【Mybatis源码分析】05-SqlSession执行过程之获取Mapper代理对象

当我们通过DefaultSqlSession的Mapper方式操作数据库时使用如下api:

 T getMapper(Class type);

此方法返回一个实现了type接口的实现类的实力，我们分析一下此实力的创建过程。

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

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

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 {
    return mapperProxyFactory.newInstance(sqlSession);
  } catch (Exception e) {
    throw new BindingException("Error getting mapper instance. Cause: " + e, e);
  }
}

可以看到getMapper方法实际是调用MapperRegistry的getMapper方法，MapperRegistry我们在前面已经分析过，它保存了Mapper接口与MapperProxyFactory的映射。通过Mapper接口取得对应的MapperProxyFactory，此类的目的是为了创建  Mapper接口的代理对象MapperProxy，对代理对象的调用委托给了MapperMethod对象的execute方法，具体过程在我前面分析过的一篇文章《Mapper映射的解析过程》里，这里不再叙述。 我们详细分析一下MapperMethod。

private final SqlCommand command;
private final MethodSignature method;

public MapperMethod(Class mapperInterface, Method method, Configuration config) {
  this.command = new SqlCommand(config, mapperInterface, method);
  this.method = new MethodSignature(config, mapperInterface, method);
}

通过MapperMethod的构造方法传入了Mapper的接口类，所有执行的Mapper方法和Mybatis配置中心。构造方法中会新创建两个对象SqlCommand和MethodSignature。下面分析一下这两个对象具体是什么东西。

SqlCommand:这个类会保存两个成员变量，一个是MappedStatement的id属性，一个是所代表执行sql的类型。

public SqlCommand(Configuration configuration, Class mapperInterface, Method method) {
  final String methodName = method.getName();
  final Class declaringClass = method.getDeclaringClass();
  MappedStatement 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);
    }
  }
}

内部有一个resolveMappedStatement方法，此方法返回在解析xml映射文件保存在Configuration中的MappedStatement，将其id属性保存赋值给name成员变量。将其sqlCommandType赋值给type成员变量。

MethodSignature:包含了关于执行的Mapper方法的参数类型和返回类型。

public MethodSignature(Configuration configuration, Class mapperInterface, Method method) {
  Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface);
  if (resolvedReturnType instanceof Class) {
    this.returnType = (Class) resolvedReturnType;
  } else if (resolvedReturnType instanceof ParameterizedType) {
    this.returnType = (Class) ((ParameterizedType) resolvedReturnType).getRawType();
  } else {
    this.returnType = method.getReturnType();
  }
  this.returnsVoid = void.class.equals(this.returnType);
  this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();
  this.returnsCursor = Cursor.class.equals(this.returnType);
  this.mapKey = getMapKey(method);
  this.returnsMap = this.mapKey != null;
  this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
  this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
  this.paramNameResolver = new ParamNameResolver(configuration, method);
}

其中包括方法的返回类型，返回类型是否为void，返回类型是否为Collection或数组或游标，如果参数中含有RowBounds类型保存其在参数列表的索引值，ResultHandler同理，最后还new了一个ParamNameResolver对象,MethodSignature通过此对象可以对外提供获取参数索引值或参数名的方法，因为此对象内部缓存了方法参数列表中除了RowBounds和ResultHandler类型参数的索引值与参数名的键值对。举个列子:

aMethod(@Param("M") int a, @Param("N") int b) -> {{0, "M"}, {1, "N"}}
aMethod(int a, int b) -> {{0, "0"}, {1, "1"}}
aMethod(int a, RowBounds rb, int b) -> {{0, "0"}, {2, "1"}}

其中参数名由@Param注解指定若没有此注解会通过java8的Parameter的获取真实参数名，前提Mapper类是使用java8编译的并且开启了--parameters编译选项。否则参数名为arg0，arg1...的形式了。

接下来着重讲一下execute方法。

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()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      } else if (method.returnsCursor()) {
        result = executeForCursor(sqlSession, args);
      } else {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
      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;
}

可以看到execute的执行分为增删改查和刷新，除了FLUSH类型操作，都会调用MethodSignature的convertArgsToSqlCommandParam方法将Object[] args的参数类型转换为null，或者单个参数对象或ParamMap三种类型中的一种。

public Object convertArgsToSqlCommandParam(Object[] args) {
  return paramNameResolver.getNamedParams(args);
}

public Object getNamedParams(Object[] args) {
  final int paramCount = names.size();
  if (args == null || paramCount == 0) {
    return null;
  } else if (!hasParamAnnotation && paramCount == 1) {
    return args[names.firstKey()];
  } else {
    final Map param = new ParamMap