Java/Spring Boot – How to autowire bean to a static field of a class or UtilityClass

There are situations in witch you need to access Spring ApplicationContext or any other Spring bean from a static method.

At the beginning I would like to emphasise, that wiring a bean into a static class is strongly discouraged. This article is for those who insist on taking going ahead with it and are aware of what they are doing.

Resources

Watch the tutorial on YouTube

The problem

We have a static utility class called StaticUtils which looks like below:

import com.bettercoding.spring.config.MyConfig;
import org.springframework.beans.factory.annotation.Autowired;

public final class StaticUtils {

    @Autowired
    private static MyConfig myConfig;

    private StaticUtils() {
    }

    public static void printFoo() {
        System.out.println("Foo:" + myConfig.getFoo());
    }
}

As you can see there is an @Autowired annotation in the 6th line and we expect that Spring inject a proper bean here. But pay attention to that the wired field is static.

Below is an example MyConfig class used in the utility class. It requires to pass foo property.

import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;

@Configuration
public class MyConfig {
    @Value("${foo}")
    private String foo;

    public String getFoo() {
        return foo;
    }
}

Let’s write an application class and to test it. Remember to pass -Dfoo=bar property.

import com.bettercoding.spring.utils.StaticUtils;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class MyApp {
    public static void main(String[] args) {
        SpringApplication.run(MyApp.class, args);
    }

    @Bean
    public CommandLineRunner commandLineRunner(ApplicationContext ctx) {
        return new CommandLineRunner() {
            @Override
            public void run(String... args) throws Exception {
                System.out.println("Hello World");
                StaticUtils.printFoo();
            }
        };
    }
}

The result is NullPointerException.

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.0.2.RELEASE)

2018-06-02 13:45:47.435  INFO 8077 --- [           main] com.bettercoding.spring.MyApp            : Starting MyApp on bc-vbox with PID 8077 (/home/better-coding/Workspace/static-initializer/out/production/classes started by better-coding in /home/better-coding/Workspace/static-initializer)
2018-06-02 13:45:47.442  INFO 8077 --- [           main] com.bettercoding.spring.MyApp            : No active profile set, falling back to default profiles: default
2018-06-02 13:45:47.539  INFO 8077 --- [           main] s.c.a.AnnotationConfigApplicationContext : Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@1b68b9a4: startup date [Sat Jun 02 13:45:47 CEST 2018]; root of context hierarchy
2018-06-02 13:45:48.495  INFO 8077 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
2018-06-02 13:45:48.516  INFO 8077 --- [           main] com.bettercoding.spring.MyApp            : Started MyApp in 1.689 seconds (JVM running for 2.339)
Hello World
2018-06-02 13:45:48.519  INFO 8077 --- [           main] ConditionEvaluationReportLoggingListener : 

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2018-06-02 13:45:48.524 ERROR 8077 --- [           main] o.s.boot.SpringApplication               : Application run failed

java.lang.IllegalStateException: Failed to execute CommandLineRunner
  at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:800) [spring-boot-2.0.2.RELEASE.jar:2.0.2.RELEASE]
  at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:781) [spring-boot-2.0.2.RELEASE.jar:2.0.2.RELEASE]
  at org.springframework.boot.SpringApplication.run(SpringApplication.java:335) [spring-boot-2.0.2.RELEASE.jar:2.0.2.RELEASE]
  at org.springframework.boot.SpringApplication.run(SpringApplication.java:1255) [spring-boot-2.0.2.RELEASE.jar:2.0.2.RELEASE]
  at org.springframework.boot.SpringApplication.run(SpringApplication.java:1243) [spring-boot-2.0.2.RELEASE.jar:2.0.2.RELEASE]
  at com.bettercoding.spring.MyApp.main(MyApp.java:13) [classes/:na]
Caused by: java.lang.NullPointerException: null
  at com.bettercoding.spring.utils.StaticUtils.printFoo(StaticUtils.java:19) ~[classes/:na]
  at com.bettercoding.spring.MyApp$1.run(MyApp.java:22) ~[classes/:na]
  at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:797) [spring-boot-2.0.2.RELEASE.jar:2.0.2.RELEASE]
  ... 5 common frames omitted

2018-06-02 13:45:48.525  INFO 8077 --- [           main] s.c.a.AnnotationConfigApplicationContext : Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@1b68b9a4: startup date [Sat Jun 02 13:45:47 CEST 2018]; root of context hierarchy
2018-06-02 13:45:48.527  INFO 8077 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Unregistering JMX-exposed beans on shutdown

Process finished with exit code 1

As you can see Spring is not able to inject beans directly to static class and the NullPointerException occurred.

The solution: Use StaticContextInitializer class

The most elegant and quite safe method I’ve known is what I called StaticContextInitializer. The idea is to create component class called StaticContextInitializer, which is responsible to initialize static classes in its @PostConstruct annotated method. Let’s see it in practice.

Step 1 – change utility class

Edit StaticUtils class, remove the @Autowired annotation and add setter method for myConfig field. The class after changes should looks like below.

import com.bettercoding.spring.config.MyConfig;

public final class StaticUtils {

    private static MyConfig myConfig;

    public static void setMyConfig(MyConfig myConfig) {
        StaticUtils.myConfig = myConfig;
    }

    private StaticUtils() {
    }

    public static void printFoo() {
        System.out.println("Foo:" + myConfig.getFoo());
    }
}

Step 2 – create StaticContextInitializer compoment

Create class called StaticContextInitializer with init method annotated by @PostConstruct. Pass myConfig to the StaticUtils class.

import com.bettercoding.spring.config.MyConfig;
import com.bettercoding.spring.utils.StaticUtils;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Component;

import javax.annotation.PostConstruct;

@Component
public class StaticContextInitializer {

    @Autowired
    private MyConfig myConfig;

    @Autowired
    private ApplicationContext context;

    @PostConstruct
    public void init() {
        StaticUtils.setMyConfig(myConfig);
    }
}

Step 3 – test

Run the application once again and you will see the expected result.

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.0.2.RELEASE)

2018-06-02 14:04:21.993  INFO 8556 --- [           main] com.bettercoding.spring.MyApp            : Starting MyApp on bc-vbox with PID 8556 (/home/better-coding/Workspace/static-initializer/out/production/classes started by better-coding in /home/better-coding/Workspace/static-initializer)
2018-06-02 14:04:21.998  INFO 8556 --- [           main] com.bettercoding.spring.MyApp            : No active profile set, falling back to default profiles: default
2018-06-02 14:04:22.141  INFO 8556 --- [           main] s.c.a.AnnotationConfigApplicationContext : Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@3d36e4cd: startup date [Sat Jun 02 14:04:22 CEST 2018]; root of context hierarchy
2018-06-02 14:04:23.078  INFO 8556 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
2018-06-02 14:04:23.093  INFO 8556 --- [           main] com.bettercoding.spring.MyApp            : Started MyApp in 1.679 seconds (JVM running for 2.253)
Hello World
Foo:bar
2018-06-02 14:04:23.101  INFO 8556 --- [       Thread-2] s.c.a.AnnotationConfigApplicationContext : Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@3d36e4cd: startup date [Sat Jun 02 14:04:22 CEST 2018]; root of context hierarchy
2018-06-02 14:04:23.106  INFO 8556 --- [       Thread-2] o.s.j.e.a.AnnotationMBeanExporter        : Unregistering JMX-exposed beans on shutdown

Process finished with exit code 0

Resources

GitHub: java-sandbox/autowire-bean-to-static-class
YouTube: Better-Coding channel

That’s all what I’ve prepared for you in this tutorial, if I helped you, please consider sharing this post to help me gain a wider audience.
Thanks and I hope to see you in my next tutorial.

Resources

The problem

The solution: Use StaticContextInitializer class

Step 1 – change utility class

Step 2 – create StaticContextInitializer compoment

Step 3 – test

Resources

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