JPA标准谓词条件

Question

我有以下代码片段用于构建条件构建器where condition。

想知道是否有任何方法可以使这更好，因为我会有更多条件和相同条件将用于获取记录的数量。

任何见解都非常值得关注

private List <Product> getProducts(MultivaluedMap params) throws JSONException {

    CriteriaBuilder criteriaBuilder = getEntityManager().getCriteriaBuilder();
    CriteriaQuery <Product> criteriaQuery = criteriaBuilder.createQuery(Product.class);

    Root <Product> root = criteriaQuery.from(Product.class);

    List <Predicate> p = new ArrayList <Predicate> ();
    Predicate prodIdPredicate, prodNamePredicate;

    JSONObject inputJSON = new JSONObject(params);

    if (inputJSON.isNull("filter") == false) {
        JSONObject filter = inputJSON.getJSONObject("filter");
        JSONArray filters = filter.getJSONArray("filters");
        for (int i = 0; i < filters.length(); i++) {

            JSONObject j = (JSONObject) filters.get(i);
            if (j.getString("field").equals("prodId")) {
                prodIdPredicate = criteriaBuilder.like(root.get(Product_.prodId), j.getString("value"));
                p.add(prodIdPredicate);
            }

            if (j.getString("field").equals("prodName")) {
                prodNamePredicate = criteriaBuilder.like(root.get(Product_.prodName), j.getString("value"));
                p.add(prodNamePredicate);
            }

        }
    }
    Predicate[] pr = new Predicate[p.size()];
    p.toArray(pr);
    criteriaQuery.where(pr);    

Answer 1

首先，您必须考虑以分层方式重新构建应用程序。您至少需要3层，DAO，服务和WebService。

关于数据库和JPA的所有内容都必须在您的DAO层中。所有与json相关的东西都必须在你的WebService层中。您的服务层必须管理Web服务和dao层之间的事务和通信。

首先让我们谈谈你的Web服务层。您的JSON对象可能来自Restful Web服务。由于几乎所有框架都支持json编组/解组，因此手动解析数据传输对象是不明智的。我的意思是，你可能更喜欢声明一个FieldDto类并传递它的实例而不是JSONObject。以下是FieldDto的示例。这是一个POJO。

public class FieldDto {
    private String prodId;
    private String prodName;
    // Getters & Setters etc.
}

您可以使用GSON或Jackson轻松地对json进行编组/解组。可能你的框架默认使用其中一个来处理json转换。

下一层是服务层。在服务层中，您可以管理事务并将DTO对象转换为DAO层可以轻松理解的对象。在这种情况下，您的服务层会将fieldDto.getProdId()和fielDto.getProdName()传递给DAO图层。

您的最后一层是DAO图层。首先，让我们更改您的方法签名。

public List <Product> getProducts(String prodId, String prodName) {

    CriteriaBuilder criteriaBuilder = getEntityManager().getCriteriaBuilder();
    CriteriaQuery <Product> criteriaQuery = criteriaBuilder.createQuery(Product.class);

    Root <Product> root = criteriaQuery.from(Product.class);

    List <Predicate> p = new ArrayList <Predicate> ();

    if(prodId != null){
         p.add(criteriaBuilder.like(root.get(Product_.prodId),prodId));
    }

    if(prodName != null){
         p.add(criteriaBuilder.like(root.get(Product_.prodName), prodName));
    }

    if(!p.isEmpty()){
        Predicate[] pr = new Predicate[p.size()];
        p.toArray(pr);
        criteriaQuery.where(pr);    
    }
    return getEntityManager().createQuery(criteriaQuery).getResultList();
}

不是这样的。此代码仍需改进。在我的一个项目中，我创建了一个流畅的api来管理所有的样板部件。当你开始编写其他DAO类时，你会发现一些代码块一遍又一遍地重复。

这是一个流畅的api的例子。您可能想要构建它的版本。

import javax.persistence.EntityManager;
import javax.persistence.LockModeType;
import javax.persistence.PersistenceException;
import javax.persistence.TypedQuery;
import javax.persistence.criteria.*;
import javax.persistence.metamodel.Attribute;
import javax.persistence.metamodel.CollectionAttribute;
import javax.persistence.metamodel.PluralAttribute;
import javax.persistence.metamodel.SingularAttribute;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.Vector;

public final class SimpleSelectBuilder<E extends Entity> {

    private final EntityManager entityManager;
    private final CriteriaBuilder criteriaBuilder;
    private final CriteriaQuery<E> criteriaQuery;
    private final Root<E> root;
    private final Collection<Predicate> predicates;

    private Integer first = null;
    private Integer max = null;
    private LockModeType lockModeType = null;

    public SimpleSelectBuilder(final EntityManager entityManager, final Class<E> entityClazz) {
        this.entityManager = entityManager;
        this.criteriaBuilder = entityManager.getCriteriaBuilder();
        this.criteriaQuery = this.criteriaBuilder.createQuery(entityClazz);
        this.root = criteriaQuery.from(entityClazz);
        this.predicates = new Vector<>();
    }

    public SimpleSelectBuilder<E> and(final Attribute attribute, final Object value) {
        final Expression expression = this.getExpression(attribute, root);
        this.predicates.add(criteriaBuilder.equal(expression, value));
        return this;
    }

    public SimpleSelectBuilder<E> andNotIn(final Attribute attribute, final Collection<Object> values) {
        final Expression expression = this.getExpression(attribute, root);
        this.predicates.add(criteriaBuilder.not(expression.in(values)));
        return this;
    }

    public SimpleSelectBuilder<E> andIn(final Attribute attribute, final Collection<Object> values) {
        final Expression expression = this.getExpression(attribute, root);
        this.predicates.add(expression.in(values));
        return this;
    }


    public SimpleSelectBuilder<E> andContains(final Attribute attribute, final Object value) {

        final Expression expression = this.getExpression(attribute, root);
        this.predicates.add(criteriaBuilder.isMember(value, expression));
        return this;
    }

    public SimpleSelectBuilder<E> orderByAsc(final Attribute attribute) {
        final List<Order> orders = new ArrayList<>();
        if (this.criteriaQuery.getOrderList() != null) {
            orders.addAll(this.criteriaQuery.getOrderList());
        }
        orders.add(criteriaBuilder.asc(this.getExpression(attribute, root)));
        this.criteriaQuery.orderBy(orders.toArray(new Order[orders.size()]));
        return this;
    }

    public SimpleSelectBuilder<E> orderByDesc(final Attribute attribute) {
        List<Order> orders = this.criteriaQuery.getOrderList();
        if (orders == null) {
            orders = new ArrayList<>();
        }
        orders.add(criteriaBuilder.desc(this.getExpression(attribute, root)));
        this.criteriaQuery.orderBy(orders.toArray(new Order[orders.size()]));
        return this;
    }

    public SimpleSelectBuilder<E> setFirst(Integer first) {
        this.first = first;
        return this;
    }

    public SimpleSelectBuilder<E> setMax(Integer max) {
        this.max = max;
        return this;
    }

    public SimpleSelectBuilder<E> setLockModeType(LockModeType lockModeType) {
        this.lockModeType = lockModeType;
        return this;
    }

    public List<E> getResultList() {
        final TypedQuery<E> query = this.prepareQuery();

        if (lockModeType != null) {
            query.setLockMode(lockModeType);
        }

        if (first != null) {
            query.setFirstResult(first);
        }

        if (max != null) {
            query.setMaxResults(max);
        }

        return query.getResultList();
    }

    public List<E> getCacheableResultList() {
        final TypedQuery<E> query = this.prepareQuery();

        if (lockModeType != null) {
            query.setLockMode(lockModeType);
        }

        if (first != null) {
            query.setFirstResult(first);
        }

        if (max != null) {
            query.setMaxResults(max);
        }

        query.setHint("org.hibernate.cacheable", true);
        query.setHint("org.hibernate.cacheMode", "NORMAL");
        return query.getResultList();
    }

    public E getSingleResult() {
        final TypedQuery<E> query = this.prepareQuery();

        if (lockModeType != null) {
            query.setLockMode(lockModeType);
        }

        return query.getSingleResult();
    }

    public E getCacheableSingleResult() {
        final TypedQuery<E> query = this.prepareQuery();

        if (lockModeType != null) {
            query.setLockMode(lockModeType);
        }

        query.setHint("org.hibernate.cacheable", true);
        query.setHint("org.hibernate.cacheMode", "NORMAL");
        return query.getSingleResult();
    }

    private TypedQuery<E> prepareQuery() {
        this.criteriaQuery.where(this.predicates.toArray(new Predicate[this.predicates.size()]));
        return this.entityManager.createQuery(criteriaQuery);
    }

    private <T> Expression<T> getExpression(final Attribute attribute, final From<E, T> from) {
        if (attribute instanceof SingularAttribute) {
            SingularAttribute singularAttribute = (SingularAttribute) attribute;
            return from.get(singularAttribute);
        } else if (attribute instanceof PluralAttribute) {
            PluralAttribute pluralAttribute = (PluralAttribute) attribute;
            return from.get(pluralAttribute);
        } else {
            throw new PersistenceException("Attribute type of '" + attribute
                    + "' must be one of [SingularAttribute, PluralAttribute].");
        }
    }

    private <T> Join<E, T> getJoinExpression(final Attribute attribute, final From<E, T> from) {
        if (attribute instanceof SingularAttribute) {
            final SingularAttribute singularAttribute = (SingularAttribute) attribute;
            return from.join(singularAttribute);
        } else if (attribute instanceof CollectionAttribute) {
            final CollectionAttribute collectionAttribute = (CollectionAttribute) attribute;
            return from.join(collectionAttribute);
        } else {
            throw new PersistenceException("Attribute type of '" + attribute
                    + "' must be one of [SingularAttribute, PluralAttribute].");
        }
    }

    public SimpleSelectBuilder<E> joinAnd(final Attribute attribute, final Object value, final Attribute... joinOn) {
        Join tableJoin = null;
        for (final Attribute join : joinOn) {
            if (tableJoin == null) {
                tableJoin = this.getJoinExpression(join, root);
            } else {
                tableJoin = this.getJoinExpression(join, tableJoin);
            }

        }

        if (tableJoin == null) {
            throw new PersistenceException("SelectBuilder cannot construct your join statement");
        }

        final Expression expression = this.getExpression(attribute, tableJoin);
        this.predicates.add(criteriaBuilder.equal(expression, value));
        return this;
    }
}

如果你使用它。比你的方法变成了这个。

public List <Product> getProducts(String prodId, String prodName) {

    // TODO add like statement to SimpleSelectBuilder
    return new SimpleSelectBuilder<Product>(this.getEntityManager(), Product.class)
           .and(Product_.prodId, prodId))
           .and(Product_.prodName, prodName))
           .getResultList();

}

如果您编写自己的SimpleSelectBuilder来处理样板代码块并提高可重用性，那将会更好。例如，您需要在上面的代码中添加like语句。

管理所有图层，交易，连接池等需要花费很多时间。相反，您可能需要考虑使用中间件来管理所有这些。在我的项目中，我更喜欢Spring。