本文分享自华为云社区《一文彻底吃透MyBatis源码！！》，作者：冰 河。

写在前面

随着互联网的发展，越来越多的公司摒弃了Hibernate，而选择拥抱了MyBatis。而且，很多大厂在面试的时候喜欢问MyBatis底层的原理和源码实现。总之，MyBatis几乎成为了Java开发人员必须深入掌握的框架技术，今天，我们就一起来深入分析MyBatis源码。文章有点长，建议先收藏后慢慢研究。整体三万字左右，全程高能，小伙伴们可慢慢研究。

MyBatis源码解析

大家应该都知道Mybatis源码也是对Jbdc的再一次封装,不管怎么进行包装,还是会有获取链接、preparedStatement、封装参数、执行这些步骤的。

配置解析过程

String resource = "mybatis-config.xml";
//1.读取resources下面的mybatis-config.xml文件
InputStream inputStream = Resources.getResourceAsStream(resource);
//2.使用SqlSessionFactoryBuilder创建SqlSessionFactory
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//3.通过sqlSessionFactory创建SqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();

Resources.getResourceAsStream(resource)读取文件

public static InputStream getResourceAsStream(String resource) throws IOException {
    return getResourceAsStream(null, resource);
} 
//loader赋值为null
public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException {
    InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);
    if (in == null) {
        throw new IOException("Could not find resource " + resource);
    } 
    return in;
}
//classLoader为null
public InputStream getResourceAsStream(String resource, ClassLoader classLoader) {
    return getResourceAsStream(resource, getClassLoaders(classLoader));
} 
//classLoader类加载
InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
    for (ClassLoader cl : classLoader) {
        if (null != cl) {
            //加载指定路径文件流
            InputStream returnValue = cl.getResourceAsStream(resource);
            // now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource
            if (null == returnValue) {
                returnValue = cl.getResourceAsStream("/" + resource);
            } 
            if (null != returnValue) {
                return returnValue;
            }
        }
    } 
    return null;
}

总结：主要是通过ClassLoader.getResourceAsStream()方法获取指定的classpath路径下的Resource 。

通过SqlSessionFactoryBuilder创建SqlSessionFactory

//SqlSessionFactoryBuilder是一个建造者模式
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
public SqlSessionFactory build(InputStream inputStream) {
    return build(inputStream, null, null);
}
//XMLConfigBuilder也是建造者模式
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
        XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
        return build(parser.parse());
    } catch (Exception e) {
        throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
        ErrorContext.instance().reset();
        try {
            inputStream.close();
        } catch (IOException e) {
            // Intentionally ignore. Prefer previous error.
        }
    }
}
//接下来进入XMLConfigBuilder构造函数
public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {
    this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);
}
//接下来进入this后,初始化Configuration
private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
    super(new Configuration());
    ErrorContext.instance().resource("SQL Mapper Configuration");
    this.configuration.setVariables(props);
    this.parsed = false;
    this.environment = environment;
    this.parser = parser;
}
//其中parser.parse()负责解析xml,build(configuration)创建SqlSessionFactory
return build(parser.parse());

parser.parse()解析xml

public Configuration parse() {
    //判断是否重复解析
    if (parsed) {
        throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    } 
    parsed = true;
    //读取配置文件一级节点configuration
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
}
private void parseConfiguration(XNode root) {
    try {
        //properties 标签,用来配置参数信息，比如最常见的数据库连接信息
        propertiesElement(root.evalNode("properties"));
        Properties settings = settingsAsProperties(root.evalNode("settings"));
        loadCustomVfs(settings);
        loadCustomLogImpl(settings);
        //实体别名两种方式:1.指定单个实体;2.指定包
        typeAliasesElement(root.evalNode("typeAliases"));
        //插件
        pluginElement(root.evalNode("plugins"));
        //用来创建对象(数据库数据映射成java对象时)
        objectFactoryElement(root.evalNode("objectFactory"));
        objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
        reflectorFactoryElement(root.evalNode("reflectorFactory"));
        settingsElement(settings);
        // read it after objectFactory and objectWrapperFactory issue #631
        //数据库环境
        environmentsElement(root.evalNode("environments"));
        databaseIdProviderElement(root.evalNode("databaseIdProvider"));
        //数据库类型和Java数据类型的转换
        typeHandlerElement(root.evalNode("typeHandlers"));
        //这个是对数据库增删改查的解析
        mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
        throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
}

总结:parseConfiguration完成的是解析configuration下的标签

private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
            for (XNode child : parent.getChildren()) {
            //解析<package name=""/>
            if ("package".equals(child.getName())) {
                String mapperPackage = child.getStringAttribute("name");
                //包路径存到mapperRegistry中
                configuration.addMappers(mapperPackage);
            } else {
                //解析<mapper url="" class="" resource=""></mapper>
                String resource = child.getStringAttribute("resource");
                String url = child.getStringAttribute("url");
                String mapperClass = child.getStringAttribute("class");
                if (resource != null && url == null && mapperClass == null) {
                    ErrorContext.instance().resource(resource);
                    //读取Mapper.xml文件
                    InputStream inputStream = Resources.getResourceAsStream(resource);
                    XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream,
                    configuration, resource, configuration.getSqlFragments());
                    mapperParser.parse();
                } else if (resource == null && url != null && mapperClass == null) {
                    ErrorContext.instance().resource(url);
                    InputStream inputStream = Resources.getUrlAsStream(url);
                    XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream,
                    configuration, url, configuration.getSqlFragments());
                    mapperParser.parse();
                } else if (resource == null && url == null && mapperClass != null) {
                    Class<?> mapperInterface = Resources.classForName(mapperClass);
                    configuration.addMapper(mapperInterface);
                } else {
                    throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
                }
            }
        }
    }
}

总结: 通过解析configuration.xml文件,获取其中的Environment、Setting,重要的是将下的所有解析出来之后添加到
Configuration,Configuration类似于配置中心,所有的配置信息都在这里。

mapperParser.parse()对 Mapper 映射器的解析

public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
        //解析所有的子标签
        configurationElement(parser.evalNode("/mapper"));
        configuration.addLoadedResource(resource);
        //把namespace（接口类型）和工厂类绑定起来
        bindMapperForNamespace();
    }
    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
} 
//这里面解析的是Mapper.xml的标签
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);
        //对其他命名空间缓存配置的引用
        cacheRefElement(context.evalNode("cache-ref"));
        //对给定命名空间的缓存配置
        cacheElement(context.evalNode("cache"));
        parameterMapElement(context.evalNodes("/mapper/parameterMap"));
        //是最复杂也是最强大的元素，用来描述如何从数据库结果集中来加载对象
        resultMapElements(context.evalNodes("/mapper/resultMap"));
        //可被其他语句引用的可重用语句块
        sqlElement(context.evalNodes("/mapper/sql"));
        //获得MappedStatement对象(增删改查标签)
        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);
    }
}
//获得MappedStatement对象(增删改查标签)
private void buildStatementFromContext(List<XNode> list) {
    if (configuration.getDatabaseId() != null) {
        buildStatementFromContext(list, configuration.getDatabaseId());
    } 
    buildStatementFromContext(list, null);
}
//获得MappedStatement对象(增删改查标签)
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    //循环增删改查标签
    for (XNode context : list) {
        final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
        try {
            //解析insert/update/select/del中的标签
            statementParser.parseStatementNode();
        } catch (IncompleteElementException e) {
            configuration.addIncompleteStatement(statementParser);
        }
    }
}
public void parseStatementNode() {
    //在命名空间中唯一的标识符,可以被用来引用这条语句
    String id = context.getStringAttribute("id");
    //数据库厂商标识
    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;
    //flushCache和useCache都和二级缓存有关
    //将其设置为true后,只要语句被调用,都会导致本地缓存和二级缓存被清空,默认值:false
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    //将其设置为 true 后，将会导致本条语句的结果被二级缓存缓存起来，默认值：对 select 元素为 true
    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());
    //会传入这条语句的参数类的完全限定名或别名
    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");
    //从这条语句中返回的期望类型的类的完全限定名或别名
    String resultType = context.getStringAttribute("resultType");
    Class<?> resultTypeClass = resolveClass(resultType);
    //外部resultMap的命名引用
    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);
}
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();
        //持有在configuration中
        configuration.addMappedStatement(statement);
        return statement;
}
public void addMappedStatement(MappedStatement ms){
//ms.getId = mapper.UserMapper.getUserById
//ms = MappedStatement等于每一个增删改查的标签的里的数据
    mappedStatements.put(ms.getId(), ms);
}
//最终存放到mappedStatements中,mappedStatements存放的是一个个的增删改查
protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection").conflictMessageProducer((savedValue, targetValue) ->
". please check " + savedValue.getResource() + " and " + targetValue.getResource());

解析bindMapperForNamespace()方法

把 namespace（接口类型）和工厂类绑定起来

private void bindMapperForNamespace() {
    //当前Mapper的命名空间
    String namespace = builderAssistant.getCurrentNamespace();
    if (namespace != null) {
        Class<?> boundType = null;
        try {
            //interface mapper.UserMapper这种
            boundType = Resources.classForName(namespace);
        } catch (ClassNotFoundException e) {
        } 
        if (boundType != null) {
            if (!configuration.hasMapper(boundType)) {
                configuration.addLoadedResource("namespace:" + namespace);
                configuration.addMapper(boundType);
            }
        }
    }
}
public <T> void addMapper(Class<T> type) {
    mapperRegistry.addMapper(type);
} 
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 {
            //接口类型(key)->工厂类
            knownMappers.put(type, new MapperProxyFactory<>(type));
            MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
            parser.parse();
            loadCompleted = true;
        } finally {
            if (!loadCompleted) {
                knownMappers.remove(type);
            }
        }
    }
}

生成SqlSessionFactory对象

XMLMapperBuilder.parse()方法,是对 Mapper 映射器的解析里面有两个方法：

（1）configurationElement()解析所有的子标签,最终解析Mapper.xml中的insert/update/delete/select标签的id(全路径)组成key和整个标签和数据连接组成MappedStatement存放到Configuration中的 mappedStatements这个map里面。

（2）bindMapperForNamespace()是把接口类型(interface mapper.UserMapper)和工厂类存到放MapperRegistry中的knownMappers里面。

SqlSessionFactory的创建

public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
}

直接把Configuration当做参数,直接new一个DefaultSqlSessionFactory。

SqlSession会话的创建过程

mybatis操作的时候跟数据库的每一次连接,都需要创建一个会话,我们用openSession()方法来创建。这个会话里面需要包含一个Executor用来执行 SQL。Executor又要指定事务类型和执行器的类型。

创建Transaction(两种方式)

  • 如果配置的是 JDBC,则会使用Connection 对象的 commit()、rollback()、close()管理事务。
  • 如果配置成MANAGED,会把事务交给容器来管理,比如 JBOSS,Weblogic。
SqlSession sqlSession = sqlSessionFactory.openSession();
public SqlSession openSession() {
    //configuration中有默认赋值protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
<environments default="development">
    <environment id="development">
        <transactionManager type="JDBC"/>
        <dataSource type="POOLED">
            <property name="driver" value="${driver}"/>
            <property name="url" value="${url}"/>
            <property name="username" value="${username}"/>
            <property name="password" value="${password}"/>
        </dataSource>
    </environment>
</environments>

创建Executor

//ExecutorType是SIMPLE,一共有三种SIMPLE(SimpleExecutor)、REUSE(ReuseExecutor)、BATCH(BatchExecutor)
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
        //xml中的development节点
        final Environment environment = configuration.getEnvironment();
        //type配置的是Jbdc所以生成的是JbdcTransactionFactory工厂类
        final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
        //Jdbc生成JbdcTransactionFactory生成JbdcTransaction
        tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
        //创建CachingExecutor执行器
        final Executor executor = configuration.newExecutor(tx, execType);
        //创建DefaultSqlSession属性包括 Configuration、Executor对象
        return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
        closeTransaction(tx); // may have fetched a connection so lets call
        close()
        throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
    } finally {
        ErrorContext.instance().reset();
    }
}

获得Mapper对象

UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
public <T> T getMapper(Class<T> type) {
    return configuration.getMapper(type, this);
}

mapperRegistry.getMapper是从MapperRegistry的knownMappers里面取的,knownMappers里面存的是接口类型(interface mapper.UserMapper)和工厂类(MapperProxyFactory)。

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
}

从knownMappers的Map里根据接口类型(interface mapper.UserMapper)取出对应的工厂类。

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);
    }
}
public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
}

这里通过JDK动态代理返回代理对象MapperProxy(org.apache.ibatis.binding.MapperProxy@6b2ea799)

protected T newInstance(MapperProxy<T> mapperProxy) {
    //mapperInterface是interface mapper.UserMapper    
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new
    Class[] { mapperInterface }, mapperProxy);
}
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

执行SQL

User user = userMapper.getUserById(1);

调用invoke代理方法

由于所有的 Mapper 都是 MapperProxy 代理对象，所以任意的方法都是执行MapperProxy 的invoke()方法

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
        //判断是否需要去执行SQL还是直接执行方法
        if (Object.class.equals(method.getDeclaringClass())) {
            return method.invoke(this, args);
            //这里判断的是接口中的默认方法Default等
        } else if (isDefaultMethod(method)) {
            return invokeDefaultMethod(proxy, method, args);
        }
    } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
    } 
    //获取缓存,保存了方法签名和接口方法的关系
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
}

调用execute方法

这里使用的例子用的是查询所以走的是else分支语句。

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //根据命令类型走不行的操作command.getType()是select
    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 {
                //将参数转换为SQL的参数
                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;
}

调用selectOne其实是selectList

selectOne查询一个和查询多个其实是一样的。

public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    List<T> list = this.selectList(statement, parameter);
    if (list.size() == 1) {
        return list.get(0);
    } else if (list.size() > 1) {
        throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
        return null;
    }
}
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
        //从Configuration里的mappedStatements里根据key(id的全路径)获取MappedStatement 对象
        MappedStatement ms = configuration.getMappedStatement(statement);
        return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
        throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
    } finally {
        ErrorContext.instance().reset();
    }
}

mappedStatements对象如图

20210103215416283.jpg

MappedStatement对象如图

20210103215433263.jpg

执行query方法

创建CacheKey

从 BoundSql 中获取SQL信息,创建 CacheKey。这个CacheKey就是缓存的Key。

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //创建缓存Key
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    //key = -575461213:-771016147:mapper.UserMapper.getUserById:0:2147483647:select * from test_user where id = ?:1:development
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {
        flushCacheIfRequired(ms);
        if (ms.isUseCache() && resultHandler == null) {
            ensureNoOutParams(ms, boundSql);
            @SuppressWarnings("unchecked")
            List<E> list = (List<E>) tcm.getObject(cache, key);
            if (list == null) {
                list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                tcm.putObject(cache, key, list); // issue #578 and #116
            } 
            return list;
        }
    }
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

清空本地缓存

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
        throw new ExecutorException("Executor was closed.");
    } 
    //queryStack 用于记录查询栈，防止递归查询重复处理缓存
    //flushCache=true 的时候，会先清理本地缓存（一级缓存）
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
        //清空本地缓存
        clearLocalCache();
    } 
    List<E> list;
    try {
        queryStack++;
        list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
        if (list != null) {
            handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
        } else {
            //如果没有缓存,会从数据库查询：queryFromDatabase()
            list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }
    } finally {
        queryStack--;
    } 
    if (queryStack == 0) {
        for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
        } 
        // issue #601
        deferredLoads.clear();
        //如果 LocalCacheScope == STATEMENT,会清理本地缓存
        if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
            // issue #482
            clearLocalCache();
        }
    } 
    return list;
}

从数据库查询

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    //先在缓存用占位符占位
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
        //执行Executor 的 doQuery(),默认是SimpleExecutor
        list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
        //执行查询后,移除占位符
        localCache.removeObject(key);
    } 
    //从新放入数据
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
        localOutputParameterCache.putObject(key, parameter);
    } 
    return list;
}

执行doQuery

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        stmt = prepareStatement(handler, ms.getStatementLog());
        return handler.query(stmt, resultHandler);
    } finally {
        closeStatement(stmt);
    }
}

源码总结

总体上来说，MyBatis的源码还是比较简单的，只要大家踏下心来，花个两三天仔细研究下，基本上都能弄明白源码的主体脉络。

点击关注，第一时间了解华为云新鲜技术~