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20 KiB
20 KiB
StatementHandler接口是MyBatis的核心接口之一，它完成了MyBatis中最核心的工作，也是Executor 接口实现的基础。
StatementHandler接口中的功能很多，例如创建Statement对象，为SQL语句绑定实参，执行select、insert、update、delete等多种类型的SQL语句，批量执行SQL语句，将结果集映射成结果对象。
public interface StatementHandler {
// 从连接中获取一个Statement
Statement prepare(Connection connection, Integer transactionTimeout)
throws SQLException;
// 绑定statement执行时所需的实参
void parameterize(Statement statement)
throws SQLException;
// 批量执行SQL语句
void batch(Statement statement)
throws SQLException;
// 执行update/insert/delete语句
int update(Statement statement)
throws SQLException;
// 执行select语句
<E> List<E> query(Statement statement, ResultHandler resultHandler)
throws SQLException;
<E> Cursor<E> queryCursor(Statement statement)
throws SQLException;
BoundSql getBoundSql();
// 获取参数处理器
ParameterHandler getParameterHandler();
}
RoutingStatementHandler
RoutingStatementHandler使用了策略模式，RoutingStatementHandler是策略类，而SimpleStatementHandler、PreparedStatementHandler、CallableStatementHandler则是实现了具体算法的实现类，RoutingStatementHandler对象会根据MappedStatement对象的StatementType属性值选择使用相应的策略去执行。
public class RoutingStatementHandler implements StatementHandler {
// 持有的真正实现StatementHandler接口功能的对象
private final StatementHandler delegate;
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
// RoutingStatementHandler的作用就是根据ms的配置，生成一个相对应的StatementHandler对象
// 并设置到持有的delegate属性中，本对象的所有方法都是通过调用delegate的相应方法实现的
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);
}
@Override
public void parameterize(Statement statement) throws SQLException {
delegate.parameterize(statement);
}
@Override
public void batch(Statement statement) throws SQLException {
delegate.batch(statement);
}
@Override
public int update(Statement statement) throws SQLException {
return delegate.update(statement);
}
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
return delegate.query(statement, resultHandler);
}
@Override
public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
return delegate.queryCursor(statement);
}
@Override
public BoundSql getBoundSql() {
return delegate.getBoundSql();
}
@Override
public ParameterHandler getParameterHandler() {
return delegate.getParameterHandler();
}
}
BaseStatementHandler
看它以Base开头，就可以猜到 它是一个实现了StatementHandler接口的抽象类，这个类只提供了一些参数绑定相关的方法，并没有实现操作数据库的方法。
public abstract class BaseStatementHandler implements StatementHandler {
// 持有的这些属性都是通过构造方法完成初始化的，typeHandlerRegistry、
// objectFactory、parameterHandler等则是通过configuration属性获得的
protected final Configuration configuration;
protected final ObjectFactory objectFactory;
protected final TypeHandlerRegistry typeHandlerRegistry;
protected final ResultSetHandler resultSetHandler;
// parameterHandler的功能主要是为SQL语句绑定实参，也就是使用传入的实参
// 替换SQL语句中的占位符"?"
protected final ParameterHandler parameterHandler;
// 用来执行SQL语句的执行器
protected final Executor executor;
protected final MappedStatement mappedStatement;
// 记录了用户设置的offset和limit，用于在结果集中定位
// 映射的起始位置和结束位置
protected final RowBounds rowBounds;
protected BoundSql boundSql;
// BaseStatementHandler的构造方法主要用于属性的初始化
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
// 其中调用了KeyGenerator的processBefore()方法
// 用于初始化SQL语句的主键
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);
}
protected void generateKeys(Object parameter) {
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
ErrorContext.instance().store();
keyGenerator.processBefore(executor, mappedStatement, null, parameter);
ErrorContext.instance().recall();
}
@Override
public BoundSql getBoundSql() {
return boundSql;
}
@Override
public ParameterHandler getParameterHandler() {
return parameterHandler;
}
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
ErrorContext.instance().sql(boundSql.getSql());
Statement statement = null;
try {
// 这是一个抽象方法，用于初始化java.sql.Statement对象
statement = instantiateStatement(connection);
// 为Statement对象设置超时时间及fetchSize
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);
}
}
protected abstract Statement instantiateStatement(Connection connection) throws SQLException;
protected void setStatementTimeout(Statement stmt, Integer transactionTimeout) throws SQLException {
Integer queryTimeout = null;
if (mappedStatement.getTimeout() != null) {
queryTimeout = mappedStatement.getTimeout();
} else if (configuration.getDefaultStatementTimeout() != null) {
queryTimeout = configuration.getDefaultStatementTimeout();
}
if (queryTimeout != null) {
stmt.setQueryTimeout(queryTimeout);
}
StatementUtil.applyTransactionTimeout(stmt, queryTimeout, transactionTimeout);
}
protected void setFetchSize(Statement stmt) throws SQLException {
Integer fetchSize = mappedStatement.getFetchSize();
if (fetchSize != null) {
stmt.setFetchSize(fetchSize);
return;
}
Integer defaultFetchSize = configuration.getDefaultFetchSize();
if (defaultFetchSize != null) {
stmt.setFetchSize(defaultFetchSize);
}
}
protected void closeStatement(Statement statement) {
try {
if (statement != null) {
statement.close();
}
} catch (SQLException e) {
//ignore
}
}
}
BaseStatementHandler主要实现了StatementHandler接口中的prepare()方法，BaseStatementHandler依赖两个重要的组件，ParameterHandler和ResultSetHandler。
ParameterHandler系列组件
我们要执行的SQL语句中可能包含占位符"?"，而每个"?"都对应了BoundSql中parameterMappings集合中的一个元素，在该ParameterMapping对象中记录了对应的参数名称以及该参数的相关属性。ParameterHandler接口定义了一个非常重要的方法setParameters()，该方法主要负责调用PreparedStatement的set＊()系列方法，为SQL语句绑定实参。MyBatis只为ParameterHandler接口提供了唯一一个实现类DefaultParameterHandler。
public interface ParameterHandler {
// 获取用户传入的实参对象
Object getParameterObject();
// 本方法主要负责调用PreparedStatement.set*()方法，为SQL语句绑定实参。
void setParameters(PreparedStatement ps)
throws SQLException;
}
public class DefaultParameterHandler implements ParameterHandler {
// 管理mybatis中所有的TypeHandler对象
private final TypeHandlerRegistry typeHandlerRegistry;
// 其中记录了SQL节点相应的配置信息
private final MappedStatement mappedStatement;
// 用户传入的实参对象
private final Object parameterObject;
// 其中记录了要执行的SQL语句，及参数信息
private final BoundSql boundSql;
private final Configuration configuration;
// 构造方法主要为持有的属性 进行初始化
public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
this.mappedStatement = mappedStatement;
this.configuration = mappedStatement.getConfiguration();
this.typeHandlerRegistry = mappedStatement.getConfiguration().getTypeHandlerRegistry();
this.parameterObject = parameterObject;
this.boundSql = boundSql;
}
@Override
public Object getParameterObject() {
return parameterObject;
}
// 为PreparedStatement对象要执行的SQL语句中的占位符 设置对应的参数值
@Override
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
// 获取参数列表
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings != null) {
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
// 过滤掉存储过程中的输出参数
if (parameterMapping.getMode() != ParameterMode.OUT) {
// 记录绑定的实参
Object value;
// 获取参数对应的属性名
String propertyName = parameterMapping.getProperty();
// 根据属性名 获取 实参值
if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
value = boundSql.getAdditionalParameter(propertyName);
// 整个实参为空
} else if (parameterObject == null) {
value = null;
// 如果实参可以直接通过TypeHandler转换成JdbcType
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else {
// 获取对象中相应的属性值 或查找Map对象中的值
MetaObject metaObject = configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
// 获取当前parameterMapping中的TypeHandler对象 及JdbcType对象
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
// TypeHandler的setParameter()方法会调用PreparedStatement对象的
// set*()系列方法，为SQL语句绑定相应的实参
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException | SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
}
}
为SQL语句绑定完实参之后，就可以调用Statement对象 相应的execute方法，将SQL语句交给数据库执行了。
SimpleStatementHandler
SimpleStatementHandler继承了BaseStatementHandler抽象类。其底层使用java.sql.Statement来完成数据库的相关操作，所以SQL语句中不存在占位符，所以SimpleStatementHandler的parameterize()方法是空实现。SimpleStatementHandler的instantiateStatement()方法直接通过JDBC Connection创建Statement对象。
public class SimpleStatementHandler extends BaseStatementHandler {
// 构造方法主要用于属性的初始化
public SimpleStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}
// 直接通过Connection创建Statement对象
@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {
// 如果结果集类型是DEFAULT默认的，则直接用connection创建Statement对象
return connection.createStatement();
} else {
// 否则，设置结果集类型，设置结果集 只读
return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
}
}
// 上面创建的Statement对象会被本方法用于完成数据库查询操作
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
// 获取SQL语句
String sql = boundSql.getSql();
// 发送请求 执行SQL语句
statement.execute(sql);
// 从statement中获取结果集，并进行映射处理
return resultSetHandler.handleResultSets(statement);
}
// 下面的batch()及queryCursor()方法的实现与上面的query()方法非常类似
@Override
public void batch(Statement statement) throws SQLException {
String sql = boundSql.getSql();
statement.addBatch(sql);
}
@Override
public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
String sql = boundSql.getSql();
statement.execute(sql);
return resultSetHandler.handleCursorResultSets(statement);
}
// 本方法用于执行insert、delete、update等类型的SQL语句，并且会根据配置的
// KeyGenerator获取数据库生成的主键
@Override
public int update(Statement statement) throws SQLException {
// 获取SQL语句 及parameterObject
String sql = boundSql.getSql();
Object parameterObject = boundSql.getParameterObject();
// 获取配置的KeyGenerator 数据库主键生成器
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
int rows;
if (keyGenerator instanceof Jdbc3KeyGenerator) {
// 执行SQL语句
statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
// 获取更新的条数
rows = statement.getUpdateCount();
// 将数据库生成的主键添加到parameterObject中
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else if (keyGenerator instanceof SelectKeyGenerator) {
// 执行SQL语句
statement.execute(sql);
// 获取更新的条数
rows = statement.getUpdateCount();
// 执行<selectKey>节点中配置的SQL语句，将从数据库获取到的主键 添加到parameterObject中
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else {
statement.execute(sql);
rows = statement.getUpdateCount();
}
return rows;
}
@Override
public void parameterize(Statement statement) {
// N/A
}
}
PreparedStatementHandler
PreparedStatementHandler底层依赖于java.sql.PreparedStatement来完成数据库的相关操作。其中的parameterize()方法中，会调用前面介绍的ParameterHandler的setParameters()方法 完成 SQL语句的参数绑定。instantiateStatement()方法直接调用JDBC Connection的prepareStatement()方法创建PreparedStatement对象。
public class PreparedStatementHandler extends BaseStatementHandler {
// 构造方法主要用于属性的初始化
public PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}
@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
// 获取SQL语句
String sql = boundSql.getSql();
// 根据mappedStatement持有的KeyGenerator的类型进行不同的处理
if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
// 获取主键列
String[] keyColumnNames = mappedStatement.getKeyColumns();
if (keyColumnNames == null) {
// 返回数据库生成的主键
return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
} else {
// 在insert语句执行完后，会将keyColumnNames指定的列返回
return connection.prepareStatement(sql, keyColumnNames);
}
} else if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {
// 如果结果集类型是DEFAULT默认的，则直接通过connection获取PreparedStatement对象
return connection.prepareStatement(sql);
} else {
// 否则，设置结果集类型，设置结果集为只读
return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
}
}
// 因为是PrepareStatement对象，所以需要处理占位符"?"
// 使用了前面介绍的ParameterHandler组件完成
@Override
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
}
// 下面的这些方法，除了多了一步 将Statement对象强转成PreparedStatement对象
// 其它的几乎与SimpleStatementHandler一样
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return resultSetHandler.handleResultSets(ps);
}
@Override
public void batch(Statement statement) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.addBatch();
}
@Override
public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return resultSetHandler.handleCursorResultSets(ps);
}
@Override
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;
}
}
另外，StatementHandler接口还有一个CallableStatementHandler的实现。其底层依赖于java.sql.CallableStatement调用指定的存储过程，其parameterize()方法也会调用ParameterHandler的setParameters()方法完成SQL语句的参数绑定，并指定输出参数的索引位置和JDBC类型。其余方法与前面介绍的ResultSetHandler实现类似，唯一区别是会调用ResultSetHandler的handleOutputParameters()方法 处理输出参数。
看到这里，我们可以发现StatementHandler组件依赖ParameterHandler组件 和 ResultSetHandler组件 完成了MyBatis的核心功能，它控制着参数绑定、SQL语句执行、结果集映射等一系列核心流程。