MyBatis中一个SQL语句的执行过程解析
MyBatis 是一款优秀的持久层框架，它支持自定义 SQL、存储过程以及高级映射。MyBatis 免除了几乎所有的 JDBC 代码以及设置参数和获取结果集的工作。MyBatis 可以通过简单的 XML 或注解来配置和映射原始类型、接口和 Java POJO（Plain Old Java Objects，普通老式 Java 对象）为数据库中的记录。
平时用MyBatis框架开发时，配置好config.xml和mapper.xml映射文件和定义好java接口，就可以操作数据库了，
当然也可以像spring一样基于注解配置，看个人情况。
mybatis配置文件
mybatis mapper映射文件（由于模块众多，mapper映射文件很多，只列举一个）
然后只需定义好java接口，就能操作数据库了
public interface MenuDao {
//要和mapper.xml一一对应上
public List<Menu> findByParentIdsLike(Menu menu);
public List<Menu> findByUserId(Menu menu);
public int updateParentIds(Menu menu);
public int updateSort(Menu menu);
}测试代码：
/**
*
* @author dgm
* 探究mybatis源码
*/
public class MyBatisTest {
public static void main(String[] args) {
// TODO Auto-generated method stub
try {
System.out.println("开始mybatis实验");
UserDao userDao;
SqlSession sqlSession;
String resource = "conf/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
.build(inputStream);
sqlSession = sqlSessionFactory.openSession();
userDao = sqlSession.getMapper(UserDao.class);//.selectUserById(1);
System.out.println("其实是代理对象："+userDao+",类型："+userDao.getClass());
User user = userDao.selectUserById(1);
//userDao = new MybatisDaoImpl(sqlSession);
//user = userDao.selectUserById(2);
System.out.println(user);
System.out.println(userDao.getClass().getName());
user.setUsername("dongguangming");
//sqlSession.commit();
sqlSession.close();
System.out.println("结束mybatis实验");
} catch (IOException e) {
e.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
}
}
}执行结果：
实在简单，我就简单介绍下mybatis怎么一步一步运行起来的，下面才是重点
1. xml文件解析阶段，怎么把xml文件解析成Configuration对象（很重要的一个全局性对象）
1.1 MyBatis Config xml配置文件
它是一种符合DTD约束的一种配置文件，可以详细看它如何规范的http://mybatis.org/dtd/mybatis-3-config.dtd，
<!ELEMENT configuration (properties?, settings?, typeAliases?, typeHandlers?, objectFactory?, objectWrapperFactory?, reflectorFactory?, plugins?, environments?, databaseIdProvider?, mappers?)>具体表现可在config.xml配置文件中查看其组成部分 https://mybatis.org/mybatis-3/zh/configuration.html，现在流行注解，不知道能不能很好的表现这种结构，dom树结构
Mybatis要把该xml文件解析成一个全局性的Configurationl类型的java对象，它的类定义诸多属性，该文件在org.apache.ibatis.session包下
public class Configuration {
protected Environment environment;
protected boolean safeRowBoundsEnabled;
protected boolean safeResultHandlerEnabled = true;
protected boolean mapUnderscoreToCamelCase;
protected boolean aggressiveLazyLoading;
protected boolean multipleResultSetsEnabled = true;
protected boolean useGeneratedKeys;
protected boolean useColumnLabel = true;
protected boolean cacheEnabled = true;
protected boolean callSettersOnNulls;
protected boolean useActualParamName = true;
protected boolean returnInstanceForEmptyRow;
protected String logPrefix;
protected Class<? extends Log> logImpl;
protected Class<? extends VFS> vfsImpl;
protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
protected Set<String> lazyLoadTriggerMethods = new HashSet<>(Arrays.asList("equals", "clone", "hashCode", "toString"));
protected Integer defaultStatementTimeout;
protected Integer defaultFetchSize;
protected ResultSetType defaultResultSetType;
protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;
protected AutoMappingUnknownColumnBehavior autoMappingUnknownColumnBehavior = AutoMappingUnknownColumnBehavior.NONE;
protected Properties variables = new Properties();
protected ReflectorFactory reflectorFactory = new DefaultReflectorFactory();
protected ObjectFactory objectFactory = new DefaultObjectFactory();
protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();
protected boolean lazyLoadingEnabled = false;
protected ProxyFactory proxyFactory = new JavassistProxyFactory(); // #224 Using internal Javassist instead of OGNL
protected String databaseId;
/**
* Configuration factory class.
* Used to create Configuration for loading deserialized unread properties.
*
* @see <a href='https://code.google.com/p/mybatis/issues/detail?id=300'>Issue 300 (google code)</a>
*/
protected Class<?> configurationFactory;
protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
protected final InterceptorChain interceptorChain = new InterceptorChain();
protected final TypeHandlerRegistry typeHandlerRegistry = new TypeHandlerRegistry(this);
protected final TypeAliasRegistry typeAliasRegistry = new TypeAliasRegistry();
protected final LanguageDriverRegistry languageRegistry = new LanguageDriverRegistry();
protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")
.conflictMessageProducer((savedValue, targetValue) ->
". please check " + savedValue.getResource() + " and " + targetValue.getResource());
protected final Map<String, Cache> caches = new StrictMap<>("Caches collection");
protected final Map<String, ResultMap> resultMaps = new StrictMap<>("Result Maps collection");
protected final Map<String, ParameterMap> parameterMaps = new StrictMap<>("Parameter Maps collection");
protected final Map<String, KeyGenerator> keyGenerators = new StrictMap<>("Key Generators collection");
protected final Set<String> loadedResources = new HashSet<>();
protected final Map<String, XNode> sqlFragments = new StrictMap<>("XML fragments parsed from previous mappers");
。。。。。。下面开始对配置文件进行处理，希望你有java和xml数据绑定的开发经历，可以参考Java and XML Data Binding.pdf https://github.com/dongguangming/java/blob/master/O'Reilly%20-%20Java%20and%20XML%20Data%20Binding.pdf
String resource = "conf/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
.build(inputStream);首先，我们使用 MyBatis 自带的工具类 Resources 读取加载配置文件，得到一个输入流。
然后再通过 SqlSessionFactoryBuilder 对象的build方法构建 SqlSessionFactory 对象。
/**
* 通过Configuration对象创建SqlSessionFactory
* 然后调用该方法创建SqlSessionFactory
*/
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
} 从图片上看到MyBatis 配置文件是通过XMLConfigBuilder进行parse（）解析的，继续一层一层往下看
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//调用解析方法，还记得xml文档的根节点configuration吗,直接子元素11个级别
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}接着调用parseConfiguration（），刚好11个直接子节点，一个个分别解析处理
这样，一个 MyBatis 的解析过程就出来了，每个配置的解析逻辑都封装在了相应的方法中。由于解析模块众多，我就选几个了，要不然写不完
1.1.1 解析 properties 配置
mysql.properties属性文件，内容如下
#MySQL for mybatis
mybatis.driver=com.mysql.jdbc.Driver
mybatis.url=jdbc:mysql://192.168.8.200:3306/demo?useUnicode=true&characterEncoding=utf8&autoReconnect=true
mybatis.username=root
mybatis.password=123456<properties resource="conf/mysql.properties" />
解析properties节点是由propertiesElement这个方法完成的，在上面的配置中， properties 节点配置了一个 resource 属性。下面我们参照上面的配置，来分析一下 propertiesElement 的逻辑。
// -☆- XMLConfigBuilder
private void propertiesElement(XNode context) throws Exception {
if (context != null) {
// 解析 propertis 的子节点，并将这些节点内容转换为属性对象 Properties
Properties defaults = context.getChildrenAsProperties();
// 获取 propertis 节点中的 resource 和 url 属性值
String resource = context.getStringAttribute("resource");
String url = context.getStringAttribute("url");
// 两者都不用空，则抛出异常
if (resource != null && url != null) {
throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference. Please specify one or the other.");
}
if (resource != null) {
// 从文件系统中加载并解析属性文件
defaults.putAll(Resources.getResourceAsProperties(resource));
} else if (url != null) {
// 通过 url 加载并解析属性文件
defaults.putAll(Resources.getUrlAsProperties(url));
}
Properties vars = configuration.getVariables();
if (vars != null) {
defaults.putAll(vars);
}
parser.setVariables(defaults);
// 将属性值设置到 configuration 中
configuration.setVariables(defaults);
}
}
public Properties getChildrenAsProperties() {
Properties properties = new Properties();
// 获取并遍历子节点
for (XNode child : getChildren()) {
// 获取 property 节点的 name 和 value 属性
String name = child.getStringAttribute("name");
String value = child.getStringAttribute("value");
if (name != null && value != null) {
// 设置属性到属性对象中
properties.setProperty(name, value);
}
}
return properties;
}
// -☆- XNode
public List<XNode> getChildren() {
List<XNode> children = new ArrayList<XNode>();
// 获取子节点列表
NodeList nodeList = node.getChildNodes();
if (nodeList != null) {
for (int i = 0, n = nodeList.getLength(); i < n; i++) {
Node node = nodeList.item(i);
if (node.getNodeType() == Node.ELEMENT_NODE) {
// 将节点对象封装到 XNode 中，并将 XNode 对象放入 children 列表中
children.add(new XNode(xpathParser, node, variables));
}
}
}
return children;
}properties 节点解析的主要过程主要包含三个步骤，一是解析 properties 节点的子节点，并将解析结果设置到 Properties 对象中。二是从文件系统或通过网络读取属性配置，这取决于 properties 节点的 resource 和 url 是否为空。最后一步则是将解析出的属性对象设置到 XPathParser 和 Configuration 对象中。
需要注意的是，propertiesElement 方法是先解析 properties 节点的子节点内容，后再从文件系统或者网络读取属性配置，并将所有的属性及属性值都放入到 defaults 属性对象中。这就会存在同名属性覆盖的问题，也就是从文件系统，或者网络上读取到的属性及属性值会覆盖掉 properties 子节点中同名的属性和及值。
1.1.2 解析 settings 配置
1.1.2.1 settings 节点的解析过程
settings 配置是 MyBatis 中非常重要的配置，这些配置用于调整 MyBatis 运行时的行为。settings 配置繁多，在对这些配置不熟悉的情况下，保持默认配置即可。关于 settings 相关配置，MyBatis 官网上进行了比较详细的描述，https://mybatis.org/mybatis-3/zh/configuration.html#settings,我就以我的配置举例
<settings>
<setting name="cacheEnabled" value="true"/>
<setting name="lazyLoadingEnabled" value="true"/>
</settings>接下来，对照上面的配置，来分析源码。如下：
// -☆- XMLConfigBuilder
private Properties settingsAsProperties(XNode context) {
if (context == null) {
return new Properties();
}
// 获取 settings 子节点中的内容，getChildrenAsProperties 方法前面已分析过，这里不再赘述
Properties props = context.getChildrenAsProperties();
// 创建 Configuration 类的“元信息”对象
MetaClass metaConfig = MetaClass.forClass(Configuration.class, localReflectorFactory);
for (Object key : props.keySet()) {
// 检测 Configuration 中是否存在相关属性，不存在则抛出异常
if (!metaConfig.hasSetter(String.valueOf(key))) {
throw new BuilderException("The setting " + key + " is not known. Make sure you spelled it correctly (case sensitive).");
}
}
return props;
}注意由于节点多，导致xml dom解析逻辑判断也多，就不一一举例细节了，但逻辑不是技术，只需要知道会生成一个Configurationl类型的java对象即可。
2. SQL语句的执行流程
再次贴下代码
SqlSession sqlSession;
String resource = "conf/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
.build(inputStream);
sqlSession = sqlSessionFactory.openSession();
MybatisDao ud= sqlSession.getMapper(MybatisDao.class);
System.out.println("其实是代理对象："+ud+",类型："+ud.getClass());
User user = ud.selectUserById(1);
System.out.println(user);
MybatisDao ud= sqlSession.getMapper(MybatisDao.class);会返回一个代理对象，继续追踪sqlSession.getMapper(MybatisDao.class)是如何实现的，
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
。。。
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}而knowMappers实际上存放的是
knownMappers.put(type, new MapperProxyFactory<>(type));MapperProxyFactory定义如下
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();
public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class<T> getMapperInterface() {
return mapperInterface;
}
public Map<Method, MapperMethod> getMethodCache() {
return methodCache;
}
//
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}
MapperProxy定义如下，实现了InvocationHandler接口
public class MapperProxy<T> implements InvocationHandler, Serializable {
private static final long serialVersionUID = -6424540398559729838L;
private final SqlSession sqlSession;
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache;
public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
然后通过调用mapperProxyFactory.newInstance(sqlSession)返回代理对象
这下明白很多文章说为啥说mybatis只定义接口就能调用方法了。
此时控制台输出
紧接着就是调用接口方法（注意是代理对象调用方法）：User user = ud.selectUserById(1);
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}怎么拿方法不一一细看，又是一大堆逻辑，这里只看最后一句执行命令：mapperMethod.execute(sqlSession, args);
具体代码如下
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional()
&& (result == null || !method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}此例子中其实会执行select
result = sqlSession.selectOne(command.getName(), param);session是DefaultSqlSession类型的，因为sqlSessionFactory默认生成的SqlSession是DefaultSqlSession类型。selectOne()会调用selectList()。
// DefaultSqlSession类
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
// CURD操作是交给Excetor去处理的
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}如图
在DefaultSqlSession.selectList中的各种CURD操作都是通多Executor进行的，这里executor的类型是CachingExecutor，接着跳转到其中的query方法中。
// CachingExecutor 类
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject); // 获取绑定的sql命令，比如"SELECT * FROM xxx"
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}getBoundSql为了获取绑定的sql命令，在创建完cacheKey之后，就进入到CachingExecutor 类中的另一个query方法中。
// CachingExecutor 类
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
Cache cache = ms.getCache();
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, parameterObject, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}这里真正执行query操作的是SimplyExecutor代理来完成的，接着就进入到了SimplyExecutor的父类BaseExecutor的query方法中。
// SimplyExecutor的父类BaseExecutor类
@SuppressWarnings("unchecked")
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
if (closed) {
throw new ExecutorException("Executor was closed.");
}
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
queryStack++;
/**
* localCache是一级缓存，如果找不到就调用queryFromDatabase从数据库中查找
*/
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}此时可以断定是第一次SQL查询操作，
所以会调用queryFromDatabase方法来执行查询。
// SimplyExecutor的父类BaseExecutor类
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}从数据库中查询数据，调用doQuery方法，进入到SimplyExecutor中进行操作。
// SimplyExecutor类
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);
// 子流程1：SQL查询参数的设置
stmt = prepareStatement(handler, ms.getStatementLog());
// StatementHandler封装了Statement
// 子流程2：SQL查询操作和结果集的封装
return handler.<E>query(stmt);
} finally {
closeStatement(stmt);
}
}特别注意，在prepareStatement方法中会进行SQL查询参数的设置，也就是咱们最开始传递进来的参数，其值为1。handler.<E>query(stmt)方法中会进行实际的SQL查询操作和结果集的封装（封装成Java对象）。
prepareStatement方法阶段（即设置SQL查询参数）：
// 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）。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);
}
}
}
}
}此时已经给Statement设置了最初传递进去的参数。
那么接着分析流程2：
handler.<E>query(stmt)方法阶段（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);
}
}
// 封装数据，DefaultResultSetHandler类
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。
至此，分析完毕。和spring有点类似，只是方向不一样，都是大量的dom解析（现在是java注解比较多了）成全局java文件，然后结合逻辑编码实现相应的功能。
但记住：逻辑往往不是技术，如何构思、组装才是重点！！！
注意： 你如果只是想使用mybatis（写下配置文件和mapper文件）就不需要看此文了，也没什么必要！
课题：留给你们一个分页插件，构思设想和编码如何实现？？？
后记：
请你们务必灵活运用这些器：分发器，过滤器，拦截器，监听器，反应堆器，特别注意这跟语言、框架无关。
当然还有操作系统相关知识，希望其他人早意识到cpu、内存分配、io模型、进程/线程等是很重要的(会让你更好的理解一些库比如libevent和中间件的实现原理)，也跟语言、库、框架无关，而不管你用java、scala还是golang实现。
因为很少有网课关于操作系统和网络的系统化培训，那还是让我董广明告诉要学什么，如下
可能市面上做crud的开发者居多，那数据库要留意下。
已把mybatis电子书上传，很简单 https://github.com/dongguangming/java/blob/master/MyBatis/Java%20Persistence%20with%20MyBatis%203(%E4%B8%AD%E6%96%87%E7%89%88).pdf
参考：
0. MyBatis事务 https://blog.csdn.net/dong19891210/article/details/105672535
-
SpringBoot : Working with MyBatis https://www.sivalabs.in/2016/03/springboot-working-with-mybatis/
-
mybatis配置 https://mybatis.org/mybatis-3/zh/configuration.html
-
Mybatis source code analysis https://developpaper.com/mybatis-source-code-analysis/
-
Mybatis Source Code Analysis https://programming.vip/docs/mybatis-source-code-analysis.html
-
MyBatis source code analysis https://programmer.group/mybatis-source-code-analysis.html
