MyBatis 原码解析（version:3.2.7）

首先，我们看使用原生的JDBC来操作数据库的方式：

// 1. 获取JDBC Connection
Connection connection = DbManager.getConnectoin();
// 2. 组装sql语句
String sql = "insert into temp(id, name) values(?,?)";
// 3. 构造 PreparedStatement
PreparedStatement ps = connection.prepareStatement(sql);
// 4. 为 PreparedStatement 设置参数
ps.setLong(1, 1L);
ps.setString(2, "aaa");
// 5. 执行 PreparedStatement
int count = ps.executeUpdate();

那么，MyBatis是如何对上面的过程进行封装的呢？

我们以update为例，看MyBatis是如何封装这几个步骤的：

sql执行的入口：

DefaultSqlSession.update(String statement, Object parameter) --> Executor.update(MappedStatement ms, Object parameter) -->
 SimpleExecutor.doUpdate(MappedStatement ms, Object parameter)

DefaultSqlSession.update(String, Object)

public int update(String statement, Object parameter) {
    try {
      dirty = true;
      MappedStatement ms = configuration.getMappedStatement(statement); // 将mapper中的sql解析成原生的JDBC能够识别的sql，其中包括MyBatis标签的解析，"#{key,JdbcType=XXX}"占位符替换成"?"
      return executor.update(ms, wrapCollection(parameter)); // 构造java.sql.PreparedStatement --> 往java.sql.PreparedStatement设置sql的参数 --> 执行sql语句
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error updating database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
}

可以看出，MyBatis将这个过程封装成了两步：

1. sql语句解析（解析sql，替换占位符为"?"）

2. sql语句执行（设置sql中的参数，执行sql）

### 1. sql语句解析

configuration.getMappedStatement(statement)会去解析MyBatis的标签（比如：<foreach>、<if>等），并且将"#{key, JdbcType=XXX}"替换成原生的JDBC看的懂的占位符"?"。
流程如下：

Configuration.getMappedStatement(String, boolean)  --> Configuration.buildAllStatements() --> MapperAnnotationBuilder.parseStatement(Method) -->
 MapperAnnotationBuilder.buildSqlSourceFromStrings(String[], Class<?>, LanguageDriver) --> RawLanguageDriver.createSqlSource(Configuration, String, Class<?>)
 --> XMLLanguageDriver.createSqlSource(Configuration, String, Class<?>)

20170721更新：（xml应该是如下）

Configuration.getMappedStatement(String, boolean)  --> Configuration.buildAllStatements() --> XMLStatementBuilder.parseStatementNode() -->
 LanguageDriver.createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) --> RawLanguageDriver.createSqlSource(Configuration, String, Class<?>)
 --> XMLLanguageDriver.createSqlSource(Configuration, String, Class<?>)

可以看出，MyBatis中的动态SQL解析是通过别名为 xml 语言驱动器 org.apache.ibatis.scripting.xmltags.XmlLanguageDriver 驱动解析的。

public class XMLLanguageDriver implements LanguageDriver {

  public ParameterHandler createParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    return new DefaultParameterHandler(mappedStatement, parameterObject, boundSql);
  }

  public SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) {
    XMLScriptBuilder builder = new XMLScriptBuilder(configuration, script, parameterType);
    return builder.parseScriptNode();
  }

  public SqlSource createSqlSource(Configuration configuration, String script, Class<?> parameterType) {
    if (script.startsWith("<script>")) { // issue #3
      XPathParser parser = new XPathParser(script, false, configuration.getVariables(), new XMLMapperEntityResolver());
      return createSqlSource(configuration, parser.evalNode("/script"), parameterType);
    } else {
      script = PropertyParser.parse(script, configuration.getVariables()); // issue #127
      TextSqlNode textSqlNode = new TextSqlNode(script);
      if (textSqlNode.isDynamic()) { // 动态sql解析入口
        return new DynamicSqlSource(configuration, textSqlNode);
      } else {
        return new RawSqlSource(configuration, script, parameterType);
      }
    }
  }

}

DynamicSqlSource.java

public class DynamicSqlSource implements SqlSource {

  private Configuration configuration;
  private SqlNode rootSqlNode;

  public DynamicSqlSource(Configuration configuration, SqlNode rootSqlNode) {
    this.configuration = configuration;
    this.rootSqlNode = rootSqlNode;
  }

  public BoundSql getBoundSql(Object parameterObject) {
    DynamicContext context = new DynamicContext(configuration, parameterObject); // 构造DynamicContext，并将DynamicContext中的属性ContextMap bindings赋值
    rootSqlNode.apply(context); // 处理sql中的标签，比如：<foreach>、<if>等
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
    SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings()); // 将占位符"#{key, jdbcType=XXX}"替换成占位符"?"。也就是原生的java.sql.PreparedStatement支持的占位符。（具体是通过GenericTokenParser#parse(String text)来实现的）
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject); // 构造BoundSql
    for (Map.Entry<String, Object> entry : context.getBindings().entrySet()) {
      boundSql.setAdditionalParameter(entry.getKey(), entry.getValue()); // 将参数的key和value存放至BoundSql的MetaObject metaParameters;中
    }
    return boundSql;
  }

}

####2. sql语句执行

通过SimpleExecutor.java 去执行sql语句。其中，最重要的两步是 1. prepareStatement(handler, ms.getStatementLog()); 2. handler.update(stmt);

public class SimpleExecutor extends BaseExecutor {

  public SimpleExecutor(Configuration configuration, Transaction transaction) {
    super(configuration, transaction);
  }

  // 所有的数据库更新操作，都会调用doUpdate()，包括insert、update、delete
  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null); // 构造执行Statement的StatementHandler
      stmt = prepareStatement(handler, ms.getStatementLog()); // 通过java.sql.Connection拿到java.sql.PreparedStatement，并且为 java.sql.PreparedStatement 设置sql中的参数
      return handler.update(stmt); // 代理执行 java.sql.PreparedStatement （PreparedStatementHandler.update(Statement)）
    } finally {
      closeStatement(stmt);
    }
  }

  // 所有的数据库查询操作，都用调用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.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

  public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
    return Collections.emptyList();
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection); // 通过 java.sql.Connection 拿到 java.sql.PreparedStatement （PreparedStatementHandler.instantiateStatement(Connection)）
    handler.parameterize(stmt); // 为 java.sql.PreparedStatement 设置sql中的参数。（DefaultParameterHandler.setParameters(PreparedStatement)）
    return stmt;
  }

}

至此，MyBatis的封装就一目了然了。^_^

附：
MyBatis开发文档： http://www.mybatis.org/mybatis-3/zh/dynamic-sql.html