引言

只要你用 Spring boot，一定会用到各种 spring-boot-starter。其实写一个spring-boot-starter，仅需4步。

下面我们就写一个starter，它将实现，在日志中打印方法执行时间。

第一步创建maven项目

在使用spring-boot-starter，会发现，有的项目名称是 XX-spring-boot-starter，有的是spring-boot-starter-XX，这个项目的名称有什么讲究呢？

从springboot官方文档摘录如下：

Do not start your module names with spring-boot, even if you use a different Maven groupId. We may offer official support for the thing you auto-configure in the future.

As a rule of thumb, you should name a combined module after the starter.

从这段话可以看出spring-boot-starter命名的潜规则。

spring-boot-starter-XX是springboot官方的starter

XX-spring-boot-starter是第三方扩展的starter

打印方法执行时间的功能，需要用到aop，咱们的项目就叫做

aspectlog-spring-boot-starter吧。

项目的pom文件如下：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>aspectlog-spring-boot-starter</artifactId>
    <version>1.0.2</version>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.15.RELEASE</version>
    </parent>
    
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-aop</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
            <optional>true</optional>
        </dependency>
    </dependencies>

</project>

关于spring-boot-configuration-processor的说明，引自springBoot官方文档：

Spring Boot uses an annotation processor to collect the conditions on auto-configurations in a metadata file ( META-INF/spring-autoconfigure-metadata.properties ). If that file is present, it is used to eagerly filter auto-configurations that do not match, which will improve startup time. It is recommended to add the following dependency in a module that contains auto-configurations:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-autoconfigure-processor</artifactId>
  <optional>true</optional>
</dependency>

简单说就是写starter时，在pom中配置 spring-boot-autoconfigure-processor，

在编译时会自动收集配置类的条件，写到一个 META-INF/spring-autoconfigure-metadata.properties中。

不熟悉 Spring Boot 基础的可以看下这个仓库：https://github.com/javastacks/spring-boot-best-practice

第二步写自动配置逻辑

各种condition

本次我们就选用@ConditionalOnProperty。即配置文件中有aspectLog.enable=true，才加载我们的配置类。

不熟悉 Spring Boot 基础的可以看下这个仓库：https://github.com/javastacks/spring-boot-best-practice

下面开始写自动配置类

2.1.定义AspectLog注解，该注解用于标注需要打印执行时间的方法。

package com.shanyuan.autoconfiguration.aspectlog; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; /** * class_name: ScheduleManage * describe:   用于控制定时任务的开启与关闭 * 对应切面 * creat_user: wenl * creat_time:  2018/11/10 18:45 **/ @Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) public @interface   AspectLog { }

2.2定义配置文件对应类

package com.shanyuan.autoconfiguration.aspectlog; import org.springframework.boot.context.properties.ConfigurationProperties; @ConfigurationProperties("aspectLog") public class AspectLogProperties {     private boolean enable;     public boolean isEnable() {         return enable;     }     public void setEnable(boolean enable) {         this.enable = enable;     } }

2.3定义自动配置类

package com.shanyuan.autoconfiguration.aspectlog; import org.aspectj.lang.ProceedingJoinPoint; import org.aspectj.lang.annotation.Around; import org.aspectj.lang.annotation.Aspect; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.boot.autoconfigure.condition.*; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.EnableAspectJAutoProxy; import org.springframework.core.PriorityOrdered; @Aspect @EnableAspectJAutoProxy(exposeProxy = true, proxyTargetClass = true) @Configuration @ConditionalOnProperty(prefix = "aspectLog", name = "enable",                      havingValue = "true", matchIfMissing = true) public class AspectLogAutoConfiguration implements PriorityOrdered {     protected Logger logger = LoggerFactory.getLogger(getClass()); @Around("@annotation(com.shanyuan.autoconfiguration.aspectlog.AspectLog) ")     public Object isOpen(ProceedingJoinPoint thisJoinPoint)                                         throws Throwable {         //执行方法名称         String taskName = thisJoinPoint.getSignature()             .toString().substring(                 thisJoinPoint.getSignature()                     .toString().indexOf(" "),                     thisJoinPoint.getSignature().toString().indexOf("("));         taskName = taskName.trim();         long time = System.currentTimeMillis();         Object result = thisJoinPoint.proceed();         logger.info("method:{} run :{} ms", taskName,                             (System.currentTimeMillis() - time));         return result;     }     @Override     public int getOrder() {         //保证事务等切面先执行         return Integer.MAX_VALUE;     } }

配置类简要说明：

@ConditionalOnProperty(prefix = "aspectLog", name = "enable",havingValue = "true", matchIfMissing = true)

当配置文件有aspectLog.enable=true时开启，如果配置文件没有设置aspectLog.enable也开启。基础知识不熟悉的可以关注公众号Java技术栈回复boot获取一份完整教程。

第三步META-INF/spring.factories

META-INF/spring.factories是spring的工厂机制，在这个文件中定义的类，都会被自动加载。多个配置使用逗号分割，换行用\

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\ com.shanyuan.autoconfiguration.aspectlog.AspectLogAutoConfiguration

第四步打包测试

这是我们最终的目录结构

在IDEA中，进行mvn intall