在静态方法中调用Spring @Autowired注入的实例对象

Spring加载类

• spring启动时资源的加载顺序：静态方法->类的属性–>无参构造–>注入@Autowired修饰的依赖–>执行@postConstruct修饰的init()方法。

在静态方法中调用@Autowired注入的实例对象

• 了解完spring加载资源的顺序后，可知如下编码是不能顺利使用的，虽然能编译过，但在执行的时候会报空指针异常

@Component
public class Application {
 	@Autowired
    private static BookDao bookDao;
    
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        for (int i = 10; i < 20; i++) {
            Book book = new Book();
            book.setName("java编程思想" + i);
            book.setNumber(i);
            book.setDetail("java书籍" + i);
            bookDao.addBook(book);
        }
    }
}

方法一：使用set方法

@Component
public class Application {
    // 方式1: 使用set方法
   private static BookDao bookDao;
    @Autowired
    public void setBookDao(BookDao bookDao) {
        Application.bookDao = bookDao;
    }
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        for (int i = 10; i < 20; i++) {
            Book book = new Book();
            book.setName("java编程思想" + i);
            book.setNumber(i);
            book.setDetail("java书籍" + i);
            bookDao.addBook(book);
        }
    }
}

方法二：使用构造方法

@Component
public class Application {
    //方式2:使用构造方法
    private static BookDao bookDao;
    @Autowired
    public Application(BookDao bookDao) {
        Application.bookDao = bookDao;
    }
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        for (int i = 10; i < 100; i++) {
            Book book = new Book();
            book.setName("java编程思想" + i);
            book.setNumber(i);
            book.setDetail("java书籍" + i);
            bookDao.addBook(book);
        }
    }
}

方法三：使用@PostConstruct注解
注解详情请见：@PostConstruct

@Component
public class Application {
    //方式3:使用@PostConståruct注解(意义不是很大,主要了解下@PostConstruct注解)
    private static BookDao bookDao;
    @Autowired
    private BookDao bookDao1;

    @PostConstruct //该注解在构造方法之后会被执行
    private void beforeInit() {
        this.bookDao = bookDao1;
    }
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        for (int i = 10; i < 100; i++) {
            Book book = new Book();
            book.setName("java编程思想" + i);
            book.setNumber(i);
            book.setDetail("java书籍" + i);
            bookDao.addBook(book);
        }
    }
}