persistence

问题 I'm using Hibernate 3.5.2-FINAL with annotations to specify my persistence mappings. I'm struggling with modelling a relationship between an Application and a set of Platforms. Each application is available for a set of platforms. From all the reading and searching I've done, I think I need to have the platform enum class be persisted as an Entity, and to have a join table to represent the many-to-many relationship. I want the relationship to be unidirectional at the object level, that is, I

问题 if I have multiple threads, each use injector to get the EntityManager object, each use the em object to select a list of other class objects. Ready to be used in a for loop. If a thread finishes first and calls clear(), will that affect the other threads? Like the for loop will have exception? How about close()? If the answer is "It depends", what (class definition? method call?) and where (java code? annotation? xml?) should I look at to find out how is it depended? I did not write the

问题 I was wondering if there were a way to save one or more javascript variables to the local machine and then call on them later? var a = b and then when you go back to the webpage it remembers the value that B was? 回答1: What you are looking for is probably how to make and use Cookies. A cookie is a variable that is stored on the visitor's computer. Each time the same computer requests a page with a browser, it will send the cookie too. With JavaScript, you can both create and retrieve cookie

问题 I need to store some data in a Django model. These data are not equal to all instances of the model. At first I thought about subclassing the model, but I’m trying to keep the application flexible. If I use subclasses, I’ll need to create a whole class each time I need a new kind of object, and that’s no good. I’ll also end up with a lot of subclasses only to store a pair of extra fields. I really feel that a dictionary would be the best approach, but there’s nothing in the Django

问题 Let's say I have something like this: d = { "abc" : [1, 2, 3], "qwerty" : [4,5,6] } What's the easiest way to progammatically get that into a file that I can load from python later? Can I somehow save it as python source (from within a python script, not manually!), then import it later? Or should I use JSON or something? 回答1: Use the pickle module. import pickle d = { "abc" : [1, 2, 3], "qwerty" : [4,5,6] } afile = open(r'C:\d.pkl', 'wb') pickle.dump(d, afile) afile.close() #reload object

问题 Background I am trying to encode a String-style enum using the NSCoding protocol, but I am running into errors converting to and back from String. I get the following errors while decoding and encoding: String is not convertible to Stage Extra argument ForKey: in call Code enum Stage : String { case DisplayAll = "Display All" case HideQuarter = "Hide Quarter" case HideHalf = "Hide Half" case HideTwoThirds = "Hide Two Thirds" case HideAll = "Hide All" } class AppState : NSCoding, NSObject {

问题 I have queries as below: What is the difference of these two? Are both of these supported by all databases? Are JPA TransactionManager and JTA TransactionManager different? 回答1: JPA implementations have the choice of managing transactions themselves ( RESOURCE_LOCAL ), or having them managed by the application server's JTA implementation. In most cases, RESOURCE_LOCAL is fine. This would use basic JDBC-level transactions. The downside is that the transaction is local to the JPA persistence

问题 I was on #hibernate IRC and somebody shared the following (partial) pattern with me @Entity public MyEntity() { ... primary key, object properties, getters/setters go here ... @Column(nullable=false) private int hashCode; public MyEntity() { hashCode += id; } private final Set<String> tags = Sets.newHashSet(); public void addTag(String tag) { if(tags.add(tag)) { hashCode += tag.hashCode(); } } public void removeTag(String tag) { if(tags.remove(tag) { hashCode -= tag.hashCode(); } } public

问题 This is in continuation of my related question: Error when trying to interactively load data file saved by paused batch script. I decided to present my question with a reproducible example separately to avoid making the already large description in the previous question even bigger. In the following reproducible example , I expect to retrieve the value of the stored object ("Important data"), but instead, as you see, I retrieve the name of the object itself ("sf.data.devLinks"). I suspected

问题 The JPA 2 maven packages I've met so far was only complete realizations either from hibernate or eclipse. But I want to build my package agains vendor agnostic api. Is it available public? 回答1: The JPA spec group were too lazy to bother releasing a vendor neutral API jar for JPA 2.0 and JPA 2.1. See for example https://java.net/jira/browse/JPA_SPEC-60 You simply have to pick a vendor's own one and hope they didn't put vendor-specifics in there. Naff 回答2: Take a look at org.hibernate.javax