Why Are @Autowired Beans Null in Constructors?

Understanding Null @Autowired Beans in Constructors

In Java-based Spring applications, @Autowired dependencies are typically injected after bean instantiation. This means that if you reference an @Autowired bean in a constructor, it will be null.

In the provided code snippet, you attempt to use the @Autowired applicationProperties bean in the DocumentManager constructor, but it initially returns null. This is expected behavior because @Autowired beans are not yet available during construction.

To circumvent this issue, consider moving any initialization logic that depends on @Autowired beans to a separate post-construction method annotated with @PostConstruct. The @PostConstruct method is executed after bean instantiation and before the bean is ready for use.

In the corrected code below, the startOOServer() method has been moved to a @PostConstruct method:

@Component
public class DocumentManager implements IDocumentManager {

  private Log logger = LogFactory.getLog(this.getClass());
  private OfficeManager officeManager = null;
  private ConverterService converterService = null;

  @Autowired
  private IApplicationProperties applicationProperties;


  // Constructors may not access @Autowired beans, so move this logic to a @PostConstruct method

  public DocumentManager() {
  }

  @PostConstruct
  public void startOOServer() {
    if (applicationProperties != null) {
      if (applicationProperties.getStartOOServer()) {
        try {
          if (this.officeManager == null) {
            this.officeManager = new DefaultOfficeManagerConfiguration()
              .buildOfficeManager();
            this.officeManager.start();
            this.converterService = new ConverterService(this.officeManager);
          }
        } catch (Throwable e){
          logger.error(e);  
        }
      }
    }
  }

  public byte[] convert(byte[] inputData, String sourceExtension, String targetExtension) {
    byte[] result = null;

    startOOServer();
    ...
}

By using the @PostConstruct method, initialization code can now safely rely on @Autowired dependencies being available.

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