Mybatis执行mapper过程(二)

发表于2019-03-11更新于2021-12-12阅读次数

案例

先看下一次完成的Mybatis查询过程示例代码：

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String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper userMapper = session.getMapper(UserMapper.class);  
//调用代理对象方法  
User user = userMapper.findUserById(27);  
//关闭session  
session.close();

本文先重点分析，以下代码背后的原理

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UserMapper userMapper = session.getMapper(UserMapper.class);  
User user = userMapper.findUserById(27);

思考一个问题，为什么userMapper没有接口实现类也能成功执行findUserById()？

创建UserMapper代理对象

首先我们看下getMapper()是怎样获取代理对象的。

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

------>>>>>>

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

------>>>>>>

MapperRegistry.java
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
  //一个Mapper接口类对应一个MapperProxyFactory
  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);
  }
}

------>>>>>>

MapperProxyFactory.java
public T newInstance(SqlSession sqlSession) {
  final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
  return newInstance(mapperProxy);
}
protected T newInstance(MapperProxy<T> mapperProxy) {
  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

创建Mapper代理对象过程.png

大致过程，先获取MapperProxyFactory，然后使用JDK动态代理方式构建出代理对象。在MapperRegistry.java中我们可以知道，mapperProxyFactory由knownMappers.get(type)得到，那么knownMappers中的数据什么时候维护进去的呢？我们看下下面的MapperRegistry.java完整的代码。

MapperRegistry

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//这个类通过名字就可以看出 是用来注册Mapper接口与获取生成代理类实例的工具类 
public class MapperRegistry {
  //全局配置文件对象
  private Configuration config;
  //一个HashMap Key是mapper的类型对象, Value是MapperProxyFactory对象
  //这个MapperProxyFactory是创建Mapper代理对象的工厂 我们一会再分析
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
  public MapperRegistry(Configuration config) {
    this.config = config;
  }
  //获取生成的代理对象
  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //通过Mapper的接口类型 去Map当中查找 如果为空就抛异常
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null)
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    try {
      //否则创建一个当前接口的代理对象 并且传入sqlSession
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

  public <T> boolean hasMapper(Class<T> type) {
    return knownMappers.containsKey(type);
  }

  //注册Mapper接口！！！！
  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 {
        knownMappers.put(type, new MapperProxyFactory<T>(type));
       
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }
  public Collection<Class<?>> getMappers() {
    return Collections.unmodifiableCollection(knownMappers.keySet());
  }
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }
  //通过包名扫描下面所有接口
  public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
  }

}

MapperRegistry 类是注册Mapper接口与获取代理类实例的工具类。从上述代码可以看到，addMapper(Class type)方法中维护了knownMappers。而XMLConfigBuilder.mapperElement(XNode) 是在读取配置文件时[XMLConfigBuilder 118行]调用了该方法addMapper(Class type)，即在加载配置文件时候为每一个Mapper类（UserMapper）维护了对应的MapperProxyFactory。

MapperProxyFactory

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//这个类负责创建具体Mapper接口代理对象的工厂类
public class MapperProxyFactory<T> {
  //具体Mapper接口的Class对象
  private final Class<T> mapperInterface;
  //该接口下面方法的缓存 key是方法对象 value是对接口中方法对象的封装
  private Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
  //构造参数没啥好说的
  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) {
    //创建了一个代理类并返回
    //关于Proxy的API 可以查看java官方的API
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
  //在这里传入sqlSession 创建一个Mapper接口的代理类
  public T newInstance(SqlSession sqlSession) {
    //在这里创建了MapperProxy对象 这个类实现了JDK的动态代理接口 InvocationHandler
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    //调用上面的方法 返回一个接口的代理类
    return newInstance(mapperProxy);
  }
}

从上述代码可以看到在newInstance(MapperProxy mapperProxy)中，利用mapperProxy作为InvocationHandler来创建代理对象。

MapperProxy

MapperProxy的源码如下：

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//实现了JDK动态代理的接口 InvocationHandler
//在invoke方法中实现了代理方法调用的细节
public class MapperProxy<T> implements InvocationHandler, Serializable {
  private static final long serialVersionUID = -6424540398559729838L;
  //SqlSession
  private final SqlSession sqlSession;
  //接口的类型对象
  private final Class<T> mapperInterface;
  //接口中方法的缓存 有MapperProxyFactory传递过来的。
  private final Map<Method, MapperMethod> methodCache;
  //构造参数
  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }
  //接口代理对象所有的方法调用 都会调用该方法
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    //判断是不是基础方法 比如toString() hashCode()等，这些方法直接调用不需要处理
    if (Object.class.equals(method.getDeclaringClass())) {
      return method.invoke(this, args);
    }
    //这里进行缓存
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //调用mapperMethod.execute 核心的地方就在这个方法里，这个方法对才是真正对SqlSession进行的包装调用
    return mapperMethod.execute(sqlSession, args);
  }
  //缓存处理
  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;
  }
}

从上述代码可以看到，invoke()方法中的逻辑是执行相应的 SQL 语句并将结果集返回，而没有相应的去执行接口实现类方法的代码。
这就是为什么mybatis的mapper没有实现类的原因？

MapperMethod

MapperMethod是MapperProxyFactory类在创建MapperProxy对象时传进来的。我们看下源码：

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//这个类是整个代理机制的核心类，对Sqlsession当中的操作进行了封装
public class MapperMethod {
  //一个内部封 封装了SQL标签的类型 insert update delete select
  private final SqlCommand command;
  //一个内部类 封装了方法的参数信息 返回类型信息等
  private final MethodSignature method;
  //构造参数
  public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, method);
  }
  //这个方法是对SqlSession的包装调用
  public Object execute(SqlSession sqlSession, Object[] args) {
    //定义返回结果
    Object result;
    //如果是INSERT操作
    if (SqlCommandType.INSERT == command.getType()) {
      //处理参数
      Object param = method.convertArgsToSqlCommandParam(args);
      //调用sqlSession的insert方法
      result = rowCountResult(sqlSession.insert(command.getName(), param));
      //如果是UPDATE操作 同上
    } else if (SqlCommandType.UPDATE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
      //如果是DELETE操作 同上
    } else if (SqlCommandType.DELETE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
      //如果是SELECT操作 那么情况会多一些 但是也都和sqlSession的查询方法一一对应
    } else if (SqlCommandType.SELECT == command.getType()) {
      //如果返回void 并且参数有resultHandler
      //则调用 void select(String statement, Object parameter, ResultHandler handler);方法
      if (method.returnsVoid() && method.hasResultHandler()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      //如果返回多行结果这调用 <E> List<E> selectList(String statement, Object parameter);
      //executeForMany这个方法调用的
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      //如果返回类型是MAP 则调用executeForMap方法
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      } else {
        //否则就是查询单个对象
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else {
      //如果全都不匹配 说明mapper中定义的方法不对
      throw new BindingException("Unknown execution method for: " + command.getName());
    }
    //如果返回值为空 并且方法返回值类型是基础类型 并且不是VOID 则抛出异常
    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;
  }
  private Object rowCountResult(int rowCount) {
    final Object result;
    if (method.returnsVoid()) {
      result = null;
    } else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) {
      result = rowCount;
    } else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) {
      result = (long) rowCount;
    } else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) {
      result = (rowCount > 0);
    } else {
      throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: " + method.getReturnType());
    }
    return result;
  }
  private void executeWithResultHandler(SqlSession sqlSession, Object[] args) {
    MappedStatement ms = sqlSession.getConfiguration().getMappedStatement(command.getName());
    if (void.class.equals(ms.getResultMaps().get(0).getType())) {
      throw new BindingException("method " + command.getName() 
          + " needs either a @ResultMap annotation, a @ResultType annotation," 
          + " or a resultType attribute in XML so a ResultHandler can be used as a parameter.");
    }
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      sqlSession.select(command.getName(), param, rowBounds, method.extractResultHandler(args));
    } else {
      sqlSession.select(command.getName(), param, method.extractResultHandler(args));
    }
  }
  //返回多行结果 调用sqlSession.selectList方法
  private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    //如果参数含有rowBounds则调用分页的查询
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
    } else {
      //没有分页则调用普通查询
      result = sqlSession.<E>selectList(command.getName(), param);
    }
    // issue #510 Collections & arrays support
    if (!method.getReturnType().isAssignableFrom(result.getClass())) {
      if (method.getReturnType().isArray()) {
        return convertToArray(result);
      } else {
        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
      }
    }
    return result;
  }
  private <E> Object convertToDeclaredCollection(Configuration config, List<E> list) {
    Object collection = config.getObjectFactory().create(method.getReturnType());
    MetaObject metaObject = config.newMetaObject(collection);
    metaObject.addAll(list);
    return collection;
  }
  @SuppressWarnings("unchecked")
  private <E> E[] convertToArray(List<E> list) {
    E[] array = (E[]) Array.newInstance(method.getReturnType().getComponentType(), list.size());
    array = list.toArray(array);
    return array;
  }
  private <K, V> Map<K, V> executeForMap(SqlSession sqlSession, Object[] args) {
    Map<K, V> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey(), rowBounds);
    } else {
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey());
    }
    return result;
  }
  public static class ParamMap<V> extends HashMap<String, V> {
    private static final long serialVersionUID = -2212268410512043556L;
    @Override
    public V get(Object key) {
      if (!super.containsKey(key)) {
        throw new BindingException("Parameter '" + key + "' not found. Available parameters are " + keySet());
      }
      return super.get(key);
    }
  }

在核心方法execute(SqlSession sqlSession, Object[] args)中，最后还是根据类型去选择到底执行sqlSession中的哪个方法。
SqlCommand来封装底层的增删改查操作。我们看下SqlCommand源码：

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//一个内部类 封装了具体执行的动作
  public static class SqlCommand {
    //xml标签的id
    private final String name;
    //insert update delete select的具体类型
    private final SqlCommandType type;
    public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) throws BindingException {
      //拿到全名 比如 org.mybatis.example.BlogMapper.selectBlog
      String statementName = mapperInterface.getName() + "." + method.getName();
      MappedStatement ms = null;
      //获取MappedStatement对象 这个对象封装了XML当中一个标签的所有信息 比如下面
      //<select id="selectBlog" resultType="Blog">
      //select * from Blog where id = #{id}
      //</select>
      if (configuration.hasStatement(statementName)) {
        ms = configuration.getMappedStatement(statementName);
      } else if (!mapperInterface.equals(method.getDeclaringClass().getName())) { // 这里是一个BUG
        String parentStatementName = method.getDeclaringClass().getName() + "." + method.getName();
        if (configuration.hasStatement(parentStatementName)) {
          ms = configuration.getMappedStatement(parentStatementName);
        }
      }
      //为空抛出异常
      if (ms == null) {
        throw new BindingException("Invalid bound statement (not found): " + statementName);
      }
      //配置文件节点的ID
      name = ms.getId();
      //CRUD 
      type = ms.getSqlCommandType();
      //判断SQL标签类型 未知就抛异常
      if (type == SqlCommandType.UNKNOWN) {
        throw new BindingException("Unknown execution method for: " + name);
      }
    }
    public String getName() {
      return name;
    }
    public SqlCommandType getType() {
      return type;
    }
  }

我们都会知道节点上有id属性值。那么MyBatis框架会把每一个节点(如：select节点、delete节点）生成一个MappedStatement类。在SqlCommand中，要找到MappedStatement类就必须通过接口类全限定名+id(方法名)来获得。
以selectOne()为例，看下底层是怎么执行？
1、selectOne()方法

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public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    //会调用selectList()方法
    List<T> list = this.<T>selectList(statement, parameter);
    if (list.size() == 1) {
      //只返回一个，取第一个即可
      return list.get(0);
    } else if (list.size() > 1) {
      //这是MyBatis中很常见的一种异常
      throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
      return null;
    }
  }

2、selectList()方法

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@Override
  public <E> List<E> selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
  }

@Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      //委托，单一职责
      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();
    }
  }

Executor是一个非常重要的接口：
excutor
其中BaseExecutor是一个抽象实现类，这里也使用了模板设计模式，这里将一些共性抽取到了BaseExecutor中。
进入了org.apache.ibatis.executor.BaseExecutor#query

BoundSql是对SQL语句的封装，ms(MapperStatment)由前面接口类全限定名+接口方法从Configuration对象中获取。
继续看下一步的query方法

执行querFormData()

localCache会先putObject()一下，这行代码的意义是声明一个占位符，当发送一次查询数据的请求时，设置该占位符告诉其他请求正在查询数据库,请其他请求先阻塞或休眠。当这次请求查询到数据之后，将真正的数据放到占位符的位置，缓存数据。如果其他请求与该次请求查询的数据时一样的，直接从一级缓存中拿数据减少了查询请求对数据库的压力 （org.apache.ibatis.executor.BaseExecutor.DeferredLoad#load org.apache.ibatis.executor.BaseExecutor.DeferredLoad#canLoad），接下来会执行doQuery()方法，doQuery()方法是BaseExecutor中的一个模板方法：

这里会有一个拦截器链去执行Plugins的拦截：

当sqlsessionFactory获取sqlsession时，产生的ParameterHandler、ResultSetHandler、StatementHandler、Executor都是由org.apache.ibatis.session.Configuration 类的方法 newParameterHandler、newResultSetHandler、newStatementHandler、newExecutor产生的代理对象，而这些代理对象是经过适用规则的plugin层层代理的 ，最后会返回一个StatementHandler。

在prepareStatement()方法中，获取了Connection：

接下来会执行handler.query(stmt, resultHandler)：

返回结果处理

执行完成后会由ResultSetHandler处理结果

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public List<Object> handleResultSets(Statement stmt) throws SQLException {
    //又看到了熟悉的ErrorContext，不过这个activity的名称与之前的不一样
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
 
    final List<Object> multipleResults = new ArrayList<Object>();
 
    int resultSetCount = 0;
    ResultSetWrapper rsw = getFirstResultSet(stmt);
 
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) {
      ResultMap resultMap = resultMaps.get(resultSetCount);
      //循环封装好每个查询结果对象
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }
 
    String[] resultSets = mappedStatement.getResultSets();
    if (resultSets != null) {
      while (rsw != null && resultSetCount < resultSets.length) {
        ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
        if (parentMapping != null) {
          String nestedResultMapId = parentMapping.getNestedResultMapId();
          ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
          handleResultSet(rsw, resultMap, null, parentMapping);
        }
        rsw = getNextResultSet(stmt);
        cleanUpAfterHandlingResultSet();
        resultSetCount++;
      }
    }
 
    return collapseSingleResultList(multipleResults);
  }

由handleResultSet()方法逐层深入最后来到getRowValue()方法。

执行org.apache.ibatis.executor.resultset.DefaultResultSetHandler#createResultObject(org.apache.ibatis.executor.resultset.ResultSetWrapper, org.apache.ibatis.mapping.ResultMap, org.apache.ibatis.executor.loader.ResultLoaderMap, java.lang.String)方法，首先会循环遍历为ResultMapping属性赋值。如果是嵌套查询而且配置了延迟加载，其中这里的createProxy()方法会生成一个具有延迟加载功能的代理对象（这里采用CGLIB代理生成！！！）。

这里返回的是一个所有属性都为空的结果：

需要在前文getRowValue()中，给各个属性赋值：

大体的执行过程如下：

整体过程

参考资料