java qr code generator library free Handling primary keys in Java

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Handling primary keys
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We ve already mentioned that we think natural primary keys can be a bad idea. Natural keys often make it difficult to refactor the data model when business requirements change. They may even, in extreme cases, impact performance. Unfortunately, many legacy schemas use (natural) composite keys heavily and, for the reason we discourage the use of composite keys, it may be difficult to change the legacy schema to use noncomposite natural or surrogate keys. Therefore, Hibernate supports the use of natural keys. If the natural key is a composite key, support is via the <composite-id> mapping. Let s map both a composite and a noncomposite natural primary key. Mapping a natural key If you encountered a USERS table in a legacy schema, it s likely that USERNAME is the actual primary key. In this case, you have no surrogate identifier that is automatically generated. Instead, you enable the assigned identifier generator strategy to indicate to Hibernate that the identifier is a natural key assigned by the application before the object is saved:
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<class name="User" table="USERS"> <id name="username" column="USERNAME" length="16"> <generator class="assigned"/> </id> ... </class>
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The code to save a new User is as follows:
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User user = new User(); user.setUsername("johndoe"); // Assign a primary key value user.setFirstname("John"); user.setLastname("Doe"); session.saveOrUpdate(user); // Will result in an INSERT // System.out.println( session.getIdentifier(user) ); session.flush();
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How does Hibernate know that saveOrUpdate() requires an INSERT and not an UPDATE It doesn t, so a trick is needed: Hibernate queries the USERS table for the given username, and if it s found, Hibernate updates the row. If it isn t found, insertion of a new row is required and done. This is certainly not the best solution, because it triggers an additional hit on the database. Several strategies avoid the SELECT:
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Add a <version> or a <timestamp> mapping, and a property, to your entity. Hibernate manages both values internally for optimistic concurrency control (discussed later in the book). As a side effect, an empty timestamp or a 0 or NULL version indicates that an instance is new and has to be inserted, not updated. Implement a Hibernate Interceptor, and hook it into your Session. This extension interface allows you to implement the method isTransient() with any custom procedure you may need to distinguish old and new objects.
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On the other hand, if you re happy to use save() and update() explicitly instead of saveOrUpdate(), Hibernate doesn t have to distinguish between transient and detached instances you do this by selecting the right method to call. (This issue is, in practice, the only reason to not use saveOrUpdate() all the time, by the way.) Mapping natural primary keys with JPA annotations is straightforward:
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@Id private String username;
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If no identifier generator is declared, Hibernate assumes that it has to apply the regular select-to-determine-state-unless-versioned strategy and expects the application to take care of the primary key value assignment. You can again avoid the SELECT by extending your application with an interceptor or by adding a versioncontrol property (version number or timestamp). Composite natural keys extend on the same ideas.
Legacy databases and custom SQL
Mapping a composite natural key Suppose that the primary key of the USERS table consists of a USERNAME and DEPARTMENT_NR. You can add a property named departmentNr to the User class and create the following mapping:
<class name="User" table="USERS"> <composite-id> <key-property name="username" column="USERNAME"/> <key-property name="departmentNr" column="DEPARTMENT_NR"/> </composite-id> ... </class>
The code to save a new User looks like this:
User user = new User(); // Assign a primary key value user.setUsername("johndoe"); user.setDepartmentNr(42); // Set property values user.setFirstname("John"); user.setLastname("Doe"); session.saveOrUpdate(user); session.flush();
Again, keep in mind that Hibernate executes a SELECT to determine what saveOrUpdate() should do unless you enable versioning control or a custom Interceptor. But what object can/should you use as the identifier when you call load() or get() Well, it s possible to use an instance of the User class, for example:
User user = new User(); // Assign a primary key value user.setUsername("johndoe"); user.setDepartmentNr(42); // Load the persistent state into user session.load(User.class, user);
In this code snippet, User acts as its own identifier class. It s more elegant to define a separate composite identifier class that declares just the key properties. Call this class UserId:
public class UserId implements Serializable { private String username;
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