在阅读这篇文章之前，建议先阅读一下我之前写的两篇文章，对理解这篇文章很有帮助，特别是Mybatis新手：

写给mybatis小白的入门指南

mybatis底层原理学习（一）：SqlSessionFactory和SqlSession的创建过程

如果你想获得更好的阅读体验，可以点击这里:Mybatis底层原理学习（二）：从源码角度分析一次查询操作过程

（1）在使用Mybatis操作数据库的时候，每一次的CRUD操作都会去获取一次映射配置文件（mapper xml文件）对应的sql映射。每一个sql映射在内存缓存中（创建SqlSessionFactory之前就缓存在内存中了）都会有唯一ID，就是sql映射所在xml文件的命名空间加上sql映射配置节点的id值。
（2）Mapper xml文件的命名空间使用的是类的全路径名，这样做的好处是可以全局唯一，又可以通过反射获取对应的Mapper类。可以理解成每一个mapper xml文件对应一个Mapper类。
（3）mapper xml文件每一个sql映射节点的id属性值对应类的一个方法。我们在配置sql映射的时候也必须这样做，因为Mybatis的底层就是使用反射机制来获取执行方法的全路径作为ID来获取sql的映射配置的。
（4）每一个和mapper xml文件关联的类，都是Mapper类，在执行过程，通过动态代理，执行对应的方法。Mybatis是如何判断哪些类是Mapper类的呢？其实只有在运行时才会知道。在加载Mybatis配置文件中，通过解析mapper xml文件缓存了所有的sql映射配置，在调用SqlSession的getMapper方法获取Mapper类的时候才会生成代理类。

现在，我们来从源码角度分析Mapper代理类的创建过程，demo源码在后面给出 demo示例：

public class Main {private static final Logger LOGGER = LoggerFactory.getLogger(Main.class);public static void main(String[] args) {SqlSession sqlSession = MyBatisUtil.getSqlSession();ArticleMapper mapper = sqlSession.getMapper(ArticleMapper.class);Article article = mapper.selectOne(1);LOGGER.info("title:" + article.getTitle() + " " + "content:" + article.getContent());}}复制代码

我们在这行代码处搭上断点：

ArticleMapper mapper = sqlSession.getMapper(ArticleMapper.class);
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Debug进去，执行下面代码：

public <T> T getMapper(Class<T> type) {return configuration.<T>getMapper(type, this);}
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configuration持有Mybatis的基本配置信息，继续看看getMapper方法的执行：

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {return mapperRegistry.getMapper(type, sqlSession);}
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mapperRegistry缓存了所有的SQL映射配置信息，在加载解析Mybatis配置文件（例子是mybatis）和mapper xml文件的时候完成缓存的，继续看getMapper的执行：

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {// 这里首先会获取Mapper代理类工厂，拿到代理工厂就创建代理类final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);if (mapperProxyFactory == null) {throw new BindingException("Type " + type + " is not known to the MapperRegistry.");}try {// 创建Mapper代理类return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}}
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通过动态代理机制创建Mapper代理类

protected T newInstance(MapperProxy<T> mapperProxy) {return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);}
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到这里，动态代理类创建完成。 通过分析了源码执行过程，Mapper代理类的创建过程弄清楚了，大体就是通过从缓存中获取sql映射配置的id（类全路径名+方法名）来通过动态代理机制创建代理类，实际执行的CRUD是执行动态代理类的方法。 执行CRUD操作的时候，我们都会执行到动态代理类的invoke方法：

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);}
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最后找到映射的方法，执行mapperMethod.execute(sqlSession, args)。 通过代码我们可以看到，会根据执行方法的操作类型（CRUD）执行不同的逻辑处理。

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);}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;}
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我们分析一下查询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);}
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首先根据方法返回类型的不同执行不同的逻辑，最终会调用SqlSession的selectXXX方法，

public <T> T selectOne(String statement, Object parameter) {// Popular vote was to return null on 0 results and throw exception on too many.List<T> list = this.<T>selectList(statement, parameter);if (list.size() == 1) {return list.get(0);} else if (list.size() > 1) {throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());} else {return null;}}
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List<T> list = this.<T>selectList(statement, parameter);这行代码逻辑处理：

public <E> List<E> selectList(String statement, Object parameter) {return this.selectList(statement, parameter, RowBounds.DEFAULT);}
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继续进去：

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {MappedStatement ms = configuration.getMappedStatement(statement);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();}}
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到这一步，是调用执行器Executor的query方法：

 public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {BoundSql boundSql = ms.getBoundSql(parameterObject);CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}复制代码

进去query方法：

 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);}
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继续进去query方法：

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++;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 #601deferredLoads.clear();if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache();}}return list;}
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真正访问数据库的是这行代码：list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);

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;}
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查询操作由doQuery方法处理，这段代码就接近原生JDBC操作了，首先会获取语句处理器，然后开始执行语句，执行完，还会对结果进行结果集处理，返回处理的结果集，这里就不多分析了

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.<E>query(stmt, resultHandler);} finally {closeStatement(stmt);}}
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我们在使用Mybatis进行CRUD操作的时候，大体过程是这样：

• 解析基本配置文件和Sql映射配置文件(mapper xml)文件，缓存配置文件节点内容在内存（一般此步骤只会执行一次，多次调用都会复用缓存结果）
• 获取SqlSession，通过SqlSession来获取Mapper类，生成Mapper类的代理
• 执行CRUED操作

当然，这个过程Mybatis还做了很多事情，Sql的解析，结果集的处理……等操作我们在这篇文章不分析，后面会有文章分析。这篇文章目的是分析Mapper代理类的创建过程和简单分析一个查询操作的过程。

源码地址

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