Configuration的newStatementHandler分析
SimpleExecutor的doUpdate方法上文有分析过：
1 public int doUpdate(MappedStatement ms, Object parameter) throws SQLException { 2 Statement stmt = null; 3 try { 4 Configuration configuration = ms.getConfiguration(); 5 StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null); 6 stmt = prepareStatement(handler, ms.getStatementLog()); 7 return handler.update(stmt); 8 } finally { 9 closeStatement(stmt);10 }11 }这两天重看第5行的newStatementHandler方法的时候，发现方法上文在这个方法中分析地太简略了，这里过一遍一下Configuration的newStatementHandler方法，方法的实现为：
1 public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {2 StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);3 statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);4 return statementHandler;5 }第3行的代码是加入插件没什么好看的，看下第2行的代码，StatementHandler接口真正实例化出来的是RoutingStatementHandler，构造方法实现为：
1 public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { 2 3 switch (ms.getStatementType()) { 4 case STATEMENT: 5 delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql); 6 break; 7 case PREPARED: 8 delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql); 9 break;10 case CALLABLE:11 delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);12 break;13 default:14 throw new ExecutorException("Unknown statement type: " + ms.getStatementType());15 }16 17 }RoutingStatementHandler同样是装饰器模式的实现，实现了StatementHandler接口并持有StatementHandler接口引用delegate。这里StatementType的为PREPARED，因此执行的第7行的判断，实例化出PreparedStatementHandler，实例化的过程为：
1 protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { 2 this.configuration = mappedStatement.getConfiguration(); 3 this.executor = executor; 4 this.mappedStatement = mappedStatement; 5 this.rowBounds = rowBounds; 6 7 this.typeHandlerRegistry = configuration.getTypeHandlerRegistry(); 8 this.objectFactory = configuration.getObjectFactory(); 9 10 if (boundSql == null) { // issue #435, get the key before calculating the statement11 generateKeys(parameterObject);12 boundSql = mappedStatement.getBoundSql(parameterObject);13 }14 15 this.boundSql = boundSql;16 17 this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);18 this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);19 }这里的重点是BoundSql，它可以通过MappedStatement获取到，BoundSql中存储了几个重要的内容：
参数对象本身
参数列表
待执行的SQL语句
有些基于MyBatis二次开发的框架通常都会拿到BoundSql中的SQL语句进行修改并重新设置进BoundSql中。
生成Statement
上文已经写了生成Connection的流程，本文继续看，首先还是再贴一下SimpleExecutor的prepareStatement方法：
1 private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {2 Statement stmt;3 Connection connection = getConnection(statementLog);4 stmt = handler.prepare(connection, transaction.getTimeout());5 handler.parameterize(stmt);6 return stmt;7 }接着就是第4行的代码，生成Statement了，第4行的代码实现为：
Statement prepare(Connection connection, Integer transactionTimeout)
delegate上文是装饰器模式中的被装饰角色，其接口类型为StatementHandler，真实类型为PreparedStatementHandler，这个在最开头的部分已经分析过了。看一下prepare方法实现：
1 public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException { 2 ErrorContext.instance().sql(boundSql.getSql()); 3 Statement statement = null; 4 try { 5 statement = instantiateStatement(connection); 6 setStatementTimeout(statement, transactionTimeout); 7 setFetchSize(statement); 8 return statement; 9 } catch (SQLException e) {10 closeStatement(statement);11 throw e;12 } catch (Exception e) {13 closeStatement(statement);14 throw new ExecutorException("Error preparing statement. Cause: " + e, e);15 }16 }第6行的代码设置的是查询超时时间、第7行的代码设置的是接收的数据大小，就不跟进去看了，接着看下第6行的instantiateStatement方法实现：
1 protected Statement instantiateStatement(Connection connection) throws SQLException { 2 String sql = boundSql.getSql(); 3 if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) { 4 String[] keyColumnNames = mappedStatement.getKeyColumns(); 5 if (keyColumnNames == null) { 6 return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS); 7 } else { 8 return connection.prepareStatement(sql, keyColumnNames); 9 }10 } else if (mappedStatement.getResultSetType() != null) {11 return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);12 } else {13 return connection.prepareStatement(sql);14 }15 }第2行，从boundSql中获取真正的SQL语句，第一部分已经分析过了。拿到SQL语句之后，执行第3行与第5行的判断，这里就是我们熟悉的通过Connection拿Statement的代码了，通过prepareStatement方法获取到PreparedStatement，其真实类型为com.mysql.jdbc.JDBC4PreparedStatement，是PreparedStatement的子类。
Statement参数设置
获取了Statement后，下一步就是设置参数了，看一下设置参数的代码，还是回到SimpleExecutor的prepareStatement方法：
1 private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {2 Statement stmt;3 Connection connection = getConnection(statementLog);4 stmt = handler.prepare(connection, transaction.getTimeout());5 handler.parameterize(stmt);6 return stmt;7 }跟第5行的代码：
1 public void parameterize(Statement statement) throws SQLException { 2 parameterHandler.setParameters((PreparedStatement) statement); 3 }继续跟第2行的代码：
1 public void setParameters(PreparedStatement ps) { 2 ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId()); 3 List<ParameterMapping> parameterMappings = boundSql.getParameterMappings(); 4 if (parameterMappings != null) { 5 for (int i = 0; i < parameterMappings.size(); i++) { 6 ParameterMapping parameterMapping = parameterMappings.get(i); 7 if (parameterMapping.getMode() != ParameterMode.OUT) { 8 Object value; 9 String propertyName = parameterMapping.getProperty();10 if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params11 value = boundSql.getAdditionalParameter(propertyName);12 } else if (parameterObject == null) {13 value = null;14 } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {15 value = parameterObject;16 } else {17 MetaObject metaObject = configuration.newMetaObject(parameterObject);18 value = metaObject.getValue(propertyName);19 }20 TypeHandler typeHandler = parameterMapping.getTypeHandler();21 JdbcType jdbcType = parameterMapping.getJdbcType();22 if (value == null && jdbcType == null) {23 jdbcType = configuration.getJdbcTypeForNull();24 }25 try {26 typeHandler.setParameter(ps, i + 1, value, jdbcType);27 } catch (TypeException e) {28 throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);29 } catch (SQLException e) {30 throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);31 }32 }33 }34 }35 }最终执行的是第26行的代码，从26行的代码我们可以知道，参数设置到最后都是通过参数的TypeHandler来执行的，JDBC为我们预定义了很多TypeHandler，比如int值的TypeHandler就是IntegerTypeHandler，当然我们也可以定义自己的TypeHandler，通常来说继承BaseTypeHandler就可以了。
但是在此之前，会获取到Statement（setParameters方法形参）、占位符位置号（for循环的遍历参数i）、参数值（通过属性名获取）与jdbcType（配置在配置文件中，默认为null），最终执行TypeHandler的setParameters方法，这是BaseTypeHandler中的一个方法：
1 public void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException { 2 if (parameter == null) { 3 if (jdbcType == null) { 4 throw new TypeException("JDBC requires that the JdbcType must be specified for all nullable parameters."); 5 } 6 try { 7 ps.setNull(i, jdbcType.TYPE_CODE); 8 } catch (SQLException e) { 9 throw new TypeException("Error setting null for parameter #" + i + " with JdbcType " + jdbcType + " . " +10 "Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. " +11 "Cause: " + e, e);12 }13 } else {14 try {15 setNonNullParameter(ps, i, parameter, jdbcType);16 } catch (Exception e) {17 throw new TypeException("Error setting non null for parameter #" + i + " with JdbcType " + jdbcType + " . " +18 "Try setting a different JdbcType for this parameter or a different configuration property. " +19 "Cause: " + e, e);20 }21 }22 }这里的参数不为null，走13行的else，执行setNonNullParameter方法，这是IntegerTypeHandler中的一个方法：
setNonNullParameter(PreparedStatement ps,
这里的代码就比较熟悉了，PreparedStatement的setInt方法。
执行更新操作并处理结果
最后一步，执行更新操作并对结果进行处理，回到SimpleExecuto的doUpdate方法：
1 public int doUpdate(MappedStatement ms, Object parameter) throws SQLException { 2 Statement stmt = null; 3 try { 4 Configuration configuration = ms.getConfiguration(); 5 StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null); 6 stmt = prepareStatement(handler, ms.getStatementLog()); 7 return handler.update(stmt); 8 } finally { 9 closeStatement(stmt);10 }11 }第6行已经准备好了Statement，第7行执行update操作并对结果进行处理并返回：
update(Statement statement)
这里的委托delegate前面已经说过了，其真实类型是PreparedStatementHandler，update方法的实现为：
1 public int update(Statement statement) throws SQLException {2 PreparedStatement ps = (PreparedStatement) statement;3 ps.execute();4 int rows = ps.getUpdateCount();5 Object parameterObject = boundSql.getParameterObject();6 KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();7 keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);8 return rows;9 }第3行的execute方法是PreparedStatement中的方法，execute方法执行操作，然后第4行通过getUpdateCount()方法获取本次操作更新了几条数据，作为最终的值返回给用户。
第5行的代码通过BoundSql获取参数对象，这里是MailDO对象，因为我们知道在插入场景下，开发者是有这种需求的，需要返回插入的主键id，此时会将主键id设置到MailDO中。
第6行的代码通过MappedStatement获取KeyGenerator，一个主键生成器。
第7行的代码做了一个操作完毕的后置处理：
<span style="color: #008080"> 1</span> <span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span><span style="color: #000000"> processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {</span><span style="color: #008080"> 2</span> <span style="color: #000000"> processBatch(ms, stmt, getParameters(parameter));</span><span style="color: #008080"> 3</span> <span style="color: #000000">}</span><span style="color: #008080"><br/></span>首先将对象包装成集合类型，然后跟第2行的代码processBatch方法：
1 public void processBatch(MappedStatement ms, Statement stmt, Collection<Object> parameters) { 2 ResultSet rs = null; 3 try { 4 rs = stmt.getGeneratedKeys(); 5 final Configuration configuration = ms.getConfiguration(); 6 final TypeHandlerRegistry typeHandlerRegistry = configuration.getTypeHandlerRegistry(); 7 final String[] keyProperties = ms.getKeyProperties(); 8 final ResultSetMetaData rsmd = rs.getMetaData(); 9 TypeHandler<?>[] typeHandlers = null;10 if (keyProperties != null && rsmd.getColumnCount() >= keyProperties.length) {11 for (Object parameter : parameters) {12 // there should be one row for each statement (also one for each parameter)13 if (!rs.next()) {14 break;15 }16 final MetaObject metaParam = configuration.newMetaObject(parameter);17 if (typeHandlers == null) {18 typeHandlers = getTypeHandlers(typeHandlerRegistry, metaParam, keyProperties, rsmd);19 }20 populateKeys(rs, metaParam, keyProperties, typeHandlers);21 }22 }23 } catch (Exception e) {24 throw new ExecutorException("Error getting generated key or setting result to parameter object. Cause: " + e, e);25 } finally {26 if (rs != null) {27 try {28 rs.close();29 } catch (Exception e) {30 // ignore31 }32 }33 }34 }简单说这里就是遍历集合，通过JDBC4PreparedStatement的getGeneratedKeys获取ResultSet，然后从ResultSet中使用getLong方法获取生成的主键，设置到MailDO中。完成整个操作。
最后，本文演示的是insert数据的update方法流程，前文已经说过insert、update、delete在MyBatis中都是一样的，因此update、delete也是一样的操作，这里就不再赘述了。
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