mybatis随笔四之MapperProxy

在上一篇文章我们已经得到了mapper的代理对象，接下来我们对demoMapper.getDemo(1)这种语句进行分析。
由于返回的mapper是个代理对象，因此会进入invoke方法，接下来我们来看看MapperProxy的invoke方法。

@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

Object.class.equals(method.getDeclaringClass())的意思是如果定义方法的类是个具体类就使用具体类的实现，如果是接口则往下执行。

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;
  }

methodCache是个Map<Method, MapperMethod>对象，第一次取时为空会进入MapperMethod构造方法。

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

public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
      String statementName = mapperInterface.getName() + "." + method.getName();
      MappedStatement ms = null;
      if (configuration.hasStatement(statementName)) {
        ms = configuration.getMappedStatement(statementName);
      } else if (!mapperInterface.equals(method.getDeclaringClass())) { // issue #35
        String parentStatementName = method.getDeclaringClass().getName() + "." + method.getName();
        if (configuration.hasStatement(parentStatementName)) {
          ms = configuration.getMappedStatement(parentStatementName);
        }
      }
      if (ms == null) {
        if(method.getAnnotation(Flush.class) != null){
          name = null;
          type = SqlCommandType.FLUSH;
        } else {
          throw new BindingException("Invalid bound statement (not found): " + statementName);
        }
      } else {
        name = ms.getId();
        type = ms.getSqlCommandType();
        if (type == SqlCommandType.UNKNOWN) {
          throw new BindingException("Unknown execution method for: " + name);
        }
      }
    }

这里主要做了这几件事，根据方法名以及接口名的组合从configuration中取得对应的MappedStatement，然后从中取出name和type。

MethodSignature构造方法如下

public MethodSignature(Configuration configuration, Method method) {
      this.returnType = method.getReturnType();
      this.returnsVoid = void.class.equals(this.returnType);
      this.returnsMany = (configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray());
      this.mapKey = getMapKey(method);
      this.returnsMap = (this.mapKey != null);
      this.hasNamedParameters = hasNamedParams(method);
      this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
      this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
      this.params = Collections.unmodifiableSortedMap(getParams(method, this.hasNamedParameters));
    }

这里主要是标记下入参中的RowBounds、ResultHandler类型参数，以及对返回值进行些标记。
MethodSignature与SqlCommand初始化后MapperMethod也就构造完成，然后methodCache将method与mapperMethod关系保留。
接下来就是屌用mapperMethod的execute方法来执行。

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    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()) {
      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 {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else if (SqlCommandType.FLUSH == command.getType()) {
        result = sqlSession.flushStatements();
    } 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;
  }

我们的mapper方法定义如下，返回的不是集合也不为空，因此进入convertArgsToSqlCommandParam方法。
convertArgsToSqlCommandParam对入参进行转换，如果没有入参返回null如果一个入参对象则直接返回，多个入参则封装成个map对象返回。

public Demo getDemo(long id);

现在进入到sqlSession的selectOne方法，使用ibatis的同学应该相当熟悉。
在selectOne内部调用了selectList方法，然后返回集合对象的第一个元素，如果集合对象大于1个则抛错。

@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();
    }
  }

rowBounds是用来分页的，暂时不管该对象，默认的初始值如下

public static final int NO_ROW_OFFSET = 0;
public static final int NO_ROW_LIMIT = Integer.MAX_VALUE;

selectList也是根据statement从configuration中取得mappedStatement，然后交由executor来执行，sqlSessionFactory构建的时候默认使用的是simpleExecutor

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到这里我们分析了mapper接口的方法最终是交到executor来执行。