persistence

问题 As there are 3 states of objects in hibernate Transient persistent detached I know Transient state object can be converted to persistent once it it persisted by attaching it to a session Can it be done with detached objects also? or detached object has no life in hibernate context as there is no record of it in the cache? 来源： https://stackoverflow.com/questions/41229034/converting-detached-object-to-persistent

问题 persistence.xml looks like this: <persistence-unit name="testPU" transaction-type="RESOURCE_LOCAL"> <provider>org.hibernate.ejb.HibernatePersistence</provider> <non-jta-data-source>jdbc/test</non-jta-data-source> <jar-file>../../lib/app-services-1.0.jar</jar-file> <exclude-unlisted-classes>false</exclude-unlisted-classes> </persistence-unit> It is a web project, so the deployment unit is a war file. The jar file I tried to refer is in WEB-INF/lib/ folder , persistence.xml is in WEB-INF

问题 I am getting NonUniqueObjectException when i persist an object using JPA. The object where i make persisent, have other objects and in some cases the objects have the same id. Make a merge in object where is already saved in DB is a possibility but i only can make persist at one object (this object have all information) I can put any annotation on my id to make a merge when this id is already saved in MySQL database? 回答1: Use entityManager.merge(..) or leave the id field empty. Otherwise

问题 I'm using JPA and I am connecting to multiple databases. Below is the persistence.xml - <persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0"> <persistence-unit name="myRead" transaction-type="RESOURCE_LOCAL"> <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider> <properties> <property name="hibernate.dialect" value="org.hibernate.dialect.Oracle10gDialect" /> <property name="hibernate.show_sql" value="true" /> </properties> </persistence-unit> <persistence

问题 I have an entity (representing a music file) that I store in Hibernate. The user can modify attributes of the entity and at some point the changes they make can be saved back to the original file. But at any point in time before they save the changes to the file I would like to able to see differences between the entity that I originally loaded and any changes made since, I want these differences to be stored in the database not the class representing entity because the main reason Im using

问题 I am using mosquitto server for MQTT protocol. Using persistence setting in a configuration file with -c option, I am able to save the data. However the file generated is binary one. How would one be able to read that file? Is there any specific tool available for that? Appreciate your views. Thanks! Amit 回答1: Why do you want to read it? The data is only kept there while messages (QOS1 or QOS2) are in flight to ensure they are not lost in transit while waiting for a response from the

问题 For this question, consider the following sample: @Entity public class File { public static enum Permission { READABLE, WRITEABLE, EXECUTABLE } @ElementCollection @Enumerated(EnumType.ORDINAL) Set<Permission> permissions; // Omitted } Assuming that the enum values are stored in the ordinal form, does JPA always create an extra table for this set? Can I alter this in order to make this not to be an one-to-many relationship, i.e., using a column instead of an extra table? Thanks. 回答1: "one-to

问题 In my iphone application I am storing values in an array and I want to retrieve it when I reload the application.That is,When the user exits my app, I want this array to be saved so that when the app is loaded again by the user it want to be accessed. 回答1: You can make use of the UIApplicationDelegate methods applicationWillTerminate and applicationDidEnterBackground to save your data. If you created your project from a template they should be implemented with some comments in your

问题 I need to store images and I have 2 options: store the image into GAE datastore. store the image somewhere (maybe also on Dropbox or another website) and store its link into GAE datastore. What's the best practice when we need to store an image into DB, in the hypotesis that each image is bijectivelly linked to a specific element of the datastore? 回答1: I think it depends heavily on the use case. I have a small company website running on appengine and the content images are all stored in the

问题 when I try to run a JPA project, I get the following error message : Exception in thread "main" java.lang.ExceptionInInitializerError at org.eclipse.persistence.internal.jpa.EntityManagerFactoryProvider.<clinit>(EntityManagerFactoryProvider.java:55) at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactoryImpl(PersistenceProvider.java:92) at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactory(PersistenceProvider.java:188) at javax.persistence