如何在运行时根据条件使用不同的EntityManager？

Question

我的配置如下：

抽象类：

public abstract class AbstractFacade<T> {

    private Class<T> entityClass;

    public AbstractFacade(Class<T> entityClass) {
        this.entityClass = entityClass;
    }

    protected abstract EntityManager getEntityManager();

    public T find(Object id) {
        return getEntityManager().find(entityClass, id);
    }

    // other methods create(T), edit(T), ...

扩展抽象类的Ejb（我有许多其他EJB和大约12个不同的持久性单元）：

@Stateless
public class FilesDao extends AbstractFacade<Files> {

    @PersistenceContext(unitName = "myfirstPU")
    private EntityManager firstEm;

    @PersistenceContext(unitName = "mysecondPU")
    private EntityManager secondEm;

    // i have more than two persistenceUnit ... 

    @Override
    protected EntityManager getEntityManager() {
         return firstEm; // or secondEm based on condition
    }

    public FilesDao() {
         super(Files.class);
    }

    public Files findByFileref(String inFileRef) {
         try {
             Query q = firstEm.createNamedQuery("Files.findByFileref"); // or secondEm based on condition
             q.setParameter("fileref", inFileRef);
             Files file = (Files) q.getSingleResult();
             return file;
        } catch (NoResultException e) {
             return null;
        }
    }

我想像这样使用FilesDao：

@Stateless
@LocalBean
public class FileBusiness {

    @EJB
    FilesDao fileDao;

    public void myMethod(){
    if(condition1){
         //use the FileDao with the EnityManager **firstEm**
    }
    else if(condition2){
        //use the FileDao with the EnityManager **secondtEm**
    }
    ...

}

有没有办法实现这一目标？

我读到了使用CDI和produce方法。

Answer 1

CDI生产者并不那么难，请参阅以下示例。

以下类是CDI-Qualifier注释，用于区分实现。

@Retention(RetentionPolicy.RUNTIME)
@Target({ TYPE, METHOD, FIELD, PARAMETER, CONSTRUCTOR }) 
@Qualifier 
public @interface MyfirstPUQualifier {  }

@Retention(RetentionPolicy.RUNTIME)
@Target({ TYPE, METHOD, FIELD, PARAMETER, CONSTRUCTOR }) 
@Qualifier 
public @interface MysecondPUQualifier {  }

以下是CDI-Bean，它注入不同的EntityManagers并实现生产者方法，使两个EntityManagers可用于CDI，通过CDI-Qualifiers区分

@ApplicationScoped
class MYProducer {
    @PersistenceContext(unitName = "myfirstPU")
    private EntityManager firstEm;

    @PersistenceContext(unitName = "mysecondPU")
    private EntityManager secondEm;

    @Produces
    @RequestScoped
    @MyfirstPUQualifier
    public EntityManager produceMyfirstPU(){
        return firstEm;
    }


    @Produces
    @RequestScoped
    @MysecondPUQualifier
    public EntityManager produceMysecondPU(){
        return secondEm;
    }
}

以下显示了注入两个EntityManagers的CDI-Bean。这可以提取到抽象基类，因为其他DAO也可能需要它。

@Stateless
public class FileDao {

    @Inject
    @MyfirstPUQualifier
    private EntityManager emFirst;

    @Inject
    @MysecondPUQualifier
    private EntityManager emSecond;

    public void myDaoMEthod(){
        final EntityManager em = getEntityManager();
        ...
    }

    private EntityManager getEntityManager(){
        if(condition1){
            return emFirst;
        }
        else if(condition2){
            return emSecond;
        }
    }
}

没有可以使用FileDao bean而无需关心EntityManager使用什么，因为无论如何都不应该在这个bean中决定

@Stateless
@LocalBean
public class FileBusiness {

    @EJB
    FilesDao fileDao;

    public void myMethod(){
        // FileDao will decide what entity manager to use
        fileDao.doStruff();
    }
}