Java Notes

Spring Core Container

Deep dive into Spring Core Container - BeanFactory, ApplicationContext, bean lifecycle, bean scopes, profiles, and Spring Expression Language (SpEL).

The IoC Container

The Spring IoC container is the core of the Spring Framework. It creates objects, wires them together, configures them, and manages their complete lifecycle.

Container Types

Container	Interface	Description
BeanFactory	`BeanFactory`	Basic container, lazy initialization
ApplicationContext	`ApplicationContext`	Advanced container, eager init, events

java example

// BeanFactory (basic — rarely used directly)
BeanFactory factory = new XmlBeanFactory(new ClassPathResource("beans.xml"));

// ApplicationContext (standard — always use this)
ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
ApplicationContext ctx2 = new ClassPathXmlApplicationContext("beans.xml");
ApplicationContext ctx3 = new FileSystemXmlApplicationContext("/path/beans.xml");

ApplicationContext Extra Features

Feature	Description
Internationalization	MessageSource for i18n
Event Publishing	ApplicationEvent system
Resource Loading	Access files, URLs, classpath
AOP Integration	Auto-proxy creation
Eager Initialization	Catches config errors at startup

Bean Scopes

Scope	Description	Lifecycle
singleton	One instance per container (default)	Container lifetime
prototype	New instance per request	Created each time, not destroyed by container
request	One per HTTP request	Request lifetime
session	One per HTTP session	Session lifetime
application	One per ServletContext	Application lifetime

java example

@Component
@Scope("singleton")  // Default — one instance shared
public class ConfigService { }

@Component
@Scope("prototype")  // New instance every time
public class ShoppingCart { }

@Component
@Scope(value = "request", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class RequestLogger { }

@Component
@Scope(value = "session", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class UserSession { }

Singleton vs Prototype

java example

ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);

// Singleton — same instance
ConfigService c1 = ctx.getBean(ConfigService.class);
ConfigService c2 = ctx.getBean(ConfigService.class);
System.out.println(c1 == c2);  // true

// Prototype — different instances
ShoppingCart s1 = ctx.getBean(ShoppingCart.class);
ShoppingCart s2 = ctx.getBean(ShoppingCart.class);
System.out.println(s1 == s2);  // false

Output

true
false

Bean Configuration Methods

Method 1: Component Scanning

java example

@Configuration
@ComponentScan(basePackages = "com.example")
public class AppConfig { }

@Service
public class UserService { }  // Auto-discovered

Method 2: @Bean Methods

java example

@Configuration
public class AppConfig {

    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }

    @Bean(name = "emailSender")
    @Profile("production")
    public EmailService emailService() {
        return new SmtpEmailService();
    }

    @Bean
    @ConditionalOnProperty(name = "cache.enabled", havingValue = "true")
    public CacheManager cacheManager() {
        return new ConcurrentMapCacheManager("users", "products");
    }
}

Method 3: XML (Legacy)

xml example

<bean id="userService" class="com.example.UserService">
    <property name="repository" ref="userRepository" />
    <property name="maxRetries" value="3" />
</bean>

Profiles

java example

@Configuration
public class DataSourceConfig {

    @Bean
    @Profile("dev")
    public DataSource devDataSource() {
        // H2 in-memory database for development
        return new EmbeddedDatabaseBuilder()
            .setType(EmbeddedDatabaseType.H2)
            .addScript("schema.sql")
            .build();
    }

    @Bean
    @Profile("prod")
    public DataSource prodDataSource() {
        HikariDataSource ds = new HikariDataSource();
        ds.setJdbcUrl("jdbc:mysql://prod-server:3306/mydb");
        return ds;
    }
}

// Activate profile
// java -Dspring.profiles.active=prod -jar app.jar
// Or: context.getEnvironment().setActiveProfiles("prod");

Spring Expression Language (SpEL)

Example

@Component public class SystemConfig { @Value("${app.name}") // Property placeholder private String appName; @Value("#{systemProperties['user.home']}") // SpEL private String userHome; @Value("#{T(java.lang.Math).PI}") // Static field access private double pi; @Value("#{${app.timeout} * 2}") // Expression private int doubleTimeout; @Value("#{'Hello World'.toUpperCase()}") // Method call private String greeting; @Value("#{userService.getDefaultUser()}") // Bean method private User defaultUser; }

Property Sources

java example

@Configuration
@PropertySource("classpath:application.properties")
@PropertySource("classpath:database.properties")
public class AppConfig {

    @Autowired
    private Environment env;

    @Bean
    public DataSource dataSource() {
        HikariDataSource ds = new HikariDataSource();
        ds.setJdbcUrl(env.getProperty("db.url"));
        ds.setUsername(env.getProperty("db.username"));
        ds.setPassword(env.getProperty("db.password"));
        return ds;
    }
}

application.properties:

properties example

app.name=MyApplication
app.timeout=30
db.url=jdbc:mysql://localhost:3306/mydb
db.username=root
db.password=secret

BeanPostProcessor

Allows custom modification of beans before/after initialization:

java example

@Component
public class LoggingBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) {
        if (bean.getClass().isAnnotationPresent(Service.class)) {
            System.out.println("Initializing service: " + beanName);
        }
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) {
        // Could create proxies here (AOP does this)
        return bean;
    }
}

Interview Key Points

ApplicationContext is preferred over BeanFactory (eager init, events, i18n)
Default scope is singleton — ONE instance per container
Bean lifecycle: Instantiate → DI → Aware → PostProcessor → Init → Ready → Destroy
@PostConstruct runs after DI; @PreDestroy runs before destruction
Prototype beans are NOT destroyed by the container
Profiles allow environment-specific configuration
SpEL (#{expr}) evaluates expressions; ${prop} reads properties
BeanPostProcessor can modify any bean (AOP uses this internally)

Summary

In this chapter, we learned about Spring Core Container in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Spring Core Container.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, core

Continue learning this concept

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Java Notes

Spring Core Container

Deep dive into Spring Core Container - BeanFactory, ApplicationContext, bean lifecycle, bean scopes, profiles, and Spring Expression Language (SpEL).

The IoC Container

The Spring IoC container is the core of the Spring Framework. It creates objects, wires them together, configures them, and manages their complete lifecycle.

Container Types

Container	Interface	Description
BeanFactory	`BeanFactory`	Basic container, lazy initialization
ApplicationContext	`ApplicationContext`	Advanced container, eager init, events

java example

// BeanFactory (basic — rarely used directly)
BeanFactory factory = new XmlBeanFactory(new ClassPathResource("beans.xml"));

// ApplicationContext (standard — always use this)
ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
ApplicationContext ctx2 = new ClassPathXmlApplicationContext("beans.xml");
ApplicationContext ctx3 = new FileSystemXmlApplicationContext("/path/beans.xml");

ApplicationContext Extra Features

Feature	Description
Internationalization	MessageSource for i18n
Event Publishing	ApplicationEvent system
Resource Loading	Access files, URLs, classpath
AOP Integration	Auto-proxy creation
Eager Initialization	Catches config errors at startup

Bean Scopes

Scope	Description	Lifecycle
singleton	One instance per container (default)	Container lifetime
prototype	New instance per request	Created each time, not destroyed by container
request	One per HTTP request	Request lifetime
session	One per HTTP session	Session lifetime
application	One per ServletContext	Application lifetime

java example

@Component
@Scope("singleton")  // Default — one instance shared
public class ConfigService { }

@Component
@Scope("prototype")  // New instance every time
public class ShoppingCart { }

@Component
@Scope(value = "request", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class RequestLogger { }

@Component
@Scope(value = "session", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class UserSession { }

Singleton vs Prototype

java example

ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);

// Singleton — same instance
ConfigService c1 = ctx.getBean(ConfigService.class);
ConfigService c2 = ctx.getBean(ConfigService.class);
System.out.println(c1 == c2);  // true

// Prototype — different instances
ShoppingCart s1 = ctx.getBean(ShoppingCart.class);
ShoppingCart s2 = ctx.getBean(ShoppingCart.class);
System.out.println(s1 == s2);  // false

Output

true
false

Bean Configuration Methods

Method 1: Component Scanning

java example

@Configuration
@ComponentScan(basePackages = "com.example")
public class AppConfig { }

@Service
public class UserService { }  // Auto-discovered

Method 2: @Bean Methods

java example

@Configuration
public class AppConfig {

    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }

    @Bean(name = "emailSender")
    @Profile("production")
    public EmailService emailService() {
        return new SmtpEmailService();
    }

    @Bean
    @ConditionalOnProperty(name = "cache.enabled", havingValue = "true")
    public CacheManager cacheManager() {
        return new ConcurrentMapCacheManager("users", "products");
    }
}

Method 3: XML (Legacy)

xml example

<bean id="userService" class="com.example.UserService">
    <property name="repository" ref="userRepository" />
    <property name="maxRetries" value="3" />
</bean>

Profiles

java example

@Configuration
public class DataSourceConfig {

    @Bean
    @Profile("dev")
    public DataSource devDataSource() {
        // H2 in-memory database for development
        return new EmbeddedDatabaseBuilder()
            .setType(EmbeddedDatabaseType.H2)
            .addScript("schema.sql")
            .build();
    }

    @Bean
    @Profile("prod")
    public DataSource prodDataSource() {
        HikariDataSource ds = new HikariDataSource();
        ds.setJdbcUrl("jdbc:mysql://prod-server:3306/mydb");
        return ds;
    }
}

// Activate profile
// java -Dspring.profiles.active=prod -jar app.jar
// Or: context.getEnvironment().setActiveProfiles("prod");

Spring Expression Language (SpEL)

Example

Property Sources

java example

@Configuration
@PropertySource("classpath:application.properties")
@PropertySource("classpath:database.properties")
public class AppConfig {

    @Autowired
    private Environment env;

    @Bean
    public DataSource dataSource() {
        HikariDataSource ds = new HikariDataSource();
        ds.setJdbcUrl(env.getProperty("db.url"));
        ds.setUsername(env.getProperty("db.username"));
        ds.setPassword(env.getProperty("db.password"));
        return ds;
    }
}

application.properties:

properties example

app.name=MyApplication
app.timeout=30
db.url=jdbc:mysql://localhost:3306/mydb
db.username=root
db.password=secret

BeanPostProcessor

Allows custom modification of beans before/after initialization:

java example

@Component
public class LoggingBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) {
        if (bean.getClass().isAnnotationPresent(Service.class)) {
            System.out.println("Initializing service: " + beanName);
        }
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) {
        // Could create proxies here (AOP does this)
        return bean;
    }
}

Interview Key Points

ApplicationContext is preferred over BeanFactory (eager init, events, i18n)
Default scope is singleton — ONE instance per container
Bean lifecycle: Instantiate → DI → Aware → PostProcessor → Init → Ready → Destroy
@PostConstruct runs after DI; @PreDestroy runs before destruction
Prototype beans are NOT destroyed by the container
Profiles allow environment-specific configuration
SpEL (#{expr}) evaluates expressions; ${prop} reads properties
BeanPostProcessor can modify any bean (AOP uses this internally)

Summary

In this chapter, we learned about Spring Core Container in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Spring Core Container.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, core

Continue learning this concept

PreviousValidation in Spring Boot NextSpring Framework Introduction