Understand the five ways to obtain beans in Spring

To understand the five ways to obtain beans in Spring, you need specific code examples

Spring is an open source lightweight Java development framework, one of its core features It is IoC (Inversion of Control, inversion of control). In Spring, Bean is the basic unit that constitutes an application, and there are many ways to obtain Beans. This article will introduce the five ways to obtain Beans in Spring and provide specific code examples.

1. Obtain through the configuration file

In Spring, we can define the Bean through the configuration file, and then obtain it through the BeanFactory or ApplicationContext. The following is an example:

<!-- 定义一个Bean -->
<bean id="myBean" class="com.example.MyBean">
    <!-- 设置属性值 -->
    <property name="name" value="Alice" />
</bean>

// 获取BeanFactory
BeanFactory beanFactory = new XmlBeanFactory(new ClassPathResource("applicationContext.xml"));

// 获取Bean
MyBean myBean = (MyBean) beanFactory.getBean("myBean");

// 使用Bean
myBean.sayHello();

2. Obtain through annotations

In addition to using configuration files to define beans, Spring also supports the use of annotations to define beans. You can use @Component or custom annotations to define beans, and then obtain dependent beans through @Autowired or @Resource annotations. The following is an example:

@Component
public class MyBean {

    @Value("Alice")
    private String name;

    public void sayHello() {
        System.out.println("Hello, " + name + "!");
    }
}

@Service
public class MyService {

    @Autowired
    private MyBean myBean;

    public void doSomething() {
        myBean.sayHello();
    }
}

// 获取ApplicationContext
ApplicationContext applicationContext = new AnnotationConfigApplicationContext(AppConfig.class);

// 获取Bean
MyService myService = applicationContext.getBean(MyService.class);

// 使用Bean
myService.doSomething();

3. Obtain through FactoryBean

FactoryBean in Spring is a special Bean. It does not return itself directly, but returns an actual object through the getObject() method. Beans. The following is an example:

public class MyFactoryBean implements FactoryBean<MyBean> {

    @Override
    public MyBean getObject() throws Exception {
        MyBean myBean = new MyBean();
        myBean.setName("Alice");
        return myBean;
    }

    @Override
    public Class<?> getObjectType() {
        return MyBean.class;
    }

    @Override
    public boolean isSingleton() {
        return false;
    }
}

// 获取BeanFactory
BeanFactory beanFactory = new XmlBeanFactory(new ClassPathResource("applicationContext.xml"));

// 获取FactoryBean
MyBean myBean = (MyBean) beanFactory.getBean("myFactoryBean");

// 使用Bean
myBean.sayHello();

4. Obtain through AOP

In Spring, you can use AOP to enhance the Bean, such as adding logs, permission control, etc. before and after method execution. When getting a Bean through AOP, Spring will return a proxy object instead of the real Bean object. The following is an example:

@Component
public class MyBean {

    public void sayHello() {
        System.out.println("Hello!");
    }
}

@Aspect
@Component
public class MyAspect {

    @Before("execution(* com.example.MyBean.sayHello())")
    public void beforeSayHello() {
        System.out.println("Before sayHello");
    }
}

// 获取ApplicationContext
ApplicationContext applicationContext = new AnnotationConfigApplicationContext(AppConfig.class);

// 获取Bean
MyBean myBean = applicationContext.getBean(MyBean.class);

// 使用Bean
myBean.sayHello();

5. Obtaining through JNDI

In addition to using Spring's built-in container to obtain Beans, you can also obtain Beans through JNDI. This method is suitable for using Spring in a Java EE environment. The following is an example:

// 获取InitialContext
Context context = new InitialContext();

// 查找Bean
MyBean myBean = (MyBean) context.lookup("java:comp/env/myBean");

// 使用Bean
myBean.sayHello();

Through the above five methods, we can flexibly obtain Bean objects in Spring. Whether using configuration files, annotations, FactoryBean, AOP or JNDI, Spring can help us implement dependency injection and control inversion, improving the readability and maintainability of the code. I hope the code examples in this article can help readers better understand how to obtain beans in Spring.

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