0

我想在我的应用程序的不同位置使用一项服务：

@Service
public class ActionListMain  { /* .... */ }

首先，我想在实体上下文中使用：

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@Table(name = "myTable")
public class myTable {
    @Autowired
    @Transient
    private static ActionListMain allActions;
    /* .... */
}

我还想在其他非注释类中使用它，例如：

public class Action {
    @Autowired
    private ActionListMain actionListMain;
}

另一方面，我有一个 StartupComponent ，它按预期连接：

@Component
public class StartupComponent {
    @Autowired
    private ActionListMain actionListMain;
}

为什么它在所有其他类中都是 NULL ？

4

2 回答 2

2

Spring 只能将 bean 自动装配到 Spring 管理的类中。由于 Action 和 MyTable 类不是由 Spring 管理的，因此 ActionListMain 不能在那里自动装配。

有一个（hackish）解决方法，包括创建一个 Spring 管理的 bean 并在其中自动装配 applicationContext，然后从静态 applicationContext 获取 bean。

@Component
public class SpringContext implements ApplicationContextAware {

    //Has to be static to have access from non-Spring-managed beans
    private static ApplicationContext context;

    public static <T extends Object> T getBean(Class<T> beanClass) {
        return context.getBean(beanClass);
    }

    @Override
    // Not static
    public void setApplicationContext(ApplicationContext context) throws BeansException {
        SpringContext.context = context;
    }
}
于 2020-03-29T10:46:56.003 回答
1

正如@Janar所提到的，Spring 只能将 bean 自动装配到 Spring 管理的类中。

您可以添加这个棘手的实现，让您可以ActionListMain脱离@Component.

我们将把所有 bean 注入（Spring + Custom）集中在BeansManager.

赞成/继续

  • 所有新服务都将自动实例化
  • 集中启动解决Circular Dependencies in Spring
  • 您需要调用.getBean手动启动

依赖项

<!-- https://mvnrepository.com/artifact/org.reflections/reflections -->
<dependency>
    <groupId>org.reflections</groupId>
    <artifactId>reflections</artifactId>
    <version>0.9.12</version>
</dependency>

实现的类Injectable需要有没有参数的构造函数

Injectable获取您的@Service/@Component实例的接口

public interface Injectable {

    /**
    * Initiate all beans dependencies.
    * @param manager Beans manager allows to get singleton instances
    */
    void init(BeansManager manager);
}

BeansManager 类

import java.lang.reflect.Method;
import java.util.List;
import java.util.Set;

import javax.annotation.PostConstruct;

import org.reflections.Reflections;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class BeansManager {

    @Autowired
    private List<Injectable> injectables;

    /**
     * This method will make sure all the injectable classes will get the
     * BeansManager in its steady state, where it's class members are ready to be
     * set.
     */
    @PostConstruct
    protected void inject() throws Exception {

        // Init all Spring @Component classes
        for (Injectable injectableItem : injectables) {
            injectableItem.init(this);
        }

        //Setup your package to scan
        Reflections reflections = new Reflections("com.example.demo");
        Set<Class<? extends Injectable>> classes = reflections.getSubTypesOf(Injectable.class);

        for (Class<? extends Injectable> clazz : classes) {

            // Avoid calling twicely if clazz already initialized by Spring
            if (getBean(clazz, Boolean.FALSE) == null) {
                Method init = clazz.getDeclaredMethod("init", BeansManager.class);
                init.invoke(clazz.newInstance(), this);
            }

        }
    }

    /**
     * Get singleton from BeansManager.
     * 
     * @param <T>   Spring service / component class
     * @param clazz singleton class
     * @return Singleton / throws exception
     * @throws NoSuchBeanDefinitionException If bean not found (required=true)
     */
    public <T> T getBean(Class<T> clazz) {
        return getBean(clazz, Boolean.TRUE);
    }

    /**
     * Get singleton from BeansManager.
     * 
     * @param <T>      Component service / component class
     * @param clazz    singleton class
     * @param required If bean not found, it throw exception (true) or returns null
     *                 (false)
     * @return Singleton / null / throws exception
     * @throws NoSuchBeanDefinitionException If bean not found (required=true) 
     */
    public <T> T getBean(Class<T> clazz, boolean required) {
        Object bean = null;
        for (Injectable injectableItem : injectables) {
            if (clazz.isInstance(injectableItem)) {
                bean = clazz.cast(injectableItem);
                return clazz.cast(bean);
            }
        }

        if (required) {
            throw new NoSuchBeanDefinitionException(clazz);
            
        } else {
            return null;
        }
    }
}

Injectable为您的课程实施

@Service
public class ActionListMain implements Injectable
...

public class MyTable implements Injectable
...

    @Override
    public void init(BeansManager manager) {
        allActions = manager.getBean(ActionListMain.class);
    }
...

public class Action implements Injectable

    private static ActionListMain actionListMain;

    @Override
    public void init(BeansManager manager) {
        actionListMain = manager.getBean(ActionListMain.class);
    }

@Component
public class StartupComponent implements Injectable
...
    @Override
    public void init(BeansManager manager) {
        actionListMain = manager.getBean(ActionListMain.class);     
    }

初始化您的应用程序

@SpringBootApplication
public class DemoApplication implements CommandLineRunner {

    @Autowired
    ActionListMain actionListMain;
    
    @Autowired
    StartupComponent startupComponent;
    
    Action action   = new Action();
    MyTable myTable = new MyTable();
    
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @Override
    public void run(String... args) throws Exception {
        actionListMain.sayHello();
        startupComponent.sayHello();
        action.sayHello();
        myTable.sayHello();
    }
}

com.example.demo.ActionListMain：就绪
类 com.example.demo.StartupComponent com.example.demo.ActionListMain@d28c214
类 com.example.demo.Action com.example.demo.ActionListMain@d28c214
类 com.example.demo。 MyTable com.example.demo.ActionListMain@d28c214

于 2020-03-29T16:48:11.747 回答