1.项目结构

2. 源码分析

2.1 Mapper代理 MapperProxy#invoke

当调用BlogMapper.getById(id)时，由于BlogMapper时JDK代理的，会回调MapperProxy#invoke方法

@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, args);} else {return cachedInvoker(method).invoke(proxy, method, args, sqlSession);}} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}}

2.2 创建MapperMethod

public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {this.command = new SqlCommand(config, mapperInterface, method);this.method = new MethodSignature(config, mapperInterface, method);
}public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) {Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface);if (resolvedReturnType instanceof Class<?>) {this.returnType = (Class<?>) resolvedReturnType;} else if (resolvedReturnType instanceof ParameterizedType) {this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType();} else {this.returnType = method.getReturnType();}this.returnsVoid = void.class.equals(this.returnType);this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();this.returnsCursor = Cursor.class.equals(this.returnType);this.returnsOptional = Optional.class.equals(this.returnType);this.mapKey = getMapKey(method);this.returnsMap = this.mapKey != null;this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);this.paramNameResolver = new ParamNameResolver(configuration, method);
}

创建MapperMethod对象，其中的方法签名MethodSignature解析了方法的信息，我们最常用的@Param注解就是在这个时候解析的–ParamNameResolver。

2.2.1 方法名称解析器ParamNameResolve

ParamNameResolver#ParamNameResolver

public ParamNameResolver(Configuration config, Method method) {final Class<?>[] paramTypes = method.getParameterTypes();final Annotation[][] paramAnnotations = method.getParameterAnnotations();final SortedMap<Integer, String> map = new TreeMap<>();int paramCount = paramAnnotations.length;// get names from @Param annotationsfor (int paramIndex = 0; paramIndex < paramCount; paramIndex++) {if (isSpecialParameter(paramTypes[paramIndex])) {// skip special parameterscontinue;}String name = null;for (Annotation annotation : paramAnnotations[paramIndex]) {if (annotation instanceof Param) {hasParamAnnotation = true;name = ((Param) annotation).value();break;}}if (name == null) {// @Param was not specified.if (config.isUseActualParamName()) {name = getActualParamName(method, paramIndex);}if (name == null) {// use the parameter index as the name ("0", "1", ...)// gcode issue #71name = String.valueOf(map.size());}}map.put(paramIndex, name);}names = Collections.unmodifiableSortedMap(map);}

1. 获取方法参数数组，它是一个二阶数组，一个方法多个参数，一个参数多个注解
2. 如果有@Param注解就将hasParamAnnotation属性设置为true。并且将该参数在方法中的索引与@Param设置的别名绑定起来（key为索引，value为@Param的值）最终封装到names。以便后期对参数的名称和实际参数的值进行绑定。

ParamNameResolver#getNamedParams

public Object getNamedParams(Object[] args) {final int paramCount = names.size();if (args == null || paramCount == 0) {return null;} else if (!hasParamAnnotation && paramCount == 1) {return args[names.firstKey()];} else {final Map<String, Object> param = new ParamMap<>();int i = 0;for (Map.Entry<Integer, String> entry : names.entrySet()) {param.put(entry.getValue(), args[entry.getKey()]);// add generic param names (param1, param2, ...)final String genericParamName = GENERIC_NAME_PREFIX + (i + 1);// ensure not to overwrite parameter named with @Paramif (!names.containsValue(genericParamName)) {param.put(genericParamName, args[entry.getKey()]);}i++;}return param;}}

1. 参数args为Mapper接口的参数，为数组。
2. 遍历方法names，key为索引，value为@Param的名称。最后将@Param的名称找到参数的索引，通过索引可以找到args里面对象的值。最后将他们绑定返回。这个方法返回的也就是parameterObject，后面会提到。

2.2.2 MapperMethod#execute

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;}

1. method.convertArgsToSqlCommandParam(args)调用了ParamNameResolver#getNamedParams获取到已经绑定的参数信息
2. 目前为止，对方法的签名已经处理完毕，接下来就交给SqlSession了。result = sqlSession.selectOne(command.getName(), param)

2.3 DefaultSqlSession

  @Overridepublic <E> List<E> selectList(String statement, Object parameter) {return this.selectList(statement, parameter, RowBounds.DEFAULT);}@Overridepublic <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();}}

1. 从Configiration#mappedStatements里面取出Satement对象，然后交给执行器Executor执行
2. 执行器是在SqlSession里面创建，这里可以对Executor代理

2.4 CachingExecutor

  @Overridepublic <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);}

1. BoundSql boundSql = ms.getBoundSql(parameterObject)，这个方法对Sql节点进行了解析，如果是动态Sql，这里会很复杂，后面专题在讲。只需要记住，这个方法返回了原始Sql，参数值（parameterObject），参数映射关系(parameterMappings)
   
2. createCacheKey这个方法创建了缓存key

  @Overridepublic <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, boundSql);@SuppressWarnings("unchecked")List<E> list = (List<E>) tcm.getObject(cache, key);if (list == null) {list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);tcm.putObject(cache, key, list); // issue #578 and #116}return list;}}return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}

如果有缓存，就从缓存里面取；没有缓存，就从数据库查询，这里是委托SimpleExecutor查询

@SuppressWarnings("unchecked")@Overridepublic <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;}

这里又定义了一个localCache缓存，缓存中没有，就queryFromDatabase从数据库中查询

2.5 SimpleExecutor#doQuery

  @Overridepublic <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);}}

1. 创建SatementHandler对象，这里会代理
2. prepareStatement方法主要负责创建连接，通过ParameterHandler设置参数
3. 通过StatementHandler执行Satement
4. 通过ResultSetHandler处理结果集

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {this.configuration = mappedStatement.getConfiguration();this.executor = executor;this.mappedStatement = mappedStatement;this.rowBounds = rowBounds;this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();this.objectFactory = configuration.getObjectFactory();if (boundSql == null) { // issue #435, get the key before calculating the statementgenerateKeys(parameterObject);boundSql = mappedStatement.getBoundSql(parameterObject);}this.boundSql = boundSql;this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);}

获取连接对象 Connection

BaseExecutor#getConnection

protected Connection getConnection(Log statementLog) throws SQLException {Connection connection = transaction.getConnection();if (statementLog.isDebugEnabled()) {return ConnectionLogger.newInstance(connection, statementLog, queryStack);} else {return connection;}
}ConnectionLogger#invoke
@Overridepublic Object invoke(Object proxy, Method method, Object[] params)throws Throwable {try {if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, params);}if ("prepareStatement".equals(method.getName())) {if (isDebugEnabled()) {debug(" Preparing: " + removeBreakingWhitespace((String) params[0]), true);}PreparedStatement stmt = (PreparedStatement) method.invoke(connection, params);stmt = PreparedStatementLogger.newInstance(stmt, statementLog, queryStack);return stmt;} else if ("prepareCall".equals(method.getName())) {if (isDebugEnabled()) {debug(" Preparing: " + removeBreakingWhitespace((String) params[0]), true);}PreparedStatement stmt = (PreparedStatement) method.invoke(connection, params);stmt = PreparedStatementLogger.newInstance(stmt, statementLog, queryStack);return stmt;} else if ("createStatement".equals(method.getName())) {Statement stmt = (Statement) method.invoke(connection, params);stmt = StatementLogger.newInstance(stmt, statementLog, queryStack);return stmt;} else {return method.invoke(connection, params);}} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}}

通过transaction获取连接对象，如果开启了日志，那么就创建代理对象，可打印日志

ManagedTransaction#openConnection

protected void openConnection() throws SQLException {if (log.isDebugEnabled()) {log.debug("Opening JDBC Connection");}connection = dataSource.getConnection();if (level != null) {connection.setTransactionIsolation(level.getLevel());}setDesiredAutoCommit(autoCommit);
}@Override
public Connection getConnection() throws SQLException {return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();
}public PooledConnection(Connection connection, PooledDataSource dataSource) {this.hashCode = connection.hashCode();this.realConnection = connection;this.dataSource = dataSource;this.createdTimestamp = System.currentTimeMillis();this.lastUsedTimestamp = System.currentTimeMillis();this.valid = true;this.proxyConnection = (Connection) Proxy.newProxyInstance(Connection.class.getClassLoader(), IFACES, this);}

1. 通过数据源的信息获取连接对象，PooledConnection对Connection对象进行代理，便于连接池对连接对象的管理，返回的是连接对象
2. 可设置事务隔离等级