why is this simple spring boot application giving a nullpointer exception?

Question

I keep getting a nullpointer exception when trying to invoke a method on an autowired object and I don't know why. I am doing everything by the book. (spring in action in this case). I know that it is included in the application context correctly because I can see a print appearing whenever a singleton instance of the injectable object is created. Here is the code and the pom.xml (its a Netbeans maven spring boot project). Please help!

main class

package com.example;

import java.util.List;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;

@SpringBootApplication
public class BasicApplication {

    public static void main(String[] args) {             
        SpringApplication.run(BasicApplication.class, args);
        new UsingAutoWired();
    }
}

configuration file (with unused imports I know) /* * To change this license header, choose License Headers in Project Properties. * To change this template file, choose Tools | Templates * and open the template in the editor. */ package com.example;

import java.util.Properties;
import javax.sql.DataSource;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.context.annotation.PropertySource;
import org.springframework.context.annotation.PropertySources;

/**
 *
 * @author maurice
 */
@Configuration
public class ComponentScanConfig {

  @Bean
  public testclass testclass(){
      return new testclass();
  }

}

the test class that gets injected

package com.example;

import org.springframework.stereotype.Component;

/**
 *
 * @author maurice
 */
@Component
public class testclass {

    public void hoi(){
        System.out.println(" ----------moiiii");
    }

    public testclass(){
        System.out.println(" ----------------hoiii");
    }
}

the POM file

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>basic</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>basic</name>
    <description>Basic project for Spring Boot</description>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.3.RELEASE</version>
        <relativePath/>
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.7</java.version>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
            <optional>true</optional>
        </dependency>

        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>jcl-over-slf4j</artifactId>
            <version>1.7.25</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <fork>true</fork>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

usingAutoWired class

package com.example;

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

/**
 *
 * @author maurice
 */
public class UsingAutoWired {

    @Autowired
    testclass testclass;

    public UsingAutoWired(){
        testclass.hoi();
    }
}

As you can see this is as simple as it can get and yet I get an error on line 26 of the basic application class. Can anyone please tell me why?

Thank you

EDIT: I've changed the example and removed the static variable, it still gives me a nullpointer exception!

Answer 1

You cannot autowire a static variable. For more information:

Can you use @Autowired with static fields?

EDIT: After your edits, you can access your bean like these:

@SpringBootApplication
public class BasicApplication implements ApplicationContextAware {

    private static ApplicationContext ac;

    public static void main(String[] args) {
        SpringApplication.run(BasicApplication.class, args);
        testclass bean = ac.getBean(testclass.class);
        bean.hoi();
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.ac = applicationContext;
    }
}

You get the ApplicationContext of Spring in a static context and then you get the Bean, which is going to be autowired.

Answer 2

new UsingAutoWired();

from you main class and

@Autowired
    testclass testclass;

from your UsingAutoWired class will not work together. You need to get an instance of UsingAutoWired from Spring container and testclass will be injected.