Mybatis - Mybatis源码分析 - 《Java 架构师之路》

01｜核心类
02｜配置解析

01｜核心类

SqlSessionFactoryBuilder：负责构建SqlSessionFactory
SqlSessionFactory：负责构建SqlSession
SqlSession：mybatis操作数据库的顶层接口
Executor：执行增删改查的具体执行器
ParameterHandler：参数解析器
StatementHandler：Statement的处理器
ResultSetHandler：结果集处理器
Configuration：全局配置类
MappedStatement：映射信息描述类
02｜配置解析
```java public class SqlSessionFactoryBuilder{ public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
```
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
return build(parser.parse());
```
} }

public class XMLConfigBuilder{ public Configuration parse() { if (parsed) { throw new BuilderException(“Each XMLConfigBuilder can only be used once.”); } parsed = true; // 解析XML配置文件中的所有配置属性并存入Configuration对象中 parseConfiguration(parser.evalNode(“/configuration”)); return configuration; }

private void parseConfiguration(XNode root) { try { //依次解析所有配置项信息 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); environmentsElement(root.evalNode(“environments”)); databaseIdProviderElement(root.evalNode(“databaseIdProvider”)); typeHandlerElement(root.evalNode(“typeHandlers”)); // 解析mapper配置信息，并封装为MappedStatement放入Configuration对象中 // MapperdStatement对象中包含的sql语句执行的所有细节信息 mapperElement(root.evalNode(“mappers”)); } catch (Exception e) { throw new BuilderException(“Error parsing SQL Mapper Configuration. Cause: “ + e, e); } } }

当配置解析完毕，有关mybatis所需的所有配置项项目全部存入到了Configuration对象当中。
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## 03｜SQL执行流程
以SqlSession.selectList()为例进行源码分析
```java
    @Test
    public void query() throws IOException {
        final SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml"));
        final SqlSession sqlSession = factory.openSession();
        final List<Object> results = sqlSession.selectList("statement id");
    }

SqlSession默认实现类为DefaultSqlSession

public class DefaultSqlSession implements SqlSession {
  @Override
  public <E> List<E> selectList(String statement) {
    return this.selectList(statement, null);
  }
  @Override
  public <E> List<E> selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
  }
  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      // 根据statementid从configuration配置中获取对应的MapperdStatement对象
      MappedStatement ms = configuration.getMappedStatement(statement);
      // 调用Executor执行具体的查询逻辑
      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();
    }
  }
}

Executor拥有一个抽象子类BaseExecutor，Mybatis此处使用模版设计模式在BaseExecutor中封装一些列模版方法

public abstract class BaseExecutor implements Executor {
  @Override
  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++;
      // 尝试从缓存中查找数据，如存在则不再从数据库查询
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        // 从数据库获取数据，调用模版方法，由子类去实现
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    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;
  }
  protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
      throws SQLException;
}

BaseExecutor的默认实现类为SimpleExecutor

public class SimpleExecutor extends BaseExecutor {
  @Override
  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();
      // 创建StatementHandler，此处创建的是RoutingStatementHadnler，会依据StatementType创建对应的StatementHandler
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      // 真正执行查询操作
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }
}
public class SimpleStatementHandler extends BaseStatementHandler {
  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    String sql = boundSql.getSql();
    statement.execute(sql);
    // 返回由ResultSetHandler处理之后的查询结果
    return resultSetHandler.handleResultSets(statement);
  }
}

Mybatis源码分析

01｜核心类

02｜配置解析