mybatis查询语句的背后揭秘

 public static SqlSessionFactory getSessionFactory() throws IOException {   Reader reader = Resources.getResourceAsReader("mybatis/mybatis-config.xml");   SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();   return builder.build(reader);  }

public Configuration parse() { if (parsed) {  throw new BuilderException("Each XMLConfigBuilder can only be used once."); } parsed = true; parseConfiguration(parser.evalNode("/configuration")); return configuration; } private void parseConfiguration(XNode root) { try {  // 解析properties节点信息  propertiesElement(root.evalNode("properties"));  // 解析settings节点配置信息，其中二级缓存的总开关就是这里配置,当然mybatis默认是开启的，详细见Configuration类中的cacheEnabled属性  Properties settings = settingsAsProperties(root.evalNode("settings"));  loadCustomVfs(settings);  loadCustomLogImpl(settings);  // 解析别名  typeAliasesElement(root.evalNode("typeAliases"));  // 解析插件  pluginElement(root.evalNode("plugins"));  // 这个节点一般不进行配置，myabtis也提供了一个默认实现类DefaultObjectFactory，除非自定义对象工厂实现，才需配置  objectFactoryElement(root.evalNode("objectFactory"));  objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));  reflectorFactoryElement(root.evalNode("reflectorFactory"));  settingsElement(settings);  // read it after objectFactory and objectWrapperFactory issue #631  environmentsElement(root.evalNode("environments"));  databaseIdProviderElement(root.evalNode("databaseIdProvider"));  // 处理java类型和数据库类型的转换,mybatis提供了许多默认实现，详细见TypeHandlerRegistry类，如果需自定义，可在此节点中进行配置  typeHandlerElement(root.evalNode("typeHandlers"));  // 这也是一个核心的配置，mapperElement方法会对mapper.xml文件内容进行一个解析  mapperElement(root.evalNode("mappers")); } catch (Exception e) {  throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e); } }

public void parse() { // 也就是检测配置文件配置的mapper节点有没有加载到configuration类中，防止重复加载 if (!configuration.isResourceLoaded(resource)) {  configurationElement(parser.evalNode("/mapper"));  configuration.addLoadedResource(resource);  // 这个是绑定,mapper接口的,当处理成功,在configuration类中的mapper注册器中，会添加一个mapper  bindMapperForNamespace(); } parsePendingResultMaps();// 解析resultMap节点 parsePendingCacheRefs(); // 解析缓存节点，如<cache-ref/> parsePendingStatements();// 解析select|update等节点,并封装成mappedStatement类 }

/**  * 通过mapper接口形式查询数据  */ @Test public void testSelectByMapper() throws IOException {  SqlSession sqlSession = MybatisUtil.getSessionFactory().openSession();  UserMapper mapper = sqlSession.getMapper(UserMapper.class);  User user = mapper.selectByPrimaryKey(10);  System.out.println(user);  sqlSession.close(); } /**  * 通过mapper接口的全限定名来进行查询  * @throws IOException  */ @Test public void testSelectByString() throws IOException {  SqlSessionFactory sessionFactory = MybatisUtil.getSessionFactory();  SqlSession sqlSession = sessionFactory.openSession();  User user = sqlSession.selectOne("com.mybatis.demo.mybatisdemo.mapper.UserMapper.selectByPrimaryKey",10);  System.out.println(user);  sqlSession.close(); }

// defaultSqlSession类public <T> T getMapper(Class<T> type) { return configuration.<T>getMapper(type, this); }//configuration类public <T> T getMapper(Class<T> type, SqlSession sqlSession) { return mapperRegistry.getMapper(type, sqlSession); }// 最终获取mapper对象的方法,其主要是创建一个mapper代理工厂，我们都知道mybatis的mapper接口是没有实现类的，// 但是我们直接查询是能获取数据，这里起作用的就是代理(采用的是jdk动态代理)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); } }

public interface TestMapper { User findByUserId(Integer id);}

public class MapperProxyTest implements InvocationHandler { private Class<?> target; public MapperProxyTest(Class<?> target) {  this.target = target; } public Object getProxyInstances(){  return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),new Class[]{target},this); } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {  if (Object.class.equals(method.getDeclaringClass())) {   return method.invoke(this, args);  }  User user = new User();  user.setPassword("123");  user.setUsername("李四");  user.setAddress("123");  user.setRegistertime(new Date());  user.setCellphone("1111111");  user.setAge(25);  return user; }}

public class MapperTest { public static void main(String[] args){  MapperProxyTest proxyTest = new MapperProxyTest(TestMapper.class);  TestMapper testMapper = (TestMapper) proxyTest.getProxyInstances();  System.out.println(testMapper.findByUserId(10)); }}

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); } // 交给了mpperMethod类去处理 final MapperMethod mapperMethod = cachedMapperMethod(method); return mapperMethod.execute(sqlSession, args); }

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()) {// 返回值为void类型，但是有ResultHandler参数，并且只能有一个，不然会报错   executeWithResultHandler(sqlSession, args);   result = null;  } else if (method.returnsMany()) {// 处理返回值类型为集合类型或者数组类型   result = executeForMany(sqlSession, args);  } else if (method.returnsMap()) {//处理返回值类型为Map类型   result = executeForMap(sqlSession, args);  } else if (method.returnsCursor()) {//返回值是否为cursor类型   result = executeForCursor(sqlSession, args);  } else {//其他类型   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; }

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) { List<E> result; // 将mapper接口的参数名称和args整成一个map结构，最后在会将值赋给sql中对应的变量 // 在3.5版本中，默认的mapper结构(假如没使用@param注解或者处于jdk1.8版本中在代码编译时加上 -parameters 参数)，结构为 // param1 -> args[0] param2 -> args[1] // arg0 -> args[0] arg1 -> args[1] mybatis之前有些版本不是arg0 而是0 1 。。数字代替。 Object param = method.convertArgsToSqlCommandParam(args); if (method.hasRowBounds()) {// 处理参数中带有rowBounds参数  RowBounds rowBounds = method.extractRowBounds(args);  result = sqlSession.<E>selectList(command.getName(), param, rowBounds); } else {// 其它情况  result = sqlSession.<E>selectList(command.getName(), param); } // issue #510 Collections & arrays support // 说明返回类型不是集合List类型，而是数组类型或其它集合类型。 if (!method.getReturnType().isAssignableFrom(result.getClass())) {  if (method.getReturnType().isArray()) {  return convertToArray(result);  } else {  return convertToDeclaredCollection(sqlSession.getConfiguration(), result);  } } return result; }

@Override 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(); } }

public Executor newExecutor(Transaction transaction, ExecutorType executorType) { executorType = executorType == null ? defaultExecutorType : executorType; executorType = executorType == null ? ExecutorType.SIMPLE : executorType; Executor executor; if (ExecutorType.BATCH == executorType) {  executor = new BatchExecutor(this, transaction); } else if (ExecutorType.REUSE == executorType) {  executor = new ReuseExecutor(this, transaction); } else {  executor = new SimpleExecutor(this, transaction); } // 如果cacheEnabled为true,其实这个属性默认为true的， // 则由CachingExecutor进行包装，也就是常说的装饰设计模式 if (cacheEnabled) {  executor = new CachingExecutor(executor); } executor = (Executor) interceptorChain.pluginAll(executor); return executor; }

@Override public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)  throws SQLException { // 如果不为空，则启用了二级缓存 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); }