(七)Mybatis Sql如何解析执行StatementHandler(3)--MyBatis源码解析

这篇博客我们来学习下StatementHandler是如何解析执行sql的，带你一起看下整个执行过程，一起学习。

SimpleExecutor执行sql过程

当BoundSql加载完成之后，下一步就是执行select/update/insert/delete语句了。在SimpleExecutor中执行语句最终会由doQuery和doUpdate方法完成。

public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
	Statement stmt = null;
	try {
		Configuration configuration = ms.getConfiguration();
		//生成一个StatementHandler
		StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, 
                                                                     RowBounds.DEFAULT, null, null);
		//执行之前的准备
		stmt = prepareStatement(handler, ms.getStatementLog());
		//执行sql
		return handler.update(stmt);
	}finally {
		closeStatement(stmt);
	}
}
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();
		//生成一个StatementHandler
		StatementHandler handler = configuration.newStatementHandler(wrapper, ms, 
        parameter, rowBounds, resultHandler,  boundSql);
		//执行之前的准备
		stmt = prepareStatement(handler, ms.getStatementLog());
		//执行sql
		return handler.<E>query(stmt, resultHandler);
	}finally {
		closeStatement(stmt);
	}
}

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    //获取一个连接
    Connection connection = getConnection(statementLog);
    //由StatementHandler从connection获取一个StatementHandler
    stmt = handler.prepare(connection);
    //设置执行参数
    handler.parameterize(stmt);
    return stmt;
}

从以上代码中可以到出Executor是怎么利用StatementHandler执行SQL的

1. 获取一个数据库连接
2. 调用StatementHandler.prepare()方法获取一个statement
3. 调用StatementHandler.parameterize()方法设置sql执行时所需要的参数
4. 调用StatementHandler.update或query方法执行sql

StatementHandler

这个接口定义了执行sql的基本操作

public interface StatementHandler {
  //从连接中获取一个Statement
  Statement prepare(Connection connection)
      throws SQLException;
 
 
  //设置statement执行里所需的参数
  void parameterize(Statement statement)
      throws SQLException;
 
  //批量
  void batch(Statement statement)
      throws SQLException;
 
  //更新：update/insert/delete语句
  int update(Statement statement)
      throws SQLException;
 
  //执行查询
  <E> List<E> query(Statement statement, ResultHandler resultHandler)
      throws SQLException;
 
  BoundSql getBoundSql();
 
  ParameterHandler getParameterHandler();
 
}

RoutingStatementHandler

这个类只是根据MappedStatement的配置，生成一个对应的StatementHandler(delegate),然后所有的实现都由delegate完成。

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());
  }
}

BaseStatementHandler

这个类是一个模板类。其中有两个主要的属性和一个主要的方法。先来看两个属性。

//处理结果的Handler
protected final ResultSetHandler resultSetHandler;
//设置参数的Handler
protected final ParameterHandler parameterHandler;


public interface ResultSetHandler {
  //转换结果集
  <E> List<E> handleResultSets(Statement stmt) throws SQLException;
 
  void handleOutputParameters(CallableStatement cs) throws SQLException;
 
}
public interface ParameterHandler {
 
  Object getParameterObject();
  //设置SQL执行的参数
  void setParameters(PreparedStatement ps)
      throws SQLException;
 
}

这ParameterHandler和ResultSetHandler在Mybatis中都只有一个实现：DefaultParameterHandler和DefaultResultSetHandler。之后会有详细有分析。Prepare方法

public Statement prepare(Connection connection) throws SQLException {
    ErrorContext.instance().sql(boundSql.getSql());
    Statement statement = null;
    try {
      //这个方法是抽象的，由子类实现
      statement = instantiateStatement(connection);
      //设置执行的超时时间
      setStatementTimeout(statement);
      //fetchSize，对大的结果集才有明显的效果
      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);
    }
}

BaseStatementHandler子类

1. PreparedStatementHandler,处理PreparedStatement对象，即带参数运行的SQL,这个类以后详细分析。
2. CallableStatementHandler,处理CallableStatement对象，即执行过程的SQL,不做讨论。
3. SimpleStatementHandler,处理Statement对象，即不带参数运行的SQL,不做讨论。