来自灵魂的一问：这两个文件有什么区别?
答：如果pom.xml中引入了spring-boot-configuration-processor包，则会自动生成spring-configuration-metadata.json。
如果需要手动修改里面的元数据，则可以在additional-spring-configuration-metadata.json中编辑，最终两个文件中的元数据会合并到一起。
MybatisProperties类是属性实体类：
@ConfigurationProperties(prefix = MybatisProperties.MYBATIS_PREFIX)
public class MybatisProperties {
public static final String MYBATIS_PREFIX = "mybatis";
private String configLocation;
private String[] mapperLocations;
private String typeAliasesPackage;
private String typeHandlersPackage;
private boolean checkConfigLocation = false;
private ExecutorType executorType;
private Properties configurationProperties;
@NestedConfigurationProperty
private Configuration configuration;
public String getConfigLocation() {
return this.configLocation;
}
public void setConfigLocation(String configLocation) {
this.configLocation = configLocation;
}
@Deprecated
public String getConfig() {
return this.configLocation;
}
@Deprecated
public void setConfig(String config) {
this.configLocation = config;
}
public String[] getMapperLocations() {
return this.mapperLocations;
}
public void setMapperLocations(String[] mapperLocations) {
this.mapperLocations = mapperLocations;
}
public String getTypeHandlersPackage() {
return this.typeHandlersPackage;
}
public void setTypeHandlersPackage(String typeHandlersPackage) {
this.typeHandlersPackage = typeHandlersPackage;
}
public String getTypeAliasesPackage() {
return this.typeAliasesPackage;
}
public void setTypeAliasesPackage(String typeAliasesPackage) {
this.typeAliasesPackage = typeAliasesPackage;
}
public boolean isCheckConfigLocation() {
return this.checkConfigLocation;
}
public void setCheckConfigLocation(boolean checkConfigLocation) {
this.checkConfigLocation = checkConfigLocation;
}
public ExecutorType getExecutorType() {
return this.executorType;
}
public void setExecutorType(ExecutorType executorType) {
this.executorType = executorType;
}
public Properties getConfigurationProperties() {
return configurationProperties;
}
public void setConfigurationProperties(Properties configurationProperties) {
this.configurationProperties = configurationProperties;
}
public Configuration getConfiguration() {
return configuration;
}
public void setConfiguration(Configuration configuration) {
this.configuration = configuration;
}
public Resource[] resolveMapperLocations() {
ResourcePatternResolver resourceResolver = new PathMatchingResourcePatternResolver();
List<Resource> resources = new ArrayList<Resource>();
if (this.mapperLocations != null) {
for (String mapperLocation : this.mapperLocations) {
try {
Resource[] mappers = resourceResolver.getResources(mapperLocation);
resources.addAll(Arrays.asList(mappers));
} catch (IOException e) {
// ignore
}
}
}
return resources.toArray(new Resource[resources.size()]);
}
}
可以看到Mybatis初始化所需要的很多属性都在这里，相当于一个JavaBean。
下面重点看一下MybatisAutoConfiguration的代码：
@org.springframework.context.annotation.Configuration
@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })
@ConditionalOnBean(DataSource.class)
@EnableConfigurationProperties(MybatisProperties.class)
@AutoConfigureAfter(DataSourceAutoConfiguration.class)
public class MybatisAutoConfiguration {
private static final Logger logger = LoggerFactory.getLogger(MybatisAutoConfiguration.class);
private final MybatisProperties properties;
private final Interceptor[] interceptors;
private final ResourceLoader resourceLoader;
private final DatabaseIdProvider databaseIdProvider;
private final List<ConfigurationCustomizer> configurationCustomizers;
public MybatisAutoConfiguration(MybatisProperties properties,
ObjectProvider<Interceptor[]> interceptorsProvider,
ResourceLoader resourceLoader,
ObjectProvider<DatabaseIdProvider> databaseIdProvider,
ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider) {
this.properties = properties;
this.interceptors = interceptorsProvider.getIfAvailable();
this.resourceLoader = resourceLoader;
this.databaseIdProvider = databaseIdProvider.getIfAvailable();
this.configurationCustomizers = configurationCustomizersProvider.getIfAvailable();
}
@PostConstruct
public void checkConfigFileexists() {
if (this.properties.isCheckConfigLocation() && StringUtils.hasText(this.properties.getConfigLocation())) {
Resource resource = this.resourceLoader.getResource(this.properties.getConfigLocation());
Assert.state(resource.exists(), "Cannot find config location: " resource
" (please add config file or check your Mybatis configuration)");
}
}
@Bean
@ConditionalOnMissingBean
public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
factory.setDataSource(dataSource);
factory.setVfs(SpringBootVFS.class);
if (StringUtils.hasText(this.properties.getConfigLocation())) {
factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));
}
Configuration configuration = this.properties.getConfiguration();
if (configuration == null && !StringUtils.hasText(this.properties.getConfigLocation())) {
configuration = new Configuration();
}
if (configuration != null && !CollectionUtils.isEmpty(this.configurationCustomizers)) {
for (ConfigurationCustomizer customizer : this.configurationCustomizers) {
customizer.customize(configuration);
}
}
factory.setConfiguration(configuration);
if (this.properties.getConfigurationProperties() != null) {
factory.setConfigurationProperties(this.properties.getConfigurationProperties());
}
if (!ObjectUtils.isEmpty(this.interceptors)) {
factory.setPlugins(this.interceptors);
}
if (this.databaseIdProvider != null) {
factory.setDatabaseIdProvider(this.databaseIdProvider);
}
if (StringUtils.hasLength(this.properties.getTypeAliasesPackage())) {
factory.setTypeAliasesPackage(this.properties.getTypeAliasesPackage());
}
if (StringUtils.hasLength(this.properties.getTypeHandlersPackage())) {
factory.setTypeHandlersPackage(this.properties.getTypeHandlersPackage());
}
if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {
factory.setMapperLocations(this.properties.resolveMapperLocations());
}
return factory.getObject();
}
@Bean
@ConditionalOnMissingBean
public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
ExecutorType executorType = this.properties.getExecutorType();
if (executorType != null) {
return new SqlSessionTemplate(sqlSessionFactory, executorType);
} else {
return new SqlSessionTemplate(sqlSessionFactory);
}
}
public static class AutoConfiguredMapperScannerRegistrar
implements BeanFactoryAware, ImportBeanDefinitionRegistrar, ResourceLoaderAware {
private BeanFactory beanFactory;
private ResourceLoader resourceLoader;
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
try {
if (this.resourceLoader != null) {
scanner.setResourceLoader(this.resourceLoader);
}
List<String> packages = AutoConfigurationPackages.get(this.beanFactory);
if (logger.isDebugEnabled()) {
for (String pkg : packages) {
logger.debug("Using auto-configuration base package '{}'", pkg);
}
}
scanner.setAnnotationClass(Mapper.class);
scanner.registerFilters();
scanner.doScan(StringUtils.toStringArray(packages));
} catch (IllegalStateException ex) {
logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled.", ex);
}
}
@Override
public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
this.beanFactory = beanFactory;
}
@Override
public void setResourceLoader(ResourceLoader resourceLoader) {
this.resourceLoader = resourceLoader;
}
}
@org.springframework.context.annotation.Configuration
@Import({ AutoConfiguredMapperScannerRegistrar.class })
@ConditionalOnMissingBean(MapperFactoryBean.class)
public static class MapperScannerRegistrarNotFoundConfiguration {
@PostConstruct
public void afterPropertiesSet() {
logger.debug("No {} found.", MapperFactoryBean.class.getName());
}
}
}
这个类就是一个Configuration（配置类），它里面定义很多bean，其中最重要的就是SqlSessionFactory的bean实例，该实例是Mybatis的核心功能，用它创建SqlSession，对数据库进行CRUD操作。
除此之外，MybatisAutoConfiguration类还包含了：
- @ConditionalOnClass 配置了只有包含SqlSessionFactory.class和SqlSessionFactoryBean.class，该配置类才生效。
- @ConditionalOnBean 配置了只有包含dataSource实例时，该配置类才生效。
- @EnableConfigurationProperties 该注解会自动填充MybatisProperties实例中的属性。
- AutoConfigureAfter 配置了该配置类在DataSourceAutoConfiguration类之后自动配置。
这些注解都是一些辅助功能，决定Configuration是否生效，当然这些注解不是必须的。
接下来，重点看看spring.factories文件有啥内容：
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration
里面只有一行配置，即key为EnableAutoConfiguration，value为MybatisAutoConfiguration。
好了，介绍了这么多东西，现在我们来总结一下，
starter几个要素如下图所示：
那么，编写starter需要哪些步骤?
- 1.需要定义一个名称为xxx-spring-boot-starter的空项目，里面不包含任何代码，可以有pom.xml和pom.properties文件。
- 2.pom.xml文件中包含了名称为xxx-spring-boot-autoconfigure的项目。
- 3.xxx-spring-boot-autoconfigure项目中包含了名称为xxxAutoConfiguration的类，该类可以定义一些bean实例。当然，Configuration类上可以打一些如：ConditionalOnClass、ConditionalOnBean、EnableConfigurationProperties等注解。
- 4.需要在spring.factories文件中增加key为EnableAutoConfiguration，value为xxxAutoConfiguration。
我们试着按照这四步，自己编写一个starter看看能否成功，验证一下总结的内容是否正确。
3 如何定义自己的starter？3.1 先创建一个空项目该项目名称为id-generate-starter，注意为了方便我把项目重命名了，原本应该是叫id-generate-spring-boot-starter的，如下图所示：
pom.xml文件定义如下：
<?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>
<version>1.3.1</version>
<groupId>com.sue</groupId>
<artifactId>id-generate-spring-boot-starter</artifactId>
<name>id-generate-spring-boot-starter</name>
<dependencies>
<dependency>
<groupId>com.sue</groupId>
<artifactId>id-generate-spring-boot-autoconfigure</artifactId>
<version>1.3.1</version>
</dependency>
</dependencies>
</project>
我们看到，它只引入了id-generate-spring-boot-autoconfigure。当然如果有需要这里还可以引入多个autoconfigure或者多个其他jar包或者。
3.2 创建id-generate-autoconfigure同样为了方便我把项目重命名了，原本是叫id-generate-spring-boot-autoconfigure，如下图所示：
该项目当中包含：pom.xml、spring.factories、IdGenerateAutoConfiguration、IdGenerateService 和 IdProperties 这5个关键文件，下面我们逐一看看。
先从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">
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.0.4.RELEASE</version>
</parent>
<modelVersion>4.0.0</modelVersion>
<version>1.3.1</version>
<groupId>com.sue</groupId>
<artifactId>id-generate-spring-boot-autoconfigure</artifactId>
<name>id-generate-spring-boot-autoconfigure</name>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-autoconfigure</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-configuration-processor</artifactId>
<optional>true</optional>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</build>
</project>
我们可以看到，这个文件比较简单就引入了：
- spring-boot-starter：springboot的相关jar包。
- spring-boot-autoconfigure：springboot自动配置相关jar包。
- spring-boot-configuration-processor：springboot生成IDE提示功能相关jar包。
重点看看spring.factories文件：
org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.sue.IdGenerateAutoConfiguration
它里面只包含一行配置，其中key是EnableAutoConfiguration，value是IdGenerateAutoConfiguration。
再重点看一下IdGenerateAutoConfiguration
@ConditionalOnClass(IdProperties.class)
@EnableConfigurationProperties(IdProperties.class)
@Configuration
public class IdGenerateAutoConfiguration {
@Autowired
private IdProperties properties;
@Bean
public IdGenerateService idGenerateService() {
return new IdGenerateService(properties.getWorkId());
}
}
该类是一个使用了@Configuration注解标记为了配置类，生效的条件是@ConditionalOnClass注解中检测到包含IdProperties.class。并且使用@EnableConfigurationProperties注解会自动注入IdProperties的实例。
此外，最关键的点是该类里面创建了idGenerateService的bean实例，这是自动配置的精髓。
再看看IdGenerateService
public class IdGenerateService {
private Long workId;
public IdGenerateService(Long workId) {
this.workId = workId;
}
public Long generate() {
return new Random().nextInt(100) this.workId;
}
}
我们可以看到它是一个普通的类，甚至都没有使用@Service注解，里面有个generate方法，根据workId的值和随机数动态生成id。
最后看看IdProperties
@ConfigurationProperties(prefix = IdProperties.PREFIX)
public class IdProperties {
public static final String PREFIX = "sue";
private Long workId;
public Long getWorkId() {
return workId;
}
public void setWorkId(Long workId) {
this.workId = workId;
}
}
它是一个配置实体类，里面包含了相关的配置文件。使用@ConfigurationProperties注解，会自动把application.properties文件中以sue开通的，参数名称跟IdProperties中一样的参数值，自动注入到IdProperties对象中。
3.3 创建id-generate-test这个项目主要用于测试。
