Mybatis原理分析
到目前为止在国内使用最广泛和最流行的持久层框架非Mybatis莫属,但是从最近一次jvm生态报告中,mybatis在java体系的使用率并不高:
可以清晰地看到mybatis不占上风，作为开发人员的常识,貌似国内java开发人员占比比较高,但是其实并不然,我们通过另一张统计报告图来看一下java开发人员在全球范围内的占比:
很明显北美的活跃度最高,老外更喜欢优雅的jpa编程方式,也可能他们对mybatis的认知还停留在ibatis时代,另外国内之所以流行,应该有一大部分原因是大厂带的节奏。比如某云曾说过:某里每年向社会输出1000名在某里工作十年以上的人才。那么他们从大厂出来后到其他中小型企业也都是中高层管理者,把大厂的规则辐射到中小厂也就无可厚非了。
一
使用
为了全面拥抱springboot和降低接入复杂度,mybatis也推出了mybatis-spring-boot-starter,是开发者通过很少的配置就能使用mybatis能力.
1.引入依赖
<dependency>
<groupId>org.mybatis.spring.boot</groupId>
<artifactId>mybatis-spring-boot-starter</artifactId>
</dependency>2.添加配置
spring.datasource.url=jdbc:mysql://localhost:3306/test?serverTimezone=UTC&useUnicode=true&characterEncoding=utf-8&useSSL=true
spring.datasource.username=root
spring.datasource.password=root
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
mybatis.config-location=classpath:mybatis/mybatis-config.xml
mybatis.mapper-locations=classpath:mybatis/mapper/*.xml3.开启Mapper接口扫描
在启动类上添加@MapperScan注解:
@SpringBootApplication
@MapperScan("xxx.xxx.mapper")
public class Application {
//....
}4.编写Mapper接口和xml实现
public interface UserInfoMapper {
UserInfoDO findByUid(@Param("uid") String uid);
}<mapper namespace="xxx.UserInfoMapper">
<resultMap id="BaseResultMap" type="xxx.UserInfoDO">
<id property="id" column="ID"/><!--自增ID-->
<result property="uid" column="UID"/>
....
</resultMap>
<select id="findByUid" resultMap="BaseResultMap">
SELECT <include refid="BaseColumnList"/>
FROM user_info
where UID = #{uid}
</select>
</mapper>这样我们就可以想使用其他bean一样,直接将Mapper接口注入到业务中正常使用了,那我们就应该思考一个问题,为什么我明明定义的是一个接口,而具体操作数据的sql实现在xml文件中,但是可以直接调用Mapper接口并且实现数据操作？我们带着问题继续分析.
二
源码&原理分析
接着前一节的问题,我们可以做出以下猜想：
- Mapper接口在应用启动的时候被解析成具体的实现,并注册到spring容器中
- Xml文件里边的sql实现在启动的时候被解析,然后放到一个大Map中
- 在应用启动完成或者Mapper接口在调用的时候把具体方法实现和Xml里边的数据操作进行绑定
- bean注入的时候进行一些初始化或者绑定操作
1.Mapper初始化
基于以上三个点,也给我们提供了思路,我们直接从@MapperScan注解入手:
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)
@Repeatable(MapperScans.class)
public @interface MapperScan {
//包路径,等价于basePackages
String[] value() default {};
//包路径
String[] basePackages() default {};
...
}注解定义了要扫描的包路径,并且导入了MapperScannerRegistrar,看一下实现:
public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {
@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) {
BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
builder.addPropertyValue("processPropertyPlaceHolders", true);
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));
}
builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(basePackages));
registry.registerBeanDefinition(beanName, builder.getBeanDefinition());
}
...
}MapperScannerRegistrar实现了ImportBeanDefinitionRegistrar接口,用来动态注册bean,被ConfigurationClassPostProcessor调用,在当前类实现中定义了MapperScannerConfigurer并注册到BeanDefinitionRegistry,接着看一下MapperScannerConfigurer实现:
public class MapperScannerConfigurer
implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {
...
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
// left intentionally blank
}
@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));
}
}MapperScannerConfigurer实现了BeanDefinitionRegistryPostProcessor接口,而BeanDefinitionRegistryPostProcessor又继承了BeanFactoryPostProcessor接口,BeanFactoryPostProcessor的作用是在bean的定义信息已经加载但还没有初始化的时候执行方法postProcessBeanFactory(）方法，而BeanDefinitionRegistryPostProcessor是在BeanFactoryPostProcessor的前面执行，MapperScannerConfigurer中postProcessBeanFactory没有实现,实现了postProcessBeanDefinitionRegistry方法,创建了ClassPathMapperScanner扫描器,注册过滤器并且扫描包路径下的候选Mapper接口.
接着看一下ClassPathMapperScanner实现:
public class ClassPathMapperScanner extends ClassPathBeanDefinitionScanner {
...
@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);
}
}
}在Spring IOC 容器初始化阶段就会调用ClassPathBeanDefinitionScanner扫描包下所有类，并将符合过滤条件的类注册到IOC 容器内,ClassPathMapperScanner继承ClassPathBeanDefinitionScanner并重写了doScan方法,用来扫描mybatis定义的路径下的包并注册到容器中。
需要注意的是这里的GenericBeanDefinition定义的BeanClass是MapperFactoryBean,也就是说这里调用父类的doScan扫描到Mapper接口的定义之后,把BeanDefinition修改成了MapperFactoryBean,而MapperFactoryBean本质上是FactoryBean在调用getObject()的时候才会返回原始的bean,看一下MapperFactoryBean继承关系和实现:
public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {
private Class<T> mapperInterface;
@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);
}
}到这里Mapper接口的初始化操作就完成了,但是注入到Spring Ioc容器中的是包装了Mapper的MapperFactoryBean,而如果某个Bean被定义成FactoryBean的话,在通过@Autowired方式注入的时候会在AutowiredAnnotationBeanPostProcessor#postProcessProperties注入：
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
metadata.inject(bean, beanName, pvs);
}
catch (BeanCreationException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}具体注入原理和时机此篇不做描述,注入的时候会调用getObject方法返回原始bean.
Mapper的注册流程如下:
2.XML操作指令解析
前边分析了Mapper的注册流程和原理,但是发现Mapper注册成MapperFactoryBean之后并没有和具体的数据操作指令关联起来,如果不关联只是一个孤零零的空壳子Mapper肯定无法实现数据操作,既然我们基于mybatis-spring-boot-starter实现和分析,那么基本确定里边肯定会有一些配置,没错那就是MybatisAutoConfiguration：
@org.springframework.context.annotation.Configuration
@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })
@ConditionalOnSingleCandidate(DataSource.class)
@EnableConfigurationProperties(MybatisProperties.class)
@AutoConfigureAfter(DataSourceAutoConfiguration.class)
public class MybatisAutoConfiguration implements InitializingBean {
@Override
public void afterPropertiesSet() {
checkConfigFileExists();
}
private 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()));
}
applyConfiguration(factory);
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 (this.properties.getTypeAliasesSuperType() != null) {
factory.setTypeAliasesSuperType(this.properties.getTypeAliasesSuperType());
}
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();
}
...
}MybatisAutoConfiguration在DataSourceAutoConfiguration数据源配置之后配置,检查有没有mybatis相关属性配置,并且创建SqlSessionFactory,SqlSessionFactory是通过SqlSessionFactoryBean配置生成的,看一下实现:
public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {
@Override
public void afterPropertiesSet() throws Exception {
notNull(dataSource, "Property 'dataSource' is required");
notNull(sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");
state((configuration == null && configLocation == null) || !(configuration != null && configLocation != null),
"Property 'configuration' and 'configLocation' can not specified with together");
this.sqlSessionFactory = buildSqlSessionFactory();
}
@Override
public SqlSessionFactory getObject() throws Exception {
if (this.sqlSessionFactory == null) {
afterPropertiesSet();
}
return this.sqlSessionFactory;
}
protected SqlSessionFactory buildSqlSessionFactory() throws Exception {
final Configuration targetConfiguration;
XMLConfigBuilder xmlConfigBuilder = null;
if (this.configuration != null) {
targetConfiguration = this.configuration;
if (targetConfiguration.getVariables() == null) {
targetConfiguration.setVariables(this.configurationProperties);
} else if (this.configurationProperties != null) {
targetConfiguration.getVariables().putAll(this.configurationProperties);
}
} else if (this.configLocation != null) {
xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
targetConfiguration = xmlConfigBuilder.getConfiguration();
} else {
LOGGER.debug(() -> "Property 'configuration' or 'configLocation' not specified, using default MyBatis Configuration");
targetConfiguration = new Configuration();
Optional.ofNullable(this.configurationProperties).ifPresent(targetConfiguration::setVariables);
}
Optional.ofNullable(this.objectFactory).ifPresent(targetConfiguration::setObjectFactory);
Optional.ofNullable(this.objectWrapperFactory).ifPresent(targetConfiguration::setObjectWrapperFactory);
Optional.ofNullable(this.vfs).ifPresent(targetConfiguration::setVfsImpl);
if (hasLength(this.typeAliasesPackage)) {
scanClasses(this.typeAliasesPackage, this.typeAliasesSuperType)
.forEach(targetConfiguration.getTypeAliasRegistry()::registerAlias);
}
if (!isEmpty(this.typeAliases)) {
Stream.of(this.typeAliases).forEach(typeAlias -> {
targetConfiguration.getTypeAliasRegistry().registerAlias(typeAlias);
LOGGER.debug(() -> "Registered type alias: '" + typeAlias + "'");
});
}
if (!isEmpty(this.plugins)) {
Stream.of(this.plugins).forEach(plugin -> {
targetConfiguration.addInterceptor(plugin);
LOGGER.debug(() -> "Registered plugin: '" + plugin + "'");
});
}
if (hasLength(this.typeHandlersPackage)) {
scanClasses(this.typeHandlersPackage, TypeHandler.class).stream()
.filter(clazz -> !clazz.isInterface())
.filter(clazz -> !Modifier.isAbstract(clazz.getModifiers()))
.filter(clazz -> ClassUtils.getConstructorIfAvailable(clazz) != null)
.forEach(targetConfiguration.getTypeHandlerRegistry()::register);
}
if (!isEmpty(this.typeHandlers)) {
Stream.of(this.typeHandlers).forEach(typeHandler -> {
targetConfiguration.getTypeHandlerRegistry().register(typeHandler);
LOGGER.debug(() -> "Registered type handler: '" + typeHandler + "'");
});
}
if (this.databaseIdProvider != null) {//fix #64 set databaseId before parse mapper xmls
try {
targetConfiguration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
} catch (SQLException e) {
throw new NestedIOException("Failed getting a databaseId", e);
}
}
Optional.ofNullable(this.cache).ifPresent(targetConfiguration::addCache);
if (xmlConfigBuilder != null) {
try {
xmlConfigBuilder.parse();
LOGGER.debug(() -> "Parsed configuration file: '" + this.configLocation + "'");
} catch (Exception ex) {
throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
} finally {
ErrorContext.instance().reset();
}
}
targetConfiguration.setEnvironment(new Environment(this.environment,
this.transactionFactory == null ? new SpringManagedTransactionFactory() : this.transactionFactory,
this.dataSource));
if (this.mapperLocations != null) {
if (this.mapperLocations.length == 0) {
LOGGER.warn(() -> "Property 'mapperLocations' was specified but matching resources are not found.");
} else {
for (Resource mapperLocation : this.mapperLocations) {
if (mapperLocation == null) {
continue;
}
try {
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments());
xmlMapperBuilder.parse();
} catch (Exception e) {
throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
} finally {
ErrorContext.instance().reset();
}
LOGGER.debug(() -> "Parsed mapper file: '" + mapperLocation + "'");
}
}
} else {
LOGGER.debug(() -> "Property 'mapperLocations' was not specified.");
}
return this.sqlSessionFactoryBuilder.build(targetConfiguration);
}
}SqlSessionFactoryBean也是一个FactoryBean,通过getObject方法返回SqlSessionFactory,SqlSessionFactoryBean实现了InitializingBean接口通过afterPropertiesSet方法创建SqlSessionFactory,在buildSqlSessionFactory方法中执行了解析xml的逻辑:
try {
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments());
xmlMapperBuilder.parse();
} catch (Exception e) {
throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
} finally {
ErrorContext.instance().reset();
}继续看XMLMapperBuilder#parse实现:
public void parse() {
if (!configuration.isResourceLoaded(resource)) {
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
bindMapperForNamespace();
}
parsePendingResultMaps();
parsePendingCacheRefs();
parsePendingStatements();
}先检查xml资源是否解析加载,如果没有则执行解析并置为加载,并执行绑定操作,然后解析通过集成用到的父文件的节点信息,我们主要看看configurationElement和bindMapperForNamespace, 先看configurationElement：
private void configurationElement(XNode context) {
try {
String namespace = context.getStringAttribute("namespace");
if (namespace == null || namespace.equals("")) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
builderAssistant.setCurrentNamespace(namespace);
cacheRefElement(context.evalNode("cache-ref"));
cacheElement(context.evalNode("cache"));
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
resultMapElements(context.evalNodes("/mapper/resultMap"));
sqlElement(context.evalNodes("/mapper/sql"));
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
}
}获取namespace属性,也就是对应的Mapper接口全路径,然后解析缓存相关节点/ResultMap节点/ParameterMap节点/sql片段,最后解析sql操作指令:
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
for (XNode context : list) {
final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
try {
statementParser.parseStatementNode();
} catch (IncompleteElementException e) {
configuration.addIncompleteStatement(statementParser);
}
}
}获取sql指令节点并执行解析:
public void parseStatementNode() {
String id = context.getStringAttribute("id");
String databaseId = context.getStringAttribute("databaseId");
if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
return;
}
String nodeName = context.getNode().getNodeName();
SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
boolean useCache = context.getBooleanAttribute("useCache", isSelect);
boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
// Include Fragments before parsing
XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
includeParser.applyIncludes(context.getNode());
String parameterType = context.getStringAttribute("parameterType");
Class<?> parameterTypeClass = resolveClass(parameterType);
String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);
// Parse selectKey after includes and remove them.
processSelectKeyNodes(id, parameterTypeClass, langDriver);
// Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
KeyGenerator keyGenerator;
String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
if (configuration.hasKeyGenerator(keyStatementId)) {
keyGenerator = configuration.getKeyGenerator(keyStatementId);
} else {
keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
}
SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
Integer fetchSize = context.getIntAttribute("fetchSize");
Integer timeout = context.getIntAttribute("timeout");
String parameterMap = context.getStringAttribute("parameterMap");
String resultType = context.getStringAttribute("resultType");
Class<?> resultTypeClass = resolveClass(resultType);
String resultMap = context.getStringAttribute("resultMap");
String resultSetType = context.getStringAttribute("resultSetType");
ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
String keyProperty = context.getStringAttribute("keyProperty");
String keyColumn = context.getStringAttribute("keyColumn");
String resultSets = context.getStringAttribute("resultSets");
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}把sql指令节点的属性和内容解析完成之后调用addMappedStatement方法转换成指令对象并维护id与其映射关系:
public MappedStatement addMappedStatement(
String id,
SqlSource sqlSource,
StatementType statementType,
SqlCommandType sqlCommandType,
Integer fetchSize,
Integer timeout,
String parameterMap,
Class<?> parameterType,
String resultMap,
Class<?> resultType,
ResultSetType resultSetType,
boolean flushCache,
boolean useCache,
boolean resultOrdered,
KeyGenerator keyGenerator,
String keyProperty,
String keyColumn,
String databaseId,
LanguageDriver lang,
String resultSets) {
if (unresolvedCacheRef) {
throw new IncompleteElementException("Cache-ref not yet resolved");
}
id = applyCurrentNamespace(id, false);
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
.resource(resource)
.fetchSize(fetchSize)
.timeout(timeout)
.statementType(statementType)
.keyGenerator(keyGenerator)
.keyProperty(keyProperty)
.keyColumn(keyColumn)
.databaseId(databaseId)
.lang(lang)
.resultOrdered(resultOrdered)
.resultSets(resultSets)
.resultMaps(getStatementResultMaps(resultMap, resultType, id))
.resultSetType(resultSetType)
.flushCacheRequired(valueOrDefault(flushCache, !isSelect))
.useCache(valueOrDefault(useCache, isSelect))
.cache(currentCache);
ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
if (statementParameterMap != null) {
statementBuilder.parameterMap(statementParameterMap);
}
MappedStatement statement = statementBuilder.build();
configuration.addMappedStatement(statement);
return statement;
}组装成MappedStatement之后放到内部维护的Map<String, MappedStatement>中备用.
然后看bindMapperForNamespace方法:
private void bindMapperForNamespace() {
String namespace = builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class<?> boundType = null;
try {
boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException e) {
//ignore, bound type is not required
}
if (boundType != null) {
if (!configuration.hasMapper(boundType)) {
// Spring may not know the real resource name so we set a flag
// to prevent loading again this resource from the mapper interface
// look at MapperAnnotationBuilder#loadXmlResource
configuration.addLoadedResource("namespace:" + namespace);
configuration.addMapper(boundType);
}
}
}
}加载namespace指定的Mapper接口并添加到ConfigurationMapperRegistry中：
public <T> void addMapper(Class<T> type) {
mapperRegistry.addMapper(type);
}接着看MapperRegistry#addMapper做了什么:
public <T> void addMapper(Class<T> type) {
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
knownMappers.put(type, new MapperProxyFactory<>(type));
// It's important that the type is added before the parser is run
// otherwise the binding may automatically be attempted by the
// mapper parser. If the type is already known, it won't try.
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}namespace指定的类是接口才处理,如果已经添加过了那么就报错终止(开发阶段可能不同的xml把namespace写成同一个接口),然后为Mapper创建MapperProxyFactory并添加到MapperRegistry的map中,然后创建MapperAnnotationBuilder解析Mapper接口方法上的注解,如果方法上使用了@Select/@Insert/@Update/@Delete，并且也同时使用了xml定义,那么会优先使用注解方式,也看一下MapperProxyFactory实现：
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();
public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class<T> getMapperInterface() {
return mapperInterface;
}
public Map<Method, MapperMethod> getMethodCache() {
return methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}其实就是对于Mapper接口的基于jdk动态代理实现,动态代理的InvocationHandler是MapperProxy,具体动态代理的实现和调用链路我们接下来分析.
XML解析流程如下：
3.Mapper注入&使用
前边有讲述到Mapper会被解析成MapperFactoryBean注册到容器中,在使用的时候通过@Autowired注入会调用MapperFactoryBean#getObject方法返回：
@Override
public T getObject() throws Exception {
return getSqlSession().getMapper(this.mapperInterface);
}然后会调用到SqlSession的默认实现类DefaultSqlSession#getMapper方法:
public <T> T getMapper(Class<T> type) {
return configuration.getMapper(type, this);
}接着调用Configuration#getMapper方法:
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}继续调用MapperRegistry#getMapper方法:
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}这里我们看到了熟悉的MapperProxyFactory,通过newInstance动态代理返回Mapper接口实例:
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}前边有描述到MapperProxy是一个InvocationHandler,继续看重载方法newInstance:
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}看到这里是不是眼前一亮,没错这就是jdk自带的动态代理,返回Mapper的动态代理类,那么动态代理类实例调用的时候会调用InvocationHandler的invoke方法,我们继续看MapperProxy的invoke实现:
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);
}
}如果Mapper是类则走if逻辑,如果是接口走else逻辑调用cachedInvoker..invoke,先看cachedInvoker实现：
private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
try {
MapperMethodInvoker invoker = methodCache.get(method);
if (invoker != null) {
return invoker;
}
return methodCache.computeIfAbsent(method, m -> {
if (m.isDefault()) {
try {
if (privateLookupInMethod == null) {
return new DefaultMethodInvoker(getMethodHandleJava8(method));
} else {
return new DefaultMethodInvoker(getMethodHandleJava9(method));
}
} catch (IllegalAccessException | InstantiationException | InvocationTargetException
| NoSuchMethodException e) {
throw new RuntimeException(e);
}
} else {
return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
}
});
} catch (RuntimeException re) {
Throwable cause = re.getCause();
throw cause == null ? re : cause;
}
}如果方法调用命中缓存,直接返回invoker,否则构造invoker放入缓存并返回,从而避免每次Mapper的方法调用都要解析构造method与具体sql的映射关系,如果方法是接口默认方法,这里的实现区分了java8和java9以上版本,主要是为了使用MethodHandle这种比传统反射更高效的方式来实现方法调用,最后封装成DefaultMethodInvoker并返回:
private static class DefaultMethodInvoker implements MapperMethodInvoker {
private final MethodHandle methodHandle;
public DefaultMethodInvoker(MethodHandle methodHandle) {
super();
this.methodHandle = methodHandle;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
return methodHandle.bindTo(proxy).invokeWithArguments(args);
}
}否则是接口抽象方法那么久封装成PlainMethodInvoker并返回:
private static class PlainMethodInvoker implements MapperMethodInvoker {
private final MapperMethod mapperMethod;
public PlainMethodInvoker(MapperMethod mapperMethod) {
super();
this.mapperMethod = mapperMethod;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
return mapperMethod.execute(sqlSession, args);
}
}很明显我们数据库操作依赖很多外部资源,基本都是定义抽象方法,然后实现交给xml的具体sql,回到前边的invoker,最终调用的是MapperMethod的execute方法：
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional()
&& (result == null || !method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}这里就看到了熟悉的crud相关指令实现,拿insert操作为例：
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}会调用DefaultSqlSession的insert方法并组装结果：
public int update(String statement, Object parameter) {
try {
dirty = true;
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.update(ms, wrapCollection(parameter));
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error updating database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}从Configuration的mappedStatements中获取对应sql并执行:
public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.update(stmt);
} finally {
closeStatement(stmt);
}
}最终会调用到jdbc定义的Statement#execute,然后返回结果.
Mapper注入和调用流程如下:
总结
mybatis使用抽象一下有几个点:
- XML文件解析,sql指令解析
- Mapper接口扫描并注册到容器
- Mapper接口注入以及与sql指令绑定
- Mapper接口动态代理与解析执行
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