java - 自动连线注释在自定义推土机转换器内不起作用

Question

我正在使用下一个推土机自定义转换器

public class MyCustomDozerConverter extends DozerConverter<MyObject, String> {

    @Autowired
    private AppConfig appConfig;

    public MyCustomDozerConverter() {
        super(MyObject.class, String.class);
    }

    @Override
    public String convertTo(MyObject source, String destination) {      
        String myProperty = appConfig.getWhatever();
        // business logic
        return destination;
    }

    @Override
    public MyObject convertFrom(String source, MyObject destination) {
        // business logic
        return null;
    }
}

我的问题是当它通过转换器内的convertTo方法时，我总是得到带有空值的appConfig实例，这当然会导致空指针异常

注意:我的 Spring Boot 类上面有这些注释:

@SpringBootApplication
@EnableAutoConfiguration
@ComponentScan({"com.xxx"})
@EntityScan("com.xxx")
@EnableJpaRepositories("com.xxx")

Answer 1

我通过下一个技巧解决了这个问题:

1- Using static with appConfig property.

2- instantiate it by spring so when dozer use default empty constructor it will find appConfig have a value already (which assigned before to it by spring)

这是我为此使用的代码:

@Component //import org.springframework.stereotype.Component;
public class MyCustomDozerConverter extends DozerConverter<MyObject, String> {

    private static AppConfig appConfig;

    // dozer needs this constructor to create an instance of converter (so it's a mandatory constructor)
    public MyCustomDozerConverter() {
        super(MyObject.class, String.class);
    }

    @Autowired // Spring will pass appConfig to constructor
    public MyCustomDozerConverter(AppConfig appConfig) {
        this();
        this.appConfig = appConfig;
    }

    @Override
    public String convertTo(MyObject source, String destination) {      
        String myProperty = appConfig.getWhatever();
        // business logic
        return destination;
    }

    @Override
    public MyObject convertFrom(String source, MyObject destination) {
        // business logic
        return null;
    }
}

更新:另一种解决方案

另一个技巧是使用 Spring ApplicationContextAware 从 getBean 方法获取单例对象:

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

@Component
public class ApplicationContextHolder implements ApplicationContextAware {
    private static ApplicationContext context;

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

    public static ApplicationContext getContext() {
        return context;
    }
}

然后在 AppConfig 类中创建一个静态方法并返回与所需类型匹配的单个 bean 的实例:

import org.springframework.context.annotation.Configuration;
import com.tripbru.ms.experiences.ApplicationContextHolder;

@Configuration
public class AppConfig {

    // Static method used to return an instatnce
    public static AppConfig getInstance() {
        return ApplicationContextHolder.getContext().getBean(AppConfig.class);
    }

    // Properties
}

然后通过 AppConfig.getInstance(); 在推土机转换器内直接调用它

public class MyCustomDozerConverter extends DozerConverter<MyObject, String> {

    private static AppConfig appConfig;

    public MyCustomDozerConverter() {
        super(MyObject.class, String.class);
        appConfig = AppConfig.getInstance(); // Here are we intializing it by calling the static method we created.
    }

    @Override
    public String convertTo(MyObject source, String destination) {      
        String myProperty = appConfig.getWhatever();
        // business logic
        return destination;
    }

    @Override
    public MyObject convertFrom(String source, MyObject destination) {
        // business logic
        return null;
    }
}