SpringBoot自动化配置之一：SpringBoot内部的一些自动化配置入门介绍

@RestController
class App {
  @RequestMapping("/")
  String home() {
    "hello"
  }
}

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>1.3.5.RELEASE</version>
</parent>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-freemarker</artifactId>
    <version>1.3.5.RELEASE</version>
</dependency>

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter</artifactId>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
  <groupId>org.freemarker</groupId>
  <artifactId>freemarker</artifactId>
</dependency>
<dependency>
  <groupId>org.springframework</groupId>
  <artifactId>spring-context-support</artifactId>
</dependency>

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>1.3.5.RELEASE</version>
</parent>

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-dependencies</artifactId>
    <version>1.3.5.RELEASE</version>
    <relativePath>../../spring-boot-dependencies</relativePath>
</parent>

@Configuration // 使用Configuration注解，自动构造一些内部定义的bean
@ConditionalOnClass({ freemarker.template.Configuration.class,
        FreeMarkerConfigurationFactory.class }) // 需要freemarker.template.Configuration和FreeMarkerConfigurationFactory这两个类存在在classpath中才会进行自动配置
@AutoConfigureAfter(WebMvcAutoConfiguration.class) // 本次自动配置需要依赖WebMvcAutoConfiguration这个配置类配置之后触发。这个WebMvcAutoConfiguration内部会配置很多Wen基础性的东西，比如RequestMappingHandlerMapping、RequestMappingHandlerAdapter等
@EnableConfigurationProperties(FreeMarkerProperties.class) // 使用FreeMarkerProperties类中的配置
public class FreeMarkerAutoConfiguration {

    private static final Log logger = LogFactory
            .getLog(FreeMarkerAutoConfiguration.class);

    @Autowired
    private ApplicationContext applicationContext;

    @Autowired
    private FreeMarkerProperties properties;

    @PostConstruct // 构造之后调用的方法，组要检查模板位置是否存在
    public void checkTemplateLocationExists() {
        if (this.properties.isCheckTemplateLocation()) {
            TemplateLocation templatePathLocation = null;
            List<TemplateLocation> locations = new ArrayList<TemplateLocation>();
            for (String templateLoaderPath : this.properties.getTemplateLoaderPath()) {
                TemplateLocation location = new TemplateLocation(templateLoaderPath);
                locations.add(location);
                if (location.exists(this.applicationContext)) {
                    templatePathLocation = location;
                    break;
                }
            }
            if (templatePathLocation == null) {
                logger.warn("Cannot find template location(s): " + locations
                        + " (please add some templates, "
                        + "check your FreeMarker configuration, or set "
                        + "spring.freemarker.checkTemplateLocation=false)");
            }
        }
    }

    protected static class FreeMarkerConfiguration {

        @Autowired
        protected FreeMarkerProperties properties;

        protected void applyProperties(FreeMarkerConfigurationFactory factory) {
            factory.setTemplateLoaderPaths(this.properties.getTemplateLoaderPath());
            factory.setPreferFileSystemAccess(this.properties.isPreferFileSystemAccess());
            factory.setDefaultEncoding(this.properties.getCharsetName());
            Properties settings = new Properties();
            settings.putAll(this.properties.getSettings());
            factory.setFreemarkerSettings(settings);
        }

    }

    @Configuration
    @ConditionalOnNotWebApplication // 非Web项目的自动配置
    public static class FreeMarkerNonWebConfiguration extends FreeMarkerConfiguration {

        @Bean
        @ConditionalOnMissingBean
        public FreeMarkerConfigurationFactoryBean freeMarkerConfiguration() {
            FreeMarkerConfigurationFactoryBean freeMarkerFactoryBean = new FreeMarkerConfigurationFactoryBean();
            applyProperties(freeMarkerFactoryBean);
            return freeMarkerFactoryBean;
        }

    }

    @Configuration // 自动配置的类
    @ConditionalOnClass(Servlet.class) // 需要运行在Servlet容器下
    @ConditionalOnWebApplication // 需要在Web项目下
    public static class FreeMarkerWebConfiguration extends FreeMarkerConfiguration {

        @Bean
        @ConditionalOnMissingBean(FreeMarkerConfig.class)
        public FreeMarkerConfigurer freeMarkerConfigurer() {
            FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
            applyProperties(configurer);
            return configurer;
        }

        @Bean
        public freemarker.template.Configuration freeMarkerConfiguration(
                FreeMarkerConfig configurer) {
            return configurer.getConfiguration();
        }

        @Bean
        @ConditionalOnMissingBean(name = "freeMarkerViewResolver") // 没有配置freeMarkerViewResolver这个bean的话，会自动构造一个freeMarkerViewResolver
        @ConditionalOnProperty(name = "spring.freemarker.enabled", matchIfMissing = true) // 配置文件中开关开启的话，才会构造
        public FreeMarkerViewResolver freeMarkerViewResolver() {
            // 构造了freemarker的ViewSolver，这就是一开始我们分析的为什么没有设置ViewResolver，但是最后却还是存在的原因
            FreeMarkerViewResolver resolver = new FreeMarkerViewResolver();
            this.properties.applyToViewResolver(resolver);
            return resolver;
        }

    }
}

@ConfigurationProperties(prefix = "spring.freemarker") // 使用配置文件中以spring.freemarker开头的配置
public class FreeMarkerProperties extends AbstractTemplateViewResolverProperties {
    public static final String DEFAULT_TEMPLATE_LOADER_PATH = "classpath:/templates/"; // 默认路径

    public static final String DEFAULT_PREFIX = ""; // 默认前缀

    public static final String DEFAULT_SUFFIX = ".ftl"; // 默认后缀

    ...

}