Spring Boot学习--3 AutoConfiguration

1. 自动配置原理：

spring boot项目的入口类的注解@SpringBootApplication引入了@EnableAutoConfiguration。后者又通过@Import引入了EnableAutoConfigurationImportSelector类, EnableAutoConfigurationImportSelector类通过方法调用链selectImports()->getCandidateConfigurations->SpringFactoriesLoader.loadFactoryNames()，获得了在spring-boot-autoconfig jar包中META-INF/spring.factories文件的org.springframework.boot.autoconfigure.EnableAutoConfiguration属性所要求的所有需要自动配置的AutoConfiguration。这些自动配置的AutoConfiguration一般由@Configuration注解，由其同一目录下的相关基于属性的Bean定义属性，并且在这些AutoConfiguration中包含了返回Bean的由@Conditional+条件进行注解的方法，因此在项目中可以通过@Autowired、@Resource等方式直接进行注入。（以上原理说明均由spring boot源代码得到）。

2. 示例的步骤：

步骤一：建立自动配置的maven组件包。该包模拟autoconfig组件建立了基于properties的安全类型的Bean（即@ConfigurationProperties注解的Bean），应用该Bean的自动配置的AutoConfiguration（由@Configuration注解，使用上一Bean的属性），AutoConfiguration中包含应用上一Bean属性的服务Bean， 并在资源目录下建立META-INF/spring.factories文件，文件中为org.springframework.boot.autoconfigure.EnableAutoConfiguration属性确定了AutoCOnfiguration的全限定名。最后按照组件包（mvn:install）。

步骤二：建立一个依赖web的spring boot项目。项目还依赖了上面建立的maven组件，在入口类中通过@Autowired注入maven组件中的服务Bean，再在入口类的@RequestMapping注解的方法中使用该服务Bean，从而验证maven组件自动配置的正确性。

3. 示例

步骤一：

demo-autoconfig maven组件的pom.xml

依赖autoconfig组件的pom.xml

<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>com.example.demo</groupId>
  <artifactId>demo-autoconfig</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>demo-autoconfig</name>
  <url>http://maven.apache.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <org.springframework.version>2.0.2.RELEASE</org.springframework.version>
  </properties>

  <dependencies>

  <dependency>
  <!-- autoconfigure support -->
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-autoconfigure</artifactId>
    <version>${org.springframework.version}</version>
  </dependency>

    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</project>

基于properties的安全类型的Bean

package com.example.demo.propertybean;

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

@ConfigurationProperties(prefix = "hello")
public class DemoPropertyBean {
    private static final String MSG = "world";

    private String msg = MSG;

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }
}

服务Bean

package com.example.demo.condition;

/** Existence of this class  under the class path acts as a condition.
 * Which is used in the class annotated with @ConditionalOnClass
 * @author apple
 *
 */
public class DemoService {
    private String msg;

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }

    public String saySomething() {
        return "Hello, " + this.msg;
    }
}

AutoConfiguration

package com.example.demo.autoconfigclass;

import org.springframework.beans.factory.annotation.Autowired;
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 com.example.demo.condition.DemoService;
import com.example.demo.propertybean.DemoPropertyBean;

// check if class DemoService is under class path.
@ConditionalOnClass(DemoService.class)
// if property "hello.msg" in application.property is missing, condition is matched.
@ConditionalOnProperty(prefix="hello", value="msg", matchIfMissing=true)
// enable inject the properties from DemoPropertyBean 
@EnableConfigurationProperties(DemoPropertyBean.class)
@Configuration
public class DemoAutoConfiguration {
    @Autowired 
    DemoPropertyBean demoPropertyBean;

    @Bean
    @ConditionalOnMissingBean(DemoService.class)
    public DemoService demoService() {
        DemoService demoService = new DemoService();
        demoService.setMsg(demoPropertyBean.getMsg());
        return demoService;
    }
}

spring.factories文件

在资源目录src/main/resources下建立META-INF/spring.factories文件，文件内容为：

org.springframework.boot.autoconfig.EnableAutoConfiguration=\
com.example.demo.autoconfigclass.DemoAutoConfiguration

属性值为需要自动配置的AutoConfiguration。

步骤二：

spring boot项目的pom.xml

<?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>com.example</groupId>
    <artifactId>com-use-demo-autoconfig</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>com-use-demo-autoconfig</name>
    <description>Demo project for Spring Boot</description>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.2.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>

        <!-- demo autoconfig support -->
        <dependency>
            <groupId>com.example.demo</groupId>
            <artifactId>demo-autoconfig</artifactId>
            <version>0.0.1-SNAPSHOT</version>
        </dependency>

    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

入口类

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import com.example.demo.condition.DemoService;

@RestController
@SpringBootApplication
public class ComUseDemoAutoconfigApplication {

    /**inject demo-autoconfig组件的DemoService
     * 
     */
    @Autowired
    DemoService demoService;

    @RequestMapping({"/", "/index"})
    public String index() {
        return demoService.saySomething();
    }

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

验证结果

a、运行spring boot项目
b、地址栏输入：http://localhost:8080/index
c、浏览器显示：
Hello, world
d、修改spring boot项目的application.properties文件，增加如下一行：
hello.msg=Hongkong
e、再次运行spring boot项目浏览器显示：
Hello, Hongkong
f、因此demo-autoconfig自动配置验证成功。