自动配置

pom.xml

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-autoconfigure</artifactId>
    <version>2.0.5.RELEASE</version>
</dependency>

SayHelloProperties.java

package com.fengqing.qqspringboot.test;

import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties(prefix = "hello")
public class SayHelloProperties {
    private String helloMsg = "springboot";

    public String getHelloMsg() {
        return helloMsg;
    }

    public void setHelloMsg(String helloMsg) {
        this.helloMsg = helloMsg;
    }
}

SayHello.java

package com.fengqing.qqspringboot.test;

public class SayHello {
    private String helloMsg;

    public String getHelloMsg() {
        return helloMsg;
    }

    public void setHelloMsg(String helloMsg) {
        this.helloMsg = helloMsg;
    }

    public String sayHello(){
        return "hello " + helloMsg;
    }
}

SayHelloAutoConfiguration.java

package com.fengqing.qqspringboot.test;

import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import javax.annotation.Resource;

@Configuration
@EnableConfigurationProperties({SayHelloProperties.class})
@ConditionalOnClass({SayHello.class})
@ConditionalOnProperty(
        prefix = "hello",
        value = {"enabled"},
        matchIfMissing = true
)
public class SayHelloAutoConfiguration {
    @Resource
    private SayHelloProperties sayHelloProperties;

    @Bean
    @ConditionalOnMissingBean({SayHello.class})
    public SayHello sayHello() {
        SayHello sayHello = new SayHello();
        sayHello.setHelloMsg(sayHelloProperties.getHelloMsg());
        return sayHello;
    }
}

创建一个resources,并把它设置成source目录，再在resources下创建一个META-INF目录，再在META-INF目录下创建

一个spring.factories文件，内容如下

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.fengqing.qqspringboot.test.SayHelloAutoConfiguration

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