Wednesday, November 23, 2011

Read Gmail from Java

package com.timers;

import java.util.Properties;

import javax.mail.Flags;
import javax.mail.Folder;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.NoSuchProviderException;
import javax.mail.Session;
import javax.mail.Store;
import javax.mail.search.FlagTerm;
/**
 *
 * @author Vinay Guntaka
 * mail.jar is required to execute this program (download from java2s.com)
 * And your Gmail Credentails.
 * This Program reads only unread mails of your box
 *
 */
public class ReadGmail {
    public static void main(String args[]) {
        while (true) {
            try {
                System.out.println("Started.......");
                Start();
                System.out.println("...Read completed.......");

                try {
                    Thread.sleep(1000 * 5);
                } catch (InterruptedException e1) {

                }

            } catch (Exception e) {

                try {
                    e.printStackTrace();
                    System.out.println("..Error in connection Sleeping...");

                } catch (Exception e1) {

                }
            }
        }

    }

    public static void Start() throws Exception {
        Properties props = System.getProperties();

        props.setProperty("mail.store.protocol", "imaps");
        try {
            Session session = Session.getDefaultInstance(props, null);
            Store store = session.getStore("imaps");
            //simply username dnt include @gmail.com
            store.connect("imap.gmail.com", "username", "password");
            System.out.println(store);
            int cout = 0;

            Folder inbox = store.getFolder("Inbox");
            inbox.open(Folder.READ_ONLY);

            // / to read only unread mails
            FlagTerm ft = new FlagTerm(new Flags(Flags.Flag.SEEN), false);
            Message messages[] = inbox.search(ft);
           
            // To read all mails from inbox
            //Message messages[] = inbox.getMessages();

            for (Message message : messages) {
                System.out.println("Reading:" + (messages.length - cout));
                System.out.println("MEssage Content :: --"
                        + message.getSubject() + message.getReceivedDate());
                cout++;
            }

        } catch (NoSuchProviderException e) {
            e.printStackTrace();

        } catch (MessagingException e) {
            e.printStackTrace();

        }

    }
}

Sunday, October 30, 2011

Hibernate



1.What is ORM ?
ORM stands for object/relational mapping. ORM is the automated persistence of objects in a Java application to the tables in a relational database.

2.What does ORM consists of ?
An ORM solution consists of the followig four pieces:
  • API for performing basic CRUD operations
  • API to express queries refering to classes
  • Facilities to specify metadata
  • Optimization facilities : dirty checking,lazy associations fetching

3.What are the ORM levels ?
The ORM levels are:
  • Pure relational (stored procedure.)
  • Light objects mapping (JDBC)
  • Medium object mapping
  • Full object Mapping (composition,inheritance, polymorphism, persistence by reachability)
4.What is Hibernate?
Hibernate is a pure Java object-relational mapping (ORM) and persistence framework that allows you to map plain old Java objects to relational database tables using (XML) configuration files.Its purpose is to relieve the developer from a significant amount of relational data persistence-related programming tasks.

5.Why do you need ORM tools like hibernate?
The main advantage of ORM like hibernate is that it shields developers from messy SQL. Apart from this, ORM provides following benefits:
  • Improved productivity
    • High-level object-oriented API
    • Less Java code to write
    • No SQL to write
  • Improved performance
    • Sophisticated caching
    • Lazy loading
    • Eager loading
  • Improved maintainability
    • A lot less code to write
  • Improved portability
    • ORM framework generates database-specific SQL for you
6.What Does Hibernate Simplify?
Hibernate simplifies:
  • Saving and retrieving your domain objects
  • Making database column and table name changes
  • Centralizing pre save and post retrieve logic
  • Complex joins for retrieving related items
  • Schema creation from object model
7.What is the need for Hibernate xml mapping file?
Hibernate mapping file tells Hibernate which tables and columns to use to load and store objects. Typical mapping file look as follows:

8.What are the most common methods of Hibernate configuration?
The most common methods of Hibernate configuration are:
  • Programmatic configuration
  • XML configuration (hibernate.cfg.xml)

9.What are the important tags of hibernate.cfg.xml?
Following are the important tags of hibernate.cfg.xml:


10.What are the Core interfaces are of Hibernate framework?
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The five core interfaces are used in just about every Hibernate application. Using these interfaces, you can store and retrieve persistent objects and control transactions.
  • Session interface
  • SessionFactory interface
  • Configuration interface
  • Transaction interface
  • Query and Criteria interfaces


11.What role does the Session interface play in Hibernate?
The Session interface is the primary interface used by Hibernate applications. It is a single-threaded, short-lived object representing a conversation between the application and the persistent store. It allows you to create query objects to retrieve persistent objects.
Session session = sessionFactory.openSession();
Session interface role:
  • Wraps a JDBC connection
  • Factory for Transaction
  • Holds a mandatory (first-level) cache of persistent objects, used when navigating the object graph or looking up objects by identifier

12.What role does the SessionFactory interface play in Hibernate?
The application obtains Session instances from a SessionFactory. There is typically a single SessionFactory for the whole application—created during application initialization. The SessionFactory caches generate SQL statements and other mapping metadata that Hibernate uses at runtime. It also holds cached data that has been read in one unit of work and may be reused in a future unit of work
SessionFactory sessionFactory = configuration.buildSessionFactory();


13.What is the general flow of Hibernate communication with RDBMS?
The general flow of Hibernate communication with RDBMS is :
  • Load the Hibernate configuration file and create configuration object. It will automatically load all hbm mapping files
  • Create session factory from configuration object
  • Get one session from this session factory
  • Create HQL Query
  • Execute query to get list containing Java objects

14.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.

15.How do you map Java Objects with Database tables?
  • First we need to write Java domain objects (beans with setter and getter).
  • Write hbm.xml, where we map java class to table and database columns to Java class variables.
Example :
<hibernate-mapping>
 
<class name="com.test.User"  table="user">
  
<property  column="USER_NAME" length="255"
     
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>
16.What’s the difference between load() and get()?
load() vs. get() :-
load() 
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 get() methods. 
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. 

17.What is the difference between and merge and update ?
Use 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.

18.How do you define sequence generated primary key in hibernate?
Using <generator> tag.Example:-
<id column="USER_ID" name="id" type="java.lang.Long">
<generator class="sequence">
<param name="table">SEQUENCE_NAME</param>
  <generator>
</id>


19.Define cascade and inverse option in one-many mapping?
cascade - enable operations to cascade to child entities.
cascade="all|none|save-update|delete|all-delete-orphan"

inverse - mark this collection as the "inverse" end of a bidirectional association.
inverse="true|false"
Essentially "inverse" indicates which end of a relationship should be ignored, so when persisting a parent who has a collection of children, should you ask the parent for its list of children, or ask the children who the parents are?

20.What do you mean by Named – SQL query?
Named SQL queries are defined in the mapping xml document and called wherever required.Example:
<sql-query name = "empdetails">
  
<return alias="emp" class="com.test.Employee"/>
      SELECT emp.EMP_ID AS {emp.empid},
                 emp.EMP_ADDRESS AS {emp.address},
                 emp.EMP_NAME AS {emp.name}
FROM Employee EMP WHERE emp.NAME LIKE :name

</sql-query>

Invoke Named Query :
List people = session.getNamedQuery("empdetails")
.setString(
"TomBrady", name)
.setMaxResults(50)
.list();

21.How do you invoke Stored Procedures?
<sql-query name="selectAllEmployees_SP" callable="true">
<return alias="emp" class="employee">
 
<return-property name="empid" column="EMP_ID"/>       

<return-property name="name" column="EMP_NAME"/>  
<return-property name="address" column="EMP_ADDRESS"/>
    {
? = call selectAllEmployees() } </return>
</sql-query>


22.Explain Criteria API
Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.Example :
List employees = session.createCriteria(Employee.class)
       .add(Restrictions.like(
"name", "a%") )
         .add(Restrictions.like(
"address", "Boston"))
.addOrder(Order.asc(
"name") )
.list();

23.Define HibernateTemplate?
org.springframework.orm.hibernate.HibernateTemplate is a helper class which provides different methods for querying/retrieving data from the database. It also converts checked HibernateExceptions into unchecked DataAccessExceptions.

24.What are the benefits does HibernateTemplate provide?
The benefits of HibernateTemplate are :
  • HibernateTemplate, a Spring Template class simplifies interactions with Hibernate Session.
  • Common functions are simplified to single method calls.
  • Sessions are automatically closed.
  • Exceptions are automatically caught and converted to runtime exceptions.


25.How do you switch between relational databases without code changes?
Using Hibernate SQL Dialects , we can switch databases. Hibernate will generate appropriate hql queries based on the dialect defined.

26.If you want to see the Hibernate generated SQL statements on console, what should we do?
In Hibernate configuration file set as follows: <property name="show_sql">true</property>

27.What are derived properties?
The properties that are not mapped to a column, but calculated at runtime by evaluation of an expression are called derived properties. The expression can be defined using the formula attribute of the element.
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28.What is component mapping in Hibernate?
  • A component is an object saved as a value, not as a reference
  • A component can be saved directly without needing to declare interfaces or identifier properties
  • Required to define an empty constructor
  • Shared references not supported
Example:



29.What is the difference between sorted and ordered collection in hibernate?
sorted collection vs. order collection :-
sorted collection 
order collection 
A sorted collection is sorting a collection by utilizing the sorting features provided by the Java collections framework. The sorting occurs in the memory of JVM which running Hibernate, after the data being read from database using java comparator.  Order collection is sorting a collection by specifying the order-by clause for sorting this collection when retrieval. 
If your collection is not large, it will be more efficient way to sort it.  If your collection is very large, it will be more efficient way to sort it . 

31.What is the advantage of Hibernate over jdbc?
Hibernate Vs. JDBC :-
JDBC 
Hibernate 
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.   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. 
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.   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.  
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.   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.  
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.  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.  
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.   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.  
With JDBC, caching is maintained by hand-coding.   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.  
In JDBC there is no check that always every user has updated data. This check has to be added by the developer.   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.  


32.What are the Collection types in Hibernate ?
  • Bag
  • Set
  • List
  • Array
  • Map

33.What are the ways to express joins in HQL?
HQL provides four ways of expressing (inner and outer) joins:-
  • An implicit association join
  • An ordinary join in the FROM clause
  • A fetch join in the FROM clause.
  • A theta-style join in the WHERE clause.

34.Define cascade and inverse option in one-many mapping?
cascade - enable operations to cascade to child entities.
cascade="all|none|save-update|delete|all-delete-orphan"

inverse - mark this collection as the "inverse" end of a bidirectional association.
inverse="true|false"
Essentially "inverse" indicates which end of a relationship should be ignored, so when persisting a parent who has a collection of children, should you ask the parent for its list of children, or ask the children who the parents are?

35.What is Hibernate proxy?
The proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will initially return CGLIB proxies which implement the named interface. The actual persistent object will be loaded when a method of the proxy is invoked.


36.How can Hibernate be configured to access an instance variable directly and not through a setter method ?
By mapping the property with access="field" in Hibernate metadata. This forces hibernate to bypass the setter method and access the instance variable directly while initializing a newly loaded object.

37.How can a whole class be mapped as immutable?
Mark the class as mutable="false" (Default is true),. This specifies that instances of the class are (not) mutable. Immutable classes, may not be updated or deleted by the application.

38.What is the use of dynamic-insert and dynamic-update attributes in a class mapping?
Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.
  • dynamic-update (defaults to false): Specifies that UPDATE SQL should be generated at runtime and contain only those columns whose values have changed
  • dynamic-insert (defaults to false): Specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null.

39.What do you mean by fetching strategy ?
A fetching strategy is the strategy Hibernate will use for retrieving associated objects if the application needs to navigate the association. Fetch strategies may be declared in the O/R mapping metadata, or over-ridden by a particular HQL or Criteria query.


40.What is automatic dirty checking?
Automatic dirty checking is a feature that saves us the effort of explicitly asking Hibernate to update the database when we modify the state of an object inside a transaction.

41.What is transactional write-behind?
Hibernate uses a sophisticated algorithm to determine an efficient ordering that avoids database foreign key constraint violations but is still sufficiently predictable to the user. This feature is called transactional write-behind.
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42.What are Callback interfaces?
Callback interfaces allow the application to receive a notification when something interesting happens to an object—for example, when an object is loaded, saved, or deleted. Hibernate applications don't need to implement these callbacks, but they're useful for implementing certain kinds of generic functionality.

43.What are the types of Hibernate instance states ?
Three types of instance states:
  • Transient -The instance is not associated with any persistence context
  • Persistent -The instance is associated with a persistence context
  • Detached -The instance was associated with a persistence context which has been closed – currently not associated

44.What are the differences between EJB 3.0 & Hibernate
Hibernate Vs EJB 3.0 :-
Hibernate 
EJB 3.0 
Session–Cache or collection of loaded objects relating to a single unit of work  Persistence Context-Set of entities that can be managed by a given EntityManager is defined by a persistence unit 
XDoclet Annotations used to support Attribute Oriented Programming  Java 5.0 Annotations used to support Attribute Oriented Programming 
Defines HQL for expressing queries to the database  Defines EJB QL for expressing queries 
Supports Entity Relationships through mapping files and annotations in JavaDoc  Support Entity Relationships through Java 5.0 annotations 
Provides a Persistence Manager API exposed via the Session, Query, Criteria, and Transaction API  Provides and Entity Manager Interface for managing CRUD operations for an Entity 
Provides callback support through lifecycle, interceptor, and validatable interfaces  Provides callback support through Entity Listener and Callback methods 
Entity Relationships are unidirectional. Bidirectional relationships are implemented by two unidirectional relationships  Entity Relationships are bidirectional or unidirectional 

45.What are the types of inheritance models in Hibernate?
There are three types of inheritance models in Hibernate:
  • Table per class hierarchy
  • Table per subclass
  • Table per concrete class













Q. How will you configure Hibernate?
Answer:

The configuration files hibernate.cfg.xml (or hibernate.properties) and mapping files *.hbm.xml are used by the Configuration class to create (i.e. configure and bootstrap hibernate) the SessionFactory, which in turn creates the Session instances. Session instances are the primary interface for the persistence service.

" hibernate.cfg.xml (alternatively can use hibernate.properties): These two files are used to configure the hibernate sevice (connection driver class, connection URL, connection username, connection password, dialect etc). If both files are present in the classpath then hibernate.cfg.xml file overrides the settings found in the hibernate.properties file.

" Mapping files (*.hbm.xml): These files are used to map persistent objects to a relational database. It is the best practice to store each object in an individual mapping file (i.e mapping file per class) because storing large number of persistent classes into one mapping file can be difficult to manage and maintain. The naming convention is to use the same name as the persistent (POJO) class name. For example Account.class will have a mapping file named Account.hbm.xml. Alternatively hibernate annotations can be used as part of your persistent class code instead of the *.hbm.xml files.

Q. What is a SessionFactory? Is it a thread-safe object?
Answer:

SessionFactory is Hibernates concept of a single datastore and is threadsafe so that many threads can access it concurrently and request for sessions and immutable cache of compiled mappings for a single database. A SessionFactory is usually only built once at startup. SessionFactory should be wrapped in some kind of singleton so that it can be easily accessed in an application code.

SessionFactory sessionFactory = new Configuration().configure().buildSessionfactory();

Q. What is a Session? Can you share a session object between different theads?
Answer:

Session is a light weight and a non-threadsafe object (No, you cannot share it between threads) that represents a single unit-of-work with the database. Sessions are opened by a SessionFactory and then are closed when all work is complete. Session is the primary interface for the persistence service. A session obtains a database connection lazily (i.e. only when required). To avoid creating too many sessions ThreadLocal class can be used as shown below to get the current session no matter how many times you make call to the currentSession() method.

&
public class HibernateUtil {
&
public static final ThreadLocal local = new ThreadLocal();

public static Session currentSession() throws HibernateException {
Session session = (Session) local.get();
//open a new session if this thread has no session
if(session == null) {
session = sessionFactory.openSession();
local.set(session);
}
return session;
}
}

It is also vital that you close your session after your unit of work completes. Note: Keep your Hibernate Session API handy.

Q. What are the benefits of detached objects?
Answer:


Detached objects can be passed across layers all the way up to the presentation layer without having to use any DTOs (Data Transfer Objects). You can later on re-attach the detached objects to another session.
Q. What are the pros and cons of detached objects?
Answer:
Pros:

" When long transactions are required due to user think-time, it is the best practice to break the long transaction up into two or more transactions. You can use detached objects from the first transaction to carry data all the way up to the presentation layer. These detached objects get modified outside a transaction and later on re-attached to a new transaction via another session.

Cons

" In general, working with detached objects is quite cumbersome, and better to not clutter up the session with them if possible. It is better to discard them and re-fetch them on subsequent requests. This approach is not only more portable but also more efficient because - the objects hang around in Hibernate's cache anyway.

" Also from pure rich domain driven design perspective it is recommended to use DTOs (DataTransferObjects) and DOs (DomainObjects) to maintain the separation between Service and UI tiers.

Q. How does Hibernate distinguish between transient (i.e. newly instantiated) and detached objects?
Answer

" Hibernate uses the version property, if there is one.
" If not uses the identifier value. No identifier value means a new object. This does work only for Hibernate managed surrogate keys. Does not work for natural keys and assigned (i.e. not managed by Hibernate) surrogate keys.
" Write your own strategy with Interceptor.isUnsaved().


Q. What is the difference between the session.get() method and the session.load() method?

Both the session.get(..) and session.load() methods create a persistent object by loading the required object from the database. But if there was not such object in the database then the method session.load(..) throws an exception whereas session.get(&) returns null.

Q. What is the difference between the session.update() method and the session.lock() method?

Both of these methods and saveOrUpdate() method are intended for reattaching a detached object. The session.lock() method simply reattaches the object to the session without checking or updating the database on the assumption that the database in sync with the detached object. It is the best practice to use either session.update(..) or session.saveOrUpdate(). Use session.lock() only if you are absolutely sure that the detached object is in sync with your detached object or if it does not matter because you will be overwriting all the columns that would have changed later on within the same transaction.
Note: When you reattach detached objects you need to make sure that the dependent objects are reatched as well.
Q. How would you reatach detached objects to a session when the same object has already been loaded into the session?

You can use the session.merge() method call.

Q. What are the general considerations or best practices for defining your Hibernate persistent classes?


1.You must have a default no-argument constructor for your persistent classes and there should be getXXX() (i.e accessor/getter) and setXXX( i.e. mutator/setter) methods for all your persistable instance variables.

2.You should implement the equals() and hashCode() methods based on your business key and it is important not to use the id field in your equals() and hashCode() definition if the id field is a surrogate key (i.e. Hibernate managed identifier). This is because the Hibernate only generates and sets the field when saving the object.


3. It is recommended to implement the Serializable interface. This is potentially useful if you want to migrate around a multi-processor cluster.

4.The persistent class should not be final because if it is final then lazy loading cannot be used by creating proxy objects.

5.Use XDoclet tags for generating your *.hbm.xml files or Annotations (JDK 1.5 onwards), which are less verbose than *.hbm.xml files.