如何在 Spring-Boot 中注入静态类？

Question

This question was asked a lot of times. I am bit confused and hope your ideas.

I am integrating Google Cloud Storage in Spring-Boot.

I have config class.

@Component
@ConfigurationProperties(prefix = "gcs.credentials")
public class  GCSConfig {

    private String serviceAccountId;

    //...
}

Storage factory which implements singleton pattern.

@Component
public class StorageFactory {

    @Autowired
    private static GCSConfig gcsConfig;

    private static Storage instance = null;

    public static synchronized Storage getService() throws GeneralSecurityException, IOException {
        if (instance == null) {
            instance = buildService();
        }
        return instance;
    }

    private static Storage buildService() throws GeneralSecurityException, IOException {

        HttpTransport httpTransport = GoogleNetHttpTransport.newTrustedTransport();
        JsonFactory jsonFactory = new JacksonFactory();
        // ...
        // I use gcsConfig here
        // ...
        return new Storage.Builder(httpTransport, jsonFactory, credential)
        .setApplicationName("Google Cloud Storage App")
        .build();
    }
}

And I use StorageFactory in the service like that.

Storage client = StorageFactory.getService()

I have read that Autowired doesn't inject static members. Is there another way to implement it? Maybe here Spring-Boot's features which create singleton easily.

What should I read? Could you give me links? Can you guide me in the right direction?

Link for the GCS examples .

Answer 1

I would recommend not using static methods for this purpose. If you are using Spring Boot it would be better to use a configuration class to inject a singleton scoped Storage instance:

@Configuration
public class StorageServiceConfiguration {

  private GCSConfig gcsConfig;

  // I prefer using explicit setters for testing, YMMV
  @Autowired
  public void setGcsConfig(GCSConfig gcsConfig) {
    this.gcsConfig = gcsConfig;
  }

  @Bean
  public Storage getStorageService() {
     // Logic to create your Storage instance.
     return new Storage.Builder(...) ...;
  }
}

Depending upon what you're doing with GCSConfig you could just inject the property values or the Spring Boot Environment into the StorageServiceConfiguration class, and skip the intermediate object.

Answer 2

You do not need to inject static classes. Injection implies that an instance has been created. Your referencing class just has to use the import static ...

Answer 3

You can perform this operation as following way and it could best fit in your usecase

Step 1: create new class that will able to return instance of bean class

package com.xyx;

import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.stereotype.Component;

@Component
public class ApplicationContextProvider implements ApplicationContextAware {

    private static ApplicationContext context;

    private ApplicationContextProvider(){}

    public static ApplicationContext getApplicationContext() {
        return context;
    }

    public  static <T> T getBean(String name,Class<T> aClass){
        return context.getBean(name,aClass);
    }

    @Override
    public void setApplicationContext(ApplicationContext ctx) throws BeansException {
        context = ctx;
    }
}

Step 2: create your own configuration class where your config properties are bind

@Component
@ConfigurationProperties(prefix = "gcs.credentials")
public class  CloudConfig {
    private static CloudConfig objectInstance=null;

    private String serviceAccountId;

    public String getServiceAccountId() {
        return serviceAccountId;
    }

    public void setServiceAccountId(String serviceAccountId) {
        this.serviceAccountId = serviceAccountId;
    }

    public static CloudConfig getInstance(){
        if(objectInstance==null){
            objectInstance=ApplicationContextProvider.getBean("cloudConfig",CloudConfig.class);
        }
        return objectInstance;
    }
}

Finally now whenever you need configuration class instance just invoke CloudConfig.getInstance() method and access all the required data using gettter and setter of that class

get more detail about bean injection click here

I hope it will help you out. Thanks.