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IntelliJ IDEA 2017.2 is a new massive update for the IDE. The update brings even smarter coding assistance, neater UI, faster performance, and tons of other improvements.Optimizing fetching and caching (Hibernate). In this topic, we’ll show you how to retrieve objects from the database and how you can optimize the loading of object networks when you navigate from object to object in your application. We then enable caching; you’ll learn how to speed up data retrieval in local and distributed applications. Defining the global fetch plan. Retrieving persistent objects from the database is one of the most interesting parts of working with Hibernate. The object- retrieval options Hibernate provides the following ways to get objects out of the database: ■ Navigating the object graph, starting from an already loaded object, by accessing the associated objects through property accessor methods such as a. User. get. Address(). City(), and so on. Hibernate automatically loads (and preloads) nodes of the graph while you call accessor methods, if the persistence context is still open. Retrieval by identifier, the most convenient method when the unique identifier value of an object is known. The Hibernate Query Language (HQL), which is a full object- oriented query language. The Java Persistence query language (JPA QL) is a standardized subset of the Hibernate query language. The Hibernate Criteria interface, which provides a type- safe and object- oriented way to perform queries without the need for string manipulation. This facility includes queries based on example objects. Native SQL queries, including stored procedure calls, where Hibernate still takes care of mapping the JDBC result sets to graphs of persistent objects. In your Hibernate or JPA application, you use a combination of these techniques. We won’t discuss each retrieval method in much detail in this topic. We’re more interested in the so- called default fetch plan and fetching strategies. The default fetch plan and fetching strategy is the plan and strategy that applies to a particular entity association or collection. In other words, it defines if and how an associated object or a collection should be loaded, when the owning entity object is loaded, and when you access an associated object or collection. Each retrieval method may use a different plan and strategy—that is, a plan that defines what part of the persistent object network should be retrieved and how it should be retrieved. Your goal is to find the best retrieval method and fetching strategy for every use case in your application; at the same time, you also want to minimize the number of SQL queries for best performance. Before we look at the fetch plan options and fetching strategies, we’ll give you an overview of the retrieval methods. We also mention the Hibernate caching system sometimes, but we fully explore it later in this topic.). You saw how objects are retrieved by identifier earlier in the previous topic, so we won’t repeat it here. Let’s go straight to the more flexible query options, HQL (equivalent to JPA QL) and Criteria. Both allow you to create arbitrary queries. The Hibernate Query Language and JPA QL. The Hibernate Query Language is an object- oriented dialect of the familiar database query language SQL. HQL bears some close resemblance to ODMG OQL, but unlike OQL, it’s adapted for use with SQL databases and is easier to learn (thanks to its close resemblance to SQL) and fully implemented (we don’t know of any OQL implementation that is complete). The EJB 3. 0 standard defines the Java Persistence query language. This new JPA QL and the HQL have been aligned so that JPA QL is a subset of HQL. A valid JPA QL query is always also a valid HQL query; HQL has more options that should be considered vendor extensions of the standardized subset. HQL is commonly used for object retrieval, not for updating, inserting, or deleting data. Object state synchronization is the job of the persistence manager, not the developer. But, as we’ve shown in the previous topic, HQL and JPA QL support direct bulk operations for updating, deleting, and inserting, if required by the use case (mass data operations). Most of the time, you only need to retrieve objects of a particular class and restrict by the properties of that class. For example, the following query retrieves a user by first name. After preparing query q, you bind a value to the named parameter : fname. The result is returned as a List of User objects. HQL is powerful, and even though you may not use the more advanced features all the time, they’re needed for more difficult problems. For example, HQL supports. The ability to apply restrictions to properties of associated objects related by reference or held in collections (to navigate the object graph using query language). The ability to retrieve only properties of an entity or entities, without the overhead of loading the entity itself into the persistence context. This is sometimes called a report query; it is more correctly called projection. The ability to order the results of the query. The ability to paginate the results. Aggregation with group by, having, and aggregate functions like sum, min, and max/min. Outer joins when retrieving multiple objects per row. The ability to call standard and user- defined SQL functions. Subqueries (nested queries). Querying with a criteria The Hibernate query by criteria (QBC) API allows a query to be built by manipulation of criteria objects at runtime. This lets you specify constraints dynamically without direct string manipulations, but you don’t lose much of the flexibility or power of HQL. On the other hand, queries expressed as criteria are often much less readable than queries expressed in HQL. Retrieving a user by first name is easy with a Criteria object: A Criteria is a tree of Criterion instances. The Restrictions class provides static factory methods that return Criterion instances. Once the desired criteria tree is build, it’s executed against the database. Many developers prefer query by criteria, considering it a more object- oriented approach. They also like the fact that the query syntax may be parsed and validated at compile time, whereas HQL expressions aren’t parsed until runtime (or startup, if externalized named queries are used). The nice thing about the Hibernate Criteria API is the Criterion framework. This framework allows extension by the user, which is more difficult in the case of a query language like HQL. Note that the Criteria API is native to Hibernate; it isn’t part of the Java Persistence standard. In practice, Criteria will be the most common Hibernate extension you utilize in your JPA application.We expect that a future version of the JPA or EJB standard will include a similar programmatic query interface.Querying by example.As part of the Criteria facility, Hibernate supports query by example (QBE).The idea behind query by example is that the application supplies an instance of the queried class, with certain property values set (to nondefault values). New York Drivers License Name Format For Essay . The query returns all persistent instances with matching property values. Query by example isn’t a particularly powerful approach. However, it can be convenient for some applications, especially if it’s used in combination with Criteria. This example first creates a new Criteria that queries for User objects. Then you add an Example object, a User instance with only the firstname property set. Finally, a Restriction criterion is added before executing the query. A typical use case for query by example is a search screen that allows users to specify a range of different property values to be matched by the returned result set. This kind of functionality can be difficult to express cleanly in a query language; string manipulations are required to specify a dynamic set of constraints. You now know the basic retrieval options in Hibernate. We focus on the object- fetching plans and strategies for the rest of this section. Let’s start with the definition of what should be loaded into memory. The lazy default fetch plan. Hibernate defaults to a lazy fetching strategy for all entities and collections. This means that Hibernate by default loads only the objects you’re querying for. Let’s explore this with a few examples. If you query for an Item object (let’s say you load it by its identifier), exactly this Item and nothing else is loaded into memory. This retrieval by identifier results in a single (or possibly several, if inheritance or secondary tables are mapped) SQL statement that retrieves an Item instance. In the persistence context, in memory, you now have this item object available in persistent state, as shown in figure 1.
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