Mybatis中的设计模式

Mybatis中使用了大量的设计模式。

以下列举一些看源码时，觉得还不错的用法：

创建型模式

工厂方法模式

DataSourceFactory

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通过不同的子类工厂，实例化不同的DataSource

TransactionFactory

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通过不同的工厂，生产不同的Transaction

单例模式

ErrorContext是单例模式，但只是线程级别的：

 private static final ThreadLocal<ErrorContext> LOCAL = new ThreadLocal<>();private ErrorContext() {}public static ErrorContext instance() {ErrorContext context = LOCAL.get();if (context == null) {context = new ErrorContext();LOCAL.set(context);}return context;}

Configuration 类虽然正常情况下只有一个实例，但是它的设计并不符合单例模式

  public Configuration(Environment environment) {this();this.environment = environment;}

可以看到，它的构造器并没有私有化，我们可以new多个实例

LogFactory也不是单例模式，每次都会通过构造器实例化对象

 private LogFactory() {// disable construction}public static Log getLog(Class<?> aClass) {return getLog(aClass.getName());}public static Log getLog(String logger) {try {return logConstructor.newInstance(logger);} catch (Throwable t) {throw new LogException("Error creating logger for logger " + logger + ".  Cause: " + t, t);}}

结构型模式

代理模式

在初始化配置时，会调用MapperRegistry的addMapper方法将Mapper接口存储在knownMappers中，key为Mapper接口，value为MapperProxyFactory实例

  public <T> void addMapper(Class<T> type) {if (type.isInterface()) {if (hasMapper(type)) {throw new BindingException("Type " + type + " is already known to the MapperRegistry.");}boolean loadCompleted = false;try {knownMappers.put(type, new MapperProxyFactory<>(type));// It's important that the type is added before the parser is run// otherwise the binding may automatically be attempted by the// mapper parser. If the type is already known, it won't try.MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);parser.parse();loadCompleted = true;} finally {if (!loadCompleted) {knownMappers.remove(type);}}}}

当我们通过UserMapper mapper = sqlSession.getMapper(UserMapper.class);获取Mapper接口时，则会调用MapperRegistry#getMapper，返回Mapper接口的代理类

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);if (mapperProxyFactory == null) {throw new BindingException("Type " + type + " is not known to the MapperRegistry.");}try {return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}}

代理类的生成

  public T newInstance(SqlSession sqlSession) {final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);return newInstance(mapperProxy);}

  protected T newInstance(MapperProxy<T> mapperProxy) {return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);}

可以看到，使用的是JDK动态代理，MapperProxy实现了InvocationHandler接口，我们关注MapperProxy#invoke即可：

  @Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, args);} else if (method.isDefault()) {return invokeDefaultMethod(proxy, method, args);}} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}final MapperMethod mapperMethod = cachedMapperMethod(method);return mapperMethod.execute(sqlSession, args);}

调用mapper接口，最终执行的就是我们编写的SQL

行为型模式

模板方法模式

BaseExecutor

BaseExecutor实现了Executor接口，定义了模板方法，并将钩子方法留给子类实现：

模板方法	钩子方法
query	doQuery
update	doUpdate
flushStatements	doFlushStatements
queryCursor	doQueryCursor

BaseTypeHandler

BaseTypeHandler实现了TypeHandler接口，定义模板方法，将钩子方法留给子类实现：

setNonNullParameter
getNullableResult

模板方法	钩子方法
setParameter	setNonNullParameter
getResult(ResultSet rs, String columnName)	getNullableResult(ResultSet rs, String columnName)
getResult(ResultSet rs, int columnIndex)	getNullableResult(ResultSet rs, int columnIndex)
getResult(CallableStatement cs, int columnIndex)	getNullableResult(CallableStatement cs, int columnIndex)

策略模式

在DefaultParameterHandler#setParameters中，

  @Overridepublic 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 paramsvalue = 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 | SQLException e) {throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);}}}}}

重点关注：

          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 | SQLException e) {throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);}

根据参数映射，拿到类型处理器，就是策略模式的应用，不同的类型处理器setParameter方法实现并不一样，这就是不同的参数类型，使用不同的类型处理器处理。

在解析结果集时，也是一样的，不同的类型需要使用不同的类型处理器获取结果：

DefaultResultSetHandler#getPropertyMappingValue通过ResultMap的属性映射：

  private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)throws SQLException {if (propertyMapping.getNestedQueryId() != null) {return getNestedQueryMappingValue(rs, metaResultObject, propertyMapping, lazyLoader, columnPrefix);} else if (propertyMapping.getResultSet() != null) {addPendingChildRelation(rs, metaResultObject, propertyMapping);   // TODO is that OK?return DEFERRED;} else {final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler();final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix);return typeHandler.getResult(rs, column);}}

DefaultResultSetHandler#applyAutomaticMappings自动映射也是一样的：

  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.isEmpty()) {for (UnMappedColumnAutoMapping mapping : autoMapping) {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;}