Mybais 源码分析二 SqlSession

SqlSession 是Mybatis提供给用户与数据库交互的重要接口，我们一般调用其getMapper方法获取mapper接口的代理类，由代理类来与数据库交互，那么我们先来看看SqlSession的getMapper方法

  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

好吧，调用的是configuration的泛型方法getMapper，我们再进入Configuration.getMapper中

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

又调用的是mapperRegistry(映射注册类)的gtMapper方法，mapperRegistry是咋来的？我们把源码往上翻可以看到

protected MapperRegistry mapperRegistry = new MapperRegistry(this);

直接new出来的并把我们的Configuration传入进去了，那我们再看看MapperRegistry是怎么实现的

  private Configuration config;
  //这个hashMap存储的是接口类型和其对应的MapperproxyFactory
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }
	/**
	*	从knownMappers中取出接口类型对应的MapperProxyFactory，如果不存在则报错这个类型并没有注册，
	*	如果存在则返回一个实例
	*/
  @SuppressWarnings("unchecked")
  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);
    }
  }
	/**
	 * 	注册Mapper接口，把接口对应的mapper.xml加载到Configuration中
	 * */
	public <T> void addMapper(Class<T> type) {
		// 必须是接口
		if (type.isInterface()) {
			//如果接口已经注册,抛出异常，从这可以看出一个mapper接口只能对应一个mapper.xml文件
			if (hasMapper(type)) {
				throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
			}
			boolean loadCompleted = false;
			try {
				//加入容器中
				knownMappers.put(type, new MapperProxyFactory<T>(type));
				//把映射的Mapper.xml文件加载到Configuration中
				MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
				parser.parse();
				//加载结束
				loadCompleted = true;
			} finally {
				//如果加载异常从注册容器中删除
				if (!loadCompleted) {
					knownMappers.remove(type);
				}
			}
		}
	}
  public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }

MapperRegistry 是一个映射注册器，当XmlConfigBuild读取核心配置文件的< mappers > 节点时就调用addMappers方法(配置读取包时)或者addMapper方法把所有配置的mapper接口注册到MapperRegistry的knownMappers 当中，并加载所有的mapper.xml文件把各个节点<select|insert|update|update>等封装到Configuration。
而当我们获取代理对象的时候直接从knownMappers 取出mapperProxyFactory返回实例

public class MapperProxyFactory<T> {

  private final Class<T> mapperInterface;
  private Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

  public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  public Class<T> getMapperInterface() {
    return mapperInterface;
  }

  public Map<Method, MapperMethod> getMethodCache() {
    return methodCache;
  }

  @SuppressWarnings("unchecked")
  //用的是jdk的动态代理，根据接口的类加载器，接口的所有父接口，InvocationHandler实例，返回mapper接口实例
  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

//MapperProxy实现了InvocationHandler接口,定义了代理类的行为
  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

}

我们可以看到最终使用了JDK的动态代理返回了mapper接口的代理实例，而mapperProxy实现了InvocationHandler接口，其定义了代理类的行为

/**
 * 用于增强mapper接口的代理类
 */
public class MapperProxy<T> implements InvocationHandler, Serializable {

	private static final long serialVersionUID = -6424540398559729838L;
	private final SqlSession sqlSession;
	private final Class<T> mapperInterface;
	private final Map<Method, MapperMethod> methodCache;

	public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
		this.sqlSession = sqlSession;
		this.mapperInterface = mapperInterface;
		this.methodCache = methodCache;
	}

	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

		// 判断是否是一个类
		if (Object.class.equals(method.getDeclaringClass())) {
			try {
				return method.invoke(this, args);
			} catch (Throwable t) {
				throw ExceptionUtil.unwrapThrowable(t);
			}
		}
		// 如果不是一个类，取得mapperMethod，调用execute方法
		final MapperMethod mapperMethod = cachedMapperMethod(method);
		return mapperMethod.execute(sqlSession, args);
	}

	private MapperMethod cachedMapperMethod(Method method) {
		MapperMethod mapperMethod = methodCache.get(method);
		if (mapperMethod == null) {
			mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
			methodCache.put(method, mapperMethod);
		}
		return mapperMethod;
	}
}

代理类的主要执行逻辑定义在invoke方法里面，我们所有的mapper接口代理类都执行的是这个逻辑，这也可以理解，mapper接口所定义的功能无非就是增删改查执行的是Mapper.xml里面的Sql。
我们需要注意的是MapperMethod这个对象，在注册mapper接口的时候会把接口的所有方法以及其对应的<select|insert|update|delete>节点封装成对象存入ConcurrentHashMap中,再invoke方法中根据方法名从ConcurrentHashMap取出

public class MapperMethod {
	/**
	 * 	根据mapper接口的方法对应的节点回调SqlSession中的方法，
	 * */
	public Object execute(SqlSession sqlSession, Object[] args) {
		Object result;
		
		//如果执行增删改的操作，用rowCountResult这个方法处理返回值
		if (SqlCommandType.INSERT == command.getType()) {
			Object param = method.convertArgsToSqlCommandParam(args);
			result = rowCountResult(sqlSession.insert(command.getName(), param));
		} else if (SqlCommandType.UPDATE == command.getType()) {
			Object param = method.convertArgsToSqlCommandParam(args);
			result = rowCountResult(sqlSession.update(command.getName(), param));
		} else if (SqlCommandType.DELETE == command.getType()) {
			Object param = method.convertArgsToSqlCommandParam(args);
			result = rowCountResult(sqlSession.delete(command.getName(), param));
		} else if (SqlCommandType.SELECT == command.getType()) {
			
			//如果代理方法返回值为void，并且设置了ResultHandle,结果集封装到ResultHandle中
			if (method.returnsVoid() && method.hasResultHandler()) {
				executeWithResultHandler(sqlSession, args);
				result = null;
			} else if (method.returnsMany()) {
				//returnsMany是这样定义的     this.returnsMany = (configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray());
				//无论你方法返回值写的是Collection类型,还是arry，executeForMany都给你返回list
				result = executeForMany(sqlSession, args);
			} else if (method.returnsMap()) {
				result = executeForMap(sqlSession, args);
			} else {
				Object param = method.convertArgsToSqlCommandParam(args);
				result = sqlSession.selectOne(command.getName(), param);
			}
		} else {
			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;
	}
	/**
	 * 	根据方法返回类型取得具体的值
	 *  如果未void，返回null,
	 *  是Integr或者int 不变
	 *  是long 向上转型
	 *  是Boolean : 判断是否大于0，这也说明了可以直接设置返回值类型为boolean来判断是否修改了数据
	 * */
	private Object rowCountResult(int rowCount) {
		final Object result;
		
		if (method.returnsVoid()) {
			result = null;
		} else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) {
			result = rowCount;
		} else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) {
			result = (long) rowCount;
		} else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) {
			result = (rowCount > 0);
		} else {
			throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: "
					+ method.getReturnType());
		}
		return result;
	}
}

mapperMethod根据返回值类型以及Mapper接口所对应的节点的不同 回调了SqlSession中的方法，兜兜转转我们又回到了DefaultSqlSession,我们先来看看查询时怎样执行的，其实无论时查询多条还是单条都时调用的selectList方法，查询单条只是取第一条罢了

public class DefaultSqlSession implements SqlSession {
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
      return result;
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
}

  public <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds) {
    final List<?> list = selectList(statement, parameter, rowBounds);
    final DefaultMapResultHandler<K, V> mapResultHandler = new DefaultMapResultHandler<K, V>(mapKey,
        configuration.getObjectFactory(), configuration.getObjectWrapperFactory());
    final DefaultResultContext context = new DefaultResultContext();
    for (Object o : list) {
      context.nextResultObject(o);
      mapResultHandler.handleResult(context);
    }
    Map<K, V> selectedMap = mapResultHandler.getMappedResults();
    return selectedMap;
  }

如果返回类型是Map,MapperMethod调用selectMap，这里有两个参数mapKey,rowBounds分别是mapper接口上的注解@MapKey和mybatis自带的分页功能，这些我们以后会专门拎出来分析。我们现在可以看到selectMap方法第一句就调用了selectList方法，selectLisl执行sql委托给了executor，传入了xml中的id（方法名）所对应的节点对象，好了，SqlSession先到这里，我们下一章看看Executor是怎样执行sql放回结果集的

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