发布于 2016-10-06 13:57:29 | 370 次阅读 | 评论: 0 | 来源: PHPERZ
MyBatis 基于Java的持久层框架
MyBatis 本是apache的一个开源项目iBatis, 2010年这个项目由apache software foundation 迁移到了google code,并且改名为MyBatis 。
MyBatis 是支持定制化 SQL、存储过程以及高级映射的优秀的持久层框架。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以对配置和原生Map使用简单的 XML 或注解,将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。如何新建MyBatis源码工程请点击MyBatis源码分析-IDEA新建MyBatis源码工程。
MyBatis框架主要完成的是以下2件事情:
MyBatis框架是一种典型的交互式框架,先准备好交互的必要条件,然后构建一个交互的环境,在交互环境中划分会话,在会话中与数据库进行交互数据。
以上几个类在SQL操作中都会涉及,在SQL操作中重点关注下SQL参数什么时候写入和结果集怎么转换为Java对象,这两个过程正好对应的类是PreparedStatementHandler和ResultSetHandler类。
子流程1:SQL查询参数的设置
// SimplyExecutor类
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
// 获取一个Connection
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());
handler.parameterize(stmt); // 设置SQL查询中的参数值
return stmt;
}
通过getConnection方法来获取一个Connection,调用prepare方法来获取一个Statement(这里的handler类型是RoutingStatementHandler,RoutingStatementHandler的prepare方法调用的是PrepareStatementHandler的prepare方法,因为PrepareStatementHandler并没有覆盖其父类的prepare方法,其实最后调用的是BaseStatementHandler中的prepare方法。是不是绕晕了,那就再看一遍吧 :) )。调用parameterize方法来设置SQL的参数值(这里最后调用的是PrepareStatementHandler中的parameterize方法,而PrepareStatementHandler.parameterize方法调用的是DefaultParameterHandler中的setParameters方法)。
// PrepareStatementHandler类
@Override
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
}
// DefaultParameterHandler类
@Override
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings != null) {
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
String propertyName = parameterMapping.getProperty();
if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
value = boundSql.getAdditionalParameter(propertyName);
} else if (parameterObject == null) {
value = null;
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else {
MetaObject metaObject = configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
} catch (SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
}
到这里为止,已经给Statement设置了最初传递进去的参数(值为1)了,那么接着分析流程2:
流程2:SQL查询及结果集的设置
// RoutingStatementHandler类
@Override
public <E> List<E> query(Statement statement) throws SQLException {
return delegate.<E>query(statement);
}
// RoutingStatementHandler类
@Override
public <E> List<E> query(Statement statement) throws SQLException {
// 这里就到了熟悉的PreparedStatement了
PreparedStatement ps = (PreparedStatement) statement;
// 执行SQL查询操作
ps.execute();
// 结果交给ResultHandler来处理
return resultSetHandler.<E> handleResultSets(ps);
}
// DefaultResultSetHandler类(封装返回值,将查询结果封装成Object对象)
@Override
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
final List<Object> multipleResults = new ArrayList<Object>();
int resultSetCount = 0;
ResultSetWrapper rsw = getFirstResultSet(stmt);
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);
while (rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = resultMaps.get(resultSetCount);
handleResultSet(rsw, resultMap, multipleResults, null);
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
String[] resultSets = mappedStatement.getResultSets();
if (resultSets != null) {
while (rsw != null && resultSetCount < resultSets.length) {
ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
if (parentMapping != null) {
String nestedResultMapId = parentMapping.getNestedResultMapId();
ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
handleResultSet(rsw, resultMap, null, parentMapping);
}
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
}
return collapseSingleResultList(multipleResults);
}
ResultSetWrapper是ResultSet的包装类,调用getFirstResultSet方法获取第一个ResultSet,同时获取数据库的MetaData数据,包括数据表列名、列的类型、类序号等,这些信息都存储在ResultSetWrapper类中了。然后调用handleResultSet方法来来进行结果集的封装。
// DefaultResultSetHandler类
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
try {
if (parentMapping != null) {
handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
} else {
if (resultHandler == null) {
DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
multipleResults.add(defaultResultHandler.getResultList());
} else {
handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
}
}
} finally {
// issue #228 (close resultsets)
closeResultSet(rsw.getResultSet());
}
}
这里调用handleRowValues方法来进行值的设置:
// DefaultResultSetHandler类
public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
if (resultMap.hasNestedResultMaps()) {
ensureNoRowBounds();
checkResultHandler();
handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
} else {
// 封装数据
handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
}
}
// DefaultResultSetHandler类
// 封装数据
private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
throws SQLException {
DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
skipRows(rsw.getResultSet(), rowBounds);
while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
Object rowValue = getRowValue(rsw, discriminatedResultMap);
storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
}
// DefaultResultSetHandler类
private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
final ResultLoaderMap lazyLoader = new ResultLoaderMap();
// createResultObject为新创建的对象,数据表对应的类
Object resultObject = createResultObject(rsw, resultMap, lazyLoader, null);
if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
final MetaObject metaObject = configuration.newMetaObject(resultObject);
boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty();
if (shouldApplyAutomaticMappings(resultMap, false)) {
// 这里把数据填充进去,metaObject中包含了resultObject信息
foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
}
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
foundValues = lazyLoader.size() > 0 || foundValues;
resultObject = foundValues ? resultObject : null;
return resultObject;
}
return resultObject;
}
// DefaultResultSetHandler类(把ResultSet中查询结果填充到JavaBean中)
private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
boolean foundValues = false;
if (autoMapping.size() > 0) {
// 这里进行for循环调用,因为user表中总共有7项,所以也就调用7次
for (UnMappedColumnAutoMapping mapping : autoMapping) {
// 这里将esultSet中查询结果转换为对应的实际类型
final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
if (value != null) {
foundValues = true;
}
if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {
// gcode issue #377, call setter on nulls (value is not 'found')
metaObject.setValue(mapping.property, value);
}
}
}
return foundValues;
}
mapping.typeHandler.getResult会获取查询结果值的实际类型,比如我们user表中id字段为int类型,那么它就对应Java中的Integer类型,然后通过调用statement.getInt("id")来获取其int值,其类型为Integer。metaObject.setValue方法会把获取到的Integer值设置到Java类中的对应字段。
// MetaObject类
public void setValue(String name, Object value) {
PropertyTokenizer prop = new PropertyTokenizer(name);
if (prop.hasNext()) {
MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
if (value == null && prop.getChildren() != null) {
// don't instantiate child path if value is null
return;
} else {
metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
}
}
metaValue.setValue(prop.getChildren(), value);
} else {
objectWrapper.set(prop, value);
}
}
metaValue.setValue方法最后会调用到Java类中对应数据域的set方法,这样也就完成了SQL查询结果集的Java类封装过程。最后贴一张调用栈到达Java类的set方法中的快照:
参考:
2、《深入分析Java Web技术内幕》的iBatis章节
3、《深入理解mybatis原理》 MyBatis的架构设计以及实例分析