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Java Persistence with Hibernate phần 5

Để chứng minh sự linh hoạt của các loại bản đồ tùy chỉnh, bạn không thay đổi lớp MonetaryAmount (và liên tục các lớp học khác) tại tất cả các bạn thay đổi chỉ có các loại bản đồ tùy chỉnh, như thể hiện trong danh sách 5,5. | Polymorphic associations 315 the given instance is this would require a database hit which you try to avoid with lazy loading in the first place. To perform a proxy-safe typecast use load User user User session.get User.class userid BillingDetails bd user.getDefaultBillingDetails Narrow the proxy to the subclass doesn t hit the database Creditcard cc Creditcard session.load CreditCard.class bd.getid expiryDate cc.getExpiryDate After the call to load bd and cc refer to two different proxy instances which both delegate to the same underlying CreditCard instance. However the second proxy has a different interface and you can call methods like getExpiryDate that apply only to this interface. Note that you can avoid these issues by avoiding lazy fetching as in the following code using an eager fetch query User user User session.createCriteria User.class .add Restrictions.eq id uid .setFetchMode defaultBillingDetails FetchMode.JOIN .uniqueResult The users defaultBillingDetails have been fetched eagerly CreditCard cc CreditCard user.getDefaultBillingDetails expiryDate cc.getExpiryDate Truly object-oriented code shouldn t use instanceof or numerous typecasts. If you find yourself running into problems with proxies you should question your design asking whether there is a more polymorphic approach. Hibernate also offers bytecode instrumentation as an alternative to lazy loading through proxies we ll get back to fetching strategies in chapter 13 section 13.1 Defining the global fetch plan. One-to-one associations are handled the same way. What about many-valued associations for example the collection of billingDetails for each User 7.3.2 Polymorphic collections A User may have references to many BillingDetails not only a single default one of the many is the default . You map this with a bidirectional one-to-many association. In BillingDetails you have the following many-to-one name user class User column USER_ID 316 CHAPTER 7 Advanced entity association mappings In the .