A commonly asked question on sites like StackOverflow is “Why is the field i’m autowiring null”. When this happens it generally is an error of the user as an autowired field in Spring cannot be null. At startup Spring will try to satisfy all the dependencies of a bean, if the depenencies cannot be satisfied starting the application will stop with an UnsatisfiedDependencyException.

The following HelloWorldService is what this blog post will be using (or a slight variation thereof). It takes a name and uses a java.io.Writer to write a hello message. This java.io.Writer needs to be introduced as a dependency.

package biz.deinum.blog.autowiring.unsatisfied;

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

import java.io.IOException;
import java.io.Writer;

@Service
public class HelloWorldService {

    @Autowired
    private Writer writer;

    public void sayHello(String name) throws IOException {
        writer.write("Hello " + name);
    }
}

The following configuration enables component-scanning but does not provide a java.io.Writer dependency.

package biz.deinum.blog.autowiring.unsatisfied;

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan
public class HelloWorldConfig {}

Now to load all of this use the following clas

package biz.deinum.blog.autowiring.unsatisfied;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class HelloWorldUnsatisfied {

    public static void main(String[] args) throws Exception {
        var ctx = new AnnotationConfigApplicationContext(HelloWorldConfig.class);
    }
}

When running this class Spring will prevent it from properly starting due to not being able to find a java.io.Writer and will throw an UnsatisfiedDependencyException.

WARNING: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'helloWorldService': Unsatisfied dependency expressed through field 'writer'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'java.io.Writer' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'helloWorldService': Unsatisfied dependency expressed through field 'writer'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'java.io.Writer' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:643)
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:130)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1422)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:594)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:226)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:893)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:879)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:551)
	at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:89)
	at biz.deinum.blog.autowiring.unsatisfied.HelloWorldUnsatisfied.main(HelloWorldUnsatisfied.java:17)
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'java.io.Writer' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1714)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1270)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1224)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640)
	... 14 more

If the application starts and your field appears to be null it is generally due to one of the following issues:

  1. Using @Autowired on a static field
  2. Omitted @Autowired on a field
  3. Instance of bean not visible to Spring
  4. Using AOP and invoking a final, private or default access method
  5. Using XML and haven’t enabled annotation processing
  6. Using @Autowired fields in the constructor

Using @Autowired on a static field

Using the HelloWorldService below as a bean in a Spring application would fail, as dependency injection on static fields isn’t supported.

package biz.deinum.blog.autowiring.statik;

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

import java.io.IOException;
import java.io.Writer;

@Service
public class HelloWorldService {

    @Autowired
    private static Writer writer;

    public void sayHello(String name) throws IOException {
        writer.write("Hello " + name);
    }
}

The following configuration will load the class and setup the needed Writer dependency.

package biz.deinum.blog.autowiring.statik;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

import java.io.PrintWriter;
import java.io.Writer;

@Configuration
@ComponentScan
public class HelloWorldConfig {

    @Bean
    public Writer consoleWriter() {
        return new PrintWriter(System.out);
    }
}

The following class with a main method will load the HelloWorldConfig, obtain the HelloWorldService from the ApplicationContext and invoke the sayHello method. The result will be a NullPointerException as the field isn’t autowired.

package biz.deinum.blog.autowiring.statik;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class HelloWorldStatic {

    public static void main(String[] args) throws Exception {

        var ctx = new AnnotationConfigApplicationContext(HelloWorldConfig.class);
        var service = ctx.getBean(HelloWorldService.class);
        service.sayHello("World"); // Throws NullPointerException due to injection on static field
    }
}

The solution, as often, depends, the most obvious one would be to remove the static keyword and it would work as it is now a regular field. However there might be a compeling reason to make the field static. If that is the case you can either use constructor, setter or method injection to set the static field. Although this should be considered a hack, imho, instead of a solution.

Omitted @Autowired on a field

Using the HelloWorldService below as a bean in a Spring application would fail. As there is no @Autowired (or @Inject or @Resource) on the field, Spring doesn’t know it needs to inject a dependency into the field. So the field remains null.

package biz.deinum.blog.autowiring.statik;

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

import java.io.IOException;
import java.io.Writer;

@Service
public class HelloWorldService {

    private Writer writer;

    public void sayHello(String name) throws IOException {
        writer.write("Hello " + name);
    }
}

The following configuration will load the class and setup the needed Writer dependency.

package biz.deinum.blog.autowiring.statik;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

import java.io.PrintWriter;
import java.io.Writer;

@Configuration
@ComponentScan
public class HelloWorldConfig {

    @Bean
    public Writer consoleWriter() {
        return new PrintWriter(System.out);
    }
}

The following class with a main method will load the HelloWorldConfig, obtain the HelloWorldService from the ApplicationContext and invoke the sayHello method. The result will be a NullPointerException as the field isn’t autowired.

package biz.deinum.blog.autowiring.statik;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class HelloWorldStatic {

    public static void main(String[] args) throws Exception {

        var ctx = new AnnotationConfigApplicationContext(HelloWorldConfig.class);
        var service = ctx.getBean(HelloWorldService.class);
        service.sayHello("World"); // Throws NullPointerException due to no injection metadata present
    }
}

To fix you can add the @Autowired dependency to the field or even better use constructor based injection (which doesn’t require an annotation!).

Instance of bean not visible to Spring

For Spring to be able to do dependency injection it needs to know about the beans inside the ApplicationContext. If a bean is created outside of the context or not as bean Spring will not be able to do dependency injection.

Taking the following HelloWorldService

package biz.deinum.blog.autowiring.instance;

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

import java.io.IOException;
import java.io.Writer;

@Service
public class HelloWorldService {

    @Autowired
    private Writer writer;

    public void sayHello(String name) throws IOException {
        writer.write("Hello " + name);
    }
}

and the following starter

package biz.deinum.blog.autowiring.instance;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class HelloWorldNewInstance {

    public static void main(String[] args) throws Exception {

        var ctx = new AnnotationConfigApplicationContext(HelloWorldConfig.class);
        var service = new HelloWorldService();
        service.sayHello("World");
    }
}

Will also result in a NullPointerException. This small sample is propably quite obvious as the new instance is created outside of the Spring ApplicationContext. Here you actually have 2 instances of the HelloWorldService, 1 inside the ApplicationContext and the one freshly constructed. Sometimes it might be more subtle and the bean creation happens inside an @Bean method in an @Configuration class. Like as part of creating a FilterRegistrationBean in Spring Boot or a configuration enhancement in Spring Security.

@Bean
public FilterRegistrationBean myFilter() {
  var registration = new FilterRegistrationBean(new MyFilter());
  return registration;
}

The configuration above creates a new instance of MyFilter but this isn’t seen as a bean. If autowiring inside MyFilter is needed, it needs to have a dedicated @Bean method or be detected through component-scanning.

Using a class-based proxy and invoking a final, private or default access method

AOP with Spring is by default applied using proxies and generally there are no issues with that. However there are some things to understand about proxy based AOP. Only public and protected methods can be enhanched with this type op AOP. When using private or default access modifiers AOP won’t be applied, the same applies to final methods or classes when using class-based proxies (the default in Spring Boot!).

The aformentioned methods are an issue because when using proxy based AOP, spring will create an additional instance of the class which acts like, in this case, the HelloWorldService. This additional instance wraps the actual instance. When invoking a method on the class it will first call the different aspects, before passing it on to the actual instance. However as there is no way to proxy a final, private or default access method the method is called on the created proxy. This proxy class will not have the dependencies injected (as it doesn’t need it).

The following aspect will add a before and after logging line upon invocation of a method

package biz.deinum.blog.autowiring.aop;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.springframework.stereotype.Component;

@Aspect
@Component
public class LoggingAspect {

    @Around("execution(* biz.deinum.blog..*.*(..))")
    public Object log(ProceedingJoinPoint pjp) throws Throwable{
        try {
            System.out.println("Before method: " + pjp.getSignature().getName());
            return pjp.proceed();
        } finally {
            System.out.println("After method: " + pjp.getSignature().getName());
        }
    }
}

Given the following HelloWorldService with a final method

package biz.deinum.blog.autowiring.aop;

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

import java.io.IOException;
import java.io.Writer;

@Service
public class HelloWorldService {

    @Autowired
    private Writer writer;

    public final void sayHello(String name) throws IOException {
        writer.write("Hello " + name);
    }
}

The configuration needs to enable AspectJ proxy creation

package biz.deinum.blog.autowiring.aop;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.EnableAspectJAutoProxy;

import java.io.PrintWriter;
import java.io.Writer;

@Configuration
@ComponentScan
@EnableAspectJAutoProxy
public class HelloWorldConfig {

    @Bean
    public Writer consoleWriter() {
        return new PrintWriter(System.out);
    }

}

Finally to bootstrap all this

package biz.deinum.blog.autowiring.aop;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class HelloWorldAop {

    public static void main(String[] args) throws Exception {

        var ctx = new AnnotationConfigApplicationContext(HelloWorldConfig.class);
        var service = ctx.getBean(HelloWorldService.class);
        service.sayHello("World"); 
    }
}

Again this will through a NullPointerException due to being unable to proxy the final method.

The solution in this case is to remove the final modifier and it will run. Sometimes that might not be an option, introducing an interface and enable interface based proxies is another solution.

Common “mistakes”

  1. Using a private method in an @(Rest)Controller and the use of the Spring Security @PreAuthorize
  2. Using @Lazy on a type or @Autowired field

Using XML and not enable annotation processing

This one is an oldie but still appears at times. Historically Spring uses XML files for its configuration and as of Spring 2.0 annotation based configuration was possible. However as XML was still the leading one the annotation processing had to be enabled. To enable annotation processing you need to add <context:annotation-config /> or use <context:component-scan />. Without one of these annotations wouldn’t be processed.

package biz.deinum.blog.autowiring.xml;

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

import java.io.IOException;
import java.io.Writer;

public class HelloWorldService {

    @Autowired
    private Writer writer;

    public final void sayHello(String name) throws IOException {
        writer.write("Hello " + name);
    }
}

The service above has an @Autowired annotation but is not a component. Using an XML based application context one would need to create the bean and the needed dependencies.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:util="http://www.springframework.org/schema/util"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
       http://www.springframework.org/schema/util https://www.springframework.org/schema/util/spring-util.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">

    <bean id="helloWorldService" class="biz.deinum.blog.autowiring.xml.HelloWorldService" />

    <bean id="writer" class="java.io.PrintWriter">
        <constructor-arg>
            <util:constant static-field="java.lang.System.out" />
        </constructor-arg>
    </bean>

</beans>

This would create the 2 beans but not apply annotation based injection due to the absence of <context:annotation-config />. Loading this XML file and obtaining the service to execute the sayHello method would result, yet again, in a NullPointerException.

package biz.deinum.blog.autowiring.xml;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class HelloWorldXml {

    public static void main(String[] args) throws Exception {

        var ctx = new ClassPathXmlApplicationContext("applicationContext.xml");
        var service = ctx.getBean(HelloWorldService.class);
        service.sayHello("World");
    }
}

Using @Autowired fields in the constructor

Using the HelloWorldService below as a bean in a Spring application would fail. The @Autowired field writer hasn’t been injected yet, as the service is still being constructed by the JVM. Spring can only inject the field after the object has been constructed. If you need the dependency in the constructor use constructor based DI instead of fields, or just always use constructor injection as that should be the preferred way of doing DI.

package biz.deinum.blog.autowiring.statik;

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

import java.io.IOException;
import java.io.Writer;

@Service
public class HelloWorldService {

    @Autowired
    private Writer writer;

    public HelloWorldService() {
        writer.write("Constructed: " + getClass().getSimpleName());
    }

    public void sayHello(String name) throws IOException {
        writer.write("Hello " + name);
    }
}

The following configuration will load the class and setup the needed Writer dependency.

package biz.deinum.blog.autowiring.statik;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

import java.io.PrintWriter;
import java.io.Writer;

@Configuration
@ComponentScan
public class HelloWorldConfig {

    @Bean
    public Writer consoleWriter() {
        return new PrintWriter(System.out);
    }
}

The following class with a main method will load the HelloWorldConfig, obtain the HelloWorldService from the ApplicationContext and invoke the sayHello method. The result will be a NullPointerException as the field isn’t autowired yet.

package biz.deinum.blog.autowiring.statik;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class HelloWorldStatic {

    public static void main(String[] args) throws Exception {

        var ctx = new AnnotationConfigApplicationContext(HelloWorldConfig.class);
        var service = ctx.getBean(HelloWorldService.class);
        service.sayHello("World"); // Throws NullPointerException due to too early access to the field
    }
}

Summary

As a rule of thumb an autowired field in Spring cannot be null. If it looks like it is null the error is generally on the user side of things and can be tracked down to one of the following issues:

  1. Using @Autowired on a static field
  2. Omitted @Autowired on a field
  3. Instance of bean not visible to Spring
  4. Using AOP and invoking a final, private or default access method
  5. Using XML and haven’t enabled annotation processing
  6. Using @Autowired fields in the constructor

The code can be found on GitHub.