上一篇讲了SqlSession对象中的Executor，接下来将对SqlSession的另一个对象StatementHandler进行讲解。
一、StatementHandler介绍
StatementHandler是Mybatis中最重要的一个对象，它负责 *** 作Statement与数据库进行交流，在此过程中还会调用ParameterHandler进行参数配置，使用ResultHandler将查询结果与实体类对象进行绑定。
因而ParameterHandler与ResultHandler的创建是与StatementHandler相关联的。
StatementHandler是一个顶级接口，它的类图如下所示：
StatementHandler接口下有两个直接实现类BaseStatementHandler和RoutingStatementHandler
1、RoutingStatementHandler类RoutingStatementHandler：是一个具体实现类，在这个类中并没有对Statement对象进行具体使用，只是根据得到Executor类型决定创建何种类型StatementHandler对象。
在MyBatis工作时，使用的StatementHandler接口对象实际上就是RoutingStatementHandler对象。
可以简单理解为：
StatementHandler statmentHandler = new RountingStatementHandler();
BaseStatementHandler：是StatementHandler接口的另一个实现类，本身是一个抽象类，用于简化StatementHandler接口实现的难度，属于适配器设计模式体现。
它有三个实现类：SimpleStatementHandler，PreparedStatementHandler和CallableStatementHandler。
在RountingStatementHandler创建时,就根据接收的传递的SQL语句来创建这个三个类型的对象。
SimpleStatementHandler：管理Statement对象向数据库中推送不需要预编译的SQL语句。
PreparedStatementHandler：管理PreparedStatementHandler对象向数据库推送预编译的SQL语句。
CallableStatementHandler：管理CallableStatement对象调用数据库中构造函数，即有存储过程的SQL语句。
二、StatementHandler接口
StatementHandler是一个接口，其代码如下：
public interface StatementHandler {
Statement prepare(Connection connection, Integer transactionTimeout)throws SQLException;
void parameterize(Statement statement)throws SQLException;
void batch(Statement statement)throws SQLException;
int update(Statement statement)throws SQLException;
<E> List<E> query(Statement statement, ResultHandler resultHandler)throws SQLException;
<E> Cursor<E> queryCursor(Statement statement)throws SQLException;
BoundSql getBoundSql();
ParameterHandler getParameterHandler();
}
我们主要关注StatementHandler中的四个重要方法：prepare()、parameterize()、update()、query()
1、prepare方法prepare方法主要是在BaseStatementHandler类中实现的，其代码如下：
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
ErrorContext.instance().sql(boundSql.getSql());
Statement statement = null;
try {
statement = instantiateStatement(connection);
setStatementTimeout(statement, transactionTimeout);
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);
}
}
prepare方法用于创建一个(Statement or PreparedStatement or CallableStatement)对象，并设置Statement对象的最大工作时间和一次性读取的最大数据量，然后将生成的Statement对象返回。
prepare方法只在BaseStatementHandler被实现，在其三个子类中没有被重写。
它主要用于三个子类调用获得对应的Statement接口对象，依靠instantiateStatement(connection)方法来返回具体Statement接口对象。
instantiateStatement方法是BaseStatementHandler中定义的抽象方法,由三个子类来具体实现，这里采用了模板方法模式。
SimpleStatementHandler的instantiateStatement方法
protected Statement instantiateStatement(Connection connection) throws SQLException {
if (mappedStatement.getResultSetType() != null) {
return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
} else {
return connection.createStatement();
}
}
PreparedStatementHandler的instantiateStatement方法
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() != null) {
return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
} else {
return connection.prepareStatement(sql);
}
}
CallableStatementHandler的instantiateStatement方法
protected Statement instantiateStatement(Connection connection) throws SQLException {
String sql = boundSql.getSql();
if (mappedStatement.getResultSetType() != null) {
return connection.prepareCall(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
} else {
return connection.prepareCall(sql);
}
}
parameterize方法用于传递参数，因此只在PreparedStatementHandler和CallableStatementHandler中被重写。
PreparedStatementHandler的parameterize方法
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
}
CallableStatementHandler的parameterize方法
public void parameterize(Statement statement) throws SQLException {
registerOutputParameters((CallableStatement) statement);
parameterHandler.setParameters((CallableStatement) statement);
}
他们都调用了parameterHandler进行参数赋值。
query方法主要用于输送查询查询语句，并将查询结果转换对应的实体类对象。
SimpleStatementHandler的query方法：
public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
String sql = boundSql.getSql();
statement.execute(sql);
return resultSetHandler.<E>handleCursorResultSets(statement);
}
PreparedStatementHandler的query方法：
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return resultSetHandler.<E> handleResultSets(ps);
}
CallableStatementHandler的query方法：
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
CallableStatement cs = (CallableStatement) statement;
cs.execute();
List<E> resultList = resultSetHandler.<E>handleResultSets(cs);
resultSetHandler.handleOutputParameters(cs);
return resultList;
}
由代码可知：在得到查询结果后，都是使用ResultSetHandler对结果进行转换的。
update方法用于输送insert、update、delete语句并返回处理数据行。
SimpleStatementHandler的update方法：
public int update(Statement statement) throws SQLException {
String sql = boundSql.getSql();
Object parameterObject = boundSql.getParameterObject();
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
int rows;
if (keyGenerator instanceof Jdbc3KeyGenerator) {
statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else if (keyGenerator instanceof SelectKeyGenerator) {
statement.execute(sql);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else {
statement.execute(sql);
rows = statement.getUpdateCount();
}
return rows;
}
PreparedStatementHandler的update方法：
public int update(Statement statement) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
int rows = ps.getUpdateCount();
Object parameterObject = boundSql.getParameterObject();
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);
return rows;
}
CallableStatementHandler的update方法：
public int update(Statement statement) throws SQLException {
CallableStatement cs = (CallableStatement) statement;
cs.execute();
int rows = cs.getUpdateCount();
Object parameterObject = boundSql.getParameterObject();
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
keyGenerator.processAfter(executor, mappedStatement, cs, parameterObject);
resultSetHandler.handleOutputParameters(cs);
return rows;
}
二、StatementHandler对象创建
StatementHandler对象是在SqlSession对象接收到 *** 作命令后由Configuraion中newStatementHandler方法负责调用的。
代码如下：
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}
由代码知：statementHandler的创建调用了RoutingStatementHandler的构造方法。
RoutingStatementHandler构造方法,将会根据StatementType决定创建SimpleStatementHandler，PreparedStatementHandler，CallableStatementHandler实例对象。
public enum StatementType {
STATEMENT, PREPARED, CALLABLE
}
构造方法如下所示：
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());
}
}
而创建三种StatementHandler的构造方法都采用的是BaseStatementHandler的构造方法，其代码如下：
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);
}
这里需要注意的是最后两行代码，创建了parameterHandler和resultSetHandler，这正好和前面所说的StatementHandler、parameterHandler和resultSetHandler是在一起创建的说法保持一致。
创建好handler，执行器的方法会调用prepareStatement方法产生Statment.，代码如下：
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
BoundSql boundSql = handler.getBoundSql();
String sql = boundSql.getSql();
if (hasStatementFor(sql)) {
stmt = getStatement(sql);
applyTransactionTimeout(stmt);
} else {
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());
putStatement(sql, stmt);
}
handler.parameterize(stmt);
return stmt;
}
此过程中会调用BaseStatementHandler的prepare方法，而在方法中又会调用instantiateStatement方法，通过Connection产生相应的Statment对象。
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