Chapter 7 Java Persistence API.docVIP

Chapter 7 Java Persistence API 7.1 Persistence Basics 7.1.1 Entity Classes 7.1.2 Persistent Fields and Properties 7.1.3 Entity Ids and Primary Keys 7.2 Entity Relationships 7.2.1 Multiplicity 7.2.2 Direction in Entity Relationships 7.3 Storing and Manipulating Entities 7.3.1 Persistence Context 7.3.2 Manipulating entities 7.3.3 Finding Entities by EntityManager Your First JPA Application 7.5 Practice Labs 7.6 Summary 7.7. Self Review Questions 7.8 Exercises Java Persistence 7.1 Persistence Basics 7.1.1 Introduction Java programs consist of classes and objects. As you may recall from when you first learned Java, the classes are sort of like types in that they represent kinds of things or concepts. The objects, which are particular instances of the classes, represent specific examples of the more general class. All of this is an attempt to model real-world information in a way that computers (and programmers) can easily work with. The program starts, creates some objects of various classes, and goes on to manipulate them to achieve the desired results. When it is finished, the Java garbage collector throws away the left-over objects in order to free up precious system resources. But sometimes that isn’t what you want. Suppose you need to keep a log of every user to access your application along with the time, date, and what the user did in the application. All of a sudden classes and objects seem to come up short of the goal. What is needed is a way to make the objects stick around between the first time the program was run (when the users accesses the program) and the last time the program was run (when it is time to review the logs to make sure both the application and the users are behaving themselves). This is the problem of persistence and exactly the situation that the Java Persistence API (JPA) was designed for. There are many ways to solve t