MyBatis原理分析之获取Mapper接口的代理对象

妖狐艹你老母 2022-09-03 14:17 344阅读 0赞

**流程原理分析系列：**  
[MyBatis原理分析之获取SqlSessionFactory][MyBatis_SqlSessionFactory]  
[MyBatis原理分析之获取SqlSession][MyBatis_SqlSession]  
[MyBatis原理分析之获取Mapper接口的代理对象][MyBatis_Mapper]  
[MyBatis原理分析之查询单个对象][MyBatis]

本篇博文是原理分析的第三篇。

当使用mapper接口进行CRUD时，其实是其代理对象在发挥作用，SQLsession获取mapper接口的代理对象时序图如下：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center]

# 【1】DefaultSqlSession #

如下代码所示，这里其实是调用了configuration实例的方法。该方法是一个泛型方法，参数有`Class<T> type`表示你的接口Class对象，比如`UserMapper.class---interface com.jane.mapper.UserMapper`

@Override
      public <T> T getMapper(Class<T> type) { 
        return configuration.<T>getMapper(type, this);
      }

关于DefaultSqlSession更多详情参考博文：[MyBatis中SqlSessionFactory和SqlSession简解][MyBatis_SqlSessionFactory_SqlSession]

# 【2】Configuration #

configuration有常量成员`protected final MapperRegistry mapperRegistry = new MapperRegistry(this);`实例，mapperRegistry 引用了当前configuration。

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

**MapperRegistry属性和构造方法如下：**

public class MapperRegistry { 
    
      private final Configuration config;
      private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
    
      public MapperRegistry(Configuration config) { 
        this.config = config;
      }
    //..
    }

在[MyBatis原理分析之获取SqlSessionFactory][MyBatis_SqlSessionFactory 1]一文中我们可以得知创建SqlSessionFactory时对所有的mapper(xml和接口)进行了解析并为每一个mapper接口创建了MapperProxyFactory对象放入knownMappers 中。

**MapperRegistry的addMapper方法如下：**

public <T> void addMapper(Class<T> type) { 
        if (type.isInterface()) { 
          if (hasMapper(type)) { 
            throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
          }
          boolean loadCompleted = false;
          try { 
           //这里向knownMappers放入当前解析的mapper对应的MapperProxyFactory实例
            knownMappers.put(type, new MapperProxyFactory<>(type));
            // It's important that the type is added before the parser is run
            // otherwise the binding may automatically be attempted by the
            // mapper parser. If the type is already known, it won't try.
            MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
            parser.parse();
            loadCompleted = true;
          } finally { 
            if (!loadCompleted) { 
              knownMappers.remove(type);
            }
          }
        }
      }

# 【3】MapperRegistry #

@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);
      }
    }

**代码解释如下：**

*  ① 从knownMappers中获取当前class对象的mapperProxyFactory实例；  
    ![在这里插入图片描述][ef113baac3444beba57a50a9e667bd62.png]
 *  ② 如果不存在则抛出异常；
 *  ③ 如果存在则执行`mapperProxyFactory.newInstance(sqlSession)`来获取当前`mapper`的代理对象

# 【4】MapperProxyFactory #

**MapperProxyFactory主要属性和构造方法**

public class MapperProxyFactory<T> { 
    
      private final Class<T> mapperInterface;
      private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
    
      public MapperProxyFactory(Class<T> mapperInterface) { 
        this.mapperInterface = mapperInterface;
      }
    //...
    }

可以看到其除了接口的class对象外还维护了一个私有ConcurrentHashMap类型常量`methodCache`。

**创建实例对象MapperProxy**

public T newInstance(SqlSession sqlSession) { 
     final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
     return newInstance(mapperProxy);
    }

**代码解释如下：**

*  ① 根据`sqlsession`、`mapperInterface`(接口的class对象)，以及类型的`methodCache`创建`MapperProxy`
 *  ② 为Mapper创建代理对象

这里需要注意的是`MapperProxy`是一个`InvocationHandler`类型，需要实现`Object invoke(Object proxy, Method method, Object[] args)`方法。

**MapperProxy的主要属性和构造方法**

public class MapperProxy<T> implements InvocationHandler, Serializable { 
    
      private static final long serialVersionUID = -6424540398559729838L;
      private final SqlSession sqlSession;
      private final Class<T> mapperInterface;
      private final Map<Method, MapperMethod> methodCache;
    
      public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) { 
        this.sqlSession = sqlSession;
        this.mapperInterface = mapperInterface;
        this.methodCache = methodCache;
      }
    //...
    }

InvocationHandler是什么呢？InvocationHandler是由代理实例的调用处理程序实现的接口。也就是说一个类/接口的代理实例的调用处理程序必须实现InvocationHandler接口的invoke方法。可以理解为本文中MapperProxy就是Mapper代理实例的调用处理程序。

**InvocationHandler的invoke方法如下：**

Object invoke(Object proxy, Method method, Object[] args)
    proxy:代理实例对象
    method:目标方法
    args:方法入参

Proxy是什么？Proxy是专门完成代理的操作类，是所有动态代理类的父类，通过此类为一个或多个接口动态地生成实现类。使用 Proxy 生成一个动态代理时，往往并不会凭空产生一个动态代理，这样没有太大的意义。通常都是为指定的目标对象生成动态代理

//直接创建一个动态代理对象
    static Object newProxyInstance( ClassLoader loader, Class<?>[] interfaces,InvocationHandler h ) 
    loader ：定义代理类的类加载器
    interfaces：被代理类实现的所有接口
    h：代理实例的调用处理程序
    该方法将会返回一个代理对象，代理对象有代理调用处理程序--InvocationHandler

--------------------

**根据mapperProxy创建Mapper的代理对象**

@SuppressWarnings("unchecked")
    protected T newInstance(MapperProxy<T> mapperProxy) {
      return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
    }

可以看下最终得到的Mapper的代理对象如下(`h 表示其是一个InvocationHandler也就是调用处理程序`)：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70]  
每个代理实例都有一个关联的调用处理程序InvocationHandler。在代理实例上调用方法时，方法调用将被编码并发送到其调用处理程序的`invoke`方法。

关于java的动态代理更多参考博文：  
[Java中的代理模式与动(静)态代理][Java]  
[Java中动态代理使用与原理详解][Java 1]

--------------------

# 【5】MapperMethod #

上面我们提到了`MapperProxyFactory`有常量成员`methodCache` ，在类加载过程中就进行了初始化。

private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

根据`MapperProxyFactory`生成`MapperProxy`实例时，将`ConcurrentHashMap`类型的methodCache 传了过去。

final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);

`MapperProxy`实现了`InvocationHandler`接口的`invoke`方法，那么在使用`Mapper`进行`CRUD`时实际会调用对应的`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);
    }

如上代码可以看到其先获取了MapperMethod 实例，然后调用了MapperMethod 实例的execute方法。

## **cachedMapperMethod方法** ##

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;
      }

**代码解释如下：**

*  ① 尝试从`ConcurrentHashMap`类型的`methodCache`获取当前`method`对应的`MapperMethod`
    
     *  ② 如果①没有获取到，则新建`MapperMethod`实例
     *  ③ 将`{method=mapperMethod}`放入`methodCache`中
 *  ④ 返回MapperMethod 实例

**这里先看一下一个Method对象是个什么？**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70 1]

## `new MapperMethod(mapperInterface, method, sqlSession.getConfiguration())` ##

构造方法如下：

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

这里我们可以看到，其新建了SqlCommand和MethodSignature实例。

**MapperMethod的UML图如下所示**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 1]

**再看一下MapperMethod实例对象**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70 2]

### `new SqlCommand(config, mapperInterface, method)` ###

SqlCommand是MapperMethod静态嵌套类，主要属性是name和SqlCommandType。

public static class SqlCommand { 
    
        private final String name;
        private final SqlCommandType type;
    
        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);
            }
          }
        }
    //...
      }

**构造方法如下解释如下**

*  ① `mapperInterface.getName() + "." + method.getName()`解析获取到`statementName` ，如`com.mybatis.dao.EmployeeMapper.getEmpById`；
 *  ② 判断configuration实例的`Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")`成员对象中是否有①中的statementName；
    
     *  ③ 如果存在，则获取statementName对应的MappedStatement；
     *  ④ 如果不存在且当前接口Class不是方法的所属Class，则根据方法的所属Class的name与方法名字解析新的statementName
        
         *  ⑤ 如果configuration的`Map<String, MappedStatement> mappedStatements`成员中存在新的statementName，则返回对应的MappedStatement
 *  ⑥ 如果最终得到的MappedStatement为null，则判断方法上面是否有注解`@Flush`；
    
     *  ⑦ 如果有注解`@Flush`,则赋值`name=null，type = SqlCommandType.FLUSH`；
     *  ⑧ 如果没有注解`@Flush`,则抛出异常
 *  ⑨ 如果最终得到的MappedStatement不为null，则赋值 `name = ms.getId(); type = ms.getSqlCommandType();`。
    
     *  ⑩ 如果type为UNKNOWN，则抛出异常

**SqlCommandType 是一个枚举类，主要有值`UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH`**

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

### hasStatement(statementName)与getMappedStatement(statementName) ###

configuration.hasStatement(statementName)) ;
    configuration.getMappedStatement(statementName);

上面我们看到这样两句代码，代码表面本身很好理解，我们跟进去看：

**hasStatement代码流程片段如下：**

public boolean hasStatement(String statementName) { 
       return hasStatement(statementName, true);
     }
    public boolean hasStatement(String statementName, boolean validateIncompleteStatements) { 
       if (validateIncompleteStatements) { 
         buildAllStatements();
       }
       return mappedStatements.containsKey(statementName);
     }

**getMappedStatement代码流程片段如下：**

public MappedStatement getMappedStatement(String id) { 
      return this.getMappedStatement(id, true);
    }
    
    public MappedStatement getMappedStatement(String id, boolean validateIncompleteStatements) { 
      if (validateIncompleteStatements) { 
        buildAllStatements();
      }
      return mappedStatements.get(id);
    }

`getMappedStatement`方法在以下地方有被调用  
![在这里插入图片描述][4f01fcfa49274484aeaee73a3f954efe.png]

可以看到，其都调用了`buildAllStatements();`。那么这个方法是什么呢？如下代码所示，其实就是mybatis提供的`快速失败`机制

protected void buildAllStatements() { 
        parsePendingResultMaps();
        if (!incompleteCacheRefs.isEmpty()) { 
          synchronized (incompleteCacheRefs) { 
            incompleteCacheRefs.removeIf(x -> x.resolveCacheRef() != null);
          }
        }
        if (!incompleteStatements.isEmpty()) { 
          synchronized (incompleteStatements) { 
            incompleteStatements.removeIf(x -> { 
              x.parseStatementNode();
              return true;
            });
          }
        }
        if (!incompleteMethods.isEmpty()) { 
          synchronized (incompleteMethods) { 
            incompleteMethods.removeIf(x -> { 
              x.resolve();
              return true;
            });
          }
        }
      }

在该方法上面有如下注释：

> Parses all the unprocessed statement nodes in the cache. It is recommended to call this method once all the mappers are added as it provides fail-fast statement validation.

解析缓存中所有未处理的statement节点。建议在添加所有映射程序后调用此方法，因为它提供`fail fast`语句验证。

resolveCacheRef、parseStatementNode、resolve都会抛出异常

**那么什么是fail fast 呢？**

fail fast即快速失败。这是一种设计思想，即系统如果发现异常立即抛出异常结束任务。与快速失败对应的还有`fail save`也就是安全失败，简单解释即为系统发现异常时，并不抛出异常结束程序，而是捕获异常通常并写入到错误日志中。在java的集合里面可以看到具体实例，参考博文[浅谈从fail-fast机制到CopyOnWriteArrayList使用][fail-fast_CopyOnWriteArrayList]

[MyBatis_SqlSessionFactory]: https://janus.blog.csdn.net/article/details/119334417
[MyBatis_SqlSession]: https://editor.csdn.net/md/?articleId=119485997
[MyBatis_Mapper]: https://blog.csdn.net/J080624/article/details/119510355
[MyBatis]: https://janus.blog.csdn.net/article/details/53203688
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[MyBatis_SqlSessionFactory_SqlSession]: https://janus.blog.csdn.net/article/details/53437272
[MyBatis_SqlSessionFactory 1]: https://blog.csdn.net/J080624/article/details/119334417?spm=1001.2014.3001.5501
[ef113baac3444beba57a50a9e667bd62.png]: /images/20220829/b5ae48c55f50455bb15258cfaf5c66c7.png
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[Java]: https://janus.blog.csdn.net/article/details/78181628
[Java 1]: https://janus.blog.csdn.net/article/details/81945043
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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70_pic_center 1]: /images/20220829/1ec971e5df274b7db12726beb312b7bb.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0owODA2MjQ_size_16_color_FFFFFF_t_70 2]: /images/20220829/b73a8cfb150d4eb1877b12544768bb22.png
[4f01fcfa49274484aeaee73a3f954efe.png]: /images/20220829/e636ad0e791445e884b8c098ae57a312.png
[fail-fast_CopyOnWriteArrayList]: https://janus.blog.csdn.net/article/details/82692326