What is Hibernate Query Language (HQL) ?
Hibernate offers a query language that embodies a very powerful and flexible mechanism to query, store, update, and retrieve objects from a database. This language, the Hibernate query Language (HQL), is an object-oriented extension to SQL.
How do you map Java Objects with Database tables ?
First we need to write Java domain objects (beans with setter and getter).
hbm.xml, where we map java class to table and database columns to Java class variables.
<hibernate-mapping> <class name="com.test.User" table="user"> <property column="USER_NAME" length="30" name="userName" not-null="true" type="java.lang.String"/> <property column="USER_PASSWORD" length="255" name="userPassword" not-null="true" type="java.lang.String"/> </class> </hibernate-mapping>
If you need Java based configuration with entity class then please read here.
What are the difference between load() and get() ?
- Only use the
load()method if you are sure that the object exists. If you are not sure that the object exists, then use one of the
load()method will throw an exception if the unique id is not found in the database.
get()method will return null if the unique id is not found in the database.
load()just returns a proxy by default and database won’t be hit until the proxy is first invoked.
get()will hit the database immediately.
What is the difference between and merge and update ?
update() if you are sure that the session does not contain an already persistent instance with the same identifier, and
merge() if you want to merge your modifications at any time without consideration of the state of the session.
What are two important sub-elements of class element in Hibernate ?
<class> element includes numerous elements to fully map the Java class, but two important sub-elements:
<id> element maps the unique ID attribute added to our CD class to the primary key of the database.
<property> element is name, which is the name of the attributes in our CD class.
Hibernate uses reflection to locate the setter/getter methods for each attribute specified. Any attributes that we don’t want to be persisted aren’t listed.
What are attributes of <id> element in Hibernate ?
name: The attribute name found in the CD class.
type: The class type given to the ID attribute.
unsaved-value: Specifies whether Hibernate should persist an object. The identifier attribute of a class is called id; it holds a value of null for a new object and an integer value for an object pulled from storage. Hibernate obtains the value of the identifier and compares it to the value specified in the unsaved-value attribute to determine whether Hibernate should persist the object. If the default value of the identifier used in your application isn’t null, you need to place the default value in the unsaved-value attribute.
What are sub-elements of <id> element in Hibernate ?
<id> element also includes a couple of sub-elements:
<column> element tells Hibernate what specific column in the mapped database table relates to the identifier for the class.
<generator> element specifies the generator to be used when creating the unique identifier. The generator class is specified in the name attribute of the
How to obtain SessionFactory in Hibernate ?
The Configuration object handles all the parsing and internal processing of mapping documents. Most of the real work done by Hibernate occurs in a
To build a Session object, a
SessionFactory object needs to be instantiated from the
buildSessionFactory() method builds a factory based on the specifics of the mapping documents processed by the
SessionFactory object has been created, the
Configuration object can be discarded.
What are Hibernate JDBC properties ?
you must specify JDBC driver class with this property. You can find a listing of available JDBC drivers at http://servlet.java.sun.com/products/jdbc/drivers/ index.html.
The URL property determines where the database server is located and how to connect with it. The URL has this format: <protocol>:<subprotocol>:<subname>
The format of the <subname> is: //<host>[: <port>] [/<database>]
Each application that attaches to a database should have a unique username and password associated with it. You can specify the username using this property. A unique username is important because in most database systems, you can specify the permissions available to that username; doing so adds a level of protection when you’re running multiple databases on a single system.
This property specifies the password to be used with the corresponding username.
You can use this property to specify the total number of connections to initially establish with the database server.
If you’re working with a JNDI connection, the datasource string is specified in this property.
The URL, as defined earlier, is specified in this property when a JNDI source is being used and a URL isn’t specified with the datasource. This property is optional.
This property specifies the name of the JNDI class that implements InitialContextFactory. This property is optional.
This property specifies the name of the dialect to use for the database connection.
In SQL query strings, you provide the table or tables from which information should be pulled without specifying schema or tablespaces. If you want your table to be fully qualified, use the default_schema property to specify the schema or tablespace Hibernate can generate with its SQL calls to the database server.
If you’re using JNDI and wish to provide a binding between the SessionFactory class and a JNDI namespace, use the session factory_name property. Provide the name, and Hibernate uses the values specified in hibernate.jndi and hibernate.jndi.class to create the initial context.
In SQL, an outer join can provide a performance increase because fewer database accesses are needed. By default, the use_outer_join property is true. If you don’t want to allow outer joins, set the property to false. For each relationship in an application’s set of tables, the outer join attribute determines the correct action.
The database server can create an outer join using a graph, where the nodes in the graph are tables and the edge and relationships are join conditions. Using the graph, a depth-first search can be performed to determine which tables will be part of the outer join. You can limit the depth of the search using this property. Note that outer joins may reduce the communication with the database, but a lot of work goes into building the outer join SQL string correctly.
When JDBC performs a query against the database, some rows are returned based on the fetch size for the JDBC driver being used. You can use this property to set the total number of rows retrieved by each JDBC fetch from the database.
The JDBC driver groups some number of INSERT, UPDATE, DELETE commands together and sends them as a batch to the server, thus reducing the communication time between the client and the database. If the batch_size property is set to a nonzero value, Hibernate attempts to use batch updates. The value used in the batch_size property dictates the total number of updates to use per batch.
If you’re using a user-supplied connection and wish to have Hibernate use scrollable resultsets, set this property to true. If you’re using a Hibernate-created connection to the database server, this property has no effect.
Instead of directly copying the binary data to the query string, JDBC lets you use a stream. To make JDBC use streams for binary data, set this property to true.
By default, Hibernate uses CGLIB; but you can defeat this behavior by setting this property to false. If the property has a false value, Hibernate uses runtime reflection instead.
You can provide extraneous properties for JNDI’s InitialContextFactory class using this property. The property and its value are passed to the JNDI factory.
When a database is confronted with multiple clients making updates and queries (a situation called database concurrency), data may be changed and returned to clients with wrong values. There are three primary concurrency issues: Dirty Read, Nonrepeatable Read and Phantom Insert. You can solve all three of these situations using the isolation levels associated with a database and ANSI SQL.
If you’re using a JDBC driver that includes a custom ConnectionProvider class, you need to specify that class in this property.
This property names a specific TransactionFactory class to be used with Hibernate.
This property specifies the JNDI name for the JTATransactionFactory.
If you want a log of the SQL commands generated by Hibernate to the underlying database, set the show_sql property to true.
When you’re using the Hibernate Query Language (HQL), Hibernate formulates appropriate SQL statements according to the dialect specified for the database. If you need to help Hibernate with a query, you can specify substitutions in the Hibernate configuration file.