spring bean-- autowired的正確用法

這兩天用idea寫spring注入的時候每一次

 @Autowired Worker worker;
   
   
  
  
          

都會報黃，用過這個ide的都知道，說明你代碼需要重構了。
然后提示的信息是

Spring Team recommends: “Always use constructor based dependency injection in your beans. Always use assertions for mandatory dependencies”

大致意思是 ，每一次的依賴注入都用構造方法吧，並且每一次對強制依賴關系使用斷言，大白話就是我們這樣可以在構造方法里面做點校驗了，這樣在spring容器啟動的時候就會發現錯誤。就類似於編譯器那些在編譯器就可以發現的錯誤，這樣防止在使用的時候出現運行時異常。導致程序崩掉。強制依賴關系可以理解為，B的方法中調用A的方法

群里面還發了個例子

// Fieldinjection/NullPointerException example: 
class MyComponent {

  @Inject MyCollaborator collaborator;

  public void myBusinessMethod() {
    collaborator.doSomething(); // -> NullPointerException
  }
}
  Constructor injection should be used:  
class MyComponent {

  private final MyCollaborator collaborator;

  @Inject
  public MyComponent(MyCollaborator collaborator) {
    Assert.notNull(collaborator, "MyCollaborator must not be null!");
    this.collaborator = collaborator;
  }

  public void myBusinessMethod() {
    collaborator.doSomething(); // -> safe
  }
}
   
   
  
  
          

本來實現一個null的bean很簡單

顯示用BeanFactoryPostProcessor實現，發現如果添加一個null會直接報錯

因為在register里面會進行判斷

Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
   
   
  
  
          

后來無奈搜索引擎一發

發現了可以 factorybean一發（原理后面補，，最近在研究源碼，還沒看這一塊）

@Component
public class Worker implements FactoryBean<Void> {

    public void doWork(){
        System.out.println("do Work...");
    }

    @Override
    public Void getObject() throws Exception {
        return null;
    }

    @Override
    public Class<?> getObjectType() {
        return Worker.class;
    }

    @Override
    public boolean isSingleton() {
        return true;
    }
}
   
   
  
  
          

那么我的依賴注入先這么寫

@Component
public class Factory {
    @Autowired Worker worker;

    public void createProduct(){
        System.out.println("生產中....");
        worker.doWork();
        System.out.println("生產完成...");
    }
}
   
   
  
  
          

運行，可以運行，報NPL
打印結果

生產中....
Exception in thread "main" java.lang.NullPointerException
   
   
  
  
          

@Component
public class Factory {
    final Worker worker;

    @Autowired public Factory(Worker worker) {
        Assert.notNull(worker, "worker must not be null!");
        this.worker = worker;
    }

    public void createProduct(){
        System.out.println("生產中....");
        worker.doWork();
        System.out.println("生產完成...");
    }
}
   
   
  
  
          

結果 無法運行，容器啟動失敗

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'factory' defined in file [F:\code\github\2017-up\spring-code\target\classes\com\wuhulala\studySpring\testnull\Factory.class]: Bean instantiation via constructor failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.wuhulala.studySpring.testnull.Factory]: Constructor threw exception; nested exception is java.lang.IllegalArgumentException: worker must not be null!

   
   
  
  
          

善用斷言，在程序中啟動的時候就發現錯誤。

原文地址：https://blog.csdn.net/u013076044/article/details/70880718