spring的自动装配失败

Question

@Component
public class test {
    @Autowired
    public cat c;
    public test(){
        System.out.println("test is running");
        System.out.println(c.str);
    }
}

@Component
public class cat {
    String str="我是一只猫";
    public cat(){
        System.out.println("miao~");
    }

}

@Configuration
@ComponentScan("test01")
public class springConfig {
}

public class start {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(springConfig.class);
    }
}

为什么cat的自动装配没成功
项目结果如下

Answer 1

用Autowired注入，在执行构造器时c还未赋值，所以会报NPE，可以把test换成构造器注入如下

@Component
public class test {

    public test( cat c){
        System.out.println("test is running");
        System.out.println(c.str);
    }
}