在上一章關(guān)于Executor的解析中姜性，我們留下了關(guān)于StatementHandler的疑問，下面我們就對(duì)這個(gè)類進(jìn)行分析观挎。

1. StatementHandler源碼分析

看到這個(gè)類，你可能會(huì)和原生的Statement聯(lián)想起來玖喘，下面我們先來看下這個(gè)接口的方法有哪些：

public interface StatementHandler {

  /**
   * 準(zhǔn)備操作贡蓖，可以理解成創(chuàng)建 Statement 對(duì)象
   *
   * @param connection         Connection 對(duì)象
   * @param transactionTimeout 事務(wù)超時(shí)時(shí)間
   * @return Statement 對(duì)象
   */
  Statement prepare(Connection connection, Integer transactionTimeout)
      throws SQLException;

  /**
   * 設(shè)置 Statement 對(duì)象的參數(shù)
   *
   * @param statement Statement 對(duì)象
   */
  void parameterize(Statement statement)
      throws SQLException;

  /**
   * 添加 Statement 對(duì)象的批量操作
   *
   * @param statement Statement 對(duì)象
   */
  void batch(Statement statement)
      throws SQLException;

  /**
   * 執(zhí)行寫操作
   *
   * @param statement Statement 對(duì)象
   * @return 影響的條數(shù)
   */
  int update(Statement statement)
      throws SQLException;

  /**
   * 執(zhí)行讀操作
   *
   * @param statement Statement 對(duì)象
   * @param resultHandler ResultHandler 對(duì)象曹鸠，處理結(jié)果
   * @param <E> 泛型
   * @return 讀取的結(jié)果
   */
  <E> List<E> query(Statement statement, ResultHandler resultHandler)
      throws SQLException;

  /**
   * 執(zhí)行讀操作，返回 Cursor 對(duì)象
   *
   * @param statement Statement 對(duì)象
   * @param <E> 泛型
   * @return Cursor 對(duì)象
   */
  <E> Cursor<E> queryCursor(Statement statement)
      throws SQLException;

  BoundSql getBoundSql();

  ParameterHandler getParameterHandler();

}

再來看下繼承關(guān)系：

StatementHandler子類繼承圖

1.1 RoutingStatementHandler類解析

我們先來看下用來路由的RoutingStatementHandler類斥铺，他的邏輯比較清晰：

public class RoutingStatementHandler implements StatementHandler {

  private final StatementHandler delegate;

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

  @Override
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    return delegate.prepare(connection, transactionTimeout);
  }

 

我們就看到這個(gè)邏輯根據(jù)ms.getStatementType()來路由彻桃，方法的話套路太多，不做所有展示晾蜘。

myBatis也是老套路邻眷，實(shí)現(xiàn)了一個(gè)抽象類，包含了一些通常的操作：
我們來看下BaseStatementHandler的屬性和構(gòu)造方法：

1.2 BaseStatementHandler源碼解析

public abstract class BaseStatementHandler implements StatementHandler {

  protected final Configuration configuration;
  protected final ObjectFactory objectFactory;
  protected final TypeHandlerRegistry typeHandlerRegistry;
  protected final ResultSetHandler resultSetHandler;
  protected final ParameterHandler parameterHandler;

  protected final Executor executor;
  protected final MappedStatement mappedStatement;
  protected final RowBounds rowBounds;

  protected BoundSql boundSql;

  protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    this.configuration = mappedStatement.getConfiguration();
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.rowBounds = rowBounds;

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.objectFactory = configuration.getObjectFactory();

    if (boundSql == null) { // issue #435, get the key before calculating the statement
      generateKeys(parameterObject);
      boundSql = mappedStatement.getBoundSql(parameterObject);
    }

    this.boundSql = boundSql;

    this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
    this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
  }
 }

這里關(guān)于parameterHandler和resultSetHandler的技術(shù)債之后再講剔交，我們先看這里的generateKeys方法：
這里的generateKeys方法為：

  protected void generateKeys(Object parameter) {

    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
    ErrorContext.instance().store();
    //獲得自增主鍵肆饶，放入到parameter
    keyGenerator.processBefore(executor, mappedStatement, null, parameter);
    ErrorContext.instance().recall();
  }

這里又讓我們欠下了關(guān)于KeyGenerator的技術(shù)債，之后我們重點(diǎn)來看下它的方法：

  @Override
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      // 創(chuàng)建 Statement 對(duì)象交給子類實(shí)現(xiàn)
      statement = instantiateStatement(connection);
      // 設(shè)置超時(shí)時(shí)間
      setStatementTimeout(statement, transactionTimeout);
      // 設(shè)置 fetchSize
      setFetchSize(statement);
      return statement;
    } catch (SQLException e) {
      closeStatement(statement);
      throw e;
    } catch (Exception e) {
      closeStatement(statement);
      throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
    }
  }

這里最主要的一個(gè)創(chuàng)建Statement的方法instantiateStatement又交給了子類去實(shí)現(xiàn)省容。我們來看SimpleStatementHandler的實(shí)現(xiàn)抖拴。

1.3 SimpleStatementHandler的instantiateStatement方法

// SimpleStatementHandler.java

@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
    if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {
        return connection.createStatement();
    } else {
        return connection.createStatement(mappedStatement.getResultSetType().getValue(),  ResultSet.CONCUR_READ_ONLY);
    }
}

就是我們傳統(tǒng)上的創(chuàng)建statement，也到了我們MyBatis封裝的最底層腥椒。我們?cè)賮砜聪翽reparedStatementHandler類的實(shí)現(xiàn)阿宅。

1.4 PreparedStatementHandler的instantiateStatement方法

  @Override
  protected Statement instantiateStatement(Connection connection) throws SQLException {
    String sql = boundSql.getSql();
    if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
      String[] keyColumnNames = mappedStatement.getKeyColumns();
      if (keyColumnNames == null) {
        return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
      } else {
        return connection.prepareStatement(sql, keyColumnNames);
      }
    } else if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {
      return connection.prepareStatement(sql);
    } else {
      return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    }
  }

是不是也非常的熟悉？

剩下的就是關(guān)于傳統(tǒng)PrepareStatement和Statement如何操作數(shù)據(jù)庫的CRUD了笼蛛，我們抽一個(gè)方法來看下洒放。

1.5 PreparedStatementHandler的query方法

  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.handleResultSets(ps);
  }

這里唯一不同的是先執(zhí)行execute方法，在通過resultHandler去解析返回值滨砍。

2. 今日總結(jié)

今天我們看了關(guān)于StatementHandler的源碼分析往湿，得知了這個(gè)類的作用是用來創(chuàng)建對(duì)應(yīng)的Statement的，并且操作數(shù)據(jù)庫的CRUD與解析工作惋戏，一塊最低層的內(nèi)容就被我們啃下了领追。但是今天我們也欠下了3個(gè)技術(shù)債：

parameterHandler
resultSetHandler
KeyGenerator

在之后的章節(jié)中我們?cè)賮矸治鰚~~