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Spring - The problem of injecting a shorter-lived scoped bean into a longer-lived scoped bean

[Last Updated: Jun 12, 2021]

If same scoped beans are wired together there's no problem. For example a singleton bean A injected into another singleton bean B. But if the bean A has the shorted-liver scope say prototype scope then there's a problem.

When injecting a shorter-lived scoped bean into a longer-lived scoped bean (for example, injecting a prototype bean into a single bean or an HTTP Session-scoped bean into singleton bean), then we may want to get a new instance of shorter-lived scoped bean multiple times during the lifespan of longer-lived scoped bean.

To understand the problem let's see an example. We are going to have two beans MyPrototypeBean, scoped as prototype and MySingletonBean, scoped as singleton. We will inject the prototype bean into the singleton bean. We will also access MySingletonBean via method call context#getBean(MySingletonBean.class) multiple times. We are expecting (per prototype specifications) that a new prototype bean will be created and be injected into MySingletonBean every time.

Example

Creating a prototype bean

package com.logicbig.example;

import java.time.LocalDateTime;

public class MyPrototypeBean {

  private String dateTimeString = LocalDateTime.now().toString();

  public String getDateTime() {
      return dateTimeString;
  }
}

Creating a singleton bean and injecting the prototype bean in it

package com.logicbig.example;


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

public class MySingletonBean {

  @Autowired
  private MyPrototypeBean prototypeBean;

  public void showMessage(){
      System.out.println("Hi, the time is "+prototypeBean.getDateTime());
  }
}

Declaring beans via @Bean and initiating app

package com.logicbig.example;

import org.springframework.beans.factory.config.ConfigurableBeanFactory;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Scope;

@Configuration
public class AppConfig {

  @Bean
  @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
  public MyPrototypeBean prototypeBean() {
      return new MyPrototypeBean();
  }

  @Bean
  public MySingletonBean singletonBean() {
      return new MySingletonBean();
  }

  public static void main(String[] args) throws InterruptedException {
      AnnotationConfigApplicationContext context =
              new AnnotationConfigApplicationContext(AppConfig.class);
      MySingletonBean bean = context.getBean(MySingletonBean.class);
      bean.showMessage();
      Thread.sleep(1000);

      bean = context.getBean(MySingletonBean.class);
      bean.showMessage();
  }
}

Output

Hi, the time is 2021-05-05T00:07:09.977
Hi, the time is 2021-05-05T00:07:09.977

In above output we are expecting MySingletonBean would print two different time, with approximately one sec difference.

The problem is: spring container creates the singleton bean MySingletonBean only once, and thus only gets one opportunity to inject the dependencies into it. The container cannot provide MySingletonBean with a new instance of MyPrototypeBean every time one is needed.

Solution to the problem

Following are the solutions to the above problems

  1. Inject ApplicationContext bean into MySingletonBean to get instance of MyPrototypeBean, whenever we need it (example).
  2. Lookup method injection. This approach involves in dynamic bytecode generation (example).
  3. Using Scoped Proxy. (example here and here).
  4. Using JSR 330 Provider<T> injected by Spring (example).
  5. Injecting Spring's ObjectProvider<T> (example).
  6. By using Java 8 Functions (example).


Example Project

Dependencies and Technologies Used:

  • spring-context 5.3.8: Spring Context. Version Compatibility: 3.2.3.RELEASE - 5.3.8 Version List
    ×

    Version compatibilities of spring-context with this example:

    • 3.2.3.RELEASE
    • 3.2.4.RELEASE
    • 3.2.5.RELEASE
    • 3.2.6.RELEASE
    • 3.2.7.RELEASE
    • 3.2.8.RELEASE
    • 3.2.9.RELEASE
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    • 5.1.15.RELEASE
    • 5.1.16.RELEASE
    • 5.1.17.RELEASE
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    • 5.1.19.RELEASE
    • 5.1.20.RELEASE
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    • 5.2.1.RELEASE
    • 5.2.2.RELEASE
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    Versions in green have been tested.

  • JDK 8
  • Maven 3.8.1

Injecting a shorter-lived scoped bean into a longer-lived scoped bean Select All Download
  • narrower-scoped-bean-injection
    • src
      • main
        • java
          • com
            • logicbig
              • example
                • AppConfig.java

    See Also