Spring整合Mybatis的源码解读——@MapperScan注解与SqlSessionTemplate的线程安全实现
回顾一下Mybatis的Mapper接口实例的生成与执行流程
众所周知，我们基于MyBatis 操作数据库，实际上就是通过 SqlSession 获取一个 JDBC 连接调用api来操作。
1、SqlSession接口。SqlSession 接口有常用的实现类有：DefaultSqlSession（线程不安全）、SqlSessionTemplate （spring中线程安全类，接下来的主角）
/*
* The default implementation for {@link SqlSession}.
* Note that this class is not Thread-Safe.
*
* @author Clinton Begin
*/
public class DefaultSqlSession implements SqlSession {}
2、我们通过sqlsession获取mapper接口实例的基本过程如下：
通过SqlSession的getMapper(type)方法获取的Mapper接口实例，这个实例是通过MapperProxyFactory生成的代理实例MapperProxy同时实现了invocationhandler接口。
3、MapperProxy的方法执行，在它的invoke方法中调用SqlSession的具体api来执行的。
public class MapperProxy<T> implements InvocationHandler, Serializable {
//通过传入的sqlsession进行方法调用
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else {
return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
这个过程用到JDK的动态代理设计模式。
今天的问题：
问题1、spring整合mybatis后是如何把Mapper代理对象注入ioc容器的呢？
- 基于注解@MapperScan和@Mapper，前者是spring的注解，后者是mybatis的注解
- 基于Xml配置文件
今天我们讨论的是基于@MapperScan注解的方式！
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)
@Repeatable(MapperScans.class)
public @interface MapperScan {
我们通过@MapperScan（“com.xxx.xxx”）扫描package把Mapper接口生成实例并注入IOC容器。扫描注册的类就是通过MapperScannerRegistrar.class实现的！
- MapperScannerRegistrar.class
public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {
/**
- {@inheritDoc}
*/
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
AnnotationAttributes mapperScanAttrs = AnnotationAttributes
.fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
if (mapperScanAttrs != null) {
registerBeanDefinitions(importingClassMetadata, mapperScanAttrs, registry,
generateBaseBeanName(importingClassMetadata, 0));
}
}
void registerBeanDefinitions(AnnotationMetadata annoMeta, AnnotationAttributes annoAttrs,
BeanDefinitionRegistry registry, String beanName) {
//获取MapperScannerConfigurer.class的BeanDefinitionBuilder
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);
}
Class<? extends BeanNameGenerator> generatorClass = annoAttrs.getClass("nameGenerator");
if (!BeanNameGenerator.class.equals(generatorClass)) {
builder.addPropertyValue("nameGenerator", BeanUtils.instantiateClass(generatorClass));
}
Class<? extends MapperFactoryBean> mapperFactoryBeanClass = annoAttrs.getClass("factoryBean");
if (!MapperFactoryBean.class.equals(mapperFactoryBeanClass)) {
builder.addPropertyValue("mapperFactoryBeanClass", mapperFactoryBeanClass);
}
String sqlSessionTemplateRef = annoAttrs.getString("sqlSessionTemplateRef");
if (StringUtils.hasText(sqlSessionTemplateRef)) {
builder.addPropertyValue("sqlSessionTemplateBeanName", annoAttrs.getString("sqlSessionTemplateRef"));
}
String sqlSessionFactoryRef = annoAttrs.getString("sqlSessionFactoryRef");
if (StringUtils.hasText(sqlSessionFactoryRef)) {
builder.addPropertyValue("sqlSessionFactoryBeanName", annoAttrs.getString("sqlSessionFactoryRef"));
}
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()));
if (basePackages.isEmpty()) {
basePackages.add(getDefaultBasePackage(annoMeta));
}
String lazyInitialization = annoAttrs.getString("lazyInitialization");
if (StringUtils.hasText(lazyInitialization)) {
builder.addPropertyValue("lazyInitialization", lazyInitialization);
}
builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(basePackages));
//注册MapperScannerConfigurer.class的BeanDefinition。
registry.registerBeanDefinition(beanName, builder.getBeanDefinition());
}
1、我们知道：Spring容器初始化时，从资源中读取到bean的相关定义后，保存在BeanDefinitionMap，在实例化bean的操作就是依据这些bean的定义来做的，而在实例化之前，Spring允许我们通过自定义扩展来改变bean的定义，定义一旦变了，后面的实例也就变了。
2、而MapperScannerRegistrar注册beandefinition其实是MapperScannerConfigurer.class！
- MapperScannerConfigurer.class！
public class MapperScannerConfigurer
implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {
private String basePackage;
private boolean addToConfig = true;
private String lazyInitialization;
private SqlSessionFactory sqlSessionFactory;
private SqlSessionTemplate sqlSessionTemplate;
private String sqlSessionFactoryBeanName;
private String sqlSessionTemplateBeanName;
private Class<? extends Annotation> annotationClass;
private Class<?> markerInterface;
private Class<? extends MapperFactoryBean> mapperFactoryBeanClass;
private ApplicationContext applicationContext;
private String beanName;
private boolean processPropertyPlaceHolders;
private BeanNameGenerator nameGenerator;
@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));
}
}
在postProcessBeanDefinitionRegistry的方法中通过ClassPathMapperScanner的实例scanner调用了scan（）方法。
public int scan(String... basePackages) {
int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
doScan(basePackages);
// Register annotation config processors, if necessary.
if (this.includeAnnotationConfig) {
AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}
return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
}
scan()方法调用了ClassPathMapperScanner的doscan方法。
- ClassPathMapperScanner
public class ClassPathMapperScanner extends ClassPathBeanDefinitionScanner {
//我们省了中间的属性和setget方式，直接看doscan方法
/**
* Calls the parent search that will search and register all the candidates. Then the registered objects are post
* processed to set them as MapperFactoryBeans
*/
@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;
}
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);
}
}
doscan方法调用父类ClassPathBeanDefinitionScanner的doscan方法。
- ClassPathBeanDefinitionScanner.class
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
for (String basePackage : basePackages) {
Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
for (BeanDefinition candidate : candidates) {
ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
candidate.setScope(scopeMetadata.getScopeName());
String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
if (candidate instanceof AbstractBeanDefinition) {
postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
}
if (candidate instanceof AnnotatedBeanDefinition) {
AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
}
if (checkCandidate(beanName, candidate)) {
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
definitionHolder =
AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
beanDefinitions.add(definitionHolder);
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}
- 父类的doscan方法作用：获取basepackage下面的所有mapper的全类名生成注入beandefinition并注入ioc中并返回了。
- 子类classPathMapperScanner的doscan方法后调用了processBeanDefinitions(Set beanDefinitions)方法，该方法把遍历父类生成的beandefinitions重新进行修改，把beanclass设置为mapperFactoryBeanclass，构造参数设置成beanClassname即mapper的类名。
- processBeanDefinitions方法的作用就是所有的mapper实例都是通过mapperfactorybean的getobject方法来生成并注入ioc容器的。
至此mapper代理对象就注入了IOC容器中！
问题2、spring是如何管理SqlSession保证线程安全的呢？
public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
PersistenceExceptionTranslator exceptionTranslator) {
notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
notNull(executorType, "Property 'executorType' is required");
this.sqlSessionFactory = sqlSessionFactory;
this.executorType = executorType;
this.exceptionTranslator = exceptionTranslator;
this.sqlSessionProxy = (SqlSession) newProxyInstance(SqlSessionFactory.class.getClassLoader(),
new Class[] { SqlSession.class }, new SqlSessionInterceptor());
}
重点在于SqlSessionTemplate中的SqlSessionProxy，所有的方法调用作用都是调用SqlSessionProxy,而它只是个代理类，具体方法执行是通过SqlSessionInterceptor的invoke方法。
SqlSessionInterceptor
private class SqlSessionInterceptor implements InvocationHandler {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
SqlSession sqlSession = getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,
SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);
try {
Object result = method.invoke(sqlSession, args);
if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
// force commit even on non-dirty sessions because some databases require
// a commit/rollback before calling close()
sqlSession.commit(true);
}
return result;
} catch (Throwable t) {
Throwable unwrapped = unwrapThrowable(t);
if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
// release the connection to avoid a deadlock if the translator is no loaded. See issue #22
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
sqlSession = null;
Throwable translated = SqlSessionTemplate.this.exceptionTranslator
.translateExceptionIfPossible((PersistenceException) unwrapped);
if (translated != null) {
unwrapped = translated;
}
}
throw unwrapped;
} finally {
if (sqlSession != null) {
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
}
}
1、SqlSessionInterceptor是SqlSessionTemplate的内部类，并实现了invocationhandler接口。
2、从mybatis的mapper生成流程我们知道，mapperproxy对象最终调用sqlsession执行方法，而到sqlsessiontemplate中就是SqlSessionInterceptor来具体执行了。
3、sqlsession通过sqlsessionutils的getsqlsession方法
- sqlsessionutils
public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType,
PersistenceExceptionTranslator exceptionTranslator) {
notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);
notNull(executorType, NO_EXECUTOR_TYPE_SPECIFIED);
SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
SqlSession session = sessionHolder(executorType, holder);
if (session != null) {
return session;
}
LOGGER.debug(() -> "Creating a new SqlSession");
session = sessionFactory.openSession(executorType);
registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);
return session;
}
1、该方法先获得了一个SqlSessionHolder，这个hodler其实内部持有Sqlsession，重点在于SqlSessionHolder的获得。在后面的调用关系比较长，我们省略说结论：这个holder是从threadlocal取出的！
2、Threadlocal中存放着一个对象：一个map
3、threadLocal对象的作用就再赘述了，保证每个线程有一个自己的变量副本，所以不存在资源争抢和线程安全问题。
SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
4、继续往下，如果通过holder获取的session不为null，就返回，不然创建一个新的sqlsession并调用registrySessionHolder尝试把session放在holder并放在threadlocal中。
- registrySessionHolder
private static void registerSessionHolder(SqlSessionFactory sessionFactory, ExecutorType executorType,
PersistenceExceptionTranslator exceptionTranslator, SqlSession session) {
SqlSessionHolder holder;
if (TransactionSynchronizationManager.isSynchronizationActive()) {
Environment environment = sessionFactory.getConfiguration().getEnvironment();
if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) {
LOGGER.debug(() -> "Registering transaction synchronization for SqlSession [" + session + "]");
holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
TransactionSynchronizationManager.bindResource(sessionFactory, holder);
TransactionSynchronizationManager
.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
holder.setSynchronizedWithTransaction(true);
holder.requested();
} else {
if (TransactionSynchronizationManager.getResource(environment.getDataSource()) == null) {
LOGGER.debug(() -> "SqlSession [" + session
+ "] was not registered for synchronization because DataSource is not transactional");
} else {
throw new TransientDataAccessResourceException(
"SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");
}
}
} else {
LOGGER.debug(() -> "SqlSession [" + session
+ "] was not registered for synchronization because synchronization is not active");
}
}
1、registrySessionHolder先判断是否有事务，如果有事务就放入，不然就不放入！
2、所有每个线程中的同一个事务会使用同一个sqlsession经事务管理，而非事务的每次操作都会新建一个sqlsession。
继续会到SqlSessionInterceptor，在返回结果后，最后在finally中执行sqlsession.close方法。
public static void closeSqlSession(SqlSession session, SqlSessionFactory sessionFactory) {
notNull(session, NO_SQL_SESSION_SPECIFIED);
notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);
SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
if ((holder != null) && (holder.getSqlSession() == session)) {
LOGGER.debug(() -> "Releasing transactional SqlSession [" + session + "]");
holder.released();
} else {
LOGGER.debug(() -> "Closing non transactional SqlSession [" + session + "]");
session.close();
}
}
1、该方法会判断是否在事务中，在事务中，只是把holder中的计数器-1，并没有关闭执行Session.close方法，所以在该事务中后续方法同一个SqlSession。
2、如果不在事务中，直接执行SqlSession.close()。
总结一句话：通过threadlocal保证sql的线程安全！有事务就把sqlsession放入threadlocal通过计数器减少引用，没有事务，所有操作都要创建一个新的sqlsession。