Python – Hibernate – Jynx

Jynx 0.4 goes Hibernate

In Jynx 0.4 JPA/Hibernate annotations are supported. Although this is still work in progress some of the more complex nested annotations were tested as well as Hibernate extension annotations which cannot be single-name imported along with the corresponding JPA annotations without conflicts.

Jynx 0.4 provides other new features as well. One can now use @signature decorators to express Java method overloading. A simple Java parser is integrated. A Java parser was necessary to improve the Java class detection heuristics used to determine required imports when a Java proxy is created from a Jython class and compiled dynamically. Finally there is a new @bean_property decorator which creates a private attribute foo along with public getters and setters given a bean_property decorated method def foo(_):_. Full documentation of Jynx as well as its changes can be found here.

Using Hibernate from Jython

Starting and closing sessions and managing simple transactions is not difficult in Hibernate. In Jynx two context managers for with-statements are defined which hide open+close and begin+commit/rollback boilerplate from the programmer. Code for Hibernate sessions and transactions lives then in with-statement blocks.

class hn_session(object):
    '''
    Context manager which opens/closes hibernate sessions.
    '''
    def __init__(self, *classes):
        sessionFactory = buildSessionFactory(*classes)
        self.session   = sessionFactory.openSession()
 
    def __enter__(self):
        return self.session
 
    def __exit__(self, *exc_info):
        self.session.close()
 
class hn_transact(object):
    '''
    Context manager which begins and performs commits/rollbacks hibernate transactions.
    '''
    def __init__(self, session):
        self.tx = session.beginTransaction()
 
    def __enter__(self):
        return self.tx
 
    def __exit__(self, type, value, traceback):
        if type is None:
            self.tx.commit()
        else:
            self.tx.rollback()

A simple session using a single Entity Bean may then look like:

from __future__ import with_statement
 
from jynx.lib.hibernate import*
 
@Entity
class Course(Serializable):
    @Id
    @Column(name="COURSE_ID")
    @signature("public int _()")
    def getCourseId(self):
        return self.courseId
 
    @Column(name="COURSE_NAME", nullable = False, length=50)
    @signature("public String _()")
    def getCourseName(self):
        return self.courseName
 
    @signature("public void _(String)")
    def setCourseName(self, value):
        self.courseName = value
 
    @signature("public void _(int)")
    def setCourseId(self, value):
        self.courseId = value
 
with hn_session(Course) as session:
    course  = Course()
    course.setCourseId(121)
    course.setCourseName(str(range(5)))
    with hn_transact(session):
        session.saveOrUpdate(course)

Boilerplate Reduction

The standard class decorator for creating a Java class from a Jython class in Jynx is @JavaClass. In Jynx 0.4 some slightly extended decorators are introduced in particular @Entity and @Embeddable. Not only do they make Jython code more concise because one doesn’t have to stack @Entity and @JavaClass but translating with @Entity turns some automatically generated Java attributes into transient ones i.e. a @Transient annotation is applied which prevents those attributes to be mapped to table columns.

The massive boilerplate needed for defining a valid Entity Bean in the preceding example can be reduced using the @bean_property decorator:

@Entity
class Course(Serializable):
    @Id
    @Column(name="COURSE_ID")
    @bean_property(int)
    def courseId(self): pass
 
    @Column(name="COURSE_NAME", nullable = False, length=50)
    @bean_property(String)
    def courseName(self): pass

Applied to def courseId(self): pass the @bean_property decorator will cause the following Java code translation

    @Id @Column(name="COURSE_ID") private int courseId;
    int getCourseId() { return courseId; }
    int setCourseId(int value) { courseId = value; }

which specifies a complete Java Bean property.

Example

In the following example two Entities are associated using a one-to-one mapping between primary keys.

@Entity
class Heart(Serializable):
    @Id
    @bean_property(int)
    def id(self):pass
 
@Entity
class Body(Serializable):
    @Id
    @bean_property(int)
    def id(self):pass
 
    @OneToOne(cascade = CascadeType.ALL)
    @PrimaryKeyJoinColumn
    @bean_property(Heart)
    def heart(self):pass

Now we can check the behavior:

# session 1
with hn_session(Heart, Body) as session:
    body = Body()
    heart = Heart()
    body.heart = heart
    body.id = 1
    heart.id = body.id
    with hn_transact(session):
        session.saveOrUpdate(body)
        session.saveOrUpdate(heart)
 
# session 2
with hn_session(Heart, Body) as session:
    with hn_transact(session):
        b = session.get(Body, 1)
        assert b
        assert b.heart
        assert b.heart.id == 1

Summary

With Hibernate support in Jython we notice another clear departure from the CPython world and its web frameworks and components. Hibernate is distinctively Java and special techniques are needed to create compile time Java properties in a dynamic language. Jython has long been a second citizen in Python land. I suspect this is going to change with support of Java frameworks which alone have as many users/downloads as Python.

  1. Eugene Kondrashev says:

    Hi Kay What is the status of jynx project?

  2. kay says:

    Hi Eugene, Jynx is complete but since I don’t use Java / Jython right now, I don’t know if there are any issues with it. Occasionally I look at the issue-tracker at

    http://code.google.com/p/jynx

    and if I don’t see anyone finding bugs, I tend to believe the project is o.k. or gets ignored, which is fine for me in either way. Do you intend to use it seriously?

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