javacontinue的用法？

一、@Conditional簡介

在開發基于SpringBoot的項目時，經常看到下面的注解：

• ConditionalOnProperty
• ConditionalOnResource
• ConditionalOnBean
• ConditionalOnClass
• ConditionalOnMissingBean
• ConditionalOnMissingClass

雖然能大致上理解它們的用途，并且能簡單使用，但是還是想更加深入的學習下這些條件裝配注解。

看了B站尚硅谷雷豐陽老師的《Spring注解驅動教程》，簡單整理如下筆記。

上面注解在定義時，都有$@Conditional$注解修飾，例如下面：

@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
@Documented
@Conditional(OnPropertyCondition.class)
public @interface ConditionalOnProperty {//省略其它
}

@Conditional的定義：

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {Class<? extends Condition>[] value();
}

• $@Conditional$的作用是按照一定的條件進行判斷，滿足條件給容器注冊Bean
• 從代碼中可以看到，需要傳入一個$Class數組$，并且需要繼承Condition接口
• Condition是個$接口$，需要實現matches方法，返回true則注入Bean，false則不注入

二、示例：加載2個Bean

1. 定義Person類

@Data
public class Person {private String name;private int age;public Person(){System.out.println("初始化：Person()");}public Person(String name, int age) {System.out.println("初始化：Person(String name, int age)");this.name = name;this.age = age;}
}

2. 利用@Configuration + @Bean加載2個Bean

利用@Configuration + @Bean加載2個Bean

@Configuration
public class MyBeanConfig {@Beanpublic Person person1(){return new Person("Bill Gates", 66);}@Beanpublic Person person2(){return new Person("Linus", 50);}
}

3. 測試是否都加載到Spring容器中

@SpringBootApplication
public class DictApplication {public static void main(String[] args) {ConfigurableApplicationContext run = SpringApplication.run(DictApplication.class, args);Map<String, Person> personMap = run.getBeansOfType(Person.class);System.out.println(personMap);}
}

結果如下：

{person1=Person(name=Bill Gates, age=66), person2=Person(name=Linus, age=50)}

三、利用@Conditional根據條件加載Bean

1. 創建條件判斷的類

如果是Windows系統則加載Bean

public class WindowsCondition implements Condition {@Overridepublic boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {Environment env = context.getEnvironment();String property = env.getProperty("os.name");if (property.contains("Windows")){return true;}return false;}
}

如果是Linux系統則加載Bean

public class LinuxCondition implements Condition {@Overridepublic boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {Environment env = context.getEnvironment();String property = env.getProperty("os.name");if (property.contains("Linux")){return true;}return false;}
}

2. @Conditional放在$方法上$

@Conditional放在方法上，只能決定該單個方法返回的實例是否加載到容器

@Configuration
public class MyBeanConfig {@Bean@Conditional(WindowsCondition.class)public Person person1(){return new Person("Bill Gates", 66);}@Bean@Conditional(LinuxCondition.class)public Person person2(){return new Person("Linus", 50);}
}

因為我這是Windows 10，所以返回如下：

{person1=Person(name=Bill Gates, age=66)

3. @Conditional放在$類上$

@Conditional放在類上，則可以判斷該類下一組的Bean是否可以加載到Spring容器

@Configuration
@Conditional(WindowsCondition.class)
public class MyBeanConfig {@Bean//@Conditional(WindowsCondition.class)public Person person1(){return new Person("Bill Gates", 66);}@Bean//@Conditional(LinuxCondition.class)public Person person2(){return new Person("Linus", 50);}
}

因為我這是Windows 10，@Conditional(WindowsCondition.class)生效可用，所以返回如下：

{person1=Person(name=Bill Gates, age=66), person2=Person(name=Linus, age=50)}

4. @Conditional傳$多個$條件，是$且$關系

@Conditional接受一個Class數組，可以傳多個條件，當所有條件都是true時，才生效
可想項目的代碼是不會加載Person對象的，因為
WindowsCondition.class返回 true，
LinuxCondition.class返回 false，
所以MyBeanConfig整個類失效，不能加載它下面的Bean。

@Configuration
@Conditional({WindowsCondition.class, LinuxCondition.class})
public class MyBeanConfig {@Bean//@Conditional(WindowsCondition.class)public Person person1(){return new Person("Bill Gates", 66);}@Bean//@Conditional(LinuxCondition.class)public Person person2(){return new Person("Linus", 50);}
}