MyBatis binding 模块分析

MyBatis binding 模块分析

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binding功能代码所在包

org.apache.ibatis.binding

binding模块做用

封装ibatis编程模型 ibatis编程模型中,SqlSession做为sql执行的入口,实用方法为sqlSession.selectOne(namespace+id, 参数列表)的形式(例:sqlSession.selectOne("com.enjoylearning.mybatis.mapper.TUserMapper.selectByPrimaryKey", 2)),不易于维护和阅读

须要解决的问题

• 找到SqlSession中对应的方法(insert|update|select)
• 找到命名空间和方法名(两维坐标)
• 传递参数

核心类

• MapperRegistry:mapper接口和对应的代理工厂的注册中心;是Configuration的成员变量
• MapperProxyFactory:用于生成mapper接口动态代理的实例对象;保证mapper实例对象是局部变量(为何不缓存?见文章最后)
• MapperMethod:封装了Mapper接口中对应的方法信息,易级对应sql语句的信息.MapperMethod对象不记录任何状态,因此它能够再多个代理对象之间共享;sqlCommand:封装sql语句;MtehodSignature:封装mapper接口的入参和返回类型

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我的理解

• MapperRegistry的Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>(); key:mapper的Class对象 value:生产这个mapper的动态代理示例的工厂
• MapperProxyFactory:Map<Method, MapperMethod> methodCache key:方法的反射类 value 方法的信息(sql语句,入参,返回值)

相关代码

public class MapperRegistry {

  private final Configuration config;//config对象，mybatis全局惟一的
  //记录了mapper接口与对应MapperProxyFactory之间的关系
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }

  @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);
    }

/**
 * 
 * 用于生成mapper接口动态代理的实例对象；
 * @author Lasse Voss
 */
public class MapperProxyFactory<T> {

  //mapper接口的class对象
  private final Class<T> mapperInterface;
//key是mapper接口中的某个方法的method对象，value是对应的MapperMethod，MapperMethod对象不记录任何状态信息，因此它能够在多个代理对象之间共享
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();

public class MapperMethod {
  //从configuration中获取方法的命名空间.方法名以及SQL语句的类型
  private final SqlCommand command;
  //封装mapper接口方法的相关信息（入参，返回类型）；
  private final MethodSignature method;

getMapper的过程

public class MapperProxyFactory<T> {

  //mapper接口的class对象
  private final Class<T> mapperInterface;
//key是mapper接口中的某个方法的method对象，value是对应的MapperMethod，MapperMethod对象不记录任何状态信息，因此它能够在多个代理对象之间共享
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();

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

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

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

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
	//建立实现了mapper接口的动态代理对象
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
	 //每次调用都会建立新的MapperProxy对象
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

}

两个newInstance方法生成mapper的动态代理对象MapperProxy

public class MapperProxy<T> implements InvocationHandler, Serializable {
.......
@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {//若是是Object自己的方法不加强
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //从缓存中获取mapperMethod对象，若是缓存中没有，则建立一个，并添加到缓存中
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //调用execute方法执行sql
    return mapperMethod.execute(sqlSession, args);
  }
  
  private MapperMethod cachedMapperMethod(Method method) {
    return methodCache.computeIfAbsent(method, k -> new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
  }

从invoke方法能够看出 是mapperMethod调用了execute方法

public class MapperMethod {
 ......
  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //根据sql语句类型以及接口返回的参数选择调用不一样的
    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()) {//返回值为void
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {//返回值为集合或者数组
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {//返回值为map
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {//返回值为游标
          result = executeForCursor(sqlSession, args);
        } else {//处理返回为单一对象的状况
          //经过参数解析器解析解析参数
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
          if (method.returnsOptional() &&
              (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = OptionalUtil.ofNullable(result);
          }
        }
        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;
  }

重点看 result = sqlSession.selectOne(command.getName(), param); 这里使用了ibatis的用法

问题解答

mapperProxy作查询依赖的仍是sqlSession.按照如今的代码结构来讲,若是缓存了mapperProxy,当sqlSession失效,mapperProxy也就失效了. 若是修改代结构,那么sqlSession就要做为查询参数

// 本来
TUser user = mapper.selectByPrimaryKey(2);
// 修改后的写法
TUser user = mapper.selectByPrimaryKey(sqslSession,2);