Spring 条件装配之@ConditionalOnXXX

1. 基于 @Conditional 的注解

• @ConditionalOnBean
• @ConditionalOnMissingBean
• @ConditionalOnClass
• @ConditionalOnMissingClass

1.1 Spring 的依赖注入

@Configuration
public class UserConfig {
    @Bean
    public User user() {
        User user = new User();
        user.setOrder(order()); // 依赖 order 组件
        return user;
    }

    @Bean
    public Order order() {
        return new Order();
    }
}

测试：

public static void main(String[] args) {
    ConfigurableApplicationContext applicationContext = SpringApplication.run(ConbootStarterApplication.class, args);
    System.out.println("contains userConfig: " + applicationContext.containsBean("userConfig"));
    System.out.println("contains user: " + applicationContext.containsBean("user"));
    System.out.println("contains order: " + applicationContext.containsBean("order"));
}

若不注入 Order 组件

// @Bean
public Order order() {
    return new Order();
}

打印结果：

contains UserConfig: true
contains user: true
contains order: false

2. 条件装配 @ConditionalOnBean

• 有指定的 Bean 时，才会加载当前bean
• @ConditionalOnBean 装配顺序

2.1 @ConditionalOnBean 加到方法上

@Configuration
public class UserConfig {
    /**
     * 条件注解中的条件组件必须要先完成注册
     * Order 注入需要放到前面
     */
    @Bean
    public Order order() {
        return new Order();
    }

    // 当 Order 未注入时，User 也不注入
    @Bean
    @ConditionalOnBean(Order.class)
    // @ConditionalOnBean(name = "order")
    public User user() {
        User user = new User();
        user.setOrder(order());
        return user;
    }
}

2.2 @ConditionalOnBean 加到配置类上

• @ConditionalOnBean 加在配置类上时，若配置类注入不成功，里面的组件都不会注入成功
  方式一：属性方式

@Configuration
@ConditionalOnBean(Order.class)
public class BeanConditionOnConfig {
    @Resource
    private Order order;

    @Bean
    public User user() {
        User user = new User();
        user.setOrder(order);
        return user;
    }
}

方式二：方法参数注入

@Configuration
@ConditionalOnBean(Order.class)
public class BeanConditionOnConfig {
    @Bean
    public User user(Order order) {
        User user = new User();
        user.setOrder(order);
        return user;
    }
}

2.3 @ConditionalOnMissingBean

@Configuration
@ConditionalOnMissingBean(Order.class)
public class BeanConditionOnConfig {

    @Bean
    public User user() {
        return new User();
    }
}

4. @ConditionalOnClass

@Configuration
public class BeanConditionOnConfig {
    @Bean
    @ConditionalOnClass(value = {Order.class, OtherClass.class})
    public User user(Order order, OtherClass otherClass) {
        User user = new User();
        user.setOrder(order);
        return user;
    }
}

@Data
@Component
public class Order {
    private Long orderId;
    private String orderCode;
}
@Component
public class OtherClass {
}

4.1 其他

public @interface ConditionalOnClass {
    Class>[] value() default {};
    // 全类名
    String[] name() default {};
}

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