Spring IOC容器如何管理Bean？从入门到源码解析

static class Car {
    private Framework framework;
    public Car() {
        // 内部自己创建依赖，强耦合
        framework = new Framework();
    }
}
static class Framework {
    private Bottom bottom;
    public Framework() {
        bottom = new Bottom();
    }
}
static class Bottom {
    private Tire tire;
    public Bottom() {
        tire = new Tire(17);
    }
}

public class Main {  
    public static void main(String[] args) {  
        Tire tire = new Tire(21);  
        Bottom bottom = new Bottom(tire);  
        Framework framework = new Framework(bottom);  
        Car car = new Car(framework);  
        car.run();  
    }  
}
public class Car {  
    private Framework framework;  
    public Car(Framework framework){  
        this.framework = framework;  
        System.out.println("car init...");  
    }  
  
    public void run(){  
        System.out.println("car run");  
    }  
}
public class Framework {  
    Bottom bottom ;  
    public Framework(Bottom bottom) {  
        this.bottom = bottom;  
        System.out.println("framework init...");  
    }  
}
public class Bottom {  
    Tire tire;  
    public Bottom( Tire tire) {  
        this.tire = tire;  
        System.out.println("bottom init...");  
    }  
}
public class Tire {  
    int size;  
    public Tire(int size){  
        this.size = size;  
        System.out.println("tire init,size: "+size);  
    }  
}

@Component
public class BookService {

@Component
@Data
public class BookDao {

@RestController  
@RequestMapping("/book")  
public class BookController {  
    @Autowired  
    private BookService bookService ;  
    @RequestMapping("/getList")  
    public List<BookInfo> getList() {  
        return bookService.getList();  
    }  
}

@Component  
public class BookService {  
    @Autowired  
    BookDao bookDao ;  
    public List<BookInfo> getList() {  
        bookDao.mockData();  
        return bookDao.getBookList();  
    }  
}

private BookService bookService = new BookService();  // ❌ 不需要手动创建

BookController (@RestController)
    │  @Autowired
    ▼
BookService    (@Component)
    │  @Autowired
    ▼
BookDao        (@Component)

Field bookDao in com.daisy.springbookdemo.service.BookService required a bean of type 'com.daisy.springbookdemo.dao.BookDao' that could not be found.

java.lang.NullPointerException: Cannot invoke "com.daisy.springbookdemo.dao.BookDao.mockData()" because "this.bookDao" is null

// 1. 根据类型获取
BookService service = context.getBean(BookService.class);
// 2. 根据Bean名称获取
BookService service = (BookService) context.getBean("bookService");
// 3. 名称+类型双重匹配
BookService service = context.getBean("bookService", BookService.class);

@Configuration
public class DataSourceConfig {
    @Bean
    public DataSource dataSource(){
        DruidDataSource ds = new DruidDataSource();
        ds.setUrl("jdbc:mysql://xxx");
        return ds;
    }
}

@Component  
public class StudentComponent {  
    @Bean  
    public Student s1 () {  
        return new Student("zhangsan", 18);  
    }  
    @Bean  
    public Student s2 () {  
        return new Student("lisi", 18);  
    }  
}

// bean名称：user
@Bean
public User user(){...}

// bean别名：u1、userOne
@Bean({"u1","userOne"})//使用数组接收
public User user(){...}

// 简写
@Bean("u1")
public User user(){...}

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan // 内置了包扫描
public @interface SpringBootApplication {}

//手动指定包
@ComponentScan("com.example.demo")

// 扫描多个包
@ComponentScan({"com.example.demo", "com.example.other"})
@SpringBootApplication
public class DemoApplication {}

@RestController
public class BookController {
    // 属性注入
    @Autowired
    private BookService bookService;
}

@RestController
public class BookController {
    private final BookService bookService;
    // 类只有一个构造方法，@Autowired可省略
    public BookController(BookService bookService) {
        this.bookService = bookService;
    }
}

@RestController
public class BookController {
    private BookService bookService;
    @Autowired
    public void setBookService(BookService bookService) {
        this.bookService = bookService;
    }
}

@Component  
public class StudentComponent {  
//创建两个Bean
    @Bean  
    public Student s1 () {  
        return new Student("zhangsan", 18);  
    }  
  
    @Bean  
    public Student s2 () {  
        return new Student("lisi", 19);  
    }  
}

public class UserService {  
//Autowired按照Student类型查找
    @Autowired  
    private Student stu;  
    public void print() {  
        System.out.println("do Service!");  
        System.out.println(stu);  
    }  
}

Field student in com.daisy.springiocdemo.service.UserService required a single bean, but 2 were found:
	- s1: defined by method 's1' in class path resource [com/daisy/springiocdemo/component/StudentComponent.class]
	- s2: defined by method 's2' in class path resource [com/daisy/springiocdemo/component/StudentComponent.class]

This may be due to missing parameter name information

Action:

Consider marking one of the beans as @Primary, updating the consumer to accept multiple beans, or using @Qualifier to identify the bean that should be consumed

@Configuration
public class StudentComponent {  
    @Primary  
    @Bean    public Student s1 () {  
        return new Student("zhangsan", 18);  
    }  
  
    @Bean  
    public Student s2 () {  
        return new Student("lisi", 19);  
    }  
}

@Service
public class UserService {  
//指定Bean名称为s1的对象
    @Qualifier("s1")  
    @Autowired  
    private Student student;  
    public void print() {  
        System.out.println("do Service!");  
        System.out.println(student);  
    }  
}

@Component  
public class StudentComponent {  
    @Bean  
    public String name1() {  
        return "zhangsan";  
    }  
    @Bean  
    public String name2() {  
        return "lisi";  
    }  
    @Bean  
    public Student s1 (@Qualifier("name1") String n) {  
        return new Student(n, 18);  
    }  
  
    @Bean  
    public Student s2 () {  
        return new Student("lisi", 19);  
    }  
}

@Bean
public Student s1 (@Qualifier("name1") String n) {
    return new Student(n, 18);
}

NoUniqueBeanDefinitionException: 找到 2 个 String 类型的 Bean，不知道用哪个

@Service
public class UserService {    
    @Resource(name = "s1")  
    private Student student;  
    public void print() {  
        System.out.println("do Service!");  
        System.out.println(student);  
    }  
}