【Mybatis源码】源码分析

• （1）Mybatis的基本执行流程
• • （1）在resources目录下建立一个mybatis-config.xml配置文件
  • （2）准备UserMapper.xml文件
  • （3）使用SqlSessionFactoryBuilder.build构建Mybatis会话工厂SqlSessionFactory
  • （4）创建一个SqlSession会话，使用上一步的SqlSessionFactory开启一个SqlSession
  • （5）从sqlSession中获取我们要执行的Mapper文件
  • （6）执行流程图
• （2）Mybatis源码分析
• • （1）Mapper的接口和xml标签的绑定
  • （2）MapperProxyFactory注册
  • （3）获取Mapper实例

（1）Mybatis的基本执行流程

（1）在resources目录下建立一个mybatis-config.xml配置文件

（2）准备UserMapper.xml文件

id, user_name, pass_word,selectid,user_name,pass_wordFROM user where id = #{id}

（3）使用SqlSessionFactoryBuilder.build构建Mybatis会话工厂SqlSessionFactory

public class MybatisUtil {private final static SqlSessionFactory sqlsessionFactory;static {String resource = "mybatis-config.xml";Reader reader = null;try {reader = Resources.getResourceAsReader(resource);} catch (IOException e) {e.printStackTrace();}sqlsessionFactory = new SqlSessionFactoryBuilder().build(reader);}public static SqlSessionFactory getSqlsessionFactory(){return sqlsessionFactory;}
}

（4）创建一个SqlSession会话，使用上一步的SqlSessionFactory开启一个SqlSession

 SqlSession sqlSession = sqlSessionFactory.openSession();

（5）从sqlSession中获取我们要执行的Mapper文件

 UserMapper mapper = sqlSession.getMapper(UserMapper.class);

然后就可以通过这个Mapper执行CURD语法了

（6）执行流程图

（2）Mybatis源码分析

思考三个问题
1-我们在调用mapper接口时是如何把方法和xml文件绑定起来的？
2-调用mapper方法具体是如何执行sql？
3-执行sql语句后应该是个resultset结合，那么怎样转换成接口对应的pojo实体？

（1）Mapper的接口和xml标签的绑定

（1）XMLMapperBuilder 的 bindMapperForNamespace 方法

private void bindMapperForNamespace() {String namespace = builderAssistant.getCurrentNamespace();if (namespace != null) {Class boundType = null;try {boundType = Resources.classForName(namespace);} catch (ClassNotFoundException e) {// ignore, bound type is not required}if (boundType != null && !configuration.hasMapper(boundType)) {// Spring may not know the real resource name so we set a flag// to prevent loading again this resource from the mapper interface// look at MapperAnnotationBuilder#loadXmlResourceconfiguration.addLoadedResource("namespace:" + namespace);configuration.addMapper(boundType);}}
}

（2）Mapper 接口的方法名与 XML 文件中的 sql、select、insert、update、delete 标签的 id 参数值进行绑定，源码体现在两个部分
1-生成id和MappedStatement对象注册到configuration
XMLMapperBuilder configurationElement 方法中，XMLMapperBuilder sqlElement 方法中

//sql标签
sqlElement(context.evalNodes("/mapper/sql"));
//select、insert、update、delete标签
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));

String id = context.getStringAttribute("id");
id = builderAssistant.applyCurrentNamespace(id, false);
if (databaseIdMatchesCurrent(id, databaseId, requiredDatabaseId)) {sqlFragments.put(id, context);
}

在XMLStatementBuilder parseStatementNode 方法中获取标签的id

//获取 Mapper xml 中标签 id
String id = context.getStringAttribute("id");
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,resultSetTypeEnum, flushCache, useCache, resultOrdered,keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);

MapperBuilderAssistant addMappedStatement 方法中，最后把 MappedStatement 注册到 configuration 对象中。

configuration.addMappedStatement(statement);

上面的过程其实就是将xml文件的标签进行解析，然后封装成一个MapperedStatement；而mapper的执行核心是用了jdk的动态代理，扫描mapper文件时有个MapperRegistry的过程，其核心就是将接口封装成MapperProxyFactory的一个属性然后在添加到knownMappers中。

（2）MapperProxyFactory注册

public  void addMapper(Class 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));MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);parser.parse();loadCompleted = true;} finally {if (!loadCompleted) {knownMappers.remove(type);}}}}

上面是Mapper的添加过程，我们在调用某个mapper如前面讲到的UserMapper，其实拿到的是我们定义的接口动态代理后的结果，下面我们看我们获取某个mapper时具体是怎样执行的流程？

（3）获取Mapper实例

第一步根据类型从knowMappers中获取一个MapperProxyFactory

  public  T getMapper(Class type, SqlSession sqlSession) {final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type); //1if (mapperProxyFactory == null) {throw new BindingException("Type " + type + " is not known to the MapperRegistry.");}try {return mapperProxyFactory.newInstance(sqlSession); //2} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}}

第二步调用MapperProxyFactory.newInstance,里面具体的操作是根据MapperProxyFactory中的接口创建了一个MapperProxy对象，而MapperProxy又实现了InvocationHandler接口，从而再通过Proxy.newProxyInstance创建一个动态代理对象返回给调用方，这就是所谓的动态代理的过程。

(T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy)

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

了解了mapper动态代理的过程，就不难发现，当我们掉用mapper接口的方法时就会调用MapperProxy的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);}}

根据 Mapper 接口方法查到并调用对应的 MappedStatement，完成绑定

new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));

MapperMethod 对象的 SqlCommand 中的 name 属性根据解析设置为对应的 MappedStatement 的 id

public MapperMethod(Class mapperInterface, Method method, Configuration config) {//创建SqlCommand对象，该对象包含一些和sql相关的信息this.command = new SqlCommand(config, mapperInterface, method);//创建MethodSignature对象，由类名可知，该对象包含了被拦截方法的一些信息this.method = new MethodSignature(config, mapperInterface, method);}

在SqlCommand中保存了一些和SQL相关信息，首先会解析MappedStatement

 MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,configuration);

public SqlCommand(Configuration configuration, Class mapperInterface, Method method) {final String methodName = method.getName();final Class declaringClass = method.getDeclaringClass();//核心代码，解析MappedStatementMappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,configuration);if (ms == null) {if (method.getAnnotation(Flush.class) != null) {name = null;type = SqlCommandType.FLUSH;} else {throw new BindingException("Invalid bound statement (not found): "+ mapperInterface.getName() + "." + methodName);}} else {name = ms.getId();type = ms.getSqlCommandType();if (type == SqlCommandType.UNKNOWN) {throw new BindingException("Unknown execution method for: " + name);}}}

根据标签属性执行insert｜update｜query｜delete方法

public Object execute(SqlSession sqlSession, Object[] args) {Object result;//根据 SQL 类型执行相应的数据库操作switch (command.getType()) {case INSERT: {// 对用户传入的参数进行转换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:// 根据目标方法的返回类型进行相应的查询操作if (method.returnsVoid() && method.hasResultHandler()) {// 如果方法返回值为 void，但参数列表中包含 ResultHandler，表明// 使用者想通过 ResultHandler 的方式获取查询结果，而非通过返回值// 获取结果executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {// 执行查询操作，并返回多个结果result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {// 执行查询操作，并将结果封装在 Map 中返回result = executeForMap(sqlSession, args);} else if (method.returnsCursor()) {// 执行查询操作，并返回一个 Cursor 对象result = executeForCursor(sqlSession, args);} else {Object param = method.convertArgsToSqlCommandParam(args);// 执行查询操作，并返回一个结果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());}
// 如果方法的返回值为基本类型，而返回值却为 null，此种情况下应抛出异常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;}

下面分析下convertArgsToSqlCommandParam，该方法中主要是为了映射查询方法的参数名称与参数值。

public Object getNamedParams(Object[] args) {final int paramCount = names.size();if (args == null || paramCount == 0) {return null;} else if (!hasParamAnnotation && paramCount == 1) {Object value = args[names.firstKey()];return wrapToMapIfCollection(value, useActualParamName ? names.get(0) : null);} else {final Map param = new ParamMap<>();int i = 0;for (Map.Entry entry : names.entrySet()) {// 添加 <参数名, 参数值> 键值对到 param 中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 @Param// 检测 names 中是否包含 genericParamName，什么情况下会包含？        // 答案如下：// 使用者显式将参数名称配置为 param1，即 @Param("param1")if (!names.containsValue(genericParamName)) {// 添加  到 param 中param.put(genericParamName, args[entry.getKey()]);}i++;}return param;}}