创建自定义Spring Boot Starter

摘要

在大家使用Spring Boot进行开发的过程中，应该可以接触到Spring Boot提供的很多Starter。比如

• spring-boot-starter-web
• spring-boot-starter-jdbc
• ...

Spring Boot AutoConfiguration原理

Auto Configuration类

在Spring Boot启动的过程中，会去找classpath:META-INF/spring.factories文件，来决定加载哪些AutoConfiguration类。
让我们看一下spring-boot-autoconfigure项目中的spring.factories文件。下面列出部分AutoConfiguration类。

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
...

Spring Boot会尝试去运行所有org.springframework.boot.autoconfigure.EnableAutoConfiguration列出的AutoConfiguration类。此处大家可以会有疑问，很多我在项目里没有用到。对应组件是不是也会被初始化？答案是不会。我们用RedisAutoConfiguration为例来看一下背后发生了什么。

@Configuration
@ConditionalOnClass(RedisOperations.class)
@EnableConfigurationProperties(RedisProperties.class)
@Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class })
public class RedisAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean(name = "redisTemplate")
    public RedisTemplate<Object, Object> redisTemplate(
            RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
        RedisTemplate<Object, Object> template = new RedisTemplate<>();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }

    @Bean
    @ConditionalOnMissingBean
    public StringRedisTemplate stringRedisTemplate(
            RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
        StringRedisTemplate template = new StringRedisTemplate();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }

}

• Line 1: @Configuration定义Configuration组件
• Line 2: @ConditionalOnClass(RedisOperations.class) 在classpath中如果有RedisOperation类，则会运行此AutoConfiguration。反之则不会运行。
• Line 8: @ConditionalOnMissingBean(name = "redisTemplate") 如果在Spring容器中没有名为redisTemplate的Bean被注册，则执行redisTemplate()方法。反之则不会执行。

注：Spring Boot提供了一系列@Condition选择，同时还可以实现自定义Condition。参考Spring Boot - Condition Annotations

AutoConfiguration Hints

AutoConfiguration Hints功能主要便于开发人员在做配置时，IDE可以提示auto complete。如下截图：

Auto Complete信息包括：

• 配置名称
• 配置描述
• 配置默认值

具体信息在additional-spring-configuration-metadata.json中定义

创建自定义Starter

1. 需要封装的Service类

public class FooService {
    private final String fooMessage;
    private final String barMessage;

    public FooService(String fooMessage, String barMessage) {
        this.fooMessage = fooMessage;
        this.barMessage = barMessage;
    }

    public String getFooMessage() {
        return this.fooMessage;
    }

    public String getBarMessage() {
        return this.barMessage;
    }
}

1. AutoConfiguration类

package io.markfredchen.custom.starter.config;

import ...;

@Configuration
@PropertySource("classpath:config/foo.properties")
public class FooAutoConfiguration {
    @Value("${foo.fooMessage}")
    private String fooMessage;
    @Value("${foo.barMessage}")
    private String barMessage;

    @Bean
    @ConditionalOnMissingBean
    public FooService fooService() {
        return new FooService(fooMessage, barMessage);
    }
}

1. spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
io.markfredchen.custom.starter.config.FooAutoConfiguration

1. 默认配置

创建 resources/config/foo.properties

foo.fooMessage=Foo!
foo.barMessage=Bar!

使用自定义Starter

1. 创建新项目
2. SpringBootApplication

@SpringBootApplication
public class FooApplication {
    public static void main(String[] args) {
        SpringApplication.run(FooApplication.class, args);
    }

    @Bean
    public CommandLineRunner runner(final FooService fooService) {
        return args -> {
            System.out.println(fooService.getFooMessage());
            System.out.println(fooService.getBarMessage());
        };
    }
}

1. pom.xml添加以下依赖

<dependencies>
    <dependency>
        <groupId>io.markfredchen</groupId>
        <artifactId>foo-spring-boot-starter</artifactId>
        <version>1.0-SNAPSHOT</version>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
        <version>2.0.5.RELEASE</version>
    </dependency>
</dependencies>

1. 运行结果

2018-10-11 11:28:55.457  INFO 23978 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/webjars/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-10-11 11:28:55.457  INFO 23978 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-10-11 11:28:55.548  INFO 23978 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
2018-10-11 11:28:55.586  INFO 23978 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2018-10-11 11:28:55.590  INFO 23978 --- [           main] i.m.c.s.foo.application.FooApplication   : Started FooApplication in 1.725 seconds (JVM running for 2.44)
Foo!
Bar!

1. 覆盖默认配置

创建resources/application.properties

foo.barMessage=Updated Bar!

1. 运行结果

2018-10-11 11:28:55.457  INFO 23978 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/webjars/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-10-11 11:28:55.457  INFO 23978 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-10-11 11:28:55.548  INFO 23978 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
2018-10-11 11:28:55.586  INFO 23978 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2018-10-11 11:28:55.590  INFO 23978 --- [           main] i.m.c.s.foo.application.FooApplication   : Started FooApplication in 1.725 seconds (JVM running for 2.44)
Foo!
Updated Bar!

1. AutoConfiguration Hints

创建resources/META-INF/additional-spring-configuration-metadata.json

{
  "properties": [
    {
      "name": "foo.fooMessage",
      "type": "java.lang.String",
      "description": "Foo Message.",
      "defaultValue": "Foo!"
    },
    {
      "name": "foo.barMessage",
      "type": "java.lang.String",
      "description": "Bar Message.",
      "defaultValue": "Bar!"
    }
  ]
}

效果

参考

• Spring Boot - Condition Annotations

总结

本文介绍了spring boot starter机制以及如何创建自定义的starter。starter的目标是对现在有项目进行有效封装，减少开发人员的重复工作。
完整源代码访问GitHub