spring|spring bean-- autowired的正确用法 spring|framework

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 {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!

【spring|spring bean-- autowired的正确用法】善用断言，在程序中启动的时候就发现错误。