从Java原生的Jdbc操作数据库到MyBatis 1

一、最先我们操作数据库就是这么几步

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二、框架该如何做到优化

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三、看看具体的MyBatis

MyBatis感觉核心就两步

• 构建，就是解析我们写的xml配置（mybatis-config.xml和mapper.xml），将其变成它所需要的对象
• 执行，在构建完成的基础上，执行我们的SQL，完成与Jdbc的交互

1、mybatis-config.xml配置文件

mybatis-config.xml配置文件中包含了对 MyBatis 系统的核心设置，包括获取数据库连接实例的数据源（DataSource）以及决定事务作用域和控制方式的事务管理器（TransactionManager）、加载mapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
   <!-- 和spring整合后 environments配置将废除 -->
  <environments default="development">
    <environment id="development">
      <!-- 使用jdbc事务管理 -->
      <transactionManager type="JDBC"/>
      <!-- 数据库连接池 -->
      <dataSource type="POOLED">
        <property name="driver" value="com.mysql.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://xxxxxxx:3306/test?characterEncoding=utf8"/>
        <property name="username" value="root"/>
        <property name="password" value="root"/>
      </dataSource>
    </environment>
  </environments>

  <!-- 加载mapper.xml -->
  <mappers>
      <!-- 注意 mapper节点中，可以使用resource/url/class三种方式获取mapper-->
      <!-- 通过配置文件路径 -->
     <mapper resource="org/mybatis/example/BlogMapper.xml"/>
     <!-- 通过Java全限定类名 -->
     <mapper class="org.mybatis.example.TestMapper"/>
     <!-- 通过url， 通常是mapper不在本地时用 -->
    <mapper url=""/>
    <!-- 通过包名 -->
    <package name="org.mybatis.example"/>
  </mappers>
</configuration>

2、BlogMapper.xml

这个文件中写了我们查询数据库所用的SQL

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="org.mybatis.example.BlogMapper">
  <select id="selectBlog" resultType="Blog">
    select * from Blog where id = #{id}
  </select>
</mapper>

3、如何将这两个配置文件读取成Java对象的

• 对于mybatis-config.xml

String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

• 对于BlogMapper.xml

//第一种方式读取BlogMapper.xml
try (SqlSession session = sqlSessionFactory.openSession()) {
  Blog blog = (Blog) session.selectOne("org.mybatis.example.BlogMapper.selectBlog", 101);
}

//第二种方式读取BlogMapper.xml
try (SqlSession session = sqlSessionFactory.openSession()) {
  BlogMapper mapper = session.getMapper(BlogMapper.class);
  Blog blog = mapper.selectBlog(101);
}

第二种代码不仅更清晰，更加类型安全，还不用担心可能出错的字符串字面值以及强制类型转换。

4、最基本完整的MyBatis代码

    public static void main(String[] args) throws Exception {
        //第一步读取mybatis-config.xml
        String resource = "org/mybatis/example/mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        //第二步利用SqlSessionFactoryBuilder创建SqlSessionFactory（（这里用到了工厂模式和构建者模式）） 
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //第三步：利用sqlSessionFactory去获取 SqlSession 
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //第四步：利用SqlSession 获取Mapper
        BlogMapper mapper = session.getMapper(BlogMapper.class);
        //传入参数，进行具体的sql查询
        Blog blog = mapper.selectBlog(101);
        //关闭连接
        sqlSession.close();
        sqlSession.commit();
    }  

以上四部的流程图大概如下

image.png

四、MyBatis3.5.6源码分析

第一步：解析mybatis-config.xml

• 看SqlSessionFactoryBuilder.build()里面的方法

  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      //解析mybatis-config.xml
      //XMLConfigBuilder  构造者
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      //parse(): 解析mybatis-config.xml里面的节点
      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.
      }
    }
  }

• 进入parse()
  注意parse的返回值，Configuration，这个与mybatis-config.xml中的<configuration>节点是关联起来的

  public Configuration parse() {
    //改标识标识只能被解析一次
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //解析<configuration>节点
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

• 进入parseConfiguration()
  对比着mybatis-config.xml中的<configuration></configuration>去看，一些在就能明白了，configuration可以再去看看官网的，有些我们没配置，比如properties、settings、typeAliases、objectFactory、objectWrapperFactory、reflectorFactory、databaseIdProvider、typeHandlers等这些我们都没配置，具体的配置请参考官网

  private void parseConfiguration(XNode root) {
    try {
      // issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(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"));
     //这一步的解析看第二步：解析mapper.xml
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

第二步：解析mapper.xml

在上面的最后一步是解析mappers
mapperElement(root.evalNode("mappers"));

  private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
         //优先级 resource>url>mapperClass
          if (resource != null && url == null && mapperClass == null) {
            ErrorContext.instance().resource(resource);
           //通过XMLMapperBuilder解析XXXMapper.xml，构建的XMLMapperBuilde传入了configuration,所以之后肯定会将mapper封装到configuration对象中去
            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.");
          }
        }
      }
    }
  }

• 先看通过resource来解析mapper.xml

  public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
     //解析mapper文件节点
      configurationElement(parser.evalNode("/mapper"));
      configuration.addLoadedResource(resource);
     //绑定Namespace里面的Class对象
      bindMapperForNamespace();
    }
    //重新解析之前解析不了的节点
    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
  }

• configurationElement(parser.evalNode("/mapper"));
  拿到里面配置的配置项 ,最后封装成一个MapperedStatemanet
  [图片上传中...(image.png-3fd741-1646375471539-0)]

  private void configurationElement(XNode context) {
    try {
     //获取命名空间 namespace，这个非常重要，mybatis后面会通过namespace动态代理我们的Mapper接口
      String namespace = context.getStringAttribute("namespace");
      if (namespace == null || namespace.isEmpty()) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      builderAssistant.setCurrentNamespace(namespace);
      //解析缓存节点，平时实际写sql没写这个
      cacheRefElement(context.evalNode("cache-ref"));
      cacheElement(context.evalNode("cache"));
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      sqlElement(context.evalNodes("/mapper/sql"));
     //解析增删改查
      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);
    }
  }

上面解析的namespace、/mapper/resultMap、/mapper/sql、select|insert|update|delete就如下面

image.png

• 解析增删改查节点
  buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
  根据这个名字就可以了解到是用来构造一个Statement，用过原生Jdbc的都知道，Statement就是我们操作数据库的对象。就是我们文章最先的那个流程图的第三步（创建Statement对象）

  private void buildStatementFromContext(List<XNode> list) {
    if (configuration.getDatabaseId() != null) {
      buildStatementFromContext(list, configuration.getDatabaseId());
    }
    buildStatementFromContext(list, null);
  }

  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) {
       //xml语句有问题时 存储到集合中 等解析完能解析的再重新解析
        configuration.addIncompleteStatement(statementParser);
      }
    }
  }

• 来看真正的解析工作
  statementParser.parseStatementNode();这段代码比较长

  public void parseStatementNode() {
   //获取<select id="xxx">中的id
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");
    //这儿跟一下源码进去看可以看到MappedStatement ,MyBatis中很亲切的一个大对象
   // MappedStatement previous = this.configuration.getMappedStatement(id, false); // issue #2
    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }

    //获取节点名  select update delete insert
    String nodeName = context.getNode().getNodeName();
    // SqlCommandType 是一个枚举型，里面的值是UNKNOWN,  INSERT, UPDATE, DELETE, SELECT,FLUSH;
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
   //判断是否是查询
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
   //是否刷新缓存 默认:增删改刷新 查询不刷新
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
   //是否使用二级缓存 默认值:查询使用 增删改不使用
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);
    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

    // Include Fragments before parsing 
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    //看代码后的图  替换Includes标签为对应的sql标签里面的值
    includeParser.applyIncludes(context.getNode());
    //获取parameterType名
    String parameterType = context.getStringAttribute("parameterType");
    // value = (Class<T>) Resources.classForName(string);
    Class<?> parameterTypeClass = resolveClass(parameterType);

    //解析配置的自定义脚本语言驱动 这里为null,走进去可以看到是设置了默认的 
    //languageRegistry.getDefaultDriver(),是放到
   // private final Map<Class<? extends LanguageDriver>, LanguageDriver> LANGUAGE_DRIVER_MAP = new HashMap<>();一个HashMap中的
    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);

    // Parse selectKey after includes and remove them.
   //SelectKey在Mybatis中是为了解决Insert数据时不支持主键自动生成的问题，他可以很随意的设置生成主键的方式 
  //SelectKey需要注意order属性，像Mysql一类支持自动增长类型的数据库中，order需要设置为after才会取到正确的值。
    processSelectKeyNodes(id, parameterTypeClass, langDriver);

    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    //设置主键自增规则
    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;
    }
   
   //根据sql来判断是否需要动态解析 如果没有动态sql语句且 只有#{}参数的时候 直接静态解析，使用?占位 当有 ${} 不解析
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    //获取StatementType，可以理解为Statement和PreparedStatement
    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");
    //获取返回值类型名
    String resultType = context.getStringAttribute("resultType");
    // 实际最后就调用的这个value = (Class<T>) Resources.classForName(string);
    Class<?> resultTypeClass = resolveClass(resultType);
    //获取resultMap
    String resultMap = context.getStringAttribute("resultMap");
    String resultSetType = context.getStringAttribute("resultSetType");
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
    if (resultSetTypeEnum == null) {
      resultSetTypeEnum = configuration.getDefaultResultSetType();
    }
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    String resultSets = context.getStringAttribute("resultSets");

   //将上面获取到的属性，封装成MappedStatement对象，我们解析mappper.xml中的非常重要的类
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered,
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }

include标签

这个具体的sql解析的SqlSource ，单独放一篇文章总结，方便自己好看，现在只看下SqlSource的debug调试的结果，可以看到具体的sql以及参数

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• 封装成MappedStatement对象的代码

一个MappedStatement就是一个sql，整个项目中有很多sql，是存在Configuration中的mappedStatements 这个参数里面的,她是一个StrictMap

image.png

  protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")
      .conflictMessageProducer((savedValue, targetValue) ->
          ". please check " + savedValue.getResource() + " and " + targetValue.getResource());

 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;

   //利用构造者模式进行MappedStatement
    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();
    configuration.addMappedStatement(statement);
    return statement;
  }

至此MyBatis的查询前构建的过程大致这样子，简单地说就是
①MyBatis会在执行查询之前，对配置文件进行解析成配置对象：Configuration
②存放SQL的xml又会解析成MappedStatement对象，但是MappedStatement最后还是放入到Configuration中的一个集合中的
所以最终我们需要操作的对象就是Configuration

image.png image.png