch (condition.operator) {
            case EQUAL:
                return criteriaBuilder.equal(from.get(condition.fieldName), condition.value);
            // 可以添加更多操作符支持，如LIKE, IN, GREATER_THAN等
            default:
                throw new UnsupportedOperationException("Unsupported operator: " + condition.operator);
        }
    }


    /**
     * 连接信息
     */
    private static class JoinInfo {
        private final String sourceField;
        private final String targetField;
        
        JoinInfo(String sourceField, String targetField) {
            this.sourceField = sourceField;
            this.targetField = targetField;
        }
    }

    /**
     * 选择字段信息
     */
    private static class SelectionInfo {
        private final Class<?> entityClass;
        private final String fieldName;

        SelectionInfo(Class<?> entityClass, String fieldName) {
            this.entityClass = entityClass;
            this.fieldName = fieldName;
        }
    }
    
    private enum Operator {
        EQUAL
        // 可以添加更多操作符支持，如LIKE, IN, GREATER_THAN等
    }

    private static class Condition {
        private final String fieldName;
        private final Object value;
        private final Operator operator;

        Condition(String fieldName, Object value, Operator operator) {
            this.fieldName = fieldName;
            this.value = value;
            this.operator = operator;
        }
    }
}