一文带你理解透MyBatis源码

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

MappedStatement对象如图

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。

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