Android Studio - add .jar library

Question

This seems an old answered question, but I'm still having some issues. I created a Web Server that shares some classes as .jar. This is an example of a shared POJO class

@JsonInclude(Include.NON_NULL)
@Entity
@Table(name = "user", uniqueConstraints =
{ @UniqueConstraint(columnNames = "email"),
    @UniqueConstraint(columnNames = "nick") })
public class User implements java.io.Serializable, RecognizedServerEntities
{
  @Id
  @GeneratedValue(strategy = IDENTITY)
  @Column(name = "id", unique = true, nullable = false)
  private Integer userId;

  @OneToMany(fetch = FetchType.LAZY, mappedBy = "threadAuthor",
      orphanRemoval = false)
  @Cascade({ CascadeType.SAVE_UPDATE })
  private Set<Thread> userThreads = new HashSet<Thread>(0);
}

Now, I'm trying to create an android app that simply use these shared classes: the app doesn't need to know anything about hibernate or javax.persistence (at least, is what I think). However, when I create a new project in Android Studio , it seems to require some additional libraries. Indeed, the gradle build process generates a lot of warnings like

warning: unknown enum constant CascadeType.REMOVE
  reason: class file for org.hibernate.annotations.CascadeType not found
warning: unknown enum constant FetchType.EAGER
  reason: class file for javax.persistence.FetchType not found

and at runtime the app crashes with this log message

java.lang.NoClassDefFoundError: org/hibernate/annotations/Cascade

So, do I have to import these .jar s as library and add a compile files() to build.gradle ? I've yet developed another client-side application using Java (Swing) and there is no need to import those libraries: the shared.jar was enough. Why do Android require them?

Answer 1

CascadeType and FetchType are part of Java EE. Try use findjar to download jar(s) which contains these classes.

Even if you will add downloaded jars to classpath, RuntimeException s still migt happen because these libs can use part of Java that isn't part of Android Java.

Just try and give us a feedback :)

Answer 2

Now, I'm trying to create an android app that simply use these shared classes: the app doesn't need to know anything about hibernate or javax.persistence (at least, is what I think).

Apparently, it does. Some of those annotations may be runtime annotations, in which case, you would need the JARs. Some of those that are compile-time annotations may expand into references to things in those JARs, and that is my interpretation of your error messages.

So, do I have to import these .jars as library and add a compile files() to build.gradle?

You are certainly welcome to try using Hibernate on Android, whether through JARs or artifacts up at someplace like Maven Central. Whether there is a port of Hibernate that works on Android, I cannot say. Last I checked, Hibernate was designed for server-side use with direct access to a database, whereas Android apps are designed for client-side use without direct access to a database (eg, using a Web service).