分析Mybatis如何利用Spring的扩展点集成到框架中的，Mybatis本身的扩展点不再本次分析范畴
构建环境
上Github上下载https://github.com/mybatis/spring。通过Git的方式试了几次没成功，后来直接Down的zip包，再导入的Idea中的。
导入的过程当中会有点慢，要下载不少东西。记得一定要修改Maven的配置文件和本地仓库地址，否则可能之前你已经下过的相关包会又下载到C盘的本地仓库当中
测试代码
直接在源码目录下新建了一个目录来写测试代码
测试类
@Configuration
@MapperScan("com.jv.mapper")
@ComponentScan("com.jv.scan")
public class TestMybatis {
public static void main(String[] args) {
AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext(TestMybatis.class);
UserService bean = ac.getBean(UserService.class);
System.out.println(bean.query());
}
@Bean
public SqlSessionFactory sqlSessionFactory() throws Exception {
SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();
PooledDataSource dataSource = new PooledDataSource();
dataSource.setDriver("org.mariadb.jdbc.Driver");
dataSource.setUrl("jdbc:mysql://192.168.10.12:3306/acct?useSSL=false&serverTimezone=UTC");
dataSource.setUsername("dev01");
dataSource.setPassword("12340101");
factoryBean.setDataSource(dataSource);
return factoryBean.getObject();
}
}Service类
@Service
public class UserService {
@Autowired
private UserMapper userMapper;
public List<User> query(){
return userMapper.query();
}
}
Mapper类
public interface UserMapper {
@Select("SELECT name,age FROM user")
List<User> query();
}实体类
@ToString
public class User {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
private Integer age;
}注意：运行之前一定要修改pom.xml。因为Mybatis导入的Spring相关的依赖不在运行时生效
<scope>provided</scope>全部注释掉，否则运行的时候会报好不到类
表结构：
CREATE TABLE `user` (
`name` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
`age` int(4) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
insert into user(name,age) values("Messi",35);
commit;
源码分析
从mybatis-spring官方文档中可以找到@MapperScan的用法：http://mybatis.org/spring/mappers.html
注册BeanDefinition
既然和Spring集成是通过@MapperScan完成的，那从它入手
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)
@Repeatable(MapperScans.class)
public @interface MapperScan {
....
}其中@Import(MapperScannerRegistrar.class)是重点，再看MapperScannerRegistrar
public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {
/**
* {@inheritDoc}
*
* @deprecated Since 2.0.2, this method not used never.
*/
@Override
@Deprecated
public void setResourceLoader(ResourceLoader resourceLoader) {
// NOP
}
/**
* {@inheritDoc}
*/
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
AnnotationAttributes mapperScanAttrs = AnnotationAttributes
.fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
if (mapperScanAttrs != null) {
registerBeanDefinitions(mapperScanAttrs, registry, generateBaseBeanName(importingClassMetadata, 0));
}
}
void registerBeanDefinitions(AnnotationAttributes annoAttrs, BeanDefinitionRegistry registry, String beanName) {
/**
* 注册一个MapperScannerConfigurer的BeanDefinition,MapperScannerConfigurer实现了BeanDefinitionRegistryPostProcessor
* BeanDefinitionRegistryPostProcessor接口的实现类，一旦放入Spring容器中，那么在Spring容器启动的时候它可以担任注册自己需要BeanDefinition的功能
*/
BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
builder.addPropertyValue("processPropertyPlaceHolders", true);
Class<? extends Annotation> annotationClass = annoAttrs.getClass("annotationClass");
if (!Annotation.class.equals(annotationClass)) {
builder.addPropertyValue("annotationClass", annotationClass);
}
Class<?> markerInterface = annoAttrs.getClass("markerInterface");
if (!Class.class.equals(markerInterface)) {
builder.addPropertyValue("markerInterface", markerInterface);
}
//自定义BeanNameGenerator
Class<? extends BeanNameGenerator> generatorClass = annoAttrs.getClass("nameGenerator");
if (!BeanNameGenerator.class.equals(generatorClass)) {
builder.addPropertyValue("nameGenerator", BeanUtils.instantiateClass(generatorClass));
}
//自定义MapperFactoryBean
Class<? extends MapperFactoryBean> mapperFactoryBeanClass = annoAttrs.getClass("factoryBean");
if (!MapperFactoryBean.class.equals(mapperFactoryBeanClass)) {
builder.addPropertyValue("mapperFactoryBeanClass", mapperFactoryBeanClass);
}
//自定义sqlSessionTemplate
String sqlSessionTemplateRef = annoAttrs.getString("sqlSessionTemplateRef");
if (StringUtils.hasText(sqlSessionTemplateRef)) {
builder.addPropertyValue("sqlSessionTemplateBeanName", annoAttrs.getString("sqlSessionTemplateRef"));
}
//自定义sqlSessionFactory
String sqlSessionFactoryRef = annoAttrs.getString("sqlSessionFactoryRef");
if (StringUtils.hasText(sqlSessionFactoryRef)) {
builder.addPropertyValue("sqlSessionFactoryBeanName", annoAttrs.getString("sqlSessionFactoryRef"));
}
//生成所有的scan要扫描的基础包路径
List<String> basePackages = new ArrayList<>();
basePackages.addAll(
Arrays.stream(annoAttrs.getStringArray("value")).filter(StringUtils::hasText).collect(Collectors.toList()));
basePackages.addAll(Arrays.stream(annoAttrs.getStringArray("basePackages")).filter(StringUtils::hasText)
.collect(Collectors.toList()));
basePackages.addAll(Arrays.stream(annoAttrs.getClassArray("basePackageClasses")).map(ClassUtils::getPackageName)
.collect(Collectors.toList()));
//设置延迟加载
String lazyInitialization = annoAttrs.getString("lazyInitialization");
if (StringUtils.hasText(lazyInitialization)) {
builder.addPropertyValue("lazyInitialization", lazyInitialization);
}
builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(basePackages));
//将MapperScannerConfigurer的BeanDefinition注册
registry.registerBeanDefinition(beanName, builder.getBeanDefinition());
}
}
MapperScannerRegistrar implements ImportBeanDefinitionRegistrar
这就是它集成到Spring的关键点，关于ImportBeanDefinitionRegistrar可以参考https://my.oschina.net/u/3049601/blog/3129295
MapperScannerRegistrar 在Spring容器初始化的时候完成从外部导入MapperScannerConfigurer类对应的BeanDefinition
MapperScannerConfigurer implements BeanDefinitionRegistryPostProcessor（又是Spring的另外一个扩展点，也是扩展BeanDefinition注册功能，但它要晚于ImportBeanDefinitionRegistrar生效，因为前者的作用触发时机是Spring的ConfigurationClassPostProcessor Implements PriorityOrder。 这个类是扫描路径下所有Mapper的关键类
/**
* BeanDefinitionRegistryPostProcessor递归地从基本包中搜索接口并将它们注册为MapperFactoryBean
* MapperFactoryBean非常重要，它实现了InitializingBean，Spring会让Bean属性设置完之后调用它的抽象方法afterPropertiesSet，完成一些初始化操作
*/
public class MapperScannerConfigurer
implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {
.................省略部分代码..................
@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);
//默认是MapperFactoryBean,可以是自定义的
scanner.setMapperFactoryBeanClass(this.mapperFactoryBeanClass);
if (StringUtils.hasText(lazyInitialization)) {
scanner.setLazyInitialization(Boolean.valueOf(lazyInitialization));
}
scanner.registerFilters();
//开始扫描
scanner.scan(
StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
}
.................省略部分代码..................
}
ClassPathMapperScanner 继承自 Spring.ClassPathBeanDefinitionScanner，重写了doScan方法
public class ClassPathMapperScanner extends ClassPathBeanDefinitionScanner {
.................省略部分代码..................
/**
* 重写了ClassPathBeanDefinitionScanner的doScan方法，但是扫描工作还是由父类的doScan完成
*/
@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;
}
/**
* 重写了ClassPathBeanDefinitionScanner的processBeanDefinitions方法
* 完成BeanDefinition的属性填充
* 其中的setAutowireMode=AbstractBeanDefinition.AUTOWIRE_BY_TYPE 是Spring根据类型完成自动注入的关键。
* @param beanDefinitions
*/
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
/**
* 自定义的Mapper接口只是Bean最初的类，当Spring初始化之后Bean的Class实际上是MapperFactoryBean
*/
definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName); // issue #59
//设置BeanClass为MapperFactoryBean
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() + "'.");
// 相当的重要，这就是Spring的根据类型进行依赖注入。
// @Autowired使用的时候，其实Spring默认是没有自动注入的，也就是说autowireMode是AUTOWIRE_NO
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
}
definition.setLazyInit(lazyInitialization);
}
}
.................省略部分代码..................
}
上面的代码完成了对basePackage的类扫描，Mybatis根据扫描生成的BeanDefinition做增强，相当重要的两点：
1.BeanClass=MapperFactoryBean
也就是说：最开始Spring容器初始化完成之后，所有Mapper并没有真正的实例化（可以通过观察XXXApplicationContext.beanFactory.FactoryBeanObjectCache中是否有对象），而是它们的FactoryBean已经完成了实例化。当需要Mapper时再创建，如果是单例，则将已经实例化的Bean放到FactoryBeanObjectCache中
2.autowireMode=AUTOWIRE_BY_TYPE
按类型自动注入和@Autowired没直接联系，Spring默认是AUTOWIRE_NO，只不过Spring发现你用了@Autowired注解会自动根据类型注入而已。按类型自动注入必须要有setXXX方法。
代表了被扫描的类支持按类型实例化，为什么要设置这个值喃？我们自己写的Mapper紧紧是一个接口，为什么还要注入东西喃。。。根本原因是MapperFactoryBean extends SqlSessionDaoSupport
SqlSessionDaoSupport里面有两个set方法，分别是：
public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory)
public void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate)
可以验证一下，将“definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE)” 注释掉，报错：
代表着没有注入成功，也验证了AUTOWIRE_BY_TYPE的重要性
截止到这里，Mybatis相关类的BeanDefinition（BeanClass=MapperFactoryBean）全部完成了注册，接下来就是实例化。
实例化
实例化时有一个非常重要的点：
前面说过完成实例化的Mapper都是MapperFactoryBean，而不是真正的Mapper，为什么喃？
接下来看看MapperFactoryBean的类图
可以看到最终实现了InitializingBean，Spring针对实现了该接口的Bean，在完成属性填充之后会调用实现类的afterPropertiesSet()方法。
来看看afterPropertiesSet()方法干了什么
public final void afterPropertiesSet() throws IllegalArgumentException, BeanInitializationException {
//运行时会调用MapperFactoryBean的checkDaoConfig
this.checkDaoConfig();
try {
this.initDao();
} catch (Exception var2) {
throw new BeanInitializationException("Initialization of DAO failed", var2);
}
}MapperFactoryBean的checkDaoConfig方法
protected void checkDaoConfig() {
super.checkDaoConfig();
notNull(this.mapperInterface, "Property 'mapperInterface' is required");
Configuration configuration = getSqlSession().getConfiguration();
if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
try {
//非常重要
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();
}
}
}configuration.addMapper(this.mapperInterface)完成了MappedStatement添加。
configuration是DefaultSqlSesstionFactory（默认的）中的属性，全局唯一
MappedStatement描述了一个要执行的SQL，参数、返回类型等等
通过这一步，调用真正的query方法所需要的东西都准备好了。
总结：Mybatis利用了@Import(class implements ImportBeanDefinitionRegistrar),BeanDefinitionRegistryPostProcessor,InitializingBean三个扩展点来完成整合。
其中Mybatis的如下几个类非常重要：
MapperScan
MapperScannerRegistrar
MapperScannerConfigurer
ClassPathMapperScanner
MapperFactoryBean
DefaultSqlSessionFactory
Configuration
MapperStatement