NullPointerException when trying to use field-injected beans inside a @Configurable class instantiat-CodePudding

I have this enum, used in a dto.

public enum MetadataType {
  TRANSACTION(new TransactionMetadataHandler());

  private MetadataHandler handler;

  public MetadataHandler getHandler() {
    return handler;
  }

  MetadataType(MetadataHandler handler) {
    this.handler = handler;
  }
}

public class MetadataInfo {
  private MetadataType type;

  public MetadataType getType() {
    return this.type;
  }
}

The idea being to be able to change what handler is used for each metadata type.

This is used in a service. Note, everything is simplified here. There are many functions that I want to be able to perform on any type of metadata without having a giant switch statement.

@Service
public class MetadataService {
  public String formatMetadata(MetadataInfo info) {
    String formattedMetadata = info.getType().getHandler().format(info);
    return formattedMetadata;
  }
}

public interface MetadataHandler {
  String formatMetadata(MetadataInfo info);
}

@Configurable
public class TransactionMetadataHandler implements MetadataHandler {
  @Autowired
  private SomeOtherBean someOtherBean;

  public String formatMetadata(MetadataInfo info) {
    someOtherBean.doSomething(); // NullPointerException here
  }
}

How do I fix this NullPointerException? It's like the enum is being processed too early and Spring never gets around to autowiring the TransactionMetadataHandler. Not sure what to do here.

CodePudding user response：

When the Enums are constructed on project startup, Spring hasn't had a chance to get up and running yet. So, instead, we can do this.

public enum MetadataType {
  TRANSACTION(TransactionMetadataHandler.class);

  private Class<? extends MetadataHandler> handlerClass;

  public MetadataHandler getHandlerClass() {
    return handlerClass;
  }

  MetadataType(Class<? extends MetadataHandler> handlerClass) {
    this.handlerClass = handlerClass;
  }
}

And then in the service, use reflection to get an instance.

@Service
public class MetadataService {
  public String formatMetadata(MetadataInfo info) {
    try {
      String formattedMetadata = info.getType().getHandlerClass().newInstance().format(info);
      return formattedMetadata;
    } catch (InstantiationException | IllegalAccessException e) {
      // do whatever with the exception
    }
  }
}

This way, though nothing material changes, since the instantiation of TransactionMetadataHandler is at runtime, Spring will actually be up and running, and so will field-inject properly.

CodePudding user response：

What version of java and aspectj-maven-plugin are you using ? When i follow step mentioned in https://www.baeldung.com/aspectj i got the null pointer exception , however i had to replace codehaus plugin with `

<plugins>
            <plugin>
                <groupId>dev.aspectj</groupId>
                <artifactId>aspectj-maven-plugin</artifactId>
                <version>1.13.M3</version>
                <configuration>
                    <complianceLevel>16</complianceLevel>
                    <aspectLibraries>
                        <aspectLibrary>
                            <groupId>org.springframework</groupId>
                            <artifactId>spring-aspects</artifactId>
                        </aspectLibrary>
                    </aspectLibraries>
                </configuration>
                <executions>
                    <execution>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>

` on java version 16 and then compile team weaving worked fine for @Configurable annotation