Mybatis 自动配置原理

自动配置无非涉及几个方面

mapper 的代理注册

sql 语句的注册

查看自动配置类

路径 ：D:\maven-repository\com\baomidou\mybatis-plus-boot-starter\3.4.1\mybatis-plus-boot-starter-3.4.1.jar!\META-INF\spring.factories

代码解释

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@ConditionalOnClass({SqlSessionFactory.class, SqlSessionFactoryBean.class})
@ConditionalOnSingleCandidate(DataSource.class)
@EnableConfigurationProperties(MybatisPlusProperties.class)
@AutoConfigureAfter({DataSourceAutoConfiguration.class, MybatisPlusLanguageDriverAutoConfiguration.class})
public class MybatisPlusAutoConfiguration implements InitializingBean {
// 省略其他字段...

// 检查配置文件是否存在
private void checkConfigFileExists() {
// 当需要检查配置文件位置且位置不为空时执行
if (this.properties.isCheckConfigLocation() && StringUtils.hasText(this.properties.getConfigLocation())) {
Resource resource = this.resourceLoader.getResource(this.properties.getConfigLocation());
// 确保资源存在
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 {
// 创建 MybatisSqlSessionFactoryBean 实例，设置数据源
MybatisSqlSessionFactoryBean factory = new MybatisSqlSessionFactoryBean();
factory.setDataSource(dataSource);
// 省略配置和设置其他属性...

// 返回 SqlSessionFactory 实例
return factory.getObject();
}

// 检查 Spring 容器中的 bean，并进行消费
private <T> void getBeanThen(Class<T> clazz, Consumer<T> consumer) {
if (this.applicationContext.getBeanNamesForType(clazz, false, false).length > 0) {
consumer.accept(this.applicationContext.getBean(clazz));
}
}

// 应用 Mybatis 配置
private void applyConfiguration(MybatisSqlSessionFactoryBean factory) {
MybatisConfiguration configuration = this.properties.getConfiguration();
// 省略其他配置...
factory.setConfiguration(configuration);
}

@Bean
@ConditionalOnMissingBean
public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
// 根据属性返回 SqlSessionTemplate 实例
ExecutorType executorType = this.properties.getExecutorType();
return executorType != null ? new SqlSessionTemplate(sqlSessionFactory, executorType) : new SqlSessionTemplate(sqlSessionFactory);
}

// 省略其他内部类和方法...
}

这个类做了几件事

注册 SqlSessionFactory
这个类主要是为 SqlSessionTemplate 提供会话获取工厂 当然了 mybatisplus 进一步封装为了 SqlSessionTemplate 为了和 spring 的事务结合

注册 SqlSessionTemplate

注册 AutoConfiguredMapperScannerRegistrar

我们看看 mapper 是怎么注册到 spring 容器里面的

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public static class AutoConfiguredMapperScannerRegistrar implements BeanFactoryAware, ImportBeanDefinitionRegistrar {

private BeanFactory beanFactory;

@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {

if (!AutoConfigurationPackages.has(this.beanFactory)) {
logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled.");
return; }

logger.debug("Searching for mappers annotated with @Mapper");

List<String> packages = AutoConfigurationPackages.get(this.beanFactory);
if (logger.isDebugEnabled()) {
packages.forEach(pkg -> logger.debug("Using auto-configuration base package '{}'", pkg));
}

BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
builder.addPropertyValue("processPropertyPlaceHolders", true);
builder.addPropertyValue("annotationClass", Mapper.class);
builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(packages));
BeanWrapper beanWrapper = new BeanWrapperImpl(MapperScannerConfigurer.class);
Stream.of(beanWrapper.getPropertyDescriptors())
// Need to mybatis-spring 2.0.2+
.filter(x -> x.getName().equals("lazyInitialization")).findAny()
.ifPresent(x -> builder.addPropertyValue("lazyInitialization", "${mybatis.lazy-initialization:false}"));
registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition());
}

@Override
public void setBeanFactory(BeanFactory beanFactory) {
this.beanFactory = beanFactory;
}
}

可以看到这个类注册了一个 BeanDefinition 叫 MapperScannerConfigurer，再进去看看

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public class MapperScannerConfigurer
implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {

@Override
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
if (this.processPropertyPlaceHolders) {
processPropertyPlaceHolders();
}

ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
scanner.setAddToConfig(this.addToConfig);
scanner.setAnnotationClass(this.annotationClass);
scanner.setMarkerInterface(this.markerInterface);
scanner.setSqlSessionFactory(this.sqlSessionFactory);
scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
scanner.setResourceLoader(this.applicationContext);
scanner.setBeanNameGenerator(this.nameGenerator);
scanner.setMapperFactoryBeanClass(this.mapperFactoryBeanClass);
if (StringUtils.hasText(lazyInitialization)) {
scanner.setLazyInitialization(Boolean.valueOf(lazyInitialization));
}
scanner.registerFilters();
scanner.scan(
StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
}

可以看到 里面进一步使用 ClassPathMapperScanner 来扫描所有的 this.annotationClass 类
而上面配置 MapperScannerConfigurer 的时候是扫描这个注解

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BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
builder.addPropertyValue("processPropertyPlaceHolders", true);
builder.addPropertyValue("annotationClass", Mapper.class); //这里
builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(packages));

其中 ClassPathMapperScanner 重写了 doScan 方法

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@Override
public Set<BeanDefinitionHolder> doScan(String... basePackages) {
Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);

if (beanDefinitions.isEmpty()) {
LOGGER.warn(() -> "No MyBatis mapper was found in '" + Arrays.toString(basePackages)
+ "' package. Please check your configuration.");
} else {
processBeanDefinitions(beanDefinitions);
}

return beanDefinitions;
}

点进去 processBeanDefinitions (beanDefinitions) 看看

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private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
GenericBeanDefinition definition;
for (BeanDefinitionHolder holder : beanDefinitions) {
definition = (GenericBeanDefinition) holder.getBeanDefinition();
String beanClassName = definition.getBeanClassName();
LOGGER.debug(() -> "Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '" + beanClassName
+ "' mapperInterface");

// the mapper interface is the original class of the bean
// but, the actual class of the bean is MapperFactoryBean definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName); // issue #59
definition.setBeanClass(this.mapperFactoryBeanClass);

definition.getPropertyValues().add("addToConfig", this.addToConfig);

boolean explicitFactoryUsed = false;
if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) {
definition.getPropertyValues().add("sqlSessionFactory",
new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
explicitFactoryUsed = true;
} else if (this.sqlSessionFactory != null) {
definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
explicitFactoryUsed = true;
}

if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
if (explicitFactoryUsed) {
LOGGER.warn(
() -> "Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
}
definition.getPropertyValues().add("sqlSessionTemplate",
new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
explicitFactoryUsed = true;
} else if (this.sqlSessionTemplate != null) {
if (explicitFactoryUsed) {
LOGGER.warn(
() -> "Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
}
definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
explicitFactoryUsed = true;
}

if (!explicitFactoryUsed) {
LOGGER.debug(() -> "Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
}
definition.setLazyInit(lazyInitialization);
}
}

可以看到将扫描的 mapper 包装为

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definition.setBeanClass(this.mapperFactoryBeanClass);

这个类最后会注册一个 mapper 代理

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@Override
protected void checkDaoConfig() {
super.checkDaoConfig();

notNull(this.mapperInterface, "Property 'mapperInterface' is required");

Configuration configuration = getSqlSession().getConfiguration();
if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
try {
configuration.addMapper(this.mapperInterface);
} catch (Exception e) {
logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", e);
throw new IllegalArgumentException(e);
} finally {
ErrorContext.instance().reset();
}
}
}

/**
* {@inheritDoc}
*/@Override
public T getObject() throws Exception {
return getSqlSession().getMapper(this.mapperInterface);
}

我们进一步进去 configuration.addMapper (this.mapperInterface); 看看怎么注册的

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public <T> void addMapper(Class<T> type) {
if (type.isInterface()) {
if (hasMapper(type)) {
// TODO 如果之前注入 直接返回
return;
// TODO 这里就不抛异常了
// throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
knownMappers.put(type, new MybatisMapperProxyFactory<>(type));
MybatisMapperAnnotationBuilder parser = new MybatisMapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}

这里进一步的将 new MapperProxyFactory<>(type) 注册到 knownMappers 中，我们点进去看看

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public class MybatisMapperProxyFactory<T> {

@Getter
private final Class<T> mapperInterface;
@Getter
private final Map<Method, MybatisMapperProxy.MapperMethodInvoker> methodCache = new ConcurrentHashMap<>();
public MybatisMapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}

@SuppressWarnings("unchecked")
protected T newInstance(MybatisMapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy);
}

public T newInstance(SqlSession sqlSession) {
final MybatisMapperProxy<T> mapperProxy = new MybatisMapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}

可以看到 最后是封装为一个 MybatisMapperProxy 对象

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public class MybatisMapperProxyFactory<T> {

@Getter
private final Class<T> mapperInterface;
@Getter
private final Map<Method, MybatisMapperProxy.MapperMethodInvoker> methodCache = new ConcurrentHashMap<>();
public MybatisMapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}

@SuppressWarnings("unchecked")
protected T newInstance(MybatisMapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy);
}

public T newInstance(SqlSession sqlSession) {
final MybatisMapperProxy<T> mapperProxy = new MybatisMapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}

好了 mapper 怎么注册到 spring 的流程通了

sql 语句什么时候注册进去的呢？

我们注意到 注册 mapper 的时候下面还有两行语句

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MybatisMapperAnnotationBuilder parser = new MybatisMapperAnnotationBuilder(config, type);
parser.parse();

这里就是注册 sql 语句的地方，我们点进去看看

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public void parse() {
String resource = type.toString();
if (!configuration.isResourceLoaded(resource)) {
loadXmlResource();
configuration.addLoadedResource(resource);
String mapperName = type.getName();
assistant.setCurrentNamespace(mapperName);
parseCache();
parseCacheRef();
InterceptorIgnoreHelper.InterceptorIgnoreCache cache = InterceptorIgnoreHelper.initSqlParserInfoCache(type);
for (Method method : type.getMethods()) {
if (!canHaveStatement(method)) {
continue;
}
if (getAnnotationWrapper(method, false, Select.class, SelectProvider.class).isPresent()
&& method.getAnnotation(ResultMap.class) == null) {
parseResultMap(method);
}
try {
parseStatement(method);
InterceptorIgnoreHelper.initSqlParserInfoCache(cache, mapperName, method);
SqlParserHelper.initSqlParserInfoCache(mapperName, method);
} catch (IncompleteElementException e) {

}
}
// TODO 注入 CURD 动态 SQL , 放在在最后, because 可能会有人会用注解重写sql
try {
// https://github.com/baomidou/mybatis-plus/issues/3038
if (GlobalConfigUtils.isSupperMapperChildren(configuration, type)) {
parserInjector();
}
} catch (IncompleteElementException e) {
configuration.addIncompleteMethod(new InjectorResolver(this));
}
}
parsePendingMethods();
}

其中有一句loadXmlResource();
我们点进去看看

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private void loadXmlResource() {
// Spring may not know the real resource name so we check a flag
// to prevent loading again a resource twice // this flag is set at XMLMapperBuilder#bindMapperForNamespace if (!configuration.isResourceLoaded("namespace:" + type.getName())) {
String xmlResource = type.getName().replace('.', '/') + ".xml";
// #1347
InputStream inputStream = type.getResourceAsStream("/" + xmlResource);
if (inputStream == null) {
// Search XML mapper that is not in the module but in the classpath.
try {
inputStream = Resources.getResourceAsStream(type.getClassLoader(), xmlResource);
} catch (IOException e2) {
// ignore, resource is not required
}
}
if (inputStream != null) {
XMLMapperBuilder xmlParser = new XMLMapperBuilder(inputStream, assistant.getConfiguration(), xmlResource, configuration.getSqlFragments(), type.getName());
xmlParser.parse();
}
}
}

可以看到这里就是加载 xml 的地方String xmlResource = type.getName().replace('.', '/') + ".xml"; 这一句就是为什么 xml 要和 mapper 放在一个文件夹的关键了，直接是吧后缀替换为 xml 再去读取

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public void parse() {
if (!configuration.isResourceLoaded(resource)) {
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
bindMapperForNamespace();
}

parsePendingResultMaps();
parsePendingCacheRefs();
parsePendingStatements();
}

这里就是加载一些配置和一些 sql 语句了，接着就是注册 mybatisplus 自带的增删改查了

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// TODO 注入 CURD 动态 SQL , 放在在最后, because 可能会有人会用注解重写sql
try {
// https://github.com/baomidou/mybatis-plus/issues/3038
if (GlobalConfigUtils.isSupperMapperChildren(configuration, type)) {
parserInjector();
}
} catch (IncompleteElementException e) {
configuration.addIncompleteMethod(new InjectorResolver(this));
}

此文由 Mix Space 同步更新至 xLog
原始链接为 https://me.liuyaowen.cn/posts/codenotes/2024112