mybatis 执行流程（3）

开篇

 这篇文章的主要目的是为了讲清楚Mybatis的整个执行流程，会通过源码、流程图等多个维度进行说明，相关的细节由于涉及面比较广这里暂时先不详细展开。
 整体的思路先让大家有个宏观的概念，由了这个主轴以后我们再针对主轴上每个节点再进行细节分析，顺藤摸瓜，让我们先找到这个贯穿的藤。

Mybatis的使用模板

 首先我们使用Mybatis的时候都有一般固定的套路：

• 定义map接口，定义对外查询sql的接口。
• 配置xml文件，定义查询sql语句。
• 引入xml文件，引入xml文件进行解析。
• 调用map接口，调用查询接口提供查询服务。

1、新建一个com.kang.mapper的包，定义map接口，这里以UserMapper为例

package com.kang.mapper;  
import java.util.List;  
import com.kang.pojo.User;  
public interface UserMapper {  
    //根据用户id查询用户信息  
    public User findUserById(int id) throws Exception;  
    //查询用户列表  
    public List<User> findUserByUsername(String username) throws Exception;  
    //添加用户信息  
    public void insertUser(User user)throws Exception;   
}  

2、配置xml文件 UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>  
<!DOCTYPE mapper  
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"  
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">  
<mapper namespace="com.kang.mapper.UserMapper">  
<!-- 注意这里的 namespace必须对应着map接口的全类名-->  
    <select id="findUserById" parameterType="int" resultType="user">  
        select * from user where id = #{id}  
    </select>  

    <select id="findUserByUsername" parameterType="java.lang.String"  
        resultType="user">  
        select * from user where username like '%${value}%'  
    </select>  

    <insert id="insertUser" parameterType="user">  
        <selectKey keyProperty="id" order="AFTER" resultType="java.lang.Integer">  
            select LAST_INSERT_ID()  
        </selectKey>  
        insert into user(username,birthday,sex,address)  
        values(#{username},#{birthday},#{sex},#{address})  
    </insert>  
</mapper>  

3、在SqlMapConfig.xml中加入映射文件

<!-- 加载 映射文件 -->  
<mappers>  
  <mapper resource="map/UserMapper.xml" />  
</mappers>  

4、调用方法

//获取session  
        SqlSession session = sqlSessionFactory.openSession();  
        //获取mapper接口的代理对象  
        UserMapper userMapper = session.getMapper(UserMapper.class);  
        //调用代理对象方法  
        User user = userMapper.findUserById(27);  
        System.out.println(user);  
        //关闭session  
        session.close();  
        System.out.println("---------执行完毕-----------");  

Mybatis执行过程

Mybatis执行过程流程图

Mybatis执行流程图.png

Mybatis执行过程时序图

Mybatis执行时序图.png

Mybatis执行过程源码分析

step1-DefaultSqlSession获取代理类

UserMapper userMapper = session.getMapper(UserMapper.class)

--------------------DefaultSqlSession.java------------------------------
  public <T> T getMapper(Class<T> type) {
    //最后会去调用MapperRegistry.getMapper
    return configuration.<T>getMapper(type, this);
  }

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

step2-MapperRegistry类生成代理类

 从代码可以看出来通过addMapper方法添加knownMappers，这个方法是在解析xml配置中被调用。

------------------------MapperRegistry.java-----------------------------------------
  //返回代理类
  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);
    }
  }


  //我们在解析XML文件的时候添加了如何添加一个映射
  public <T> void addMapper(Class<T> type) {
    //mapper必须是接口！才会添加
    if (type.isInterface()) {
      if (hasMapper(type)) {
        //如果重复添加了，报错
        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 {
        //如果加载过程中出现异常需要再将这个mapper从mybatis中删除,这种方式比较丑陋吧，难道是不得已而为之？
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

step3-MapperProxyFactory生成代理类MapperProxy

--------------------MapperProxyFactory.java------------------------------------------
/**
 * 映射器代理工厂
 */
public class MapperProxyFactory<T> {

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

  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) {
    //用JDK自带的动态代理生成映射器
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

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

}

step4-MapperProxy通过invoke调用真正的接口

 MapperProxy的invoke当中执行method.invoke(this, args)调用真正的SQL查询接口，method就是查询的SQL接口的MapperMethod封装。

---------------------MapperProxy.java-----------------------
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;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    //代理以后，所有Mapper的方法调用时，都会调用这个invoke方法
    //并不是任何一个方法都需要执行调用代理对象进行执行，如果这个方法是Object中通用的方法（toString、hashCode等）无需执行
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    //这里优化了，去缓存中找MapperMethod
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //执行
    return mapperMethod.execute(sqlSession, args);
  }

  //去缓存中找MapperMethod
  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      //找不到才去new
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

}

step5-MapperMethod的execute过程

 MapperMethod我们暂时只关注result = sqlSession.selectOne(command.getName(), param)过程，command.getName()返回的是在Mybatis的mapper.xml文件对应的接口文件当中定义的。

--------------------------MapperMethod.java---------------------------------
  //执行
  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //可以看到执行时就是4种情况，insert|update|delete|select，分别调用SqlSession的4大类方法
    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()) {
        //如果有结果处理器
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        //如果结果有多条记录
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        //如果结果是map
        result = executeForMap(sqlSession, args);
      } else {
        //否则就是一条记录
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } 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;
  }

step6-DefaultSqlSession的执行过程

 selectList当中通过executor.query()方法调用进入到executor当中执行。

------------------------DefaultSqlSession.java-----------------------------------
  public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    //转而去调用selectList,很简单的，如果得到0条则返回null，得到1条则返回1条，得到多条报TooManyResultsException错
    List<T> list = this.<T>selectList(statement, parameter);
    if (list.size() == 1) {
      return list.get(0);
    } else if (list.size() > 1) {
      throw new TooManyResultsException("Expected one result (or null) 
                                         to be returned by selectOne(), but found: " + list.size());
    } else {
      return null;
    }
  }

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

  //核心selectList
  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //根据statement id找到对应的MappedStatement
      MappedStatement ms = configuration.getMappedStatement(statement);
      //转而用执行器来查询结果,注意这里传入的ResultHandler是null
      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();
    }
  }

step7-BaseExecutor的执行过程

 在executor的执行过程中，首先调用的是BaseExecutor的接口，Executor的类关系图参加文章末尾。

--------------------------BaseExecutor.java------------------------------------------
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) 
                 throws SQLException {
    //得到绑定sql
    BoundSql boundSql = ms.getBoundSql(parameter);
    //创建缓存Key
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    //查询
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
 }

  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler)
            throws SQLException {
    //得到绑定sql
    BoundSql boundSql = ms.getBoundSql(parameter);
    //创建缓存Key
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    //查询
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
 }

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, 
                         ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    //如果已经关闭，报错
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    //先清局部缓存，再查询.但仅查询堆栈为0，才清。为了处理递归调用
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      //加一,这样递归调用到上面的时候就不会再清局部缓存了
      queryStack++;
      //先根据cachekey从localCache去查
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        //若查到localCache缓存，处理localOutputParameterCache
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        //从数据库查
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      //清空堆栈
      queryStack--;
    }
    if (queryStack == 0) {
      //延迟加载队列中所有元素
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      //清空延迟加载队列
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        //如果是STATEMENT，清本地缓存
        clearLocalCache();
      }
    }
    return list;
  }

  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, 
                                        ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    //先向缓存中放入占位符？？？
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      //最后删除占位符
      localCache.removeObject(key);
    }
    //加入缓存
    localCache.putObject(key, list);
    //如果是存储过程，OUT参数也加入缓存
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

step8-SimpleExecutor的执行过程

 我们这里暂且以SimpleExecutor为例进行分析，其他还有ReuseExecutor和BatchExecutor。通过handler.<E>query(stmt, resultHandler)调用SimpleStatementHandler。
 在StatementHandler的创建过程中长链路调用过程中我们创建了ParameterHandler。

----------------------SimpleExecutor.java---------------------------------------
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, 
                             ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      //新建一个StatementHandler
      //这里看到ResultHandler传入了,这个StatementHandler 是RoutingStatementHandler
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      //准备语句，transaction获取connection，connection创建statement。
      stmt = prepareStatement(handler, ms.getStatementLog());
      //StatementHandler.query
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }


  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, 
Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    //创建路由选择语句处理器
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, 
                                                                    parameterObject, rowBounds, resultHandler, boundSql);
    //插件在这里插入
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }



  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, 
                                 ResultHandler resultHandler, BoundSql boundSql) {

    //根据语句类型，委派到不同的语句处理器(STATEMENT|PREPARED|CALLABLE)
    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }
  }


public SimpleStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, 
                              RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
  }


protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, 
                               RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    this.configuration = mappedStatement.getConfiguration();
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.rowBounds = rowBounds;

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.objectFactory = configuration.getObjectFactory();

    if (boundSql == null) { // issue #435, get the key before calculating the statement
      generateKeys(parameterObject);
      boundSql = mappedStatement.getBoundSql(parameterObject);
    }

    this.boundSql = boundSql;

    //生成parameterHandler
    this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, 

boundSql);
    //生成resultSetHandler
    this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, 
parameterHandler, resultHandler, boundSql);
  }

step9-StatementHandler的执行过程

 这里我们以SimpleStatementHandler为例进行说明，BaseStatementHandler的具体实现类还包括 CallableStatementHandler和PreparedStatementHandler。

---------------------------------SimpleStatementHandler.java---------------------------

  //select-->结果给ResultHandler
  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    String sql = boundSql.getSql();
    //这里感觉调用jdbc的接口完成查询,待研究
    statement.execute(sql);
    //先执行Statement.execute，然后交给ResultSetHandler.handleResultSets
    return resultSetHandler.<E>handleResultSets(statement);
  }

step10-ResultSetHandler的执行过程

 ResultSetHandler的实现类只有DefaultResultSetHandler。

---------------------------DefaultResultSetHandler.java---------------------------

public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
    
    final List<Object> multipleResults = new ArrayList<Object>();

    int resultSetCount = 0;
    ResultSetWrapper rsw = getFirstResultSet(stmt);

    // 处理 getResultMaps
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    //一般resultMaps里只有一个元素
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) {
      ResultMap resultMap = resultMaps.get(resultSetCount);
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }

    //处理 getResulSets
    String[] resultSets = mappedStatement.getResulSets();
    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);
  }

类关系图

StatementHandler.png Executor.png

参考文档

Mybatis中Mapper动态代理的实现原理
Mybatis3.3.x技术内幕（十一）：执行一个Sql命令的完整流程