这两天用idea写spring注入的时候每一次
@Autowired Worker worker;
- 1
- 2
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 }}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
- 23
- 24
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");
- 1
- 2
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）
@Componentpublic 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; }}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
- 17
- 18
- 19
- 20
- 21
- 22
那么我的依赖注入先这么写
@Componentpublic class Factory { @Autowired Worker worker; public void createProduct(){ System.out.println("生产中...."); worker.doWork(); System.out.println("生产完成..."); }}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
运行，可以运行，报NPL
打印结果
生产中....Exception in thread "main" java.lang.NullPointerException
- 1
- 2
@Componentpublic 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..."); }}
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 16
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!
- 1
- 2
善用断言，在程序中启动的时候就发现错误。
再分享一下我老师大神的人工智能教程吧。零基础！通俗易懂！风趣幽默！还带黄段子！希望你也加入到我们人工智能的队伍中来！https://blog.csdn.net/jiangjunshow