Spring core

SPRING IOC CONTAINER
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OUTLINE
 Inversion of Control explained
 Spring IOC Container
 Spring Bean
 Container Overview
 Spring Bean Configuration
 XML Based Spring Bean Configuration
 Annotation Based Spring Bean Configuration
 Java Based Spring Bean Configuration
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INVERSION OF CONTROL
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 Object management inverted from Application Code to the Container
 A Design Pattern that says you do not create your objects but describe how they should be created.
 You don't directly connect your components and services together in code but describe which services are needed by which components in a
configuration file.
 Help achieve loose coupling between Object Dependencies
 Dependency Injection is a specialized form of Inversion of Control.
 Object Dependencies are injected by other assembler objects.
 Example of IOC is explained in the upcoming slides
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Class Diagram for a Registration Application, which may need to connect to different Databases.

INVERSION OF CONTROL (IOC)
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package com.training.spring;
public class RegistrationApp {
public static void main(String[] args) {
DBConnect dbcon = new MySQLConnection();
dbcon.connect();
}
}
DBConnect dbcon = new OracleConnection();
dbcon.connect();
}
}
Creating object of
corresponding DB class

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DBConnect dbcon = (DBConnect)Container.getComponent(args[0]);
if(dbcon !=null)
dbcon.connect();
}
}
IOC reverse the process of Object creation.
Container is going to provide us with the
required class object

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public class Container {
private static Map<String, object> container;
public synchronized static Object getComponent(final String componentName) {
if (container == null) {
container = new HashMap<String, Object>();
}
Object result = container.get(componentName);
if (result == null) {
if ("mysql".equals(componentName)) {
result = new MySQLConnection();
} else if ("oracle".equals(componentName)) {
result = new OracleConnection();
} else if ("sqlserver".equals(componentName)) {
result = new SQLServerConnection();
}
if (result != null) {
container.put(componentName, result);
}
}
return result;
}
}
Container class Implementation

SUMMEDUP
 DI is a process whereby objects define their dependencies through constructor arguments, setters or
arguments to a factory method. The container then injects those dependencies when it creates the bean
 This process is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself
controlling the instantiation or location of its dependencies by using direct construction of classes, or a
mechanism such as the Service Locator pattern.
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WHAT DOES IOC DO?
• Create new objects
• Configure/solve dependency among objects and assemble them
• Allow objects to be retrieved by id/name
• Manage object’s lifecycle
• Allow external configuration
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WHY DO WE USE IOC?
• Achieve Loose coupling among Object Dependencies
• Reduce the amount of code in your application
• Does the plumbing work for you
• Application is more testable
• No more creating and hooking of objects together
• No more lookup
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SPRING IOC CONTAINER
 Spring IOC Container is the program that injects dependencies into an object and make it
ready for use.
 Packages for Spring IOC container
 org.springframework.beans
 org.springframework.context
 2 types of IoC container implementation
 BeanFactory
 ApplicationContext
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BEAN FACTORY
 BeanFactory interface provides an advanced configuration mechanism capable of managing any type of
objects
 Provides the underlying basis for Spring’s IOC functionality.
 It is the root container that loads all the beans and provide dependency injection to enterprise
applications
 Now largely historical in nature for most users of Spring.
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APPLICATIONCONTEXT
 ApplicationContext is a subinterface of BeanFactory
 It adds easier integration with Spring AOP features, i18n, event publication, and application-layer specific
context
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USEFUL APPLICATIONCONTEXT IMPLEMENTATIONS
 AnnotationConfigApplicationContext: If we are using Spring in standalone java applications and using
annotations for Configuration, then we can use this to initialize the container and get the bean objects.
 ClassPathXmlApplicationContext: If we have spring bean configuration xml file in standalone
application, then we can use this class to load the file and get the container object.
 FileSystemXmlApplicationContext: This is similar to ClassPathXmlApplicationContext except that the
xml configuration file can be loaded from anywhere in the file system.
 AnnotationConfigWebApplicationContext and XmlWebApplicationContext for web applications.
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BEANFACTORY OR APPLICATIONCONTEXT?
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Feature BeanFactory ApplicationContext
Bean instantiation/wiring Yes Yes
Automatic BeanPostProcessor registratio
n
No Yes
Automatic BeanFactoryPostProcessor reg
istration
No Yes
Convenient MessageSource access (for
i18n)
No Yes
ApplicationEvent publication No Yes
Use an ApplicationContext unless you have a good reason for not doing so.

SPRING BEANS
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SPRING BEANS
 The objects that form the backbone of the application and that are managed by Spring IOC Container
are called beans.
 A bean is an object that is instantiated, assembled and otherwise managed by a Spring IOC Container.
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BEAN SCOPES
 Singleton
 Default Scope
 Only one instance of the bean will be created for each container
 Prototype
 A new instance will be created every time the bean is requested
 Request
 Same as prototype scope, but is used in Web Applications.
 A new instance will be created for each HTTP request
 Session
 A new bean will be created for each HTTP Session by the container
 Global-session
 To create global session beans for Portlet applications
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SPRING BEAN CONFIGURATION
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SPRING BEAN CONFIGURATION
 Spring provides three ways to configure beans to be used in applications
 XML Based Configuration
 By creating Spring Configuration XML file to configure the beans.
 Annotation Based Configuration
 Spring 2.5 introduced support for annotation-based configuration metadata. Base Container is still XML.
 Java Based Configuration
 Starting from Spring 3.0, we can configure Spring beans using java programs. Pure Java-based configuration. No need for
having XML file for configuration Metadata
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XML-BASED CONFIGURATION METADATA
 Root element: <beans>
 The XML contains one or more <bean> elements
 id (or name) attribute to identify the bean
 class attribute to specify the fully qualified class
 By default, beans are treated as singletons
 Can also be prototypes (non singletons)
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XML-BASED CONFIGURATION METADATA
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The bean’s ID
The bean’s fully-
qualified classname

DEPENDENCY INJECTION
 Setter-Based
 Dependencies are assigned through JavaBeans properties (for example, setter methods)
 Constructor-Based
 Dependencies are provided as constructor parameters and are not exposed as JavaBeans
properties
 Method-Based
 The container is responsible for implementing methods at runtime
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SETTER INJECTION
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CONSTRUCTOR INJECTION
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CONSTRUCTOR ARGUMENT RESOLUTION
 Index
 Type
 Name
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WHICH ONE TO CHOOSE?
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POINTS IN FAVOR OF CONSTRUCTOR
• Constructor injection enforces a strong dependency contract. In short, a bean cannot be instantiated
without being given all of its dependencies. It is perfectly valid and ready to use upon instantiation.
• Because all of the bean’s dependencies are set through its constructor, there’s no need for superfluous
setter methods. This helps keep the lines of code at a minimum.
• By only allowing properties to be set through the constructor, you are, in effect, making those properties
immutable.
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POINTS IN FAVOR OF SETTER
• If a bean has several dependencies, the constructor’s parameter list can be quite lengthy.
• If there are several ways to construct a valid object, it can be hard to come up with unique constructors
since constructor signatures vary only by the number and type of parameters.
• If a constructor takes two or more parameters of the same type, it may be difficult to determine what
each parameter’s purpose is.
• Constructor injection does not lend itself readily to inheritance. A bean’s constructor will have to pass
parameters to super() in order to set private properties in the parent object.
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CONSTRUCTOR-BASED VS SETTER-BASED DI
 Tips : Use constructor arguments for mandatory dependencies and setters for optional dependencies
 More properties, more arguments to constructor
 Hence the Spring team generally advocates setter injection
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METHOD-BASED INJECTION
 Useful when a singleton bean needs to use a non-singleton bean
 Using CGLIB library, Spring generates dynamically a subclass and overrides the look up method
 Spring overrides the getEmployee() using lookup-method injection to provide a new instance of a Employee every time that method
is called
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METHOD BASED INJECTION
 Look-up method must be as follows
 <public|protected> [abstract] <return-type> theMethodName(no-arguments)
 We need an abstract method which will be configured as a lookup-method in the
configuration file.
 Spring will generate a proxy around which will implement the abstract method and return the
object of the target bean. Again used only if scopes of both the beans are different
 Also you must have the CGLIB jar(s) in your classpath
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AUTOWIRING COLLABORATORS
• Spring can resolve collaborators (other beans) automatically for your bean by
inspecting the contents of the ApplicationContext
• Advantages:
- Reduces the need to specify properties or constructor arguments
- New dependencies can be added to a class without changing the configuration
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AUTOWIRING MODES
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Mode Explanation
No (Default) No autowiring
byName Autowiring by property name. Spring looks for a bean with the
same name as the property that needs to be autowired
byType Allows a property to be autowired if exactly one bean of the
property type exists in the container. If more than one exists, a
fatal exception is thrown.
constructor Analogous to byType, but applies to constructor arguments.
autodetect If a default constructor with no argument is found, the
dependencies will be auto-wired by type. Otherwise, they will
be auto-wired by constructor
• Note : Autowiring works best when it is used consistently across a project

AUTOWIRING EXAMPLE
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Autowire by Name
Autowire by Type

AUTOWIRING EXAMPLE
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Autowire Constructor

LIFECYCLE OF BEANS
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LIFECYCLE CALLBACKS
 Three ways to interact with the container’s bean lifecycle management
 By implementing the InitializingBean and DisposableBean interfaces
 Using init-method and destroy-method attributes in the bean definition
(if you don’t want your classes coupled to Spring interfaces)
 @PostConstruct and @PreDestroy annotations (More on this later)
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INITIALIZATION CALLBACKS
• Implement the InitializingBean interface and override afterPropertiesSet() method
• Container calls this method upon initialization of your beans
• Alternatively specify a POJO initialization using the init-method attribute
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public class Employee implements InitializingBean {
public void afterPropertiesSet() {
// do some initialization work
}}
public class Employee {
public void init() {
// do some initialization work
}
}
<bean id=“emp” class=“com.example.lifecycle.Employee“ init-method="init"/>

DESTRUCTION CALLBACKS
• Implement the DisposableBean interface and override destroy() method
• Container calls this method upon destruction of your beans
 Alternatively specify a POJO initialization using destroy-method attribute
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public class Employee implements DisposableBean{
public void destroy() {
//do some destruction work (like releasing pooled connections) }
}
public class Employee {
public void cleanup() {
// do some destruction work
}
}
<bean id=“emp" class=“com.example.lifecycle.Employee“ destroy-method="cleanup"/>

ANNOTATION BASED CONTAINER CONFIGURATION
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TO CONFIGURE SPRING - ANNOTATION VS XML
 Annotations
+ More concise configuration
+ Wiring is more close to the source
- Configuration becomes decentralized and harder to control
 XML
+ Wiring done without touching source code or recompiling
+ A centralized location for configuration
- Can become verbose
 It is up to the developer to decide the strategy that suits better
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CAN WE USE BOTH?
 Spring supports mix of both
 But Annotation injection is performed before XML injection
 Hence XML-based injection will override Annotation-based injection for properties wired through both
approaches
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ANNOTATIONS INTRODUCED IN SPRING
 Spring 2.0
 @Required
 Spring 2.5
 @Autowired
 @Resource, @PostConstruct, @PreDestroy
 Spring 3.0
 @Inject, @Qualifier, @Named, and @Provider
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@REQUIRED
• Applies to bean property setter methods
• Indicates that the affected bean property must be populated at configuration time
• Throws an exception if it has not been set
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@Required use. Must use
setter injection to set the
value

@AUTOWIRED
 Can be used in the Java source code for specifying DI requirement
 Places where @Autowired can be used
 Fields
 Setter methods
 Constructor methods
 Arbitrary methods
 Need to include the below element in the bean configuration file to use this
 <context:annotation-config>
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 Because autowiring by type may lead to multiple candidates, it is necessary to have more control over
the selection process
 One way to accomplish this is with Spring's @Qualifier annotation.
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Of all the beans of Address
class, it uses one matching the
address1 id.
@Qualifier can be used with
parameter names as well.

@RESOURCE
 Spring also supports injection using the @Resource on fields or bean property setter methods
 It takes a 'name' attribute, and by default Spring will interpret that value as the bean name to be injected
i.e., it follows by-name semantics
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Must have a bean with
id – address1 of type
Address defined in
config file.

@POSTCONSTRUCT AND @PREDESTROY
 An alternative to initialization callbacks and destruction callbacks
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JAVA BASED CONTAINER CONFIGURATION
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@CONFIGURATION AND @BEAN
 Class with @Configuration indicates that the class can be used as a source of bean definitions
 @Bean-annotated methods define instantiation, configuration, and initialization logic for objects to be
managed by the Spring IoC container
 This is equivalent to
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@Configuration
public class AppConfig {
@Bean
public MyService myService() {
return new MyServiceImpl();
} }
<beans>
<bean id="myService" class=“MyServiceImpl"/>
</beans>

ANNOTATIONCONFIGAPPLICATIONCONTEXT
 An ApplicationContext implementation
 Uses @Configuration classes as input
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ApplicationContext ctx = new
AnnotationConfigApplicationContext(AppConfig.class);
MyService myService = ctx.getBean(MyService.class);
myService.doStuff();
}

@CONFIGURATION AND @BEAN
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@Configuration
public class AppConfig {
@Bean
public TransferService transferService() {
return new TransferServiceImpl(accountRepository());
}
@Bean
public AccountRepository accountRepository() {
return new InMemoryAccountRepository();
} }
<bean id = "accountRepository"
class = “InMemoryAccountRepository“></bean>
<bean id = "transferService“ class = “TransferServiceImpl">
<property name="accountRepository" ref="accountRepository"/>
</bean>
This is same as:

SCANNING COMPONENTS FROM CLASSPATH
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SCANNING COMPONENTS FROM THE CLASSPATH
 So far the “base" bean definitions are explicitly defined in the XML file, while the annotations only drive the
dependency injection
 But Component Scanning avoids manual configuration
 It can automatically scan, detect, and instantiate your components with particular stereotype annotations from
the classpath
 The basic annotation denoting a Spring-managed component is @Component
 Other stereotypes include @Repository, @Service, and @Controller denoting components in the persistence,
service, and presentation layers, respectively
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AUTOMATICALLY DETECTING CLASSES AND REGISTERING BEAN
DEFINITIONS
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To Add in Configuration XML File other than DataSource
Configuartion common parent package for the two classes

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