Mybatis的解析和运行原理中构建SqlSessionFactory过程

一、MyBatis的功能架构

我们把Mybatis的功能架构分为四层：

(1)API接口层：提供给外部使用的接口API，开发人员通过这些本地API来操纵数据库。接口层一接收到调用请求就会调用数据处理层来完成具体的数据处理。

(2)数据处理层：负责具体的SQL查找、SQL解析、SQL执行和执行结果映射处理等。它主要的目的是根据调用的请求完成一次数据库操作。

(3框架支撑层：负责最基础的功能支撑，包括连接管理、事务管理、配置加载和缓存处理，这些都是共用的东西，将他们抽取出来作为最基础的组件。为上层的数据处理层提供最基础的支撑。

(4)引导层：负责配置一些Mybatis运行环境，注册Sql映射文件

二、Mybatis的解析和运行原理

java代码：

String resource = "mybatis-config.xml";
Reader reader = Resources.getResourceAsReader(resource);
sessionFactory = new SqlSessionFactoryBuilder().build(reader);
SqlSession sqlSession = sessionFactory.openSession();
EmployeeMapperPlus mapper = sqlSession.getMapper(EmployeeMapperPlus.class);
Employee employee = mapper.getEmpByIdWithDeptName(2);
           

首先先来解析前两行代码

String resource = "mybatis-config.xml";
Reader reader = Resources.getResourceAsReader(resource);
           

这两行代码的主要作用就是读取Mybaits的主配置配置文件，并返回该文件的输入流，我们知道Mybatis所有的SQL语句都写在XML配置文件里面,所以第一步就需要读取这些XML配置文件，这个不难理解，关键是读取文件后怎么存放。

（一）、构建SqlSessionFactory过程

SqlSessionFactory的主要功能是提供创建Mybatis的核心接口SqlSession，所以首先要创建SqlSessionFactory

构建SqlSessionFactory的代码

sessionFactory = new SqlSessionFactoryBuilder().build(reader);
           

SqlSessionFactoryBuilder().build(reder)的具体实现：

public SqlSessionFactory build(Reader reader) {
  return build(reader, null, null);
}
           

最终调用的是：

public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
        XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
        return build(parser.parse());
    } catch (Exception e) {
        throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
        ErrorContext.instance().reset();
        try {
            reader.close();
        } catch (IOException e) {
            // Intentionally ignore. Prefer previous error.
        }
    }
}
           

这里需要注意的是这两行代码：

进行解析配置文件和返回SqlSessionFactory

//创建一个配置文件流的解析对象XMLConfigBuilder,其实这里是将环境和配置文件流赋予解析类 
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
// 解析类对配置文件进行解析并将解析的内容存放到Configuration对象中,并返回SqlSessionFactory
return build(parser.parse());
           

通过org.apache.ibatis.builder.xml.XMLConfigBuilderlai 的parse()方法来解析配置的XML文件，读取配置的参数

1、XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);解析配置文件

XMLConfigBuilderlai 的构造方法：

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XMLConfigBuilder 调用了父类BaseBuilder的构造方法：

protected final Configuration configuration;
protected final TypeAliasRegistry typeAliasRegistry;
protected final TypeHandlerRegistry typeHandlerRegistry;

public BaseBuilder(Configuration configuration) {
    this.configuration = configuration;
    this.typeAliasRegistry = this.configuration.getTypeAliasRegistry();
    this.typeHandlerRegistry = this.configuration.getTypeHandlerRegistry();
}
           

注意：这里重点是Configuration创建了一个Configuration 对象，而Configuration是用来封装所有配置文件的详细信息

XMLConfigBuilderlai 的parse()方法：

public Configuration parse() {
    if (parsed) {
        throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
}
           

注意：XMLConfigBuilder的parsed的属性，默认值是false，当创建了一个XMLConfigBuilder对象，并进行解析配置文件的时候，parsed的值就变成了true。如果第二次进行解析的时候就会抛出BuilderException异常，提示只能构建一次Configuration对象。确保了Configuration对象是单例的。

parseConfiguration(parser.evalNode("/configuration"));
           

parser.evalNode("/configuration")是利用XPathParser类来解析配置文件中的"/configuration"返回一个XNode对象。

private void parseConfiguration(XNode root) {
    try {
        Properties settings = settingsAsPropertiess(root.evalNode("settings"));
        //issue #117 read properties first
        propertiesElement(root.evalNode("properties"));
        loadCustomVfs(settings);
        typeAliasesElement(root.evalNode("typeAliases"));
        pluginElement(root.evalNode("plugins"));
        objectFactoryElement(root.evalNode("objectFactory"));
        objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
        reflectorFactoryElement(root.evalNode("reflectorFactory"));
        settingsElement(settings);
        // read it after objectFactory and objectWrapperFactory issue #631
        environmentsElement(root.evalNode("environments"));
        databaseIdProviderElement(root.evalNode("databaseIdProvider"));
        typeHandlerElement(root.evalNode("typeHandlers"));
        mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
        throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
}
           

可以看到parseConfiguration(XNode root)方法就是解析配置文件中的所有文件并封装到Configuration对象中去。

mapperElement(root.evalNode("mappers"));
           

这条代码主要把Sql映射文件配置到Configuration对象中去：

private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
        // 遍历所有mappers节点下的所有元素
        for (XNode child : parent.getChildren()) {
            // 如果是package引入的方式
            if ("package".equals(child.getName())) {
                String mapperPackage = child.getStringAttribute("name");
                configuration.addMappers(mapperPackage);
            // 如果是mapper引入的方式
            } else {
                String resource = child.getStringAttribute("resource");
                String url = child.getStringAttribute("url");
                String mapperClass = child.getStringAttribute("class");
                // 如果是是resource
                if (resource != null && url == null && mapperClass == null) {
                    ErrorContext.instance().resource(resource);
                    InputStream inputStream = Resources.getResourceAsStream(resource);
                    XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
                    mapperParser.parse();
                // 如果是是url 
                } 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();
                // 如果是是mapperClass 
                } else if (resource == null && url == null && mapperClass != null) {
                    Class<?> mapperInterface = Resources.classForName(mapperClass);
                    configuration.addMapper(mapperInterface);
                // 否则抛出异常
                } else {
                    throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
                }
            }
        }
    }
}
           

可以注意到所有的Sql映射文件都是用XMLMapperBuilder 类的parse()方法来进行解析：

XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
mapperParser.parse();
           

XMLMapperBuilder 的parse()会解析配置文件<mapper>下的所有配置：namespace名称空间、cache缓存策略、resultMap结果集、Sql动态sql、CRUD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);
        cacheRefElement(context.evalNode("cache-ref"));
        cacheElement(context.evalNode("cache"));
        parameterMapElement(context.evalNodes("/mapper/parameterMap"));
        resultMapElements(context.evalNodes("/mapper/resultMap"));
        sqlElement(context.evalNodes("/mapper/sql"));
        buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
        throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
    }
}
           

解析Sql映射文件的Sql语句的代码

buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
           

private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    for (XNode context : list) {
        final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
        try {
            statementParser.parseStatementNode();
        } catch (IncompleteElementException e) {
            configuration.addIncompleteStatement(statementParser);
        }
    }
}
           

public void parseStatementNode() {
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");

    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }

    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String parameterType = context.getStringAttribute("parameterType");
    Class<?> parameterTypeClass = resolveClass(parameterType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultType = context.getStringAttribute("resultType");
    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);

    Class<?> resultTypeClass = resolveClass(resultType);
    String resultSetType = context.getStringAttribute("resultSetType");
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);

    String nodeName = context.getNode().getNodeName();
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);
    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

    // Include Fragments before parsing
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    // Parse selectKey after includes and remove them.
    processSelectKeyNodes(id, parameterTypeClass, langDriver);
    
    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    String resultSets = context.getStringAttribute("resultSets");
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    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))
          ? new Jdbc3KeyGenerator() : new NoKeyGenerator();
    }

    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered, 
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }
           

注意：解析所有的Sql语句封装一个MappedStatement，MappedStatement中包含了许多我们配置的SQL、SQL的id、缓存信息、resultMap、ParameterType、resultType、resultMap等重要配置内容。最重要的是它还有一个属性sqlSource。

SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
           

SqlSource是MappedStatement的一个属性，它只是一个接口。它的主要作用是根据上下文和参数解析生成需要的Sql。

SqlSource接口的实现类有：

而SqlSource有一个BoundSql成员属性：

public interface SqlSource {
  BoundSql getBoundSql(Object parameterObject);
}
           

BoundSql是一个结果集对象，也就是SqlSource通过对SQL和参数解析得到的SQL和参数

2、return build(parser.parse());

最终会返回SqlSessionFactory的子类DefaultSqlSessionFactory的实例对象，DefaultSqlSessionFactory是SqlSessionFactory默认的实现

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

更多的源码解析会在接下来的章节进行总结，下一篇会讲解SqlSession的构建过程