准备工作
public class MainClass {
public static void main(String[] args) throws Exception {
String resources = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resources);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
Student student = sqlSession.selectOne("org.apache.ibatis.dao.StudentMapper.getStudent",1);
System.out.println(student.toString());
sqlSession.close();
}
}
MyBatis是如何获取数据源的
这是我们mybatis-config.xml中配置数据库的4个关键属性，也就是看看MyBatis是怎么来解析这个配置文件块的。
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</dataSource>
</environment>
</environments>
数据源获取，我们从上面的代码片段中开始分析，在SqlSessionFactoryBuilder().build(inputStream) 方法中入手，这里看到实例了XMLConfigBuilder类。
SqlSessionFactoryBuilder
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
//parser.parse()方法返回Configuration对象，然后调用build(Configuration config)
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
inputStream.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
XMLConfigBuilder#parse()
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
private void parseConfiguration(XNode root) {
try {
propertiesElement(root.evalNode("properties"));
Properties settings = settingsAsProperties(root.evalNode("settings"));
loadCustomVfs(settings);
loadCustomLogImpl(settings);
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectorFactoryElement(root.evalNode("reflectorFactory"));
settingsElement(settings);
environmentsElement(root.evalNode("environments"));
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
typeHandlerElement(root.evalNode("typeHandlers"));
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
看到parseConfiguration参数为XNode就知道这个方法里肯定是要解析xml节点了,可以debug查看下root参数的值，尝试debug后发现，其内容就是我们的配置文件mybatis-config.xml的文件内容。 接着我们在22行这里则看到了熟悉的environments节点，那就直接看下environmentsElement方法。
<configuration>
<properties resource="db.properties"/>
<settings>
<setting name="logImpl" value="STDOUT_LOGGING"/>
<setting name="cacheEnabled" value="true"/>
<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>
<setting name="localCacheScope" value="SESSION"/>
</settings>
<typeAliases>
<typeAlias alias="Student" type="org.apache.ibatis.domain.Student"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="mapper/StudentMapper.xml"/>
</mappers>
</configuration>
XMLConfigBuilder#environmentsElement
这个方法参数XNode内容则是配置文件里的
private void environmentsElement(XNode context) throws Exception {
if (context != null) {
if (environment == null) {
environment = context.getStringAttribute("default");
}
for (XNode child : context.getChildren()) {
String id = child.getStringAttribute("id");
if (isSpecifiedEnvironment(id)) {
TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
DataSource dataSource = dsFactory.getDataSource();
Environment.Builder environmentBuilder = new Environment.Builder(id)
.transactionFactory(txFactory)
.dataSource(dataSource);
configuration.setEnvironment(environmentBuilder.build());
break;
}
}
}
}
解析DataSourceFactory
这里看到了getDeclaredConstructor().newInstance()反射实例化一个PoolDataSourceFactory。
private DataSourceFactory dataSourceElement(XNode context) throws Exception {
if (context != null) {
String type = context.getStringAttribute("type");
Properties props = context.getChildrenAsProperties();
DataSourceFactory factory = (DataSourceFactory) resolveClass(type).getDeclaredConstructor().newInstance();
factory.setProperties(props);
return factory;
}
throw new BuilderException("Environment declaration requires a DataSourceFactory.");
}
public class PooledDataSourceFactory extends UnpooledDataSourceFactory {
public PooledDataSourceFactory() {
this.dataSource = new PooledDataSource();
}
}
解析DataSource
拿到PooledDataSourceFactory后则调用dsFactory.getDataSource()拿到数据源， 这里重要的代码是最后一句configuration.setEnvironment，这里最后会把解析后的Environment对象给Configuration对象赋值， Configuration对象也是MyBatis框架中相当重量级的一个对象。
MyBatis是如何获取SQL语句的
解析SQL语句Code Chain
文章开头我们是用sqlSession.selectOne()方法来获取SQL语句的，那么这个SQL语句是怎么解析并获取呢？上面我们分析获取数据源时提到XMLConfigBuilder.parseConfiguration方法时看到会调用mapperElement方法。
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
if ("package".equals(child.getName())) {
String mapperPackage = child.getStringAttribute("name");
configuration.addMappers(mapperPackage);
} else {
String resource = child.getStringAttribute("resource");
String url = child.getStringAttribute("url");
String mapperClass = child.getStringAttribute("class");
if (resource != null && url == null && mapperClass == null) {
ErrorContext.instance().resource(resource);
try(InputStream inputStream = Resources.getResourceAsStream(resource)) {
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
mapperParser.parse();
}
} else if (resource == null && url != null && mapperClass == null) {
ErrorContext.instance().resource(url);
try(InputStream inputStream = Resources.getUrlAsStream(url)){
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
mapperParser.parse();
}
} else if (resource == null && url == null && mapperClass != null) {
Class<?> mapperInterface = Resources.classForName(mapperClass);
configuration.addMapper(mapperInterface);
} else {
throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
}
}
}
}
}
该方法会根据配置文件依次解析package, resource, url, class4种配置mapper的方式。
<mappers>
<mapper resource="mapper/StudentMapper.xml"/>
</mappers>
这里我们是用resource方式配置的，所以这个方法程序会在Line 12 的逻辑开始执行。
XMLStatementBuilder
解析SQL语句的关键逻辑都在这个类的parseStatementNode方法里，方法最后会调用MapperBuilderAssistant#.addMappedStatement(), 这个方法会很到MyBatis里很重要的一个类MappedStatement对象，这个方法最关键的一行代码是先生成MappedStatement对象，最后会把生成的MappedStatement对象放入Configuration对象的Map字典中。
看到这个方法里的代码应该很直观，就是在解析XML中select标签的一些属性，比如useCache, flushCache等，这些标签可以查看MyBatis文档，真的很详细。
public void parseStatementNode() {
String id = context.getStringAttribute("id");
String databaseId = context.getStringAttribute("databaseId");
if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
return;
}
String nodeName = context.getNode().getNodeName();
SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
boolean useCache = context.getBooleanAttribute("useCache", isSelect);
boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
includeParser.applyIncludes(context.getNode());
String parameterType = context.getStringAttribute("parameterType");
Class<?> parameterTypeClass = resolveClass(parameterType);
String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);
// Parse selectKey after includes and remove them.
processSelectKeyNodes(id, parameterTypeClass, langDriver);
// Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
KeyGenerator keyGenerator;
String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
if (configuration.hasKeyGenerator(keyStatementId)) {
keyGenerator = configuration.getKeyGenerator(keyStatementId);
} else {
keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
}
SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
Integer fetchSize = context.getIntAttribute("fetchSize");
Integer timeout = context.getIntAttribute("timeout");
String parameterMap = context.getStringAttribute("parameterMap");
String resultType = context.getStringAttribute("resultType");
Class<?> resultTypeClass = resolveClass(resultType);
String resultMap = context.getStringAttribute("resultMap");
String resultSetType = context.getStringAttribute("resultSetType");
ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
if (resultSetTypeEnum == null) {
resultSetTypeEnum = configuration.getDefaultResultSetType();
}
String keyProperty = context.getStringAttribute("keyProperty");
String keyColumn = context.getStringAttribute("keyColumn");
String resultSets = context.getStringAttribute("resultSets");
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}
MapperBuilderAssistant
public MappedStatement addMappedStatement(
String id,
SqlSource sqlSource,
StatementType statementType,
SqlCommandType sqlCommandType,
Integer fetchSize,
Integer timeout,
String parameterMap,
Class<?> parameterType,
String resultMap,
Class<?> resultType,
ResultSetType resultSetType,
boolean flushCache,
boolean useCache,
boolean resultOrdered,
KeyGenerator keyGenerator,
String keyProperty,
String keyColumn,
String databaseId,
LanguageDriver lang,
String resultSets) {
if (unresolvedCacheRef) {
throw new IncompleteElementException("Cache-ref not yet resolved");
}
id = applyCurrentNamespace(id, false);
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
.resource(resource)
.fetchSize(fetchSize)
.timeout(timeout)
.statementType(statementType)
.keyGenerator(keyGenerator)
.keyProperty(keyProperty)
.keyColumn(keyColumn)
.databaseId(databaseId)
.lang(lang)
.resultOrdered(resultOrdered)
.resultSets(resultSets)
.resultMaps(getStatementResultMaps(resultMap, resultType, id))
.resultSetType(resultSetType)
.flushCacheRequired(valueOrDefault(flushCache, !isSelect))
.useCache(valueOrDefault(useCache, isSelect))
.cache(currentCache);
ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
if (statementParameterMap != null) {
statementBuilder.parameterMap(statementParameterMap);
}
MappedStatement statement = statementBuilder.build();
configuration.addMappedStatement(statement);
return statement;
}
MyBatis是如何操作数据库的
执行器(Executor)
执行器在MyBatis中是用来封装Statement执行JDBC操作，MyBatis共包含3种类型的执行器：SimpleExecutor, ReuseExecutor, BatchExecutor，默认值使用SimpleExecutor。
@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
MySQL查询的元数据和实体建立映射关系
那么我们的select语句查询出来的一条mysql元数据和java实体到底是怎样建立映射关系的？ 我们先从PreparedStatementHandler#query方法着手分析。
@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
//这两行代码是不是很亲切了，在JDBC中我们就是这么干的。
//1.执行SQL
PreparedStatement ps = (PreparedStatement) statement;
//2.处理结果集
ps.execute();
return resultSetHandler.handleResultSets(ps);
}
结果集映射
DefaultResultSetHandler#handleResultSets
@Override
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
//1.<select>标签的resultMap属性，用来装映射后的实体对象数据
final List<Object> multipleResults = new ArrayList<>();
int resultSetCount = 0;
//2.获取第一个结果集数据
ResultSetWrapper rsw = getFirstResultSet(stmt);
//3.这里就是取出需要映射的ResultMap
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
//4.这里就是需要映射的ResultMap的数量
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);
//5.循环处理每个ResultMap
while (rsw != null && resultMapCount > resultSetCount) {
//6.循环取出需要映射的ResultMap（id,type）实体名称
ResultMap resultMap = resultMaps.get(resultSetCount);
//7.从rsw结果集参数中获取查询结果，再根据resultMap映射信息，将查询结果装到到multipleResults中
handleResultSet(rsw, resultMap, multipleResults, null);
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
//这里会获取select标签的resultSet属性
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);
}
DefaultResultSetHandler#handleResultSet
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) {
//1.创建DefaultResultHandler 来处理结果集
DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
//2.这里就是处理映射结果集的最终方法
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
multipleResults.add(defaultResultHandler.getResultList());
} else {
handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
}
}
} finally {
closeResultSet(rsw.getResultSet());
}
}
DefaultResultSetHandler#handleRowValues
public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
if (resultMap.hasNestedResultMaps()) {
ensureNoRowBounds();
checkResultHandler();
//嵌套结果映射
handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
} else {
//简单结果映射
handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
}
}
ResultSetWrapper
这里还有一个关键的类在上述方法调用链里没有提到就是ResultSetWrapper， 它的构造器里有3个集合分别是columnNames，jdbcTypes， classNames， 当你看到这3个数组时脑海里是不是浮现出MyBatis框架的意图呢，代替你管理JDBC，并从结果集建立到java对象的映射。
public ResultSetWrapper(ResultSet rs, Configuration configuration) throws SQLException {
super();
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.resultSet = rs;
final ResultSetMetaData metaData = rs.getMetaData();
final int columnCount = metaData.getColumnCount();
for (int i = 1; i <= columnCount; i++) {
columnNames.add(configuration.isUseColumnLabel() ? metaData.getColumnLabel(i) : metaData.getColumnName(i));
jdbcTypes.add(JdbcType.forCode(metaData.getColumnType(i)));
classNames.add(metaData.getColumnClassName(i));
}
}
DefaultResultSetHandler#handleRowValuesForSimpleResultMap
private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
throws SQLException {
DefaultResultContext<Object> resultContext = new DefaultResultContext<>();
//1.因为在创建ResultSetWrapper的时候 我们将结果集封装进去了 现在将结果集取出来
ResultSet resultSet = rsw.getResultSet();
//2.分页信息
skipRows(resultSet, rowBounds);
while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {
ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);
///3.将查询结果封装到POJO中(这一行代码重点关注)
Object rowValue = getRowValue(rsw, discriminatedResultMap, null);
storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);
}
}
DefaultResultSetHandler#getRowValue
private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
final ResultLoaderMap lazyLoader = new ResultLoaderMap();
Object rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
final MetaObject metaObject = configuration.newMetaObject(rowValue);
boolean foundValues = this.useConstructorMappings;
if (shouldApplyAutomaticMappings(resultMap, false)) {
//封装结果集 将sql的结果与实体类封装对应起来
foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
}
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
foundValues = lazyLoader.size() > 0 || foundValues;
rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
}
return rowValue;
}
DefaultResultSetHandler#applyAutomaticMappings
这个方法里会循环处理行列字段并映射实体属性。
private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
//1.封装你的返回结果集对象
List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
boolean foundValues = false;
if (!autoMapping.isEmpty()) {
//2.遍历 给实体类对象赋值
for (UnMappedColumnAutoMapping mapping : autoMapping) {
//3.根据实体属性 去sql中取值，拿到SQL的结果值和实体类的属性值，并封装在metaObject对象中。
final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
if (value != null) {
foundValues = true;
}
if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {
metaObject.setValue(mapping.property, value);
}
}
}
//返回值为boolean值
return foundValues;
}
这里autoMapping集合里能看到就是存放三个字段的元数据。OK，执行到这一步，外层的getRowValue的返回值就拿到了返回的实体对象。
