spring_boot_cloud(14)Condition条件注解

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Condition

A single condition that must be matched in order for a component to be registered.
用来标示一个组件是否可以被注册。

Conditional

Condition和Conditional成对出现

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@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {

	/**
	 * All {@link Condition Conditions} that must {@linkplain Condition#matches match}
	 * in order for the component to be registered.
	 */
   //所有Condition都匹配才可以被注册进来
	Class<? extends Condition>[] value();

}

实例

我们在application.yml里边添加一个配置：

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person:
  address:
    city: dalian
````
创建一个实现了Condition的类：

public class TestCondition implements Condition {
@Override
public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
String city = context.getEnvironment().getProperty(“person.address.city”);
if(“tianjin”.equals(city)){
return true;
}
return false;
}
}

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如果城市是天津就返回true，表示加载。
接着创建一个Config:

@Configuration
public class TestConditionConfig {

//Conditional的参数TestCondition规定了是否可以加载这个bean，中间会调TestCondition的matches方法
@Bean
@Conditional(TestCondition.class)
public String getConfig(){
    return "hello";
}

//没有配置Conditional条件加载
@Bean
public String getConfig2(){
    return "world";
}


@Bean
@Conditional(TestCondition.class)
public String getConfig3(){
    return "welcome";
}

}

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最后在MyApplication里边启动测试：

@SpringBootApplication
public class MyApplication {
private static final Logger logger = LoggerFactory.getLogger(MyApplication.class);
public static void main(String[] args) {
ConfigurableApplicationContext configurableApplicationContext = SpringApplication.run(MyApplication.class,args);
String [] names = configurableApplicationContext.getBeanNamesForType(String.class);
System.out.println(names);
}
}

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输出：
[getConfig2]


因为我们配置的大连，所以getConfig和getConfig3不会被加载。

修改application.yml为:

person:
address:
city: dalian
name:
firstNamme: zhangsan

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创建新的Condition：

public class TestCondition2 implements Condition {
@Override
public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
String city = context.getEnvironment().getProperty(“person.name.firstNamme”);
if (“lisi”.equals(city)) {
return true;
}
return false;
}
}

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修改TestConditionConfig的getConfig3

@Bean
@Conditional({TestCondition.class, TestCondition2.class})
public String getConfig3(){
return “welcome”;
}

运行MyApplication输出：
[getConfig, getConfig2]

#### 小结
Conditional要求Conditional.value()返回的值都返回true，才会加载。

#### Conditional在spring框架内部的使用
ConditionalOnBean Bean存在的条件加载
ConditionalOnClass Class存在的条件加载
ConditionalOnCloudPlatform
ConditionalOnExpression
ConditionalOnJava
ConditionalOnJndi
ConditionalOnMissingBean  当Bean不存在的条件加载
ConditionalOnMissingClass 当Class不存在的条件加载
ConditionalOnNotWebApplication
ConditionalOnProperty
ConditionalOnResource
ConditionalOnSingleCandidate
ConditionalOnWebApplication
ConditionEvaluationReport

这些注解实现了springboot的按需加载。