Mybatis执行一条SQL命令的过程

Mybatis中的sql命令，在枚举类SqlCommandType中定义的：

public enum SqlCommandType {
  UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH;
}

下面以Mapper接口中一个方法为例子，看下sql命令的执行完整流程:

public interface StudentMapper {
    //参数RowBounds和ResultSetHandler是可选参数，表示分页对象和自定义结果集处理器，一般不需要。
    List<Student> findAllStudents(Map<String, Object> map, RowBounds rowBounds, ResultSetHandler rh);  
}

一条完整的sql命令，其执行的完整流程如下图所示：

MapperProxy的功能

1. 因为Mapper接口不能直接实例化，MapperProxy的作用就是使用JDK动态代理功能，间接实例化Mapper的proxy对象

2. 缓存MapperMethod对象

private final Map<Method, MapperMethod> methodCache;
@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);
        }
    }
    // 投鞭断流
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
}

    // 缓存MapperMethod
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;
}

MapperMethod的功能

1. 解析Mapper接口的方法，并且封装成MapperMethod对象

2. 将sql命令正确路由到恰当的SqlSession的方法上

public class MapperMethod {

  // 保存了Sql命令的类型和键id
  private final SqlCommand command;
  // 保存了Mapper接口方法的解析信息
  private final MethodSignature method;

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

  // 根据解析结果，路由到恰当的SqlSession方法上
  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;
  }
  // ...