[Java知识库]SpringBoot条件注解和条件注解解析类

一.SpringBoot自动配置jar包下的condition

引入的SpringBoot版本为 ：?2.1.0.RELEASE

       <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
            <version>2.1.0.RELEASE</version>
        </dependency>

二.条件注解以及条件注解解释类

1.@Conditional

这是Spring4.0添加的新注解，用来标识一个SpringBean或者Configuration配置文件 当满足指定的条件才开启配置。

package com.xiaoxiaowang.springbootDemo.condition;


import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;

/**
 * @author: xxw
 * @create: 2022-10
 */
public class MyCondition implements Condition {


    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return true;
    }
}

    @Conditional(MyCondition.class)
    public XiaoXiaoService xiaoXiaoService2() {
        System.out.println("XiaoXiaoService2======>instanced...");
        // @ConditionalOnCloudPlatform(CloudPlatform.CLOUD_FOUNDRY) 是不是运行在某个云平台
        return new XiaoXiaoService();
    }

2.@ConditionalOnBean

当ApplicationContext存在某些bean时条件才成立，组合@OnBeanCondition使用

3.@ConditionalOnMissingBean

ApplicationContext不存在某些bean时条件才成立，组合@OnBeanCondition使用

4.@ConditionalOnClass

classPath中不存在某些Class时条件才成立，组合@OnClassCondition使用

5.@ConditionalOnMissingClass

classPath中不存在某些Class时条件才成立，组合@OnClassCondition使用

6.@ConditionalOnExpression

SPEL表达式(spring的el表达式)成立时条件才成立，组合@OnExpressionCondition使用

?在application.properties配置文件中定义变量

xiaoxiao.type=server
xiaoxiao.user.enable=true
xiaoxiao.order.size=10

    @ConditionalOnExpression("'${xiaoxiao.type}'.equals('server')&&          (${xiaoxiao.user.enable})")
    @Bean
    public MultiBean m1() {
        System.out.println("=========>m1 instanced...");
        return new MultiBean();
    }

7.@ConditionalOnJava

JDK某些版本条件才成立，结合@OnJavaCondition使用

它的默认Range范围是 >= JDK的某个版本

    @ConditionalOnJava(JavaVersion.EIGHT)
    @Bean
    public MultiBean m1() {
        System.out.println("=========>m1 instanced...");
        return new MultiBean();
    }

?8.@ConditionalOnProperty

?Environment中存在某些配置项时才成立，结合@OnPropertyCondition使用

9.@ConditionalOnResource

存在某些资源时条件才成立，结合@OnResourceCondition使用

10.@ConditionalOnSingleCandidate

ApplicationContext中存在且只有一个bean时条件成立，结合@OnBeanCondition使用

11.@ConditionalOnJndi (不常用)

JNDI路径下存在时条件才成立，结合@OnJndiCondition使用

12.@ConditionalOnCloudPlatform?(不常用)

在某些云平台(Kubemetes、Heroku、Cloud Foundry)下条件才成立，结合@OnCloudPlatformCondition使用

13.@ConditionalOnWebApplication

在Web环境下条件才成立，结合@OnWebApplicationCondition使用

14.@ConditionalOnNotWebApplication

在非Web环境下条件才成立，结合@OnWebApplicationCondition使用

三.代码片段

自定义条件注解类和条件注解解析类

package com.xiaoxiaowang.springbootDemo.condition;

import org.springframework.context.annotation.Conditional;

import java.lang.annotation.*;

/**
 *
 * 自定义条件注解
 *
 *
 */
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(XiaoXiaoOnClassCondition.class)
public @interface XiaoXiaoConditionalOnClass {
    String value() default "";
}

package com.xiaoxiaowang.springbootDemo.condition;

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;

import java.util.Map;

/**
 * @author: xxw
 * @create: 2022-10-18
 *
 * 自定义条件注解解析类
 *
 */
public class XiaoXiaoOnClassCondition implements Condition {


    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        // 拿到条件注解
        Map<String, Object> annotationAttributes =
                metadata.getAnnotationAttributes(XiaoXiaoConditionalOnClass.class.getName());
        // 拿到注解上的value值
        String className = (String) annotationAttributes.get("value");
        try {
            // 类加载器加载类,加载成功,返回true,否则返回false
            context.getClassLoader().loadClass(className);
            return true;
        } catch (ClassNotFoundException e) {
            return false;
        }
    }
}

测试代码片段 ：

package com.xiaoxiaowang.springbootDemo;

import com.xiaoxiaowang.springbootDemo.bean.BeanTest;
import com.xiaoxiaowang.springbootDemo.bean.ExistBean;
import com.xiaoxiaowang.springbootDemo.bean.MultiBean;
import com.xiaoxiaowang.springbootDemo.condition.MyCondition;
import com.xiaoxiaowang.springbootDemo.condition.XiaoXiaoConditionalOnClass;
import com.xiaoxiaowang.springbootDemo.service.XiaoXiaoService;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.condition.*;
import org.springframework.boot.cloud.CloudPlatform;
import org.springframework.boot.system.JavaVersion;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

/**
 * @author: xxw
 * @create: 2022-10-18
 */
@SpringBootApplication
@RestController
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @ConditionalOnJava(JavaVersion.EIGHT)
    @Bean
    public MultiBean m1() {
        System.out.println("=========>m1 instanced...");
        return new MultiBean();
    }

    @Bean
    @XiaoXiaoConditionalOnClass("com.xiaoxiaowang.springbootDemo.bean.BeanTest")
    @ConditionalOnClass(BeanTest.class)
    @ConditionalOnBean(ExistBean.class)
    @ConditionalOnExpression("'${xiaoxiao.type}'.equals('server') && (${xiaoxiao.user.enable})")
    @ConditionalOnJava(JavaVersion.EIGHT)
    @ConditionalOnProperty(prefix = "xiaoxiao.user", name = "enable", havingValue = "true")
    @ConditionalOnResource(resources = {"jdbc.properties"})
    @ConditionalOnSingleCandidate(ExistBean.class)
    @Conditional(MyCondition.class)
    public XiaoXiaoService xiaoXiaoService2() {
        System.out.println("XiaoXiaoService2======>instanced...");
        // @ConditionalOnCloudPlatform(CloudPlatform.CLOUD_FOUNDRY) 是不是运行在某个云平台
        return new XiaoXiaoService();
    }

//    @Bean
//    @XiaoXiaoConditionalOnClass("com.xiaoxiaowang.springbootDemo.bean.BeanTest1")
//    public XiaoXiaoService xiaoXiaoService(){
//        System.out.println("XiaoXiaoService======>instanced...");
//        return new XiaoXiaoService();
//    }
//
//    @Bean
//    @XiaoXiaoConditionalOnClass("com.xiaoxiaowang.springbootDemo.bean.BeanTest")
//    @ConditionalOnClass(BeanTest.class)
//    @ConditionalOnBean(BeanTest.class)
//    public XiaoXiaoService xiaoXiaoService3(){
//        System.out.println("XiaoXiaoService3======>instanced...");
//        return new XiaoXiaoService();
//    }

    @GetMapping("/hello")
    public String hello(@RequestParam(value = "name", defaultValue = "World") String name) {
        return String.format("Hello %s!", name);
    }
}

测试结果:

四.参考文档

1.https://www.hxstrive.com/subject/spring_boot/478.htm?

2.https://blog.csdn.net/qq_42651904/article/details/120994557

3.https://blog.csdn.net/qq_40041868/article/details/123264571