@Autowired的作用是什么？

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
 
import blog.service.ArticleService;
import blog.service.TagService;
import blog.service.TypeService;
 
@Controller
public class TestController {
 
	//成员属性字段使用 @Autowired，无需字段的 set 方法
	@Autowired
	private TypeService typeService;
	
	
	//set 方法使用 @Autowired
	private ArticleService articleService;
	@Autowired
	public void setArticleService(ArticleService articleService) {
		this.articleService = articleService;
	}
 
	//构造方法使用 @Autowired
	private TagService tagService;
	@Autowired
	public TestController(TagService tagService) {
		this.tagService = tagService; 
	}
	
}