我有以下代码片段用于构建标准生成器 where 条件。
想知道是否有任何方法可以使它变得更好，因为我会有更多的条件和相同的条件将用于获取记录计数。
任何见解都是非常值得赞赏的
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);
最佳答案
首先，您必须考虑以分层方式重构您的应用程序。你至少需要 3 层，DAO，Service 和 WebService。
所有关于数据库和 JPA 的东西都必须在你的 DAO 层中。而所有与json相关的东西都必须在你的WebService层。您的服务层必须管理 Web 服务和 Dao 层之间的事务和通信。
首先让我们谈谈您的 Web 服务层。您的 JSON 对象可能来自 Restful 网络服务。由于几乎所有框架都支持 json 编码/解码，因此手动解析数据传输对象并不明智。我的意思是，您可能更愿意声明一个 FieldDto 类并传递它的实例而不是 JSONObject。下面是一个 FieldDto 的例子。它是一个 POJO。
public class FieldDto {
private String prodId;
private String prodName;
// Getters & Setters etc.
}
您可以使用 GSON 或 Jackson 轻松编码/解码为 json。可能您的框架默认使用其中之一来处理 json 转换。
下一层是服务层。在服务层中，您管理事务并将 DTO 对象转换为 DAO 层可以轻松理解的对象。在这种情况下，您的服务层将 fieldDto.getProdId() 和 fielDto.getProdName() 传递给 DAO 层。
你的最后一层是 DAO 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。此代码仍需要改进。在我的一个项目中，我创建了一个流畅的 API 来管理所有样板文件部分。当您开始编写其他 DAO 类时，您会意识到某些代码块会一遍又一遍地重复。
这是一个流畅的 api 示例。您可能想要构建您的版本。
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;
}
}
如果你用这个。比你的方法变成这样。
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会更好。例如，您需要在上面的代码中添加 like 语句。
管理所有层、事务、连接池等将花费您很多时间。相反，您可能需要考虑一个中间件来为您管理所有这些。在我的项目中，我更喜欢 Spring。
关于java - JPA 条件谓词条件，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/31700942/