【图灵干货】java高级教程第十九节:Mybatis是如何执行一条SQL命令

发布时间:2021-11-28 15:57:00


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,我们以Mapper接口中的一个方法作为例子,看看Sql命令的执行完整流程。

public interface StudentMapper {

  List<Student> findAllStudents(Map<string, Object> map, RowBounds rowBounds, ResultSetHandler rh);  

}

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。

一个完整的Sql命令,其执行的完整流程图如下:

1603187809865338.png

对于上面的流程图,如果看过前面的文章的话,大部分对象我们都比较熟悉了。一个图,就完整展示了其执行流程。

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。可参看系列的第二篇。

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;

    }

org.apache.ibatis.binding.MapperMethod.SqlCommand。

public static class SqlCommand {

    // full id, 通过它可以找到MappedStatement

    private final String name;

    private final SqlCommandType type;

org.apache.ibatis.binding.MapperMethod.MethodSignature:

public static class MethodSignature {

    private final boolean returnsMany;

    private final boolean returnsMap;

    private final boolean returnsVoid;

    private final Class<?> returnType;

    private final String mapKey;

    private final Integer resultHandlerIndex;

    private final Integer rowBoundsIndex;

    private final SortedMap<Integer, String> params;

    private final boolean hasNamedParameters;

 

    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);

      // 分页参数

      this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);

      // 自定义ResultHandler

      this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);

      this.params = Collections.unmodifiableSortedMap(getParams(method, this.hasNamedParameters));

    }

以上是对MapperMethod的补充说明。

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