Mybatis源码分析之（三）mapper接口底层原理（为什么不用写方法体就能访问到数据库）

mybatis是怎么拿sqlSession

在 上一篇的时候，我们的SqlSessionFactoryBuilder已经从xml文件中解析出了Configuration并且返回了sessionFactory。

然后我们要从sessionFactory.openSession();中拿到sqlSession

public class DefaultSqlSessionFactory implements SqlSessionFactory {

  private final Configuration configuration;
  @Override
  public SqlSession openSession() {
  //默认情况下ExecutorType是ExecutorType.SIMPLE类型的
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

 private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
    //获取配置的环境信息
      final Environment environment = configuration.getEnvironment();
      //获取environment中的TransactionFactory
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      //生成Transaction
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //生成Executor（重要，之后的查询都得靠它）
      final Executor executor = configuration.newExecutor(tx, execType);
      //返回DefaultSqlSession
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
}



//DefaultSqlSession类的组成，其实新建的时候就只是把他的字段赋值而已
public class DefaultSqlSession implements SqlSession {

  private Configuration configuration;
  private Executor executor;

  private boolean autoCommit;
  private boolean dirty;
  private List> cursorList;
}

  public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
    this.configuration = configuration;
    this.executor = executor;
    this.dirty = false;
    this.autoCommit = autoCommit;
  }

上面分析的是到拿到sqlSession为止，重点其实不是在上面这里，因为到上面为止，其实主要的功能只是将配置的信息解析成我们要的类，然后进行初始化赋值。

Mapper的实现原理

下面我们从SqlSession中拿到mapper，并执行方法其实才是，你感觉到mybatis框架开始帮我们做事的开始。

    public static void main(String[] args) {
        //拿到SqlSession
        SqlSession sqlsession = MybatisUtil.getSqlsession();
        //拿mapper
        TDemoMapper mapper = sqlsession.getMapper(TDemoMapper.class);
        //调用mapper的方法
        List all = mapper.getAll();
        for (TDemo item : all)
            System.out.println(item);

    }

因为我们在项目中的TDemoMapper只是一个接口，并没有实现这个接口方法，但是为什么我们在调用这个接口方法的时候就可以得到返回结果呢？mybatis究竟做了什么？

首先我们回到之前解析Mapper的语句

mapperElement(root.evalNode("mappers"));


 private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");

          if (resource != null && url == null && mapperClass == null) { //根据resource解析
            ErrorContext.instance().resource(resource);
            InputStream inputStream = Resources.getResourceAsStream(resource);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url != null && mapperClass == null) {//根据url 解析
            ErrorContext.instance().resource(url);
            InputStream inputStream = Resources.getUrlAsStream(url);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) {//根据mapperClass解析
          //首先通过mapperClass的路径，生成mapperClass的接口类
            Class mapperInterface = Resources.classForName(mapperClass);
            //降mapperClass加入到configuration中去
            configuration.addMapper(mapperInterface);
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }



//Configuration类下
  public  void addMapper(Class type) {
    mapperRegistry.addMapper(type);
  }

//MapperRegistry类下
public class MapperRegistry {

  private final Configuration config;
  private final Map, MapperProxyFactory> knownMappers = new HashMap, MapperProxyFactory>();

//最终调用这个方法
  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 {
      //将接口包装成MapperProxyFactory类放入knownMappers中（knownMappers就是存放我们的mapper接口的）
        knownMappers.put(type, new MapperProxyFactory<T>(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.
        //通过这个builder来解析mapper的statement。（把mapper和mapper.xml文件相关联，方法名与xml中的id相关联，为了之后调用的时候能找到的语句）
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        //开始解析
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }
}


//MapperAnnotationBuilder类中
  public void parse() {
    String resource = type.toString();
    if (!configuration.isResourceLoaded(resource)) {
    //通过xml文件解析
      loadXmlResource();
      configuration.addLoadedResource(resource);
      assistant.setCurrentNamespace(type.getName());
      parseCache();
      parseCacheRef();
      //获得接口的方法（为了获取方法上的注解，通过注解的方式来让方法于sql语句相关联）
      Method[] methods = type.getMethods();
      for (Method method : methods) {
        try {
          // issue #237
          if (!method.isBridge()) {
          //具体的解析过程
            parseStatement(method);
          }
        } catch (IncompleteElementException e) {
          configuration.addIncompleteMethod(new MethodResolver(this, method));
        }
      }
    }
    parsePendingMethods();
  }

具体的调用过程就不细跟了，无非就是获取节点，获取属性值，（或者是获取方法，然后获取注解信息），巴拉巴拉……然后设置到configuration中。
上面要注意的点是，若既配置xml又配置注解的情况下，注解会覆盖xml，原因非常简单，源码中注解解析在xml解析后面，然后覆盖的情况是，他们有相同的namespace+id。
然后我们继续我们的主线任务，就是mapper的设计架构。从上面我们可以知道，configuration中有一个MapperRegistry类型的字段mapperRegistry，其中有一个字段叫knownMappers，knownMappers里面存着key为接口类型，值为MapperProxyFactory的。

//我们在调用TDemoMapper mapper = sqlsession.getMapper(TDemoMapper.class);的时候
//先调用DefaultSqlSession类下的
  @Override
  public  T getMapper(Class type) {
    return configuration.getMapper(type, this);
  }
//然后调用Configuration类下的

  public  T getMapper(Class type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }
  //最后调用MapperRegistry类下的

  public  T getMapper(Class type, SqlSession sqlSession) {
    final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
        //实际生成是这段代码，通过mapperProxyFactory来生成实例对象
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

//实际调用类
public class MapperProxyFactory {

  private final Class mapperInterface;
  private final Map methodCache = new ConcurrentHashMap();
  public T newInstance(SqlSession sqlSession) {
      //实例化一个代理类
    final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
    //通过这个函数实例化
    return newInstance(mapperProxy);
  }
  protected T newInstance(MapperProxy mapperProxy) {
      //动态代理的基本操作（说明最终实现方式是动态代理）
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
}

public class MapperProxy implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  private final Class mapperInterface;
  private final Map methodCache;

  public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }
  //动态代理中最重要的方法invoke
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
      //如果是Object中的方法就不走下面的代理了，直接执行（比如toString，hashCode）
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
      //如果不是静态方法而且不是抽象方法，则不增强方法
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //实际我们的mapper接口的方法走的逻辑就是下面这2条
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }
}

总结

我们通过sqlSession获得mapper方法，而sqlSession从configuration中的mapperRegistry中获取MapperProxyFactory对象，在通过MapperProxyFactory对象的newInstance方法得到MapperProxy的动态代理实例对象。

我们使用的mapper其实是通过MapperProxy动态代理，在运行时候生成的一个新的对象进行方法增强的，里面的接口方法都会通过下面2个语句进行数据库的操作，以及后续对数据的处理

    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);

这两条语句其实包含对访问数据库对象的创建，访问数据库到得到数据库返回数据后的处理等内容，非常复杂，本篇就到此为止。