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作者: ndbf123 [ndbf123]    论坛用户

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从第7.1到7.2. 7.1 Executing queries The Query and Criteria interfaces both define several methods for controlling execution of a query. In addition, Query provides methods for binding concrete values to query parameters. To execute a query in your application, you need to obtain an instance of one of these interfaces, using the Session. 7.1.1 The query interfaces To create a new Query instance, call either createQuery() or createSQLQuery(). The createQuery() method prepares an HQL query: Query hqlQuery = session.createQuery("from User"); The createSQLQuery() is used to create a SQL query, using the native syntax of the underlying database: Query sqlQuery = session.createSQLQuery( "select {u.*} from USERS {u}", "u", User.class ); In both cases, Hibernate returns a newly instantiated Query object that may be used to specify exactly how a particular query should be executed, and to execute the query. To obtain a Criteria instance, call createCriteria(), passing the class of the objects you want the query to return. This is also called the root entity of the criteria query, the User in this example: Criteria crit = session.createCriteria(User.class); The Criteria instance may be used in the same way as a Query object―but it’s also used to construct the object-oriented representation of the query by adding Criterion instances and navigating associations to new Criterias. We discuss this in much more detail later; for now, let’s continue with query execution. Paging the result Pagination is a commonly used technique. Users might see the result of their search request (for example, for specific Items) as a page. This page shows only a limited subset (say, 10 Items) at a time, and users can navigate to the next and previous pages manually. Both the Query and Criteria interfaces support this pagination of the query result: Query query = session.createQuery("from User u order by u.name asc"); query.setFirstResult(0); query.setMaxResults(10); The call to setMaxResults(10) limits the query result set to the first 10 objects selected by the database. In this criteria query, the requested page starts in the “middle” of the result set: Criteria crit = session.createCriteria(User.class); crit.addOrder( Order.asc("name") ); crit.setFirstResult(40); crit.setMaxResults(20); List results = crit.list(); Starting from the fortieth object, we retrieve the next 20 objects. Note that there is no standard way to express pagination in SQL―but Hibernate knows the tricks to make this work efficiently on your particular database. You can use the method-chaining coding style (methods return the receiving object instead of void) with both the Query and Criteria interfaces, rewriting the two previous examples as follows: List results = session.createQuery("from User u order by u.name asc") .setFirstResult(0) .setMaxResults(10) .list(); List results = session.createCriteria(User.class) .addOrder( Order.asc("name") ) .setFirstResult(40) .setMaxResults(20) .list(); Chaining method calls is less verbose and is supported by many Hibernate APIs. Listing and iterating results The list() method executes the query and returns the results as a list: List result = session.createQuery("from User").list(); With some queries, we know the result will be only a single instance―for example, if we want only the highest bid. In this case, we can read it from the result list by index: result.get(0) or setMaxResult(1). We then execute the query with the uniqueResult() method, because we know only one object will be returned: Executing queries 245 Bid maxBid = (Bid) session.createQuery("from Bid b order by b.amount desc") .setMaxResults(1) .uniqueResult(); Bid bid = (Bid) session.createCriteria(Bid.class) .add( Expression.eq("id", id) ) .uniqueResult(); If the query returns more than one object, an exception will be thrown. The Query and Session interfaces also provide the iterate() method, which returns the same result as list() (or find()) but uses a different strategy for retrieving the results. When you use iterate() to execute a query, Hibernate retrieves only the primary key (identifier) values in the first SQL select; it tries to find the rest of the state of the objects in the cache, before querying again for the rest of the property values. This technique can be used to optimize loading in specific cases, as discussed in section 7.6, “Optimizing object retrieval.” FAQ Is Session.find() faster than Query.list()? The Session API provides shortcut methods for simple queries. Instead of creating a Query, you can also call Session.find("from User"). The result is the same as from Query.list(); one isn’t faster than the other. The same is true for iterate(): You’re free to choose the API. However, it’s highly likely that the query shortcut methods on the Session API will be removed in the future to reduce the bloat of session methods. We recommend the Query API. Finally, the Query interface lets you bind values to query parameters. 谢谢了

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