Question

在Spring中使用no args-constructor时，是否可以确保在初始化bean之前已经设置了一些属性？我想在创建bean之后使用InitializingBean来验证设置。例如，我想做什么：

public class HelloWorld implements InitializingBean{
   private String message;

   public HelloWorld()
   {
      //Only no-args constructor must be used
      //How do we make sure 'message' was ever set before the Bean is used?
   }

   public void setMessage(String message){
      this.message  = message;
   }

   public void getMessage(){
      System.out.println("Your Message : " + message);
   }

   public void afterPropertiesSet(){
      //Validate object, requires message to be set!
   }
}

public class MainApp {
   public static void main(String[] args) {

      AbstractApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");

      //Bean is instantiated
      HelloWorld obj = (HelloWorld) context.getBean("helloWorld");
      //Bean is initialized and thus afterPropertiesSet() is called here. It will fail because it requires 'message' to be set.

      //Right after the bean is instantiated we set the 'message', but it's already to late. afterPropertiesSet() was already called.
      obj.setMessage("Hello World!");

   }
}

Answer 1

要注入在运行时确定的内容，请考虑使用FactoryBean。它基本上是Factory模式，但本身支持Spring：

    public class SomeWeirdMessageFactoryBean implements AbstractFactoryBean {
        public Class<?> getObjectType() {
            return String.class;
        }

        protected Object createInstance() throws Exception {
            String message;
            // magic:
            [...]
            return message;
        }
    }

magic部分显然需要弄清楚要返回什么。然后你必须将它作为你的类的属性注入，而Spring将会发现它是一个工厂并为你做肮脏的工作。

使用no-args构造函数时，Spring确保已设置Bean属性

1 个答案: