spring之IoC

April 4, 2023

https://docs.spring.io/spring-framework/docs/current/reference/html/index.html

IoC

Inversion of Control

https://docs.spring.io/spring-framework/docs/current/reference/html/core.html

annotation 依赖注入

java

// AppConfig.java
@Configuration // 标记配置类
@ComponentScan // 自动搜索当前类所在的包以及子包，把所有标注为@Component的Bean自动创建出来，并根据@Autowired进行装配
public class AppConfig {

    @SuppressWarnings("resource")
    public static void main(String[] args) {
        ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class); // 通过配置类创建Context

        // 从Context获取Bean
        UserService userService = context.getBean(UserService.class);

        // 测试
        User user = userService.login("bob@example.com", "password");
        System.out.println(user.getName());
    }
}

// service/User.java
public class User {
    private long id;
    private String email;
    private String password;
    private String name;
    public User(long id, String email, String password, String name);
    // 略，javaBean
}

// service/MailService.java
@Component
public class MailService {
    ZoneId zoneId = ZoneId.systemDefault();
    public String getTime() {
        return ZonedDateTime.now(this.zoneId).format(DateTimeFormatter.ISO_ZONED_DATE_TIME); }
    public void sendLoginMail(User user) {
        System.err.println(String.format("Hi, %s! You are logged in at %s", user.getName(), getTime())); }
}

// service/UserService.java
@Component
public class UserService {
    @Autowired // 此处不必要，因为构造函数已注入
    MailService mailService;

    public UserService(@Autowired MailService mailService) {
        this.mailService = mailService;
    }

    private List<User> users = new ArrayList<>(List.of(
            new User(1, "bob@example.com", "password", "Bob"),
            new User(2, "alice@example.com", "password", "Alice"),
            new User(3, "tom@example.com", "password", "Tom")));

    public User login(String email, String password) {
        for (User user : users) {
            if (user.getEmail().equalsIgnoreCase(email) && user.getPassword().equals(password)) {
                mailService.sendLoginMail(user);
                return user;
            }
        }
        throw new RuntimeException("login failed.");
    }

    public User getUserById(long id) {
        return this.users.stream().filter(user -> user.getId() == id).findFirst().orElseThrow();
    }
}

自定义装配 Bean

装配的 bean 默认为单例，下面则声明为原型（每次返回不同的实例）

java

@Component
@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
// 或 @Scope("prototype")
public class ProtoService { }

java

interface Checker {
    public Boolean check(String str);
}

@Component
@Order(1) // 可选，声明顺序
class CheckerOne implements Checker {
    @Override
    public Boolean check(String str) {
        return str.length() > 0;
    }
}

@Component
@Order(2)
class CheckTwo implements Checker {
    @Override
    public Boolean check(String str) {
        return str.startsWith("*");
    }
}

public class ChainService {
    @Autowired // 注入列表
    List<Checker> checkers;
}

可选注入

java

@Component
public class MailService {
    @Autowired(required = false)
    ZoneId zoneId = ZoneId.systemDefault();
}

第三方 Bean（在 @Configuration 类中创建）

java

@Configuration
@ComponentScan
public class AppConfig {
    @Bean // 此方法仅调用一次
    ZoneId createZoneId() {
        return ZoneId.of("Z");
    }
}

初始化和销毁 Bean

java

@Component
public class MailService {
    @PostConstruct
    public void init() {}

    @PreDestroy
    public void shutdown() {}
}

指定别名

java

@Configuration
@ComponentScan
public class AppConfig {
    @Bean
    @Primary // 首选项
    DataSource createMasterDataSource() {
        ...
    }

    @Bean
    @Qualifier("slave") // 相当于 @Bean("slave")
    DataSource createSlaveDataSource() {
        ...
    }
}

通过工厂模式创建 Bean

java

@Component
public class ZoneIdFactoryBean implements FactoryBean<ZoneId> {
    String zone = "Z";

    @Override
    public ZoneId getObject() throws Exception { return ZoneId.of(zone); }

    @Override
    public Class<?> getObjectType() { return ZoneId.class; }
}

根据 profile 条件注入

spring 启动时可以指定 profile

-Dspring.profiles.active=test
# 可以指定多个 profile
-Dspring.profiles.active=test,master

java

@Configuration
@ComponentScan
public class AppConfig {
    @Bean
    @Profile("!test")
    ZoneId createZoneId() {
        return ZoneId.systemDefault();
    }

    @Bean
    @Profile("test")
    ZoneId createZoneIdForTest() {
        return ZoneId.of("America/New_York");
    }

    @Bean
    @Profile({ "test", "master" }) // 要求满足多个profile
    ZoneId createZoneId() {
        return ZoneId.systemDefault();
    }
}

通过实现 Condition 接口条件创建 Bean

java

public class OnSmtpEnvCondition implements Condition {
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return "true".equalsIgnoreCase(System.getenv("smtp"));
    }
}

@Component
@Conditional(OnSmtpEnvCondition.class)
public class SmtpMailService implements MailService {
    ...
}

类似的 conditional 注解还有

java

@ConditionalOnProperty(name="property_fieldname_is", havingValue="true")
@ConditionalOnProperty(name = "property_fieldname_test", havingValue = "test_string_value", matchIfMissing = true)
@ConditionalOnClass(name = "javax.mail.Transport")

注入 Resource `org.springframework.core.io.Resource`

java

@Component
public class AppService {
    @Value("classpath:/file.txt") // classpath: 相当于 src/main/resources 目录
    private Resource resource1;

    @Value("file:/path/to/file.txt") // file: 相当于项目根目录
    private Resource resource2;

    @PostConstruct
    public void init() throws IOException {
        try (var reader = new BufferedReader(new InputStreamReader(resource.getInputStream(), StandardCharsets.UTF_8))) {
            // ...
        }
    }
}

从 classpath 下的 properties 文件注入

java

@Configuration
@ComponentScan
@PropertySource("app.properties") // classpath下的app.properties文件
public class AppConfig {
    @Value("${option_key}")
    // @Value("${option_key:default_value}")
    String some_option;

    @Bean
    String injectToParam(@Value("${field_name:default_value}") String fieldName) {
        return fieldName;
    }
}

通过 javaBean 进行配置

java

// 注入
@Component
public class MyConfig {
    @Value("${fieldName}")
    private String fieldName;
}

// 读取
@Component
public class MyService {
    @Value("#{fieldName}")
    private String fieldName;
}

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This work is licensed under CC BY-NC-SA 4.0