我们知道用使用Mybatis的时候，一般有两种方式。
一是在接口上添加@Mapper注解，二是在datasource中添加@MapperScan注解
那么这两种注解有什么区别呢？
@Mapper
首先我们先看@Mapper，@Mapper的加载还需要在接口中添加@Component注解。
之后SpringBoot中会自动加载MybatisAutoConfiguration类，在该类最下面会判断，如果我们没有手动创建MapperFactoryBean，或者使用@MapperScan（MapperScan会创建MapperFactoryBean），那么会导入AutoConfiguredMapperScannerRegistrar类。
@org.springframework.context.annotation.Configuration
@Import({ AutoConfiguredMapperScannerRegistrar.class })
@ConditionalOnMissingBean(MapperFactoryBean.class)
public static class MapperScannerRegistrarNotFoundConfiguration 跟进AutoConfiguredMapperScannerRegistrar类
public static class AutoConfiguredMapperScannerRegistrar
implements BeanFactoryAware, ImportBeanDefinitionRegistrar, ResourceLoaderAware
{
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
logger.debug("Searching for mappers annotated with @Mapper");
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);
}
}
}可以看到在执行AutoConfiguredMapperScannerRegistrar的registerBeanDefinitions方法时，会创建一个ClassPathMapperScanner类，用于扫描路径下包含Mapper注解的扫描器。
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;
}
private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
GenericBeanDefinition definition;
for (BeanDefinitionHolder holder : beanDefinitions) {
definition = (GenericBeanDefinition) holder.getBeanDefinition();
if (logger.isDebugEnabled()) {
logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName()
+ "' and '" + definition.getBeanClassName() + "' mapperInterface");
}
// the mapper interface is the original class of the bean
// but, the actual class of the bean is MapperFactoryBean
definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
definition.setBeanClass(this.mapperFactoryBean.getClass());
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) {
if (logger.isDebugEnabled()) {
logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
}
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
}
}
}并将符合的Mapper设置BeanClass为MapperFactoryBean。MapperFactoryBean的父类SqlSessionDaoSupport会创建SqlSessionTemplate。SqlSessionTemplate中注入了sqlSessionFactory。
public abstract class SqlSessionDaoSupport extends DaoSupport {
public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
if (!this.externalSqlSession) {
this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
}
}
}@MapperScan
MapperScan一般注解在dataSourceConfig类上，同时也需要配置@Component注解。
如：@MapperScan(basePackages = "com.demo.transaction.BaseDao", sqlSessionFactoryRef = "test1SqlSessionFactory")
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)
public @interface MapperScan 进入MapperScannerRegistrar。
public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
AnnotationAttributes annoAttrs = AnnotationAttributes.fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
// this check is needed in Spring 3.1
if (resourceLoader != null) {
scanner.setResourceLoader(resourceLoader);
}
Class<? extends Annotation> annotationClass = annoAttrs.getClass("annotationClass");
if (!Annotation.class.equals(annotationClass)) {
scanner.setAnnotationClass(annotationClass);
}
Class<?> markerInterface = annoAttrs.getClass("markerInterface");
if (!Class.class.equals(markerInterface)) {
scanner.setMarkerInterface(markerInterface);
}
Class<? extends BeanNameGenerator> generatorClass = annoAttrs.getClass("nameGenerator");
if (!BeanNameGenerator.class.equals(generatorClass)) {
scanner.setBeanNameGenerator(BeanUtils.instantiateClass(generatorClass));
}
Class<? extends MapperFactoryBean> mapperFactoryBeanClass = annoAttrs.getClass("factoryBean");
if (!MapperFactoryBean.class.equals(mapperFactoryBeanClass)) {
scanner.setMapperFactoryBean(BeanUtils.instantiateClass(mapperFactoryBeanClass));
}
scanner.setSqlSessionTemplateBeanName(annoAttrs.getString("sqlSessionTemplateRef"));
scanner.setSqlSessionFactoryBeanName(annoAttrs.getString("sqlSessionFactoryRef"));
List<String> basePackages = new ArrayList<String>();
for (String pkg : annoAttrs.getStringArray("value")) {
if (StringUtils.hasText(pkg)) {
basePackages.add(pkg);
}
}
for (String pkg : annoAttrs.getStringArray("basePackages")) {
if (StringUtils.hasText(pkg)) {
basePackages.add(pkg);
}
}
for (Class<?> clazz : annoAttrs.getClassArray("basePackageClasses")) {
basePackages.add(ClassUtils.getPackageName(clazz));
}
scanner.registerFilters();
scanner.doScan(StringUtils.toStringArray(basePackages));
}
}MapperScannerRegistrar类中设置了扫描路径basePackages。并且这里的scanner.registerFilters()做了处理。将basePackages路径下的所有类全匹配为true。
public void registerFilters() {
boolean acceptAllInterfaces = true;
if (this.annotationClass != null) {
addIncludeFilter(new AnnotationTypeFilter(this.annotationClass));
acceptAllInterfaces = false;
}
if (acceptAllInterfaces) {
// default include filter that accepts all classes
addIncludeFilter(new TypeFilter() {
@Override
public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory) throws IOException {
return true;
}
});
}
}因为使用@MapperScan时扫描annotationClass为null，所以添加的typeFilter默认全为true。而之前使用@Mapper使用的是Mybatis默认的注入器，那里设置了annotationClass为Mapper，所以添加了注解的AnnotationTypeFilter，并置acceptAllInterfaces为false。
总结：
1、@Mapper和@MapperScan使用一个即可，即使我们没有在ApplicationDemo中配置@MapperScan，SpringBoot中的MybatisAutoConfiguration也会自动找到路径中标有@Mapper注解的接口。
2、何时使用@Mapper，何时使用@MapperScan？ 如果你的项目数据源单一，那么完全可以使用@Mapper，让SpringBoot创建默认的SqlSessionTemplate、sqlSessionFactory。如果项目中有多个数据源，那么只能使用@MapperScan，扫描不同的路径下的mapper接口，并设置不同的sqlSessionFactory。因为如果还使用@Mapper，SpringBoot只会找带@primary的sqlSessionFactory，真正执行的时候，会报表不存在的错误。另外@Mapper和@MapperScan不要混合使用，混合使用只会装载@MapperScan设置的映射关系。
3、@Mapper和@MapperScan均需配合@Component注解使用，为了让Spring扫描到。