一、含义

从注解命名上可以看出这个注解叫条件属性，就是说只有当属性满足一定条件时才会进行自动装配。

二、源码

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.springframework.boot.autoconfigure.condition;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Conditional;

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
@Documented
@Conditional({OnPropertyCondition.class})
public @interface ConditionalOnProperty {
	String[] value() default {}; //数组，获取对应property名称的值，与name不可同时使用

    String prefix() default "";//property名称的前缀，可有可无

    String[] name() default {};//数组，property完整名称或部分名称（可与prefix组合使用，组成完整的property名称），与value不可同时使用

    String havingValue() default "";//可与name组合使用，比较获取到的属性值与havingValue给定的值是否相同，相同才加载配置

    boolean matchIfMissing() default false;//缺少该property时是否可以加载。如果为true，没有该property也会正常加载；反之报错

	boolean relaxedNames() default true;//是否可以松散匹配，至今不知道怎么使用的（早期版本存在该属性，现行版本已取消）
}

三、实例

/*
 * Copyright 1999-2018 Alibaba Group Holding Ltd.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.druid.spring.boot.autoconfigure.stat;

import com.alibaba.druid.spring.boot.autoconfigure.properties.DruidStatProperties;
import com.alibaba.druid.support.http.StatViewServlet;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
import org.springframework.boot.web.servlet.ServletRegistrationBean;
import org.springframework.context.annotation.Bean;

/**
 * @author lihengming [89921218@qq.com]
 */
@ConditionalOnWebApplication
@ConditionalOnProperty(name = "spring.datasource.druid.stat-view-servlet.enabled", havingValue = "true")
public class DruidStatViewServletConfiguration {
    private static final String DEFAULT_ALLOW_IP = "127.0.0.1";

    @Bean
    public ServletRegistrationBean statViewServletRegistrationBean(DruidStatProperties properties) {
        DruidStatProperties.StatViewServlet config = properties.getStatViewServlet();
        ServletRegistrationBean registrationBean = new ServletRegistrationBean();
        registrationBean.setServlet(new StatViewServlet());
        registrationBean.addUrlMappings(config.getUrlPattern() != null ? config.getUrlPattern() : "/druid/*");
        if (config.getAllow() != null) {
            registrationBean.addInitParameter("allow", config.getAllow());
        } else {
            registrationBean.addInitParameter("allow", DEFAULT_ALLOW_IP);
        }
        if (config.getDeny() != null) {
            registrationBean.addInitParameter("deny", config.getDeny());
        }
        if (config.getLoginUsername() != null) {
            registrationBean.addInitParameter("loginUsername", config.getLoginUsername());
        }
        if (config.getLoginPassword() != null) {
            registrationBean.addInitParameter("loginPassword", config.getLoginPassword());
        }
        if (config.getResetEnable() != null) {
            registrationBean.addInitParameter("resetEnable", config.getResetEnable());
        }
        return registrationBean;
    }
}

该实例来源于druid-spring-boot-starter

参考以下素材：

• @ConditionalOnProperty注解理解
• spring-boot-autoconfigure源码
• druid-spring-boot-starter源码