【Mybatis中Mapper的实现原理解析】书到用时方恨少，事非经过不知难。这篇文章主要讲述Mybatis中Mapper的实现原理解析相关的知识，希望能为你提供帮助。
1.Mybatis的架构 1.1 Mybatis的框架分层

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1.2 MyBatis的实现原理
mybatis底层还是采用原生jdbc来对数据库进行操作的，只是通过 SqlSessionFactory，SqlSession Executor,StatementHandler，ParameterHandler,ResultHandler和TypeHandler等几个处理器封装了这些过程

执行器：Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed) 参数处理器： ParameterHandler (getParameterObject, setParameters) 结构处理器 ResultSetHandler (handleResultSets, handleOutputParameters) sql查询处理器：StatementHandler (prepare, parameterize, batch, update, query)

其中StatementHandler用通过ParameterHandler与ResultHandler分别进行参数预编译 与结果处理。而ParameterHandler与ResultHandler都使用TypeHandler进行映射。如下图： 

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2.Mybatis工作过程通过读mybatis的源码进行分析mybatis的执行操作的整个过程，我们通过debug调试就可以知道Mybatis每一步做了什么事，我先把debug每一步结果 截图，然后在分析这个流程。 
第一步:读取配置文件，形成InputStream
2.1 创建SqlSessionFacotry的过程

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从debug调试看出 返回的 sqlSessionFactory 是DefaultSesssionFactory类型的，但是configuration此时已经被初始化了。查看源码后画如下创建DefaultSessionFactory的时序图： 

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2.2 创建SqlSession的过程

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从debug调试 看出SqlSessinoFactory.openSession() 返回的sqlSession是 DefaultSession类型的，此SqlSession里包含一个Configuration的对象，和一个Executor对象。查看源码后画如下创建DefaultSession的时序图：

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2.3 创建Mapper的过程

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从debug调试可以看出，mapper是一个Mapper代理对象，而且初始化了Configuration对象，Executor的对象。查看源码后画如下创建Mapper的时序图： 

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2.4 执行CRUD过程
2.4.1 以select为例查看各步执行的源码1.mapper.selectEmployeeList()其实是MapperProxy执行invoke方法，此方法显示是判断Method的方法是不是Object的toString等方法如果不是就执行MapperMethod

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { // 判断Method的方法是不是Object的toString等方法 if(Object.class.equals(method.getDeclaringClass())) { try { return method.invoke(this, args); } catch (Throwable var5) { throw ExceptionUtil.unwrapThrowable(var5); } } else { //判断private final Map< Method, MapperMethod> methodCache; 这个map里面有没有这个方法的一级缓存，如果没 MapperMethod mapperMethod = this.cachedMapperMethod(method); return mapperMethod.execute(this.sqlSession, args); } } //查询一级缓存和设置一级缓存private MapperMethod cachedMapperMethod(Method method) { MapperMethod mapperMethod = (MapperMethod)this.methodCache.get(method); if(mapperMethod == null) { mapperMethod = new MapperMethod(this.mapperInterface, method, this.sqlSession.getConfiguration()); this.methodCache.put(method, mapperMethod); }return mapperMethod; }

经过上面的调用后进入MapperMethod里面执行

//判断sql命令类型 public Object execute(SqlSession sqlSession, Object[] args) { Object param; Object result; if(SqlCommandType.INSERT == this.command.getType()) { param = this.method.convertArgsToSqlCommandParam(args); result = this.rowCountResult(sqlSession.insert(this.command.getName(), param)); } else if(SqlCommandType.UPDATE == this.command.getType()) { param = this.method.convertArgsToSqlCommandParam(args); result = this.rowCountResult(sqlSession.update(this.command.getName(), param)); } else if(SqlCommandType.DELETE == this.command.getType()) { param = this.method.convertArgsToSqlCommandParam(args); result = this.rowCountResult(sqlSession.delete(this.command.getName(), param)); } else if(SqlCommandType.SELECT == this.command.getType()) { //我们测试的是select类型，则再判断这个方法的返回类型 if(this.method.returnsVoid() & & this.method.hasResultHandler()) { this.executeWithResultHandler(sqlSession, args); result = null; } else if(this.method.returnsMany()) { //我们是查询列表，此方法执行 result = this.executeForMany(sqlSession, args); } else if(this.method.returnsMap()) { result = this.executeForMap(sqlSession, args); } else { param = this.method.convertArgsToSqlCommandParam(args); result = sqlSession.selectOne(this.command.getName(), param); } } else { if(SqlCommandType.FLUSH != this.command.getType()) { throw new BindingException("Unknown execution method for: " + this.command.getName()); }result = sqlSession.flushStatements(); }if(result == null & & this.method.getReturnType().isPrimitive() & & !this.method.returnsVoid()) { throw new BindingException("Mapper method ‘" + this.command.getName() + " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ")."); } else { return result; } }private < E> Object executeForMany(SqlSession sqlSession, Object[] args) { //将param做处理 自动处理为param1,param2.. Object param = this.method.convertArgsToSqlCommandParam(args); List result; if(this.method.hasRowBounds()) { RowBounds rowBounds = this.method.extractRowBounds(args); //调用该对象的DefaultSqlSession的selectList方法 result = sqlSession.selectList(this.command.getName(), param, rowBounds); } else { result = sqlSession.selectList(this.command.getName(), param); }return !this.method.getReturnType().isAssignableFrom(result.getClass())?(this.method.getReturnType().isArray()?this.convertToArray(result):this.convertToDeclaredCollection(sqlSession.getConfiguration(), result)):result; }//处理参数方法 public Object convertArgsToSqlCommandParam(Object[] args) { int paramCount = this.params.size(); if(args != null & & paramCount != 0) { if(!this.hasNamedParameters & & paramCount == 1) { return args[((Integer)this.params.keySet().iterator().next()).intValue()]; } else { Map< String, Object> param = new MapperMethod.ParamMap(); int i = 0; for(Iterator i$ = this.params.entrySet().iterator(); i$.hasNext(); ++i) { Entry< Integer, String> entry = (Entry)i$.next(); param.put(entry.getValue(), args[((Integer)entry.getKey()).intValue()]); String genericParamName = "param" + String.valueOf(i + 1); if(!param.containsKey(genericParamName)) { param.put(genericParamName, args[((Integer)entry.getKey()).intValue()]); } }return param; } } else { return null; } }

调用DefaultSqlSession的selectList的方法

public < E> List< E> selectList(String statement, Object parameter, RowBounds rowBounds) { List var5; try { //获取MappedStatement对象 MappedStatement ms = this.configuration.getMappedStatement(statement); //调用cachingExecutor执行器的方法 var5 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER); } catch (Exception var9) { throw ExceptionFactory.wrapException("Error querying database.Cause: " + var9, var9); } finally { ErrorContext.instance().reset(); }return var5; }//CachingExector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 = this.createCacheKey(ms, parameterObject, rowBounds, boundSql); //调用下2代码 return this.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); } //2代码 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) { this.flushCacheIfRequired(ms); if(ms.isUseCache() & & resultHandler == null) { this.ensureNoOutParams(ms, parameterObject, boundSql); List< E> list = (List)this.tcm.getObject(cache, key); if(list == null) { //这里是调用Executor里的query方法 如果开启了缓存这掉CachingExecutor的 如果没有则是调用BaseExecutor的 list = this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); this.tcm.putObject(cache, key, list); }return list; } }return this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }

BaseExecutor的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(this.closed) { throw new ExecutorException("Executor was closed."); } else { if(this.queryStack == 0 & & ms.isFlushCacheRequired()) { this.clearLocalCache(); }List list; try { ++this.queryStack; list = resultHandler == null?(List)this.localCache.getObject(key):null; if(list != null) { this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else { //如果缓存中没有就从数据库中查询 list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally { --this.queryStack; }if(this.queryStack == 0) { Iterator i$ = this.deferredLoads.iterator(); while(i$.hasNext()) { BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)i$.next(); deferredLoad.load(); }this.deferredLoads.clear(); if(this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) { this.clearLocalCache(); } }return list; } }//从数据库中查询 private < E> List< E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { //放入缓存 this.localCache.putObject(key, ExecutionPlaceholder.EXECUTION_PLACEHOLDER); List list; try { //此处是调用子Executor的方法，ExecutorType默认是使用的SimpleExecutor list = this.doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally { this.localCache.removeObject(key); }this.localCache.putObject(key, list); if(ms.getStatementType() == StatementType.CALLABLE) { this.localOutputParameterCache.putObject(key, parameter); }return list; }

SimpleExecutor的doQuery方法

public < E> List< E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException { Statement stmt = null; List var9; try { Configuration configuration = ms.getConfiguration(); //创建StateMentHandler处理器 StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, resultHandler, boundSql); //调用下3的方法 stmt = this.prepareStatement(handler, ms.getStatementLog()); //调用no4的方法 var9 = handler.query(stmt, resultHandler); } finally { this.closeStatement(stmt); }return var9; } //下3方法 private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException { Connection connection = this.getConnection(statementLog); Statement stmt = handler.prepare(connection); //SatementHanlder 采用PreparedStatementHandler来实现此方法，而PreparedStatementHandler调用的是父接口ParameterHandler的方法 handler.parameterize(stmt); return stmt; }

ParameterHandler参数处理器的方法

public interface ParameterHandler { Object getParameterObject(); //此方法是用DefaultParameterHandler实现的 void setParameters(PreparedStatement var1) throws SQLException; }

DefaultParameterHandler默认参数处理器的方法

public void setParameters(PreparedStatement ps) { ErrorContext.instance().activity("setting parameters").object(this.mappedStatement.getParameterMap().getId()); List< ParameterMapping> parameterMappings = this.boundSql.getParameterMappings(); if(parameterMappings != null) { for(int i = 0; i < parameterMappings.size(); ++i) { ParameterMapping parameterMapping = (ParameterMapping)parameterMappings.get(i); if(parameterMapping.getMode() != ParameterMode.OUT) { String propertyName = parameterMapping.getProperty(); Object value; if(this.boundSql.hasAdditionalParameter(propertyName)) { value = https://www.songbingjia.com/android/this.boundSql.getAdditionalParameter(propertyName); } else if(this.parameterObject == null) { value = null; } else if(this.typeHandlerRegistry.hasTypeHandler(this.parameterObject.getClass())) { value = this.parameterObject; } else { MetaObject metaObject = this.configuration.newMetaObject(this.parameterObject); value = metaObject.getValue(propertyName); } //这里用调用 TypeHandler类型映射处理器来映射 TypeHandler typeHandler = parameterMapping.getTypeHandler(); JdbcType jdbcType = parameterMapping.getJdbcType(); if(value == null & & jdbcType == null) { jdbcType = this.configuration.getJdbcTypeForNull(); }try { //类型处理器设置参数映射 typeHandler.setParameter(ps, i + 1, value, jdbcType); } catch (TypeException var10) { throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var10, var10); } catch (SQLException var11) { throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var11, var11); } } } }}

no4的方法

public < E> List< E> query(Statement statement, ResultHandler resultHandler) throws SQLException { //此处调用原生sql的处理器 PreparedStatement ps = (PreparedStatement)statement; //发出原生sql命令 ps.execute(); //采用ResultHandler结果处理器对结果集封装 return this.resultSetHandler.handleResultSets(ps); }

ResultHandler代码

public interface ResultSetHandler { //此处调用的是DefaultResultSetHandler的方法 < E> List< E> handleResultSets(Statement var1) throws SQLException; void handleOutputParameters(CallableStatement var1) throws SQLException; }

DefaultResultSetHandler的方法

public List< Object> handleResultSets(Statement stmt) throws SQLException { ErrorContext.instance().activity("handling results").object(this.mappedStatement.getId()); List< Object> multipleResults = new ArrayList(); int resultSetCount = 0; ResultSetWrapper rsw = this.getFirstResultSet(stmt); List< ResultMap> resultMaps = this.mappedStatement.getResultMaps(); int resultMapCount = resultMaps.size(); this.validateResultMapsCount(rsw, resultMapCount); while(rsw != null & & resultMapCount > resultSetCount) { ResultMap resultMap = (ResultMap)resultMaps.get(resultSetCount); this.handleResultSet(rsw, resultMap, multipleResults, (ResultMapping)null); rsw = this.getNextResultSet(stmt); this.cleanUpAfterHandlingResultSet(); ++resultSetCount; }String[] resultSets = this.mappedStatement.getResulSets(); if(resultSets != null) { while(rsw != null & & resultSetCount < resultSets.length) { ResultMapping parentMapping = (ResultMapping)this.nextResultMaps.get(resultSets[resultSetCount]); if(parentMapping != null) { String nestedResultMapId = parentMapping.getNestedResultMapId(); ResultMap resultMap = this.configuration.getResultMap(nestedResultMapId); this.handleResultSet(rsw, resultMap, (List)null, parentMapping); }rsw = this.getNextResultSet(stmt); this.cleanUpAfterHandlingResultSet(); ++resultSetCount; } }return this.collapseSingleResultList(multipleResults); } //处理结果集 private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List< Object> multipleResults, ResultMapping parentMapping) throws SQLException { try { if(parentMapping != null) { this.handleRowValues(rsw, resultMap, (ResultHandler)null, RowBounds.DEFAULT, parentMapping); } else if(this.resultHandler == null) { DefaultResultHandler defaultResultHandler = new DefaultResultHandler(this.objectFactory); this.handleRowValues(rsw, resultMap, defaultResultHandler, this.rowBounds, (ResultMapping)null); multipleResults.add(defaultResultHandler.getResultList()); } else { this.handleRowValues(rsw, resultMap, this.resultHandler, this.rowBounds, (ResultMapping)null); } } finally { this.closeResultSet(rsw.getResultSet()); }}private void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler< ?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException { if(resultMap.hasNestedResultMaps()) { this.ensureNoRowBounds(); this.checkResultHandler(); this.handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping); } else { this.handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping); }}private void handleRowValuesForNestedResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler< ?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException { DefaultResultContext< Object> resultContext = new DefaultResultContext(); this.skipRows(rsw.getResultSet(), rowBounds); Object rowValue = https://www.songbingjia.com/android/null; while(this.shouldProcessMoreRows(resultContext, rowBounds) & & rsw.getResultSet().next()) { ResultMap discriminatedResultMap = this.resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, (String)null); CacheKey rowKey = this.createRowKey(discriminatedResultMap, rsw, (String)null); Object partialObject = this.nestedResultObjects.get(rowKey); if(this.mappedStatement.isResultOrdered()) { if(partialObject == null & & rowValue != null) { this.nestedResultObjects.clear(); this.storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet()); } //获取行的值 rowValue = this.getRowValue(rsw, discriminatedResultMap, rowKey, (String)null, partialObject); } else { rowValue = this.getRowValue(rsw, discriminatedResultMap, rowKey, (String)null, partialObject); if(partialObject == null) { this.storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet()); } } }if(rowValue != null & & this.mappedStatement.isResultOrdered() & & this.shouldProcessMoreRows(resultContext, rowBounds)) { this.storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet()); }} String resultMapId = resultMap.getId(); Object resultObject = partialObject; if(partialObject != null) { MetaObject metaObject = this.configuration.newMetaObject(partialObject); this.putAncestor(partialObject, resultMapId, columnPrefix); this.applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, false); this.ancestorObjects.remove(resultMapId); } else { ResultLoaderMap lazyLoader = new ResultLoaderMap(); resultObject = this.createResultObject(rsw, resultMap, lazyLoader, columnPrefix); if(resultObject != null & & !this.typeHandlerRegistry.hasTypeHandler(resultMap.getType())) { MetaObject metaObject = this.configuration.newMetaObject(resultObject); boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty(); if(this.shouldApplyAutomaticMappings(resultMap, true)) { foundValues = this.applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues; }foundValues = this.applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues; this.putAncestor(resultObject, resultMapId, columnPrefix); foundValues = this.applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, true) || foundValues; this.ancestorObjects.remove(resultMapId); foundValues = lazyLoader.size() > 0 || foundValues; resultObject = foundValues?resultObject:null; }if(combinedKey != CacheKey.NULL_CACHE_KEY) { this.nestedResultObjects.put(combinedKey, resultObject); } }return resultObject; }private boolean shouldApplyAutomaticMappings(ResultMap resultMap, boolean isNested) { return resultMap.getAutoMapping() != null?resultMap.getAutoMapping().booleanValue():(isNested?AutoMappingBehavior.FULL == this.configuration.getAutoMappingBehavior():AutoMappingBehavior.NONE != this.configuration.getAutoMappingBehavior()); } private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException { List< DefaultResultSetHandler.UnMappedColumAutoMapping> autoMapping = this.createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix); boolean foundValues = false; if(autoMapping.size() > 0) { Iterator i$ = autoMapping.iterator(); while(true) { //这里使用了内部类对参数和结果集进行映射 DefaultResultSetHandler.UnMappedColumAutoMapping mapping; Object value; do { if(!i$.hasNext()) { return foundValues; }mapping = (DefaultResultSetHandler.UnMappedColumAutoMapping)i$.next(); value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column); } while(value == null & & !this.configuration.isCallSettersOnNulls()); if(value != null || !mapping.primitive) { metaObject.setValue(mapping.property, value); }foundValues = true; } } else { return foundValues; } } private static class UnMappedColumAutoMapping { private final String column; private final String property; private final TypeHandler< ?> typeHandler; private final boolean primitive; //此处才类型器对结果进行映射 public UnMappedColumAutoMapping(String column, String property, TypeHandler< ?> typeHandler, boolean primitive) { this.column = column; this.property = property; this.typeHandler = typeHandler; this.primitive = primitive; } }

本文转自：https://blog.csdn.net/u014297148/article/details/78696096

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