I haven't worked with hibernate. I have little bit of experience in java. I was going through source of a beast of an java application created by Oracle(Retail Price Management). I was expecting a lot of sql code embedded in there as the application makes heavy use of database. But to my surprise, NO embedded SQL code! so far. I found that it was using what is called as "Hibernate" from the lot of .hbm.xml files. Is it a trademark for java programs using hibernate or maybe I haven't seen the complete codebase?. Could someone enlighten me how this is possible?. Thanks.
Hibernate, as all ORM tools, indeed lessens or eliminates the need to use raw SQL in Java code, due to the following:
many associations between various entities are recorded in the Hibernate mapping, so these are fetched automatically by Hibernate - i.e. if you have an aggregation relationshiop between two classes on the Java side, this may be mapped as a foreign key relationship in the DB, and Hibernate, whenever an instance of class A is loaded, can automatically load the associated instances of class B too,
many queries can be done in Hibernate's own HQL query language, or using its Criteria API.
Under the hood Hibernate does generate SQL to communicate with the DB, but this is not visible on the Java side. It can be seen in the logs though, if it is enabled.
Due to this, programs using Hibernate very rarely need to use JDBC or SQL directly. The exceptions are typically ralted to "tricky" legacy DB schemas which can't be fully handled by Hibernate.
Because that's the whole purpose of using Hibernate or any other object-relational mapping framework.
Hibernate solves object-relational impedance mismatch problems by replacing direct persistence-related database accesses with high-level object handling functions.
Hibernate generates SQL for all its standard database operations. It understands different SQL dialects, and the mapping files (.hbm.xml) tell it about the database structure so it knows how to construct its queries. There is a showSql setting you can turn on if you want to see it outputting its generated SQL as it runs.
Hibernate is an Object-Relational Mapper (ORM). ORMs are used to hide the ugly details of SQL incompatibility[sic] between databases from your program -- you define your tables and map them to an object hierarchy (the .hbm.xml files) and then Hibernate does the rest. Thus most programs that use Hibernate won't see a single phrase of SQL, unless there's a specific reason to execute a complicated query.
Hibernate is a tool, or technology that takes care of the interaction between the database and application for you. You have to tell the structure of the application and the database to it, this is what is in the .hbm.xml files.
The SQL is generated by Hibernate at runtime (kind of)
Say you have an Fruit class, and objects of this is persisted into a T_FRUIT table.
You say this to hibernate, via the .hbm.xml files. That there is a table T_FRUIT, this table is represented by the Fruit class, and which fields in the Fruit class correspond to which columns in th T_FRUIT table.
And then it knows whenever you are trying to save a fruit, it should insert/update to the T_FRUIT table.
When you want to create an Apple, you create an object of fruit corresponding to apple and save "save this fruit".
Hibernate takes care of persisting it.
You can have relationships defined between tables, and Hibernate is intelligent enough to persist in multiple tables.
When you fetch a fruit, hibernate fetches the details of the fruit and its children also(data from referencing tables). And you can say whether you want fetch all the children
at once, or as and when required.
And so on. Aim is to make your life easier, and code maintainable, easy to read, portable,...
With this info, let me redirect you.
Related
We have a (possibly large) custom data structure implemented in Java (8+). It has a simple and optimal API for querying pieces of data. The logical structure is roughly similar to an RDMS (it has e. g. relations, columns, primary keys, and foreign keys), but there is no SQL driver.
The main goal is to access the data via ORM (mapping logical entities to JPA annotated beans). It would be nice if we could use JPQL. Hibernate is preferred but other alternatives are welcome too.
What is the simplest way to achieve this? Which are the key parts of such an implementation?
(P. S. Directly implementing SessionImplementor, EntityManagerImplementor etc. seems to be too complicated.)
You have two possibilities.
Implement a JDBC compliant driver for your system, so you can use a JPA implementation such as Hibernate "directly" (although you may need to create a custom dialect for your system).
Program directly against the JPA specification like ObjectDB does, which bypasses the need to go through SQL and JPA implementations completely.
The latter one is probably easier, but you'd still need to implement the full JPA API. If it's a custom in-house-only system, there's very little sense in doing either one.
One idea I thought up just now, that I feel may work is this:
Use an existing database implementation like H2 and use the JPA integration with that. H2 already has a JPA integration libraries, so it should be easy.
In this database, create a Java stored procedure or function and call it from your current application through JPA. See this H2 documentation on how to create a Java stored procedure or function. (You may want to explore the section "Using a Function as a Table" also.)
Define a protocol for the service methods and encapsulate it in a model class. An instance of this model class may be passed to the function/SP and responses retrieved.
Caveat: I have never done this myself but I think it will work.
Edit: Here is a diagram representing the thought. Though the diagram show H2 separately, it will most probably be in the same JVM as "Your Java/JEE application". However, since it is not necessary to use H2, I have shown it as as separate entity.
In my java application I have some serialized entity classes with inheritance. When saving instances of these classes i am converting them to a byte array and saving to a longblob column in my database table. Is there any advantage using hibernate to implement this program. Because as far I understand hibernate is used to map entities with database tables in a proper way. But here I don't have a relational model to map attributes of entities. I am saving them as objects. Am I missing something. Please clarify me. Thanks in advance.
If you don't have a relational data model to save those objects and you can't change your schema, then you can use your current approach.
If you use PostgreSQL you might be interested in JSON storage as well. That way you can store your hierarchies using JSON objects and you can even run native SQL queries against them (although not inheritance-aware, but you can cope with that if you use some _class column to differ between object types).
The cleanest approach is to have the relation model in sync with your business domain model. That way you can benefit from:
optimistic locking (preventing lost updates phenomena)
caching (2nd level cache and query cache)
query-able hierarchies
an external DBA hierarchies could run an update on your hierarchies using mere SQL
auditing
I am working in a project which uses JPA ORM and framework provides two kinds of method to create queries.
entityManager.createQuery(query1);
entityManager.createNativeQuery(query2);
I understand the kinds of query string is to be passed to use them, but I don't know exactly why do we need to create native query? Probably we don't want to use ORM capabilities there?
You do not need to create a native query unless you want to. JPQL eventually is translated into SQL by the framework but the framework lets you call the native query also. Why would want to do that:
Low level access, which means that you can optimize and handle the mapping by yourself; with SQL you actually access the database table while with JPQL you access the entity objects;
Maybe you do not want to learn JPQL if you already know SQL
You already have the queries written in SQL, and do not have resources/time to port them to JPQL
createQuery uses JPAs own query language, you select from Class names instead of table names. This is not SQL, it is just similar, and is later transformed to real SQL. Mapping to java classes will be done automatically and actual class instances will be returned as result.
createNativeQuery uses real SQL, and will not be able to use JPA features. This method is used in general if you need to do something really odd that is not supported by JPA. A list of Object[] will be returned, and mapping to java objects will have to be done manually. In other words, its just like working with a DB before JPA came to, just slightly more convenient since connection handling is done automatically.
I have used it for optimization purposes. Using Native queries means that the ORM mapping is not in place, and instead of JPQL, you use the DB's native syntax. So, as #RasmusFranke also pointed out, if you need something that is not supported by JPA (like when you want to use DB vendor specific extensions, which is conceptually a bad idea, since JPA is all about being DB agnostic, but happens nevertheless. I know...)
The other effect of this is that by using native queries, only the supplied query is run. No eager fetching of other entities, or other unwanted stuff. This way, if you deal with huge amounts of objects, you can save some heap space.
There's an enterprise application using Java + Hibernate + PostgreSQL. Hibernate is configured via annotations in the Java source code. So far the database schema is fixed, but I faced the problem that it needs to be dynamic:I can receive data from different locations and I have to store these in different tables. This means that I have to create tables run-time.
Fortunately, it seems that all of these data coming from the different institutes can have the same schema. But I still don't know how to do that using Hibernate. There are two main problems:
How to tell to Hibernate that many different tables have the same structure? For example the "Patient" class can be mapped to not just the "patient" table, but the "patient_mayo_clinic" table, "patient_northwestern" table, etc. I can feel that this causes ambiguity: how Hibernate knows which table to access when I do operations on the Patient class? It can be any (but only one) of the former listed tables.
How can I dynamically create tables with Hibernate and bind a class to them?
Response to suggestions:
Thanks for all of the suggestions. So far all of the answers discouraged the dynamic creation of tables. I'll mark Axel's answer, since it achieves certain goals, and it is a supported solution. More specifically it's called multi-tenancy. Sometimes it's important to know some important phrases which describes our problem (or part of our problem).
Here are some links about multi-tenancy:
Multi-tenancy in Hibernate
Hibernate Chapter 16. Multi-tenancy
Multi-tenancy Design
EclipseLink JPA multi-tenancy
In real world scenario multi-tenancy also involves the area of isolating the sets of data from each other (also in terms of access and authorization by different credentials) once they are shoved into one table.
You can't do this with Hibernate.
Why not extend your patient table with an institute column?
This way you'll be able to differentiate, without running into mapping issues.
I am afraid you can't do this easily in Hibernate. You would have to generate the Java source, compile it, add it to your classpath and load it dynamically with java.reflection package. If that works, which I doubt it, it will be an ugly solution (IMHO).
Have you consider using a schema less database i.e: NoSQL databases or RDF
databases. They are much more flexible in terms of what you can store in them , basically things are not tight up against a relational schema.
In most environments it is not a good idea to create tables dynamically simply because dbas will not give you the rights to create tables in production.
Axel's answer may be right for you. Also look into Inheritance Mapping for Hibernate.
I agree that its not advisable to create tables dynamically nevertheless it's doable.
Personally i would do as Axel Fontaine proposed but if dynamic tables is a must-have for you I would consider using Partitioning.
PostgreSQL allows you to create ona main table and few child tables (partitions), records are disjunctive between child tables, but every record from any child table is visible in parent table. This means that you can insert rows into any child table you want using just simple insert statement (its not cool but has the same level of complexity as composing and persisting an entity, so its acceptable in your case) and query database using HQL
My question is regarding ORM and JDBC technologies, on what criteria would you decide to go for an ORM technology as compared to JDBC and other way round ?
Thanks.
JDBC
With JDBC, developer has to write code to map an object model's data representation to a relational data model and its corresponding database schema.
With JDBC, the automatic mapping of Java objects with database tables and vice versa conversion is to be taken care of by the developer manually with lines of code.
JDBC supports only native Structured Query Language (SQL). Developer has to find out the efficient way to access database, i.e. to select effective query from a number of queries to perform same task.
Application using JDBC to handle persistent data (database tables) having database specific code in large amount. The code written to map table data to application objects and vice versa is actually to map table fields to object properties. As table changed or database changed then it’s essential to change object structure as well as to change code written to map table-to-object/object-to-table.
With JDBC, it is developer’s responsibility to handle JDBC result set and convert it to Java objects through code to use this persistent data in application. So with JDBC, mapping between Java objects and database tables is done manually.
With JDBC, caching is maintained by hand-coding.
In JDBC there is no check that always every user has updated data. This check has to be added by the developer.
HIBERNATE.
Hibernate is flexible and powerful ORM solution to map Java classes to database tables. Hibernate itself takes care of this mapping using XML files so developer does not need to write code for this.
Hibernate provides transparent persistence and developer does not need to write code explicitly to map database tables tuples to application objects during interaction with RDBMS.
Hibernate provides a powerful query language Hibernate Query Language (independent from type of database) that is expressed in a familiar SQL like syntax and includes full support for polymorphic queries. Hibernate also supports native SQL statements. It also selects an effective way to perform a database manipulation task for an application.
Hibernate provides this mapping itself. The actual mapping between tables and application objects is done in XML files. If there is change in Database or in any table then the only need to change XML file properties.
Hibernate reduces lines of code by maintaining object-table mapping itself and returns result to application in form of Java objects. It relieves programmer from manual handling of persistent data, hence reducing the development time and maintenance cost.
Hibernate, with Transparent Persistence, cache is set to application work space. Relational tuples are moved to this cache as a result of query. It improves performance if client application reads same data many times for same write. Automatic Transparent Persistence allows the developer to concentrate more on business logic rather than this application code.
Hibernate enables developer to define version type field to application, due to this defined field Hibernate updates version field of database table every time relational tuple is updated in form of Java class object to that table. So if two users retrieve same tuple and then modify it and one user save this modified tuple to database, version is automatically updated for this tuple by Hibernate. When other user tries to save updated tuple to database then it does not allow saving it because this user does not have updated data.
Complexity.
ORM If your application is domain driven and the relationships among objects is complex or you need to have this object defining what the app does.
JDBC/SQL If your application is simple enough as to just present data directly from the database or the relationships between them is simple enough.
The book "Patterns of enterprise application architecture" by Martin Fowler explains much better the differences between these two types:
See: Domain Model and Transaction Script
I think you forgot to look at "Functional Relational Mapping"
I would sum up by saying:
If you want to focus on the data-structures, use an ORM like JPA/Hibernate
If you want to shed light on treatments, take a look at FRM libraries: QueryDSL or Jooq
If you need to tune your SQL requests to specific databases, use JDBC and native SQL requests
The strengh of various "Relational Mapping" technologies is portability: you ensure your application will run on most of the ACID databases.
Otherwise, you will cope with differences between various SQL dialects when you write manually the SQL requests.
Of course you can restrain yourself to the SQL92 standard (and then do some Functional Programming) or you can reuse some concepts of functionnal programming with ORM frameworks
The ORM strenghs are built over a session object which can act as a bottleneck:
it manages the lifecycle of the objects as long as the underlying database transaction is running.
it maintains a one-to-one mapping between your java objects and your database rows (and use an internal cache to avoid duplicate objects).
it automatically detects association updates and the orphan objects to delete
it handles concurrenty issues with optimistic or pessimist lock.
Nevertheless, its strengths are also its weaknesses:
The session must be able to compare objects so you need to implements equals/hashCode methods
But Objects equality must be rooted on "Business Keys" and not database id (new transient objects have no database ID!).
However, some reified concepts have no business equality (an operation for instance).
A common workaround relies on GUIDs which tend to upset database administrators.
The session must spy relationship changes but its mapping rules push the use of collections unsuitable for the business algorithms.
Sometime your would like to use an HashMap but the ORM will require the key to be another "Rich Domain Object" instead of another light one...
Then you have to implement object equality on the rich domain object acting as a key...
But you can't because this object has no counterpart on the business world.
So you fall back to a simple list that you have to iterate on (and performance issues result from)
The ORM API are sometimes unsuitable for real-world use.
For instance, real world web applications try to enforce session isolation by adding some "WHERE" clauses when you fetch data...
Then the "Session.get(id)" doesn't suffice and you need to turn to more complex DSL (HSQL, Criteria API) or go back to native SQL
The database objects conflicts with other objects dedicated to other frameworks (like OXM frameworks = Object/XML Mapping).
For instance, if your REST services use jackson library to serialize a business object.
But this Jackson exactly maps to an Hibernate One.
Then either you merge both and a strong coupling between your API and your database appears
Or you must implement a translation and all the code you saved from the ORM is lost there...
On the other side, FRM is a trade-off between "Object Relational Mapping" (ORM) and native SQL queries (with JDBC)
The best way to explain differences between FRM and ORM consists into adopting a DDD approach.
Object Relational Mapping empowers the use of "Rich Domain Object" which are Java classes whose states are mutable during the database transaction
Functional Relational Mapping relies on "Poor Domain Objects" which are immutable (so much so you have to clone a new one each time you want to alter its content)
It releases the constraints put on the ORM session and relies most of time on a DSL over the SQL (so portability doesn't matter)
But on the other hand, you have to look into the transaction details, the concurrency issues
List<Person> persons = queryFactory.selectFrom(person)
.where(
person.firstName.eq("John"),
person.lastName.eq("Doe"))
.fetch();
It also depends on the learning curve.
Ebean ORM has a pretty low learning curve (simple API, simple query language) if you are happy enough with JPA annotations for mapping (#Entity, #Table, #OneToMany etc).