{"id":8,"date":"2013-07-02T16:32:24","date_gmt":"2013-07-02T14:32:24","guid":{"rendered":"http:\/\/itblog.inginea.eu\/?p=8"},"modified":"2016-10-24T09:55:27","modified_gmt":"2016-10-24T07:55:27","slug":"deproxy-a-lazily-loaded-jpa-reference-using-standard-java-and-jpa-api","status":"publish","type":"post","link":"https:\/\/ondro.inginea.eu\/index.php\/deproxy-a-lazily-loaded-jpa-reference-using-standard-java-and-jpa-api\/","title":{"rendered":"Deproxy a lazily loaded JPA reference (using standard Java and JPA API)"},"content":{"rendered":"

When tried to tune our near-production application, we came to problem when a single entity reference When tried to tune our near-production application, we came to problem when a single entity reference (not a collection) is loaded lazily. We used inheritance with this entity and hibernate JPA provider (as probably any other provider) inserts a proxy object into referencing entity instead of reference to real object loaded from database (as it is not yet loaded).<\/p>\n

We came to a problem because we were casting super class to subclasses using instanceof. The problem is that proxy class is already a subclass and cannot be cast to a sibling class. It is a proxy only to super class instance, but it’s not possible to access methods of superclasses directly. The proxy never gets converted to real subclass. The problem is specified here<\/a>, here<\/a> and here<\/a>. All described solutions depend on hibernate non-standard API to retrieve deproxied instance.<\/p>\n

However, after lots of thinking…<\/p>\n

… I found a solution to deproxy a class using standard Java and JPA API. Tested with hibernate, but does not require hibernate as a dependency and should work with all JPA providers.<\/p>\n

Only one requirement – its necessary to modify parent class (Address) and add a simple helper method.<\/p>\n

General idea: add helper method to parent class which returns itself. when method called on proxy, it will forward the call to real instance and return this real instance.<\/p>\n

Implementation is a little bit more complex, as hibernate recognizes that proxied class returns itself and still returns proxy instead of real instance. Workaround is to wrap returned instance into a simple wrapper class, which has different class type than the real instance.<\/p>\n

In code:<\/p>\n

class Address {\r\n  public AddressWrapper getWrappedSelf() {\r\n    return new AddressWrapper(this);\r\n  }\r\n...\r\n}\r\n\r\nclass AddressWrapper {\r\n  private Address wrappedAddress;\r\n...\r\n}\r\n\r\n<\/pre>\n
 <\/p>\n
To cast Address proxy to real subclass, use following:<\/p>\n
Address address = dao.getSomeAddress(...);\r\nAddress deproxiedAddress = address.getWrappedSelf().getWrappedAddress();\r\nif (deproxiedAddress instanceof WorkAddress) {\r\n  WorkAddress workAddress = (WorkAddress)deproxiedAddress;\r\n}<\/pre>\n
 <\/p>\n","protected":false},"excerpt":{"rendered":"
When tried to tune our near-production application, we came to problem when a single entity reference When tried to tune our near-production application, we came to problem when a single entity reference (not a collection) is loaded lazily. 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