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每天用Mybatis，但是Mybatis的工作原理你真的知道嗎？前言第一步：建立一個sqlSessionFactory第二步：建立sqlSession第三步：執行具體的sql請求總結最後

前言

近來想寫一個mybatis的分頁插件，但是在寫插件之前肯定要了解一下mybatis具體的工作原理吧，于是邊參考别人的部落格，邊看源碼就開幹了。

核心部件：

  • SqlSession

    Executor

StatementHandler

ParameterHandler

ResultSetHandler

TypeHandler

MappedStatement

Configuration

在分析工作原理之前，首先看一下我的mybatis全局配置檔案

<?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整合後 environments配置将廢除 -->
    <environments default="development">
        <environment id="development">
            <!-- 使用jdbc事務管理 -->
            <transactionManager type="JDBC" />
            <!-- 資料庫連接配接池 -->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver" />
                <property name="url"
                    value="jdbc:mysql://localhost:3306/test?characterEncoding=utf-8" />
                <property name="username" value="root" />
                <property name="password" value="123456" />
            </dataSource>
        </environment>
    </environments>
    <mappers>
       <mapper  resource="sqlMapper/userMapper.xml"/>
    </mappers>
</configuration>

第一步：建立一個sqlSessionFactory

在了解如何建立sqlSessionFactory之前，先看一下mybatis是如何加載全局配置檔案，解析xml檔案生成Configuration的

public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }
           
private void parseConfiguration(XNode root) {
    try {
      propertiesElement(root.evalNode("properties")); //issue #117 read properties first
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      settingsElement(root.evalNode("settings"));
      environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631
      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);
    }
  }

在上面的第二段代碼中有一句

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

剛好我們的全局配置檔案中有一個mapper的配置，由此可見，mapperElemet()方法是解析mapper映射檔案的，具體代碼如下

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");
          if (resource != null && url == null && mapperClass == null) {//進入該判斷
            ErrorContext.instance().resource(resource);
            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.");
          }
        }
      }
    }
  }

根據以上代碼可以分析，在寫mapper映射檔案的位址時不僅可以寫成resource，還可以寫成url和mapperClass的形式，由于我們用的是resource，是以直接進入第一個判斷，最後解析mapper映射檔案的方法是

private void configurationElement(XNode context) {
    try {
      String namespace = context.getStringAttribute("namespace");
      if (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);
    }
  }

其中具體解析每一個sql語句節點的是

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

進入這個方法一層層深究，最後到這裡可以知道MappedStatement是由builderAssistant（即MapperBuildAssistant）建立的。

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

最後進入方法addMappedStatement()，mappedStatement最後以id為鍵儲存在了Configuration中的一個map變量mappedStatements中。

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);
    statementBuilder.resource(resource);
    statementBuilder.fetchSize(fetchSize);
    statementBuilder.statementType(statementType);
    statementBuilder.keyGenerator(keyGenerator);
    statementBuilder.keyProperty(keyProperty);
    statementBuilder.keyColumn(keyColumn);
    statementBuilder.databaseId(databaseId);
    statementBuilder.lang(lang);
    statementBuilder.resultOrdered(resultOrdered);
    statementBuilder.resulSets(resultSets);
    setStatementTimeout(timeout, statementBuilder);

    setStatementParameterMap(parameterMap, parameterType, statementBuilder);
    setStatementResultMap(resultMap, resultType, resultSetType, statementBuilder);
    setStatementCache(isSelect, flushCache, useCache, currentCache, statementBuilder);

    MappedStatement statement = statementBuilder.build();
    configuration.addMappedStatement(statement);
    return statement;
  }

最後回到我們的建立sqlSessionFactory上，之前的一切都是為了生成一個sqlSessionFactory服務的

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.
      }
    }
  }

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

從上面的代碼可以看出最後是通過以Configuration為參數build()方法生成DefautSqlSessionFactory。

第二步：建立sqlSession 

public SqlSession openSession() {
  return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    final Environment environment = configuration.getEnvironment();
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    final Executor executor = configuration.newExecutor(tx, execType);
    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();
  }
}
           
//傳回一個SqlSession，預設使用DefaultSqlSession 
 public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
    this.configuration = configuration;
    this.executor = executor;
    this.dirty = false;
    this.autoCommit = autoCommit;
  }

executor在這一步得到建立，具體的使用在下一步。

第三步：執行具體的sql請求

在我的代碼裡執行的是

User user = sqlSession.selectOne("test.findUserById", 1);

具體到裡面的方法就是

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
     //1.根據Statement Id，在mybatis 配置對象Configuration中查找和配置檔案相對應的MappedStatement
      MappedStatement ms = configuration.getMappedStatement(statement);
      //2. 将查詢任務委托給MyBatis 的執行器 Executor
      List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
      return result;
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

在這裡通過statementId拿到了我們在第一步存在map裡面的MappedStatement。在這裡引用參考部落格的一句話：

SqlSession根據Statement ID, 在mybatis配置對象Configuration中擷取到對應的MappedStatement對象，然後調用mybatis執行器來執行具體的操作。

再繼續看query()和queryFromDatabase()這兩個方法

@SuppressWarnings("unchecked")
  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.");
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      deferredLoads.clear(); // issue #601
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        clearLocalCache(); // issue #482
      }
    }
    return list;
  }
           
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

在這兩個方法裡面會為目前的查詢建立一個緩存key，如果緩存中沒有值，直接從資料庫中讀取，執行查詢後将得到的list結果放入緩存之中。

緊接着看doQuery()在SimpleExecutor類中重寫的方法

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.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

Statement連接配接對象就是在這裡建立的，是以Executor的作用之一就是建立Statement了，建立完後又把Statement丢給StatementHandler傳回List查詢結果。

接下來再看一下這裡的兩個方法prepareStatement()和query()的具體實作

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection);
    handler.parameterize(stmt);
    return stmt;
  }
           
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
  }

prepareStatement()是建立Statement的具體實作方法，調用parameterize()對建立的Statement對象設定參數，即為我們設為占位符的地方賦上指定的參數，parameterize()方法再深入進去就是調用ParameterHandler的setParameters()方法具體指派了。

歡迎大家關注我的公種浩【程式員追風】，文章都會在裡面更新，整理的資料也會放在裡面。

這裡的query()是調用了ResultSetHandler的handleResultSets(Statement) 方法。作用就是把ResultSet結果集對象轉換成List類型的集合。

總結以上步驟就是：

根據具體傳入的參數，動态地生成需要執行的SQL語句，用BoundSql對象表示

為目前的查詢建立一個緩存Key

緩存中沒有值，直接從資料庫中讀取資料

執行查詢，傳回List 結果，然後 将查詢的結果放入緩存之中

根據既有的參數，建立StatementHandler對象來執行查詢操作

将建立Statement傳遞給StatementHandler對象,調用parameterize()方法指派

調用StatementHandler.query()方法，傳回List結果集

總結

以上三個步驟所有流程大體可以用一張圖來總結

每天用Mybatis，但是Mybatis的工作原理你真的知道嗎？前言第一步：建立一個sqlSessionFactory第二步：建立sqlSession第三步：執行具體的sql請求總結最後

最後

歡迎大家一起交流，喜歡文章記得點個贊喲，感謝支援！

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