在上一章关于Executor的解析中，我们留下了关于StatementHandler的疑问，下面我们就对这个类进行分析。
1. StatementHandler源码分析
看到这个类，你可能会和原生的Statement联想起来，下面我们先来看下这个接口的方法有哪些：
public interface StatementHandler {
/**
* 准备操作，可以理解成创建 Statement 对象
*
* @param connection Connection 对象
* @param transactionTimeout 事务超时时间
* @return Statement 对象
*/
Statement prepare(Connection connection, Integer transactionTimeout)
throws SQLException;
/**
* 设置 Statement 对象的参数
*
* @param statement Statement 对象
*/
void parameterize(Statement statement)
throws SQLException;
/**
* 添加 Statement 对象的批量操作
*
* @param statement Statement 对象
*/
void batch(Statement statement)
throws SQLException;
/**
* 执行写操作
*
* @param statement Statement 对象
* @return 影响的条数
*/
int update(Statement statement)
throws SQLException;
/**
* 执行读操作
*
* @param statement Statement 对象
* @param resultHandler ResultHandler 对象，处理结果
* @param <E> 泛型
* @return 读取的结果
*/
<E> List<E> query(Statement statement, ResultHandler resultHandler)
throws SQLException;
/**
* 执行读操作，返回 Cursor 对象
*
* @param statement Statement 对象
* @param <E> 泛型
* @return Cursor 对象
*/
<E> Cursor<E> queryCursor(Statement statement)
throws SQLException;
BoundSql getBoundSql();
ParameterHandler getParameterHandler();
}
再来看下继承关系：
StatementHandler子类继承图
1.1 RoutingStatementHandler类解析
我们先来看下用来路由的RoutingStatementHandler类，他的逻辑比较清晰：
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);
}
我们就看到这个逻辑根据ms.getStatementType()来路由，方法的话套路太多，不做所有展示。
myBatis也是老套路，实现了一个抽象类，包含了一些通常的操作：
我们来看下BaseStatementHandler的属性和构造方法：
1.2 BaseStatementHandler源码解析
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);
}
}
这里关于parameterHandler和resultSetHandler的技术债之后再讲，我们先看这里的generateKeys方法：
这里的generateKeys方法为：
protected void generateKeys(Object parameter) {
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
ErrorContext.instance().store();
//获得自增主键，放入到parameter
keyGenerator.processBefore(executor, mappedStatement, null, parameter);
ErrorContext.instance().recall();
}
这里又让我们欠下了关于KeyGenerator的技术债，之后我们重点来看下它的方法：
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
ErrorContext.instance().sql(boundSql.getSql());
Statement statement = null;
try {
// 创建 Statement 对象交给子类实现
statement = instantiateStatement(connection);
// 设置超时时间
setStatementTimeout(statement, transactionTimeout);
// 设置 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);
}
}
这里最主要的一个创建Statement的方法instantiateStatement又交给了子类去实现。我们来看SimpleStatementHandler的实现。
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);
}
}
就是我们传统上的创建statement，也到了我们MyBatis封装的最底层。我们再来看下PreparedStatementHandler类的实现。
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？
剩下的就是关于传统PrepareStatement和Statement如何操作数据库的CRUD了，我们抽一个方法来看下。
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);
}
这里唯一不同的是先执行execute方法，在通过resultHandler去解析返回值。
2. 今日总结
今天我们看了关于StatementHandler的源码分析，得知了这个类的作用是用来创建对应的Statement的，并且操作数据库的CRUD与解析工作，一块最低层的内容就被我们啃下了。但是今天我们也欠下了3个技术债：
parameterHandler
resultSetHandler
KeyGenerator
在之后的章节中我们再来分析~~~
