AEM: Spring

Topics covered in this page:

A) Spring Inversion of Control (IOC)

B) Dependency injection

C) XML Configuration using p-namespace

D) Autowiring Dependencies

E) Lazy Initialization

Create Sample Project Structure from https://start.spring.io/

A) Spring Inversion of Control (IOC) - Transfers the control of objects to a container or framework
       Example: ApplicationContext

package com.sample.Spring;
public class MessagePojo {
   private String message;
   public String getMessage() { return message; }
   public void setMessage(String message) { this.message = message; }
}

package com.sample.Spring;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class MyMainClass {
   public static void main(String[] args) {
       ApplicationContext appContext = new ClassPathXmlApplicationContext("mybean.xml");
       MessagePojo pojo = (MessagePojo) appContext.getBean("messageBean");
       System.out.println(pojo.getMessage());
   }
}

xml file under /src/main/resources
<beans xmlns = "http://www.springframework.org/schema/beans"
xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation = "http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
   <bean id="messageBean" class="com.sample.Spring.MessagePojo">
       <property name="message" value="This is predefined value in xml"></property>
   </bean>
</beans>

B) Dependency injection: A pattern we can use to implement IoC
1) Constructor-Based Dependency Injection

2) Setter-Based Dependency Injection

3) Field-Based Dependency Injection

1) Constructor-Based Dependency Injection

package com.sample.Spring;
public class SpellChecker {
   public SpellChecker() { System.out.println("Inside SpellChecker constructor."); }
   public void checkSpelling() { System.out.println("Inside checkSpelling method."); }
}

package com.sample.Spring;
public class TextEditor {
private SpellChecker spellChecker;
   public TextEditor(SpellChecker spellChecker) {
       System.out.println("Inside TextEditor constructor.");
       this.spellChecker = spellChecker;
   }
   public void spellCheck() { spellChecker.checkSpelling(); }
}

package com.sample.Spring;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class MainApp {
   public static void main(String[] args) {
       ApplicationContext context = new ClassPathXmlApplicationContext("mybean.xml");
       TextEditor te = (TextEditor) context.getBean("textEditor");
       te.spellCheck();
   }
}

<beans xmlns = "http://www.springframework.org/schema/beans"
xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation = "http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
   <bean id = "textEditor" class = "com.sample.Spring.TextEditor">
       <constructor-arg ref = "spellChecker"/>
   </bean>
   <bean id = "spellChecker" class = "com.sample.Spring.SpellChecker"></bean>
</beans>

Output:

Inside SpellChecker constructor.
Inside TextEditor constructor.
Inside checkSpelling.

2) Setter-Based Dependency Injection

package com.sample.Spring;
public class TextEditor {
private SpellChecker spellChecker;
// Setter

   public void setSpellChecker(SpellChecker spellChecker) {
       System.out.println("Inside setSpellChecker.");
       this.spellChecker = spellChecker;
   }

// Getter
   public SpellChecker getSpellChecker() {
       return spellChecker;
   }
   public void spellCheck() {
       spellChecker.checkSpelling();
   }
}

Output:

Inside SpellChecker constructor.
Inside setSpellChecker.
Inside checkSpelling.

3) Field-Based Dependency Injection

package com.sample.Spring;

public class OrderService {

public String getOrderDetails(String orderId) {

return "Order details for order id=" + orderId;

}

package com.sample.Spring;

import org.springframework.beans.factory.annotation.Autowired;

public class OrderServiceClient {

@Autowired

private OrderService orderService;

public void showPendingOrderDetails() {

System.out.println(orderService.getOrderDetails("300"));

}

package com.sample.Spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Configuration;

@Configuration

public class MainApp {

@Bean

public OrderService orderService() {

return new OrderService();

}

@Bean

public OrderServiceClient orderServiceClient() {

return new OrderServiceClient();

}

public static void main(String... strings) {

AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(MainApp.class);

OrderServiceClient bean = context.getBean(OrderServiceClient.class);

bean.showPendingOrderDetails();

}

Output:

Order details for order id=300

C) XML Configuration using p-namespace

THIS

<?xml version = "1.0" encoding = "UTF-8"?>

<beans xmlns = "http://www.springframework.org/schema/beans"

xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation = "http://www.springframework.org/schema/beans

http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

<bean id = "john-classic" class = "com.example.Person">

<property name = "name" value = "John Doe"/>

<property name = "spouse" ref = "jane"/>

</bean>

<?xml version = "1.0" encoding = "UTF-8"?>

CAN BE REPLACED BY THIS

<beans xmlns = "http://www.springframework.org/schema/beans"

xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"

xmlns:p = "http://www.springframework.org/schema/p"

xsi:schemaLocation = "http://www.springframework.org/schema/beans

http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

<bean id = "john-classic" class = "com.example.Person"

p:name = "John Doe"

p:spouse-ref = "jane"/>

</bean>

</beans>

D) Autowiring Dependencies

1) The Spring framework enables automatic dependency injection. In other words, by declaring all the bean dependencies in a Spring configuration file, Spring container can autowire relationships between collaborating beans. This is called Spring bean autowiring.

2) When a bean is being constructed, the @Autowired dependencies should be available. Otherwise, if Spring cannot resolve a bean for wiring, it will throw an exception. To fix this, we need to declare a bean of the required type: @Autowired(required = false)

3) We can use @Autowired on Properties, Setters and Constructors

4) By default, Spring resolves @Autowired entries by type. If more than one bean of the same type is available in the container, the framework will throw a fatal exception. To resolve this conflict, we need to tell Spring explicitly which bean we want to inject using @Qualifier annotation

@Component("fooFormatter")

public class FooFormatter implements Formatter {

public String format() {

return "foo";

}

@Component("barFormatter")

public class BarFormatter implements Formatter {

public String format() {

return "bar";

}

public class FooService {

@Autowired

@Qualifier("fooFormatter")

private Formatter formatter;

}

5) Autowiring can be done by Custom Qualifier

@Qualifier

@Target({

ElementType.FIELD, ElementType.METHOD, ElementType.TYPE, ElementType.PARAMETER})

@Retention(RetentionPolicy.RUNTIME)

public @interface FormatterType {

String value();

}

@FormatterType("Foo")

@Component

public class FooFormatter implements Formatter {

public String format() {

return "foo";

}

@FormatterType("Bar")

@Component

public class BarFormatter implements Formatter {

public String format() {

return "bar";

}

@Component

public class FooService {

@Autowired

@FormatterType("Foo")

private Formatter formatter;

}

E) Lazy Initialization

When we configure a bean with lazy initialization, the bean will only be created, and its dependencies injected, once they're needed.

Enable Lazy Initialization: It can be enabled in any of the below following ways

1) In application.yml add

spring:

main:

lazy-initialization: true

Note: All the defined beans will use lazy initialization, except for those that we explicitly configure with @Lazy(false).

2) In application.properties add

spring.main.lazy-initialization=true

Note: All the defined beans will use lazy initialization, except for those that we explicitly configure with @Lazy(false).

3) Use the SpringApplicationBuilder method

SpringApplicationBuilder(Application.class)

.lazyInitialization(true)

.build(args)

.run();

4) Use the SpringApplication class

SpringApplication app = new SpringApplication(Application.class);

app.setLazyInitialization(true);

app.run(args);

5) Use with Example

public class Writer {

private final String writerId;

public Writer(String writerId) {

this.writerId = writerId;

System.out.println(writerId + " initialized!!!");

}

public void write(String message) {

System.out.println(writerId + ": " + message);

}

@SpringBootApplication

public class Application {

@Bean("writer1")

public Writer getWriter1() {

return new Writer("Writer 1");

}

@Bean("writer2")

public Writer getWriter2() {

return new Writer("Writer 2");

}

public static void main(String[] args) {

ApplicationContext ctx = SpringApplication.run(Application.class, args);

ctx.setLazyInitialization(true);

ctx.run(args);

System.out.println("Application context initialized!!!");

Writer writer1 = ctx.getBean("writer1", Writer.class);

writer1.write("First message");

Writer writer2 = ctx.getBean("writer2", Writer.class);

writer2.write("Second message");

}

AEM

Thursday 3 June 2021

Spring

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