spring bean-- autowired的正确用法

这两天用idea写spring注入的时候每一次

 @Autowired 
 Worker worker;

都会报黄，用过这个ide的都知道，说明你代码需要重构了。
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 ，每一次的依赖注入都用构造方法吧，并且每一次对强制依赖关系使用断言，大白话就是我们这样可以在构造方法里面做点校验了，这样在spring容器启动的时候就会发现错误。就类似于编译器那些在编译器就可以发现的错误，这样防止在使用的时候出现运行时异常。导致程序崩掉。强制依赖关系可以理解为，B的方法中调用A的方法

群里面还发了个例子

// 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
  }
}

本来实现一个null的bean很简单

显示用BeanFactoryPostProcessor实现，发现如果添加一个null会直接报错

因为在register里面会进行判断

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

后来无奈搜索引擎一发

发现了可以 factorybean一发（原理后面补，，最近在研究源码，还没看这一块）

@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;
    }
}

那么我的依赖注入先这么写

@Component
public class Factory {
    @Autowired Worker worker;

    public void createProduct(){
        System.out.println("生产中....");
        worker.doWork();
        System.out.println("生产完成...");
    }
}

运行，可以运行，报NPL
打印结果

生产中....
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("生产中....");
        worker.doWork();
        System.out.println("生产完成...");
    }
}

结果 无法运行，容器启动失败

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!

善用断言，在程序中启动的时候就发现错误。

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