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2

Paths and Types in JPQL and Criteria API

classes) are represented in JPQL by the following types of expressions: Variables - FROM ... and functions (e.g. for strings and for numbers), they can be compared by all six comparison operators ... with an instance of a user defined class (represented by a variable, parameter or prefix path expression
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2

Query Parameters in JPA

) The following method retrieves a Country object from the database by its name: public Country ... method. The setParameter method supports method chaining (by returning the same TypedQuery instance ... . Named parameters can be easily identified in a query string by their special form, which is a colon
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system exception occurred during an invocation on EJB GuestDao

$Worker.run(AbstractThreadPool.java:544) at java.lang.Thread.run(Thread.java:744) Caused by: java ... (Thread.java:744) Caused by: java.lang.IllegalStateException: Unable to retrieve ... a failure to find a persistence unit with a null name: Caused by: java.lang.IllegalStateException
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Update Entity references if we change the type of an entity

to this element. And we thought that also the real references are based on the ID. This step is done by our updateDB (we call this step profileMigration) But after this conversion, the reference from ... Embedded Systems AG References to entity objects are implemented by ObjectDB as a pair of (type + id
Manual
2

Detached Entity Objects

Detached entity objects are objects in a special state in which they are not managed by any ... ). Retrieval by navigation from detached objects is not supported, so only persistent fields ... EntityManager. In addition, in JPA 2 we can detach an entity object by using the detach method: em.detach
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java.nio.channels.OverlappingFileLockException

.wicket.RequestCycle.step(RequestCycle.java:1329) at org.apache.wicket.RequestCycle.steps ... locking conflict. In embedded mode the database cannot be opened by more than one user (process / class loader) simultaneously. Please verify that the database is not open by another process - e.g
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JPA and handling large data sets

. But exporting large binary data sets is very slow. To avoid memory problems the binary data type is read step by step during the export with the  query/setFirstResult() & setMaxResults(). But for large data ... results ordered by the primary key, use the value of the primary key of the last object in one batch as
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2

Performance and memory usage of queries

.Persistence; import javax.persistence.Query; public class MemoryUsageOfQuerys { static int steps ... .merge(myEntity);    for (int m = 0; m < steps; m++) {     MyEntityElement myEntityElement = new ... message we get by the example: [ObjectDB 2.6.1_01] Unexpected exception (Error 990)   Generated by
Manual
2

FROM clause (JPQL / Criteria API)

at the WHERE clause level by using a type expression. For example, in the following query, c iterates ... , and the same query can also be written as follows: SELECT c FROM Country c By default, the name ... name). The default name can be overridden by specifying another name explicitly in the @Entity's
Manual
2

Shared (L2) Entity Cache

an entity object that is already managed by the EntityManager returns the existing instance from ... by the EntityManagerFactory and shared by all its EntityManager objects. the broader scope ... is wider, since they exist per database and are shared by all the EntityManagerFactory and EntityManager