【MyBatis源码分析】insert方法、update方法、delete方法处理流程（下篇）

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 statement
11       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：

1. 参数对象本身
2. 参数列表
3. 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：

1 public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
2     return delegate.prepare(connection, transactionTimeout);
3 }

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 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 params
11             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：

 1 public void setNonNullParameter(PreparedStatement ps, int i, Integer parameter, JdbcType jdbcType)
 2       throws SQLException {
 3     ps.setInt(i, parameter);
 4 }

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：

 1 public int update(Statement statement) throws SQLException {
 2     return delegate.update(statement);
 3 }

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：

 1 public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
 2     processBatch(ms, stmt, getParameters(parameter));
 3 }

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