一文帶你理解透MyBatis原始碼

本文分享自華為雲社群《一文徹底吃透MyBatis原始碼！！》，作者：冰 河。

寫在前面

隨著網際網路的發展，越來越多的公司摒棄了Hibernate，而選擇擁抱了MyBatis。而且，很多大廠在面試的時候喜歡問MyBatis底層的原理和原始碼實現。總之，MyBatis幾乎成為了Java開發人員必須深入掌握的框架技術，今天，我們就一起來深入分析MyBatis原始碼。文章有點長，建議先收藏後慢慢研究。整體三萬字左右，全程高能，小夥伴們可慢慢研究。

MyBatis原始碼解析

大家應該都知道Mybatis原始碼也是對Jbdc的再一次封裝,不管怎麼進行包裝,還是會有獲取連結、preparedStatement、封裝引數、執行這些步驟的。

配置解析過程

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

Resources.getResourceAsStream(resource)讀取檔案

public static InputStream getResourceAsStream(String resource) throws IOException {
    return getResourceAsStream(null, resource);
} 
//loader賦值為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為null
public InputStream getResourceAsStream(String resource, ClassLoader classLoader) {
    return getResourceAsStream(resource, getClassLoaders(classLoader));
} 
//classLoader類載入
InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
    for (ClassLoader cl : classLoader) {
        if (null != cl) {
            //載入指定路徑檔案流
            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;
}

總結：主要是透過ClassLoader.getResourceAsStream()方法獲取指定的classpath路徑下的Resource 。

透過SqlSessionFactoryBuilder建立SqlSessionFactory

//SqlSessionFactoryBuilder是一個建造者模式
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.
        }
    }
}
//接下來進入XMLConfigBuilder建構函式
public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {
    this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);
}
//接下來進入this後,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()負責解析xml,build(configuration)建立SqlSessionFactory
return build(parser.parse());

parser.parse()解析xml

public Configuration parse() {
    //判斷是否重複解析
    if (parsed) {
        throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    } 
    parsed = true;
    //讀取配置檔案一級節點configuration
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
}

private void parseConfiguration(XNode root) {
    try {
        //properties 標籤,用來配置引數資訊，比如最常見的資料庫連線資訊
        propertiesElement(root.evalNode("properties"));
        Properties settings = settingsAsProperties(root.evalNode("settings"));
        loadCustomVfs(settings);
        loadCustomLogImpl(settings);
        //實體別名兩種方式:1.指定單個實體;2.指定包
        typeAliasesElement(root.evalNode("typeAliases"));
        //外掛
        pluginElement(root.evalNode("plugins"));
        //用來建立物件(資料庫資料對映成java物件時)
        objectFactoryElement(root.evalNode("objectFactory"));
        objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
        reflectorFactoryElement(root.evalNode("reflectorFactory"));
        settingsElement(settings);
        // read it after objectFactory and objectWrapperFactory issue #631
        //資料庫環境
        environmentsElement(root.evalNode("environments"));
        databaseIdProviderElement(root.evalNode("databaseIdProvider"));
        //資料庫型別和Java資料型別的轉換
        typeHandlerElement(root.evalNode("typeHandlers"));
        //這個是對資料庫增刪改查的解析
        mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
        throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
}

總結:parseConfiguration完成的是解析configuration下的標籤

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");
                //包路徑存到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);
                    //讀取Mapper.xml檔案
                    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.");
                }
            }
        }
    }
}

總結: 透過解析configuration.xml檔案,獲取其中的Environment、Setting,重要的是將下的所有解析出來之後新增到
Configuration,Configuration類似於配置中心,所有的配置資訊都在這裡。

mapperParser.parse()對 Mapper 對映器的解析

public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
        //解析所有的子標籤
        configurationElement(parser.evalNode("/mapper"));
        configuration.addLoadedResource(resource);
        //把namespace（介面型別）和工廠類繫結起來
        bindMapperForNamespace();
    }
    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
} 
//這裡面解析的是Mapper.xml的標籤
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);
        //對其他名稱空間快取配置的引用
        cacheRefElement(context.evalNode("cache-ref"));
        //對給定名稱空間的快取配置
        cacheElement(context.evalNode("cache"));
        parameterMapElement(context.evalNodes("/mapper/parameterMap"));
        //是最複雜也是最強大的元素，用來描述如何從資料庫結果集中來載入物件
        resultMapElements(context.evalNodes("/mapper/resultMap"));
        //可被其他語句引用的可重用語句塊
        sqlElement(context.evalNodes("/mapper/sql"));
        //獲得MappedStatement物件(增刪改查標籤)
        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);
    }
}
//獲得MappedStatement物件(增刪改查標籤)
private void buildStatementFromContext(List<XNode> list) {
    if (configuration.getDatabaseId() != null) {
        buildStatementFromContext(list, configuration.getDatabaseId());
    } 
    buildStatementFromContext(list, null);
}
//獲得MappedStatement物件(增刪改查標籤)
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    //迴圈增刪改查標籤
    for (XNode context : list) {
        final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
        try {
            //解析insert/update/select/del中的標籤
            statementParser.parseStatementNode();
        } catch (IncompleteElementException e) {
            configuration.addIncompleteStatement(statementParser);
        }
    }
}
public void parseStatementNode() {
    //在名稱空間中唯一的識別符號,可以被用來引用這條語句
    String id = context.getStringAttribute("id");
    //資料庫廠商標識
    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都和二級快取有關
    //將其設定為true後,只要語句被呼叫,都會導致本地快取和二級快取被清空,預設值:false
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    //將其設定為 true 後，將會導致本條語句的結果被二級快取快取起來，預設值：對 select 元素為 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());
    //會傳入這條語句的引數類的完全限定名或別名
    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");
    //從這條語句中返回的期望型別的類的完全限定名或別名
    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等於每一個增刪改查的標籤的裡的資料
    mappedStatements.put(ms.getId(), ms);
}
//最終存放到mappedStatements中,mappedStatements存放的是一個個的增刪改查
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（介面型別）和工廠類繫結起來

private void bindMapperForNamespace() {
    //當前Mapper的名稱空間
    String namespace = builderAssistant.getCurrentNamespace();
    if (namespace != null) {
        Class<?> boundType = null;
        try {
            //interface mapper.UserMapper這種
            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 {
            //介面型別(key)->工廠類
            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物件

XMLMapperBuilder.parse()方法,是對 Mapper 對映器的解析裡面有兩個方法：

（1）configurationElement()解析所有的子標籤,最終解析Mapper.xml中的insert/update/delete/select標籤的id(全路徑)組成key和整個標籤和資料連線組成MappedStatement存放到Configuration中的 mappedStatements這個map裡面。

（2）bindMapperForNamespace()是把介面型別(interface mapper.UserMapper)和工廠類存到放MapperRegistry中的knownMappers裡面。

SqlSessionFactory的建立

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

直接把Configuration當做引數,直接new一個DefaultSqlSessionFactory。

SqlSession會話的建立過程

mybatis操作的時候跟資料庫的每一次連線,都需要建立一個會話,我們用openSession()方法來建立。這個會話裡面需要包含一個Executor用來執行 SQL。Executor又要指定事務型別和執行器的型別。

建立Transaction(兩種方式)

• 如果配置的是 JDBC,則會使用Connection 物件的 commit()、rollback()、close()管理事務。
• 如果配置成MANAGED,會把事務交給容器來管理,比如 JBOSS,Weblogic。

SqlSession sqlSession = sqlSessionFactory.openSession();

public SqlSession openSession() {
    //configuration中有預設賦值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>

建立Executor

//ExecutorType是SIMPLE,一共有三種SIMPLE(SimpleExecutor)、REUSE(ReuseExecutor)、BATCH(BatchExecutor)
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
        //xml中的development節點
        final Environment environment = configuration.getEnvironment();
        //type配置的是Jbdc所以生成的是JbdcTransactionFactory工廠類
        final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
        //Jdbc生成JbdcTransactionFactory生成JbdcTransaction
        tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
        //建立CachingExecutor執行器
        final Executor executor = configuration.newExecutor(tx, execType);
        //建立DefaultSqlSession屬性包括 Configuration、Executor物件
        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();
    }
}

獲得Mapper物件

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

mapperRegistry.getMapper是從MapperRegistry的knownMappers裡面取的,knownMappers裡面存的是介面型別(interface mapper.UserMapper)和工廠類(MapperProxyFactory)。

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

從knownMappers的Map里根據介面型別(interface mapper.UserMapper)取出對應的工廠類。

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);
}

這裡透過JDK動態代理返回代理物件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);

執行SQL

User user = userMapper.getUserById(1);

呼叫invoke代理方法

由於所有的 Mapper 都是 MapperProxy 代理物件，所以任意的方法都是執行MapperProxy 的invoke()方法

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
        //判斷是否需要去執行SQL還是直接執行方法
        if (Object.class.equals(method.getDeclaringClass())) {
            return method.invoke(this, args);
            //這裡判斷的是介面中的預設方法Default等
        } else if (isDefaultMethod(method)) {
            return invokeDefaultMethod(proxy, method, args);
        }
    } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
    } 
    //獲取快取,儲存了方法簽名和介面方法的關係
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
}

呼叫execute方法

這裡使用的例子用的是查詢所以走的是else分支語句。

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //根據命令型別走不行的操作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 {
                //將引數轉換為SQL的引數
                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;
}

呼叫selectOne其實是selectList

selectOne查詢一個和查詢多個其實是一樣的。

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 {
        //從Configuration裡的mappedStatements里根據key(id的全路徑)獲取MappedStatement 物件
        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物件如圖

MappedStatement物件如圖

執行query方法

建立CacheKey

從 BoundSql 中獲取SQL資訊,建立 CacheKey。這個CacheKey就是快取的Key。

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //建立快取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);
}

清空本地快取

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 用於記錄查詢棧，防止遞迴查詢重複處理快取
    //flushCache=true 的時候，會先清理本地快取（一級快取）
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
        //清空本地快取
        clearLocalCache();
    } 
    List<E> list;
    try {
        queryStack++;
        list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
        if (list != null) {
            handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
        } else {
            //如果沒有快取,會從資料庫查詢：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,會清理本地快取
        if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
            // issue #482
            clearLocalCache();
        }
    } 
    return list;
}

從資料庫查詢

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    //先在快取用佔位符佔位
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
        //執行Executor 的 doQuery(),預設是SimpleExecutor
        list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
        //執行查詢後,移除佔位符
        localCache.removeObject(key);
    } 
    //從新放入資料
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
        localOutputParameterCache.putObject(key, parameter);
    } 
    return list;
}

執行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);
    }
}

原始碼總結

總體上來說，MyBatis的原始碼還是比較簡單的，只要大家踏下心來，花個兩三天仔細研究下，基本上都能弄明白原始碼的主體脈絡。

點選關注，第一時間瞭解華為雲新鮮技術~