Springboot自动整合mybatis原理

　　springboot整合mybatis 之后直接注入接口的原理。

1. pom引入

    <!-- spring-boot整合mybatis -->
    <dependency>
      <groupId>org.mybatis.spring.boot</groupId>
      <artifactId>mybatis-spring-boot-starter</artifactId>
      <version>2.1.4</version>
    </dependency>

2. 源码查看

1. spring.factories 引入如下配置

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=
org.mybatis.spring.boot.autoconfigure.MybatisLanguageDriverAutoConfiguration,
org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration

2.  org.mybatis.spring.boot.autoconfigure.MybatisLanguageDriverAutoConfiguration 源码

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.mybatis.spring.boot.autoconfigure;

import org.apache.ibatis.scripting.LanguageDriver;
import org.mybatis.scripting.freemarker.FreeMarkerLanguageDriver;
import org.mybatis.scripting.freemarker.FreeMarkerLanguageDriverConfig;
import org.mybatis.scripting.thymeleaf.ThymeleafLanguageDriver;
import org.mybatis.scripting.thymeleaf.ThymeleafLanguageDriverConfig;
import org.mybatis.scripting.thymeleaf.SqlGeneratorConfig.DialectConfig;
import org.mybatis.scripting.thymeleaf.ThymeleafLanguageDriverConfig.TemplateFileConfig;
import org.mybatis.scripting.velocity.Driver;
import org.mybatis.scripting.velocity.VelocityLanguageDriver;
import org.mybatis.scripting.velocity.VelocityLanguageDriverConfig;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
@ConditionalOnClass({LanguageDriver.class})
public class MybatisLanguageDriverAutoConfiguration {
    private static final String CONFIGURATION_PROPERTY_PREFIX = "mybatis.scripting-language-driver";

    public MybatisLanguageDriverAutoConfiguration() {
    }

    @Configuration
    @ConditionalOnClass({ThymeleafLanguageDriver.class})
    public static class ThymeleafConfiguration {
        public ThymeleafConfiguration() {
        }

        @Bean
        @ConditionalOnMissingBean
        ThymeleafLanguageDriver thymeleafLanguageDriver(ThymeleafLanguageDriverConfig config) {
            return new ThymeleafLanguageDriver(config);
        }

        @Bean
        @ConditionalOnMissingBean
        @ConfigurationProperties("mybatis.scripting-language-driver.thymeleaf")
        public ThymeleafLanguageDriverConfig thymeleafLanguageDriverConfig() {
            return ThymeleafLanguageDriverConfig.newInstance();
        }

        private static class MetadataThymeleafLanguageDriverConfig extends ThymeleafLanguageDriverConfig {
            private MetadataThymeleafLanguageDriverConfig() {
            }

            @ConfigurationProperties("mybatis.scripting-language-driver.thymeleaf.dialect")
            public DialectConfig getDialect() {
                return super.getDialect();
            }

            @ConfigurationProperties("mybatis.scripting-language-driver.thymeleaf.template-file")
            public TemplateFileConfig getTemplateFile() {
                return super.getTemplateFile();
            }
        }
    }

    @Configuration
    @ConditionalOnClass({VelocityLanguageDriver.class, VelocityLanguageDriverConfig.class})
    public static class VelocityConfiguration {
        public VelocityConfiguration() {
        }

        @Bean
        @ConditionalOnMissingBean
        VelocityLanguageDriver velocityLanguageDriver(VelocityLanguageDriverConfig config) {
            return new VelocityLanguageDriver(config);
        }

        @Bean
        @ConditionalOnMissingBean
        @ConfigurationProperties("mybatis.scripting-language-driver.velocity")
        public VelocityLanguageDriverConfig velocityLanguageDriverConfig() {
            return VelocityLanguageDriverConfig.newInstance();
        }
    }

    @Configuration
    @ConditionalOnClass({Driver.class})
    @ConditionalOnMissingClass({"org.mybatis.scripting.velocity.VelocityLanguageDriverConfig"})
    public static class LegacyVelocityConfiguration {
        public LegacyVelocityConfiguration() {
        }

        @Bean
        @ConditionalOnMissingBean
        Driver velocityLanguageDriver() {
            return new Driver();
        }
    }

    @Configuration
    @ConditionalOnClass({FreeMarkerLanguageDriver.class, FreeMarkerLanguageDriverConfig.class})
    public static class FreeMarkerConfiguration {
        public FreeMarkerConfiguration() {
        }

        @Bean
        @ConditionalOnMissingBean
        FreeMarkerLanguageDriver freeMarkerLanguageDriver(FreeMarkerLanguageDriverConfig config) {
            return new FreeMarkerLanguageDriver(config);
        }

        @Bean
        @ConditionalOnMissingBean
        @ConfigurationProperties("mybatis.scripting-language-driver.freemarker")
        public FreeMarkerLanguageDriverConfig freeMarkerLanguageDriverConfig() {
            return FreeMarkerLanguageDriverConfig.newInstance();
        }
    }

    @Configuration
    @ConditionalOnClass({FreeMarkerLanguageDriver.class})
    @ConditionalOnMissingClass({"org.mybatis.scripting.freemarker.FreeMarkerLanguageDriverConfig"})
    public static class LegacyFreeMarkerConfiguration {
        public LegacyFreeMarkerConfiguration() {
        }

        @Bean
        @ConditionalOnMissingBean
        FreeMarkerLanguageDriver freeMarkerLanguageDriver() {
            return new FreeMarkerLanguageDriver();
        }
    }
}

3. org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration 源码

package org.mybatis.spring.boot.autoconfigure;

import java.beans.FeatureDescriptor;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import javax.sql.DataSource;
import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.mapping.DatabaseIdProvider;
import org.apache.ibatis.plugin.Interceptor;
import org.apache.ibatis.scripting.LanguageDriver;
import org.apache.ibatis.session.ExecutorType;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.type.TypeHandler;
import org.mybatis.spring.SqlSessionFactoryBean;
import org.mybatis.spring.SqlSessionTemplate;
import org.mybatis.spring.mapper.MapperFactoryBean;
import org.mybatis.spring.mapper.MapperScannerConfigurer;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.BeanWrapper;
import org.springframework.beans.BeanWrapperImpl;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.beans.factory.support.BeanDefinitionBuilder;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.boot.autoconfigure.AutoConfigurationPackages;
import org.springframework.boot.autoconfigure.AutoConfigureAfter;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnSingleCandidate;
import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
import org.springframework.core.io.Resource;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.util.Assert;
import org.springframework.util.CollectionUtils;
import org.springframework.util.ObjectUtils;
import org.springframework.util.StringUtils;

@Configuration
@ConditionalOnClass({SqlSessionFactory.class, SqlSessionFactoryBean.class})
@ConditionalOnSingleCandidate(DataSource.class)
@EnableConfigurationProperties({MybatisProperties.class})
@AutoConfigureAfter({DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class})
public class MybatisAutoConfiguration implements InitializingBean {
    private static final Logger logger = LoggerFactory.getLogger(MybatisAutoConfiguration.class);
    private final MybatisProperties properties;
    private final Interceptor[] interceptors;
    private final TypeHandler[] typeHandlers;
    private final LanguageDriver[] languageDrivers;
    private final ResourceLoader resourceLoader;
    private final DatabaseIdProvider databaseIdProvider;
    private final List<ConfigurationCustomizer> configurationCustomizers;

    public MybatisAutoConfiguration(MybatisProperties properties, ObjectProvider<Interceptor[]> interceptorsProvider, ObjectProvider<TypeHandler[]> typeHandlersProvider, ObjectProvider<LanguageDriver[]> languageDriversProvider, ResourceLoader resourceLoader, ObjectProvider<DatabaseIdProvider> databaseIdProvider, ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider) {
        this.properties = properties;
        this.interceptors = (Interceptor[])interceptorsProvider.getIfAvailable();
        this.typeHandlers = (TypeHandler[])typeHandlersProvider.getIfAvailable();
        this.languageDrivers = (LanguageDriver[])languageDriversProvider.getIfAvailable();
        this.resourceLoader = resourceLoader;
        this.databaseIdProvider = (DatabaseIdProvider)databaseIdProvider.getIfAvailable();
        this.configurationCustomizers = (List)configurationCustomizersProvider.getIfAvailable();
    }

    public void afterPropertiesSet() {
        this.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()));
        }

        this.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.typeHandlers)) {
            factory.setTypeHandlers(this.typeHandlers);
        }

        if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {
            factory.setMapperLocations(this.properties.resolveMapperLocations());
        }

        Set<String> factoryPropertyNames = (Set)Stream.of((new BeanWrapperImpl(SqlSessionFactoryBean.class)).getPropertyDescriptors()).map(FeatureDescriptor::getName).collect(Collectors.toSet());
        Class<? extends LanguageDriver> defaultLanguageDriver = this.properties.getDefaultScriptingLanguageDriver();
        if (factoryPropertyNames.contains("scriptingLanguageDrivers") && !ObjectUtils.isEmpty(this.languageDrivers)) {
            factory.setScriptingLanguageDrivers(this.languageDrivers);
            if (defaultLanguageDriver == null && this.languageDrivers.length == 1) {
                defaultLanguageDriver = this.languageDrivers[0].getClass();
            }
        }

        if (factoryPropertyNames.contains("defaultScriptingLanguageDriver")) {
            factory.setDefaultScriptingLanguageDriver(defaultLanguageDriver);
        }

        return factory.getObject();
    }

    private void applyConfiguration(SqlSessionFactoryBean factory) {
        org.apache.ibatis.session.Configuration configuration = this.properties.getConfiguration();
        if (configuration == null && !StringUtils.hasText(this.properties.getConfigLocation())) {
            configuration = new org.apache.ibatis.session.Configuration();
        }

        if (configuration != null && !CollectionUtils.isEmpty(this.configurationCustomizers)) {
            Iterator var3 = this.configurationCustomizers.iterator();

            while(var3.hasNext()) {
                ConfigurationCustomizer customizer = (ConfigurationCustomizer)var3.next();
                customizer.customize(configuration);
            }
        }

        factory.setConfiguration(configuration);
    }

    @Bean
    @ConditionalOnMissingBean
    public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
        ExecutorType executorType = this.properties.getExecutorType();
        return executorType != null ? new SqlSessionTemplate(sqlSessionFactory, executorType) : new SqlSessionTemplate(sqlSessionFactory);
    }

    @Configuration
    @Import({MybatisAutoConfiguration.AutoConfiguredMapperScannerRegistrar.class})
    @ConditionalOnMissingBean({MapperFactoryBean.class, MapperScannerConfigurer.class})
    public static class MapperScannerRegistrarNotFoundConfiguration implements InitializingBean {
        public MapperScannerRegistrarNotFoundConfiguration() {
        }

        public void afterPropertiesSet() {
            MybatisAutoConfiguration.logger.debug("Not found configuration for registering mapper bean using @MapperScan, MapperFactoryBean and MapperScannerConfigurer.");
        }
    }

    public static class AutoConfiguredMapperScannerRegistrar implements BeanFactoryAware, ImportBeanDefinitionRegistrar {
        private BeanFactory beanFactory;

        public AutoConfiguredMapperScannerRegistrar() {
        }

        public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
            if (!AutoConfigurationPackages.has(this.beanFactory)) {
                MybatisAutoConfiguration.logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled.");
            } else {
                MybatisAutoConfiguration.logger.debug("Searching for mappers annotated with @Mapper");
                List<String> packages = AutoConfigurationPackages.get(this.beanFactory);
                if (MybatisAutoConfiguration.logger.isDebugEnabled()) {
                    packages.forEach((pkg) -> {
                        MybatisAutoConfiguration.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);
                Set<String> propertyNames = (Set)Stream.of(beanWrapper.getPropertyDescriptors()).map(FeatureDescriptor::getName).collect(Collectors.toSet());
                if (propertyNames.contains("lazyInitialization")) {
                    builder.addPropertyValue("lazyInitialization", "${mybatis.lazy-initialization:false}");
                }

                if (propertyNames.contains("defaultScope")) {
                    builder.addPropertyValue("defaultScope", "${mybatis.mapper-default-scope:}");
                }

                registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition());
            }
        }

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

 　　所以核心在MybatisAutoConfiguration 这个类上面。这个类注入了几个重要的对象：

(1) SqlSessionFactory  mybatis 的会话工厂

1》this.properties.resolveMapperLocations() 这个会解析到所有的Mapper.xml 文件，然后存到org.mybatis.spring.SqlSessionFactoryBean#mapperLocations 属性中

2》 factory.getObject() 会调用到org.mybatis.spring.SqlSessionFactoryBean#buildSqlSessionFactory， 这个里面会解析Mapper.xml 文件，生成MappedStatement 对象等

    protected SqlSessionFactory buildSqlSessionFactory() throws Exception {
        XMLConfigBuilder xmlConfigBuilder = null;
        Configuration targetConfiguration;
        Optional var10000;
        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(), (String)null, this.configurationProperties);
            targetConfiguration = xmlConfigBuilder.getConfiguration();
        } else {
            LOGGER.debug(() -> {
                return "Property 'configuration' or 'configLocation' not specified, using default MyBatis Configuration";
            });
            targetConfiguration = new Configuration();
            var10000 = Optional.ofNullable(this.configurationProperties);
            Objects.requireNonNull(targetConfiguration);
            var10000.ifPresent(targetConfiguration::setVariables);
        }

        var10000 = Optional.ofNullable(this.objectFactory);
        Objects.requireNonNull(targetConfiguration);
        var10000.ifPresent(targetConfiguration::setObjectFactory);
        var10000 = Optional.ofNullable(this.objectWrapperFactory);
        Objects.requireNonNull(targetConfiguration);
        var10000.ifPresent(targetConfiguration::setObjectWrapperFactory);
        var10000 = Optional.ofNullable(this.vfs);
        Objects.requireNonNull(targetConfiguration);
        var10000.ifPresent(targetConfiguration::setVfsImpl);
        Stream var24;
        if (StringUtils.hasLength(this.typeAliasesPackage)) {
            var24 = this.scanClasses(this.typeAliasesPackage, this.typeAliasesSuperType).stream().filter((clazz) -> {
                return !clazz.isAnonymousClass();
            }).filter((clazz) -> {
                return !clazz.isInterface();
            }).filter((clazz) -> {
                return !clazz.isMemberClass();
            });
            TypeAliasRegistry var10001 = targetConfiguration.getTypeAliasRegistry();
            Objects.requireNonNull(var10001);
            var24.forEach(var10001::registerAlias);
        }

        if (!ObjectUtils.isEmpty(this.typeAliases)) {
            Stream.of(this.typeAliases).forEach((typeAlias) -> {
                targetConfiguration.getTypeAliasRegistry().registerAlias(typeAlias);
                LOGGER.debug(() -> {
                    return "Registered type alias: '" + typeAlias + "'";
                });
            });
        }

        if (!ObjectUtils.isEmpty(this.plugins)) {
            Stream.of(this.plugins).forEach((plugin) -> {
                targetConfiguration.addInterceptor(plugin);
                LOGGER.debug(() -> {
                    return "Registered plugin: '" + plugin + "'";
                });
            });
        }

        if (StringUtils.hasLength(this.typeHandlersPackage)) {
            var24 = this.scanClasses(this.typeHandlersPackage, TypeHandler.class).stream().filter((clazz) -> {
                return !clazz.isAnonymousClass();
            }).filter((clazz) -> {
                return !clazz.isInterface();
            }).filter((clazz) -> {
                return !Modifier.isAbstract(clazz.getModifiers());
            });
            TypeHandlerRegistry var25 = targetConfiguration.getTypeHandlerRegistry();
            Objects.requireNonNull(var25);
            var24.forEach(var25::register);
        }

        if (!ObjectUtils.isEmpty(this.typeHandlers)) {
            Stream.of(this.typeHandlers).forEach((typeHandler) -> {
                targetConfiguration.getTypeHandlerRegistry().register(typeHandler);
                LOGGER.debug(() -> {
                    return "Registered type handler: '" + typeHandler + "'";
                });
            });
        }

        targetConfiguration.setDefaultEnumTypeHandler(this.defaultEnumTypeHandler);
        if (!ObjectUtils.isEmpty(this.scriptingLanguageDrivers)) {
            Stream.of(this.scriptingLanguageDrivers).forEach((languageDriver) -> {
                targetConfiguration.getLanguageRegistry().register(languageDriver);
                LOGGER.debug(() -> {
                    return "Registered scripting language driver: '" + languageDriver + "'";
                });
            });
        }

        var10000 = Optional.ofNullable(this.defaultScriptingLanguageDriver);
        Objects.requireNonNull(targetConfiguration);
        var10000.ifPresent(targetConfiguration::setDefaultScriptingLanguage);
        if (this.databaseIdProvider != null) {
            try {
                targetConfiguration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
            } catch (SQLException var23) {
                throw new NestedIOException("Failed getting a databaseId", var23);
            }
        }

        var10000 = Optional.ofNullable(this.cache);
        Objects.requireNonNull(targetConfiguration);
        var10000.ifPresent(targetConfiguration::addCache);
        if (xmlConfigBuilder != null) {
            try {
                xmlConfigBuilder.parse();
                LOGGER.debug(() -> {
                    return "Parsed configuration file: '" + this.configLocation + "'";
                });
            } catch (Exception var21) {
                throw new NestedIOException("Failed to parse config resource: " + this.configLocation, var21);
            } finally {
                ErrorContext.instance().reset();
            }
        }

        targetConfiguration.setEnvironment(new Environment(this.environment, (TransactionFactory)(this.transactionFactory == null ? new SpringManagedTransactionFactory() : this.transactionFactory), this.dataSource));
        if (this.mapperLocations != null) {
            if (this.mapperLocations.length == 0) {
                LOGGER.warn(() -> {
                    return "Property 'mapperLocations' was specified but matching resources are not found.";
                });
            } else {
                Resource[] var3 = this.mapperLocations;
                int var4 = var3.length;

                for(int var5 = 0; var5 < var4; ++var5) {
                    Resource mapperLocation = var3[var5];
                    if (mapperLocation != null) {
                        try {
                            XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(), targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments());
                            xmlMapperBuilder.parse();
                        } catch (Exception var19) {
                            throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", var19);
                        } finally {
                            ErrorContext.instance().reset();
                        }

                        LOGGER.debug(() -> {
                            return "Parsed mapper file: '" + mapperLocation + "'";
                        });
                    }
                }
            }
        } else {
            LOGGER.debug(() -> {
                return "Property 'mapperLocations' was not specified.";
            });
        }

        return this.sqlSessionFactoryBuilder.build(targetConfiguration);
    }

(2) SqlSessionTemplate 实际是一个SqlSession 对象，spring对mybatis的SqlSession的使用是由SqlSessionTemplate控制的，在SqlSessionTemplate类中执行SQL语句的SqlSession都是通过sqlSessionProxy来代理执行的。其源码如下：org.mybatis.spring.SqlSessionTemplate

package org.mybatis.spring;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.sql.Connection;
import java.util.List;
import java.util.Map;
import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.exceptions.PersistenceException;
import org.apache.ibatis.executor.BatchResult;
import org.apache.ibatis.reflection.ExceptionUtil;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ExecutorType;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.springframework.beans.factory.DisposableBean;
import org.springframework.dao.support.PersistenceExceptionTranslator;
import org.springframework.util.Assert;

public class SqlSessionTemplate implements SqlSession, DisposableBean {
    private final SqlSessionFactory sqlSessionFactory;
    private final ExecutorType executorType;
    private final SqlSession sqlSessionProxy;
    private final PersistenceExceptionTranslator exceptionTranslator;

    public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
        this(sqlSessionFactory, sqlSessionFactory.getConfiguration().getDefaultExecutorType());
    }

    public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType) {
        this(sqlSessionFactory, executorType, new MyBatisExceptionTranslator(sqlSessionFactory.getConfiguration().getEnvironment().getDataSource(), true));
    }

    public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {
        Assert.notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
        Assert.notNull(executorType, "Property 'executorType' is required");
        this.sqlSessionFactory = sqlSessionFactory;
        this.executorType = executorType;
        this.exceptionTranslator = exceptionTranslator;
        this.sqlSessionProxy = (SqlSession)Proxy.newProxyInstance(SqlSessionFactory.class.getClassLoader(), new Class[]{SqlSession.class}, new SqlSessionTemplate.SqlSessionInterceptor());
    }

    public SqlSessionFactory getSqlSessionFactory() {
        return this.sqlSessionFactory;
    }

    public ExecutorType getExecutorType() {
        return this.executorType;
    }

    public PersistenceExceptionTranslator getPersistenceExceptionTranslator() {
        return this.exceptionTranslator;
    }

    public <T> T selectOne(String statement) {
        return this.sqlSessionProxy.selectOne(statement);
    }

    public <T> T selectOne(String statement, Object parameter) {
        return this.sqlSessionProxy.selectOne(statement, parameter);
    }

    public <K, V> Map<K, V> selectMap(String statement, String mapKey) {
        return this.sqlSessionProxy.selectMap(statement, mapKey);
    }

    public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey) {
        return this.sqlSessionProxy.selectMap(statement, parameter, mapKey);
    }

    public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds) {
        return this.sqlSessionProxy.selectMap(statement, parameter, mapKey, rowBounds);
    }

    public <T> Cursor<T> selectCursor(String statement) {
        return this.sqlSessionProxy.selectCursor(statement);
    }

    public <T> Cursor<T> selectCursor(String statement, Object parameter) {
        return this.sqlSessionProxy.selectCursor(statement, parameter);
    }

    public <T> Cursor<T> selectCursor(String statement, Object parameter, RowBounds rowBounds) {
        return this.sqlSessionProxy.selectCursor(statement, parameter, rowBounds);
    }

    public <E> List<E> selectList(String statement) {
        return this.sqlSessionProxy.selectList(statement);
    }

    public <E> List<E> selectList(String statement, Object parameter) {
        return this.sqlSessionProxy.selectList(statement, parameter);
    }

    public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
        return this.sqlSessionProxy.selectList(statement, parameter, rowBounds);
    }

    public void select(String statement, ResultHandler handler) {
        this.sqlSessionProxy.select(statement, handler);
    }

    public void select(String statement, Object parameter, ResultHandler handler) {
        this.sqlSessionProxy.select(statement, parameter, handler);
    }

    public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
        this.sqlSessionProxy.select(statement, parameter, rowBounds, handler);
    }

    public int insert(String statement) {
        return this.sqlSessionProxy.insert(statement);
    }

    public int insert(String statement, Object parameter) {
        return this.sqlSessionProxy.insert(statement, parameter);
    }

    public int update(String statement) {
        return this.sqlSessionProxy.update(statement);
    }

    public int update(String statement, Object parameter) {
        return this.sqlSessionProxy.update(statement, parameter);
    }

    public int delete(String statement) {
        return this.sqlSessionProxy.delete(statement);
    }

    public int delete(String statement, Object parameter) {
        return this.sqlSessionProxy.delete(statement, parameter);
    }

    public <T> T getMapper(Class<T> type) {
        return this.getConfiguration().getMapper(type, this);
    }

    public void commit() {
        throw new UnsupportedOperationException("Manual commit is not allowed over a Spring managed SqlSession");
    }

    public void commit(boolean force) {
        throw new UnsupportedOperationException("Manual commit is not allowed over a Spring managed SqlSession");
    }

    public void rollback() {
        throw new UnsupportedOperationException("Manual rollback is not allowed over a Spring managed SqlSession");
    }

    public void rollback(boolean force) {
        throw new UnsupportedOperationException("Manual rollback is not allowed over a Spring managed SqlSession");
    }

    public void close() {
        throw new UnsupportedOperationException("Manual close is not allowed over a Spring managed SqlSession");
    }

    public void clearCache() {
        this.sqlSessionProxy.clearCache();
    }

    public Configuration getConfiguration() {
        return this.sqlSessionFactory.getConfiguration();
    }

    public Connection getConnection() {
        return this.sqlSessionProxy.getConnection();
    }

    public List<BatchResult> flushStatements() {
        return this.sqlSessionProxy.flushStatements();
    }

    public void destroy() throws Exception {
    }

    private class SqlSessionInterceptor implements InvocationHandler {
        private SqlSessionInterceptor() {
        }

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            SqlSession sqlSession = SqlSessionUtils.getSqlSession(SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);

            Object unwrapped;
            try {
                Object result = method.invoke(sqlSession, args);
                if (!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
                    sqlSession.commit(true);
                }

                unwrapped = result;
            } catch (Throwable var11) {
                unwrapped = ExceptionUtil.unwrapThrowable(var11);
                if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
                    SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                    sqlSession = null;
                    Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException)unwrapped);
                    if (translated != null) {
                        unwrapped = translated;
                    }
                }

                throw (Throwable)unwrapped;
            } finally {
                if (sqlSession != null) {
                    SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                }

            }

            return unwrapped;
        }
    }
}

重点解释：

1》org.mybatis.spring.SqlSessionTemplate#sqlSessionProxy 是一个代理对象，并且走的JDK的动态代理，其实现类如下：org.mybatis.spring.SqlSessionTemplate.SqlSessionInterceptor

    private class SqlSessionInterceptor implements InvocationHandler {
        private SqlSessionInterceptor() {
        }

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            SqlSession sqlSession = SqlSessionUtils.getSqlSession(SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);

            Object unwrapped;
            try {
                Object result = method.invoke(sqlSession, args);
                if (!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
                    sqlSession.commit(true);
                }

                unwrapped = result;
            } catch (Throwable var11) {
                unwrapped = ExceptionUtil.unwrapThrowable(var11);
                if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
                    SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                    sqlSession = null;
                    Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException)unwrapped);
                    if (translated != null) {
                        unwrapped = translated;
                    }
                }

                throw (Throwable)unwrapped;
            } finally {
                if (sqlSession != null) {
                    SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                }

            }

            return unwrapped;
        }
    }

　　所以最终到达sqlSessionTemplate 的请求最终会到达org.mybatis.spring.SqlSessionTemplate.SqlSessionInterceptor#invoke 方法，方法内部首先调用：org.mybatis.spring.SqlSessionUtils#getSqlSession(org.apache.ibatis.session.SqlSessionFactory, org.apache.ibatis.session.ExecutorType, org.springframework.dao.support.PersistenceExceptionTranslator) 开启一个新的SelSession：(也就是每次都会创建一个新的SqlSession对象)。

    public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {
        Assert.notNull(sessionFactory, "No SqlSessionFactory specified");
        Assert.notNull(executorType, "No ExecutorType specified");
        SqlSessionHolder holder = (SqlSessionHolder)TransactionSynchronizationManager.getResource(sessionFactory);
        SqlSession session = sessionHolder(executorType, holder);
        if (session != null) {
            return session;
        } else {
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Creating a new SqlSession");
            }

            session = sessionFactory.openSession(executorType);
            registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);
            return session;
        }
    }

(3) AutoConfiguredMapperScannerRegistrar 自动扫描配置类，这里注册了一个对象 MapperScannerConfigurer Mapper扫描类是一个配置类，会扫描所有的Mapper 注解，接下来方法会调用：

1》org.mybatis.spring.mapper.MapperScannerConfigurer#postProcessBeanDefinitionRegistry

    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        if (this.processPropertyPlaceHolders) {
            this.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(this.lazyInitialization)) {
            scanner.setLazyInitialization(Boolean.valueOf(this.lazyInitialization));
        }

        if (StringUtils.hasText(this.defaultScope)) {
            scanner.setDefaultScope(this.defaultScope);
        }

        scanner.registerFilters();
        scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ",; 	
"));
    }

2》org.springframework.context.annotation.ClassPathBeanDefinitionScanner#scan

    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);
    }

3》org.mybatis.spring.mapper.ClassPathMapperScanner#doScan

    public Set<BeanDefinitionHolder> doScan(String... basePackages) {
        Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
        if (beanDefinitions.isEmpty()) {
            LOGGER.warn(() -> {
                return "No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.";
            });
        } else {
            this.processBeanDefinitions(beanDefinitions);
        }

        return beanDefinitions;
    }

4》org.mybatis.spring.mapper.ClassPathMapperScanner#processBeanDefinitions

    private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
        BeanDefinitionRegistry registry = this.getRegistry();
        Iterator var4 = beanDefinitions.iterator();

        while(var4.hasNext()) {
            BeanDefinitionHolder holder = (BeanDefinitionHolder)var4.next();
            AbstractBeanDefinition definition = (AbstractBeanDefinition)holder.getBeanDefinition();
            boolean scopedProxy = false;
            if (ScopedProxyFactoryBean.class.getName().equals(definition.getBeanClassName())) {
                definition = (AbstractBeanDefinition)Optional.ofNullable(((RootBeanDefinition)definition).getDecoratedDefinition()).map(BeanDefinitionHolder::getBeanDefinition).orElseThrow(() -> {
                    return new IllegalStateException("The target bean definition of scoped proxy bean not found. Root bean definition[" + holder + "]");
                });
                scopedProxy = true;
            }

            String beanClassName = definition.getBeanClassName();
            LOGGER.debug(() -> {
                return "Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '" + beanClassName + "' mapperInterface";
            });
            definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName);
            definition.setBeanClass(this.mapperFactoryBeanClass);
            definition.getPropertyValues().add("addToConfig", this.addToConfig);
            definition.setAttribute("factoryBeanObjectType", beanClassName);
            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(() -> {
                        return "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(() -> {
                        return "Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.";
                    });
                }

                definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
                explicitFactoryUsed = true;
            }

            if (!explicitFactoryUsed) {
                LOGGER.debug(() -> {
                    return "Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.";
                });
                definition.setAutowireMode(2);
            }

            definition.setLazyInit(this.lazyInitialization);
            if (!scopedProxy) {
                if ("singleton".equals(definition.getScope()) && this.defaultScope != null) {
                    definition.setScope(this.defaultScope);
                }

                if (!definition.isSingleton()) {
                    BeanDefinitionHolder proxyHolder = ScopedProxyUtils.createScopedProxy(holder, registry, true);
                    if (registry.containsBeanDefinition(proxyHolder.getBeanName())) {
                        registry.removeBeanDefinition(proxyHolder.getBeanName());
                    }

                    registry.registerBeanDefinition(proxyHolder.getBeanName(), proxyHolder.getBeanDefinition());
                }
            }
        }

    }

　　mapperFactoryBeanClass 是： org.mybatis.spring.mapper.MapperFactoryBean，源码如下：

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {
    private Class<T> mapperInterface;
    private boolean addToConfig = true;

    public MapperFactoryBean() {
    }

    public MapperFactoryBean(Class<T> mapperInterface) {
        this.mapperInterface = mapperInterface;
    }

    protected void checkDaoConfig() {
        super.checkDaoConfig();
        Assert.notNull(this.mapperInterface, "Property 'mapperInterface' is required");
        Configuration configuration = this.getSqlSession().getConfiguration();
        if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
            try {
                configuration.addMapper(this.mapperInterface);
            } catch (Exception var6) {
                this.logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", var6);
                throw new IllegalArgumentException(var6);
            } finally {
                ErrorContext.instance().reset();
            }
        }

    }

    public T getObject() throws Exception {
        return this.getSqlSession().getMapper(this.mapperInterface);
    }

    public Class<T> getObjectType() {
        return this.mapperInterface;
    }

    public boolean isSingleton() {
        return true;
    }

    public void setMapperInterface(Class<T> mapperInterface) {
        this.mapperInterface = mapperInterface;
    }

    public Class<T> getMapperInterface() {
        return this.mapperInterface;
    }

    public void setAddToConfig(boolean addToConfig) {
        this.addToConfig = addToConfig;
    }

    public boolean isAddToConfig() {
        return this.addToConfig;
    }
}

　　可以看到getObject 是拿到SqlSession，然后调用 getMapper 获取代理对象。

　　checkDaoConfig 会调用到 调用到org.apache.ibatis.session.Configuration#addMapper。会走原来MapperAnnotationBuilder 扫描注解的一套机制。

　到这里就完成了解析XML，解析Mapper 注解，然后生成代理对象。最终代理对象以及调用走的也是Mybatis 的一套机制。

补充：Springboot 自动整合mybatis 导致mybatis 一级缓存失效 

　　上面代码可以看出来。mybatis和spring结合使用的时候，将原本的DefaultSqlSession替换成了SqlSessionTemplate，并且在SqlSessionTemplate将sqlSession替换成了sqlSessionProxy代理对象，当我们执行sqlSession的方法的时会调用到SqlSessionInterceptor的invoke()方法， 在invoke()方法回次会创建一个新的SqlSession 对象，并且在fianlly中调用了SqlSessionUtils.closeSqlSession()方法将SqlSession关闭了，所以一级缓存就会失效了。