什么是starter？

starter是SpringBoot中的一个新发明，它有效的降低了项目开发过程的复杂程度，对于简化开发操作有着非常好的效果。提供一个开箱即用的组件。

手写一个starter

假设我们有个需求，我们需要序列化一个对象，当工程中存在FastJson的com.alibaba.fastjson.JSON类时使用FastJson序列化，如果不存在就则进行简单的toString操作。

我们首先定义一个接口有一个格式化方法

public interface FormatProcessor {
    //定义一个序列化的方法
    <T> String format(T obj);
}

然后这个接口有两个实现类，分别做不同序列化操作。

import com.alibaba.fastjson.JSON;

public class JsonFormatProcessor implements FormatProcessor{
    @Override
    public <T> String format(T obj) {
        return "JsonFormatProcessor:"+ JSON.toJSONString(obj);
    }
}

import java.util.Objects;

public class StringFormatProcessor implements FormatProcessor{


    @Override
    public <T> String format(T obj) {
        return "StringFormatProcessor:"+Objects.toString(obj);
    }
}

定义一个Configuration类。

@Configuration
public class FormatAutoConfiguration {
    @ConditionalOnMissingClass("com.alibaba.fastjson.JSON") // 如果不存在JSON类就加载
    @Bean
    @Primary //  如果有两个实现则优先加载这个
    public FormatProcessor stringFormat() {
        return new StringFormatProcessor();
    }

    @ConditionalOnClass(name = "com.alibaba.fastjson.JSON") // 如果存在JSON类
    @Bean
    public FormatProcessor jsonFormat() {
        return new JsonFormatProcessor();
    }
}

定义一个自动配置类。

@Import(FormatAutoConfiguration.class)
@Configuration
public class HelloAutoConfiguration {

    @Bean
    public HelloFormatTemplate helloFormatTemplate( FormatProcessor formatProcessor) {
        return new HelloFormatTemplate(formatProcessor);
    }
}

template类

public class HelloFormatTemplate {

    private FormatProcessor formatProcessor;

    public HelloFormatTemplate(FormatProcessor formatProcessor) {
        this.formatProcessor = formatProcessor;
    }

    public <T> String doFormat(T obj){
        StringBuilder stringBuilder=new StringBuilder();
        stringBuilder.append("begin:Execute format").append("<br/>");
        stringBuilder.append("Obj format result:").append(formatProcessor.format(obj)).append("<br/>");
        return stringBuilder.toString();
    }
}

使用spring boot的SPI机制把 HelloAutoConfiguration自动装配进spring容器。在spring.factories文件中定义

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.yhx.starter.autoconfiguration.HelloAutoConfiguration

这样我们就写出了一个starter，实现了一个类似以RedisTemplate的HelloFormatTemplate的工具。那如果我们要使用外部化的配置也就要读取到application.yml文件的配置该怎么写呢？

首先我们写一个Properties类，并加上@ConfigurationProperties注解。

@ConfigurationProperties(prefix = HelloProperties.HELLO_FORMAT_PREFIX)
public class HelloProperties {
    public static final String HELLO_FORMAT_PREFIX = "yhx.hello.format";
    private Map<String, Object> info;
    public Map<String, Object> getInfo() {
        return info;
    }
    public void setInfo(Map<String, Object> info) {
        this.info = info;
    }
}

然后在HelloAutoConfiguration的自动装配的类中加上@EnableConfigurationProperties(HelloProperties.class)，此时HelloAutoConfiguration类为

@Import(FormatAutoConfiguration.class)
@EnableConfigurationProperties(HelloProperties.class)
@Configuration
public class HelloAutoConfiguration {

    @Bean
  // HelloProperties这时候的已经在spring容器中了，会自动去找查的
    public HelloFormatTemplate helloFormatTemplate(HelloProperties helloProperties, FormatProcessor formatProcessor) {  
        return new HelloFormatTemplate(helloProperties, formatProcessor);
    }
}

这时候我们的Template可以改写为如下的

public class HelloFormatTemplate {

    private FormatProcessor formatProcessor;

    private HelloProperties helloProperties;

    public HelloFormatTemplate(HelloProperties helloProperties,FormatProcessor formatProcessor) {
        this.helloProperties=helloProperties;
        this.formatProcessor = formatProcessor;
    }

    public <T> String doFormat(T obj){
        StringBuilder stringBuilder=new StringBuilder();
        stringBuilder.append("begin:Execute format").append("<br/>");
       stringBuilder.append("HelloProperties:").append(formatProcessor.format(helloProperties.getInfo())).append("<br/>");
        stringBuilder.append("Obj format result:").append(formatProcessor.format(obj)).append("<br/>");
        return stringBuilder.toString();

    }
}

我们在使用这个这个starter的工程中在application.yml或者是application.properties中配置，就可以配置进这个序列化的bean中

yhx.hello.format.info.country=CN
yhx.hello.format.info.provice=HuNan
yhx.hello.format.info.city=shanghai
yhx.hello.format.info.org=xingren

常见的starter之logger

我们在项目中加入logging starter的依赖

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-logging</artifactId>
    <version>2.3.4.RELEASE</version>
</dependency>

然后看一下spring-boot-starter-logging的jar包，发现里面并没有什么，这是为什么呢？

spring-boot-starter-logging-2.3.4.RELEASE.jar
 META-INF
     LICENSE.txt
     MANIFEST.MF
     NOTICE.txt

然后我们点进pom文件中的spring-boot-starter-logging看看

<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <!-- This module was also published with a richer model, Gradle metadata,  -->
  <!-- which should be used instead. Do not delete the following line which  -->
  <!-- is to indicate to Gradle or any Gradle module metadata file consumer  -->
  <!-- that they should prefer consuming it instead. -->
  <!-- do_not_remove: published-with-gradle-metadata -->
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-logging</artifactId>
  <version>2.3.4.RELEASE</version>
  <name>spring-boot-starter-logging</name>
  <description>Starter for logging using Logback. Default logging starter</description>
  <url>https://spring.io/projects/spring-boot</url>
  <organization>
    <name>Pivotal Software, Inc.</name>
    <url>https://spring.io</url>
  </organization>
  <licenses>
    <license>
      <name>Apache License, Version 2.0</name>
      <url>https://www.apache.org/licenses/LICENSE-2.0</url>
    </license>
  </licenses>
  <developers>
    <developer>
      <name>Pivotal</name>
      <email>info@pivotal.io</email>
      <organization>Pivotal Software, Inc.</organization>
      <organizationUrl>https://www.spring.io</organizationUrl>
    </developer>
  </developers>
  <scm>
    <connection>scm:git:git://github.com/spring-projects/spring-boot.git</connection>
    <developerConnection>scm:git:ssh://git@github.com/spring-projects/spring-boot.git</developerConnection>
    <url>https://github.com/spring-projects/spring-boot</url>
  </scm>
  <issueManagement>
    <system>GitHub</system>
    <url>https://github.com/spring-projects/spring-boot/issues</url>
  </issueManagement>
  <dependencies>
    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <version>1.2.3</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.apache.logging.log4j</groupId>
      <artifactId>log4j-to-slf4j</artifactId>
      <version>2.13.3</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>jul-to-slf4j</artifactId>
      <version>1.7.30</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>
</project>

里面就是一堆依赖，为什么空的也能实现自动装配呢？因为spring boot官方提供了装配，如果相关的类或者一类存在的话，所以logging的starter只需要添加一些依赖就可以。