mybatis原理

本文最后更新于:7 个月前

MyBatis工作流程简述

传统工作模式:

public static void main(String[] args) {
		InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
		SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
		SqlSession sqlSession = factory.openSession();
		String name = "tom";
		List<User> list = sqlSession.selectList("com.demo.mapper.UserMapper.getUserByName",params);
}
  1. 创建SqlSessionFactoryBuilder对象,调用build(inputstream)方法读取并解析配置文件,返回SqlSessionFactory对象
  2. 由SqlSessionFactory创建SqlSession 对象,没有手动设置的话事务默认开启
  3. 调用SqlSession中的api,传入Statement Id和参数,内部进行复杂的处理,最后调用jdbc执行SQL语句,封装结果返回。

使用Mapper接口:
由于面向接口编程的趋势,MyBatis也实现了通过接口调用mapper配置文件中的SQL语句

public static void main(String[] args) {
		//前三步都相同
		InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
		SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
		SqlSession sqlSession = factory.openSession();
		//这里不再调用SqlSession 的api,而是获得了接口对象,调用接口中的方法。
		UserMapper mapper = sqlSession.getMapper(UserMapper.class);
		List<User> list = mapper.getUserByName("tom");
}

原生MyBatis原理分析

初始化工作

解析配置文件

InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
//这一行代码正是初始化工作的开始。
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);

进入源码分析:

// 1.我们最初调用的build
public SqlSessionFactory build(InputStream inputStream) {
	//调用了重载方法
    return build(inputStream, null, null);
  }

// 2.调用的重载方法
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      //  XMLConfigBuilder是专门解析mybatis的配置文件的类
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      //这里又调用了一个重载方法。parser.parse()的返回值是Configuration对象
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } //省略部分代码
  }

下面进入对配置文件解析部分:

首先对Configuration对象进行介绍:

Configuration对象的结构和xml配置文件的对象几乎相同。

回顾一下xml中的配置标签有哪些:

properties(属性),settings(设置),typeAliases(类型别名),typeHandlers(类型处理器),objectFactory(对象工厂),mappers(映射器)等

Configuration也有对应的对象属性来封装它们:
在这里插入图片描述
也就是说,初始化配置文件信息的本质就是创建Configuration对象,将解析的xml数据封装到Configuration内部的属性中。

//在创建XMLConfigBuilder时,它的构造方法中解析器XPathParser已经读取了配置文件
//3. 进入XMLConfigBuilder 中的 parse()方法。
public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //parser是XPathParser解析器对象,读取节点内数据,<configuration>是MyBatis配置文件中的顶层标签
    parseConfiguration(parser.evalNode("/configuration"));
    //最后返回的是Configuration 对象
    return configuration;
}

//4. 进入parseConfiguration方法
//此方法中读取了各个标签内容并封装到Configuration中的属性中。
private void parseConfiguration(XNode root) {
    try {
      //issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
}

到此对xml配置文件的解析就结束了(下文会对部分解析做详细介绍),回到步骤 2. 中调用的重载build方法。

// 5. 调用的重载方法
public SqlSessionFactory build(Configuration config) {
	//创建了DefaultSqlSessionFactory对象,传入Configuration对象。
    return new DefaultSqlSessionFactory(config);
  }

配置类方式

发散一下思路,既然解析xml是对Configuration中的属性进行复制,那么我们同样可以在一个类中创建Configuration对象,手动设置其中属性的值来达到配置的效果。

执行SQL

先简单介绍SqlSession

SqlSession是一个接口,它有两个实现类:DefaultSqlSession(默认)和SqlSessionManager(弃用,不做介绍)
SqlSession是MyBatis中用于和数据库交互的顶层类,通常将它与ThreadLocal绑定,一个会话使用一个SqlSession,并且在使用完毕后需要close。
在这里插入图片描述
SqlSession中的两个最重要的参数,configuration与初始化时的相同,Executor为执行器,

Executor:

Executor也是一个接口,他有三个常用的实现类BatchExecutor(重用语句并执行批量更新),ReuseExecutor(重用预处理语句prepared statements),SimpleExecutor(普通的执行器,默认)。

SqlSession API方式

继续分析,初始化完毕后,我们就要执行SQL了:

SqlSession sqlSession = factory.openSession();
String name = "tom";
List<User> list = sqlSession.selectList("com.demo.mapper.UserMapper.getUserByName",params);

获得sqlSession

//6. 进入openSession方法。
  public SqlSession openSession() {
  	//getDefaultExecutorType()传递的是SimpleExecutor
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

//7. 进入openSessionFromDataSource。
//ExecutorType 为Executor的类型,TransactionIsolationLevel为事务隔离级别,autoCommit是否开启事务
//openSession的多个重载方法可以指定获得的SeqSession的Executor类型和事务的处理
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //根据参数创建指定类型的Executor
      final Executor executor = configuration.newExecutor(tx, execType);
      //返回的是DefaultSqlSession
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

执行sqlsession中的api

//8.进入selectList方法,多个重载方法。
public <E> List<E> selectList(String statement) {
    return this.selectList(statement, null);
}
public <E> List<E> selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
}

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //根据传入的全限定名+方法名从映射的Map中取出MappedStatement对象
      MappedStatement ms = configuration.getMappedStatement(statement);
      //调用Executor中的方法处理
      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();
    }
 }

介绍一下MappedStatement

  • 作用: MappedStatement与Mapper配置文件中的一个select/update/insert/delete节点相对应。mapper中配置的标签都被封装到了此对象中,主要用途是描述一条SQL语句。
  • 初始化过程:回顾刚开始介绍的加载配置文件的过程中,会对mybatis-config.xml中的各个标签都进行解析,其中有 mappers标签用来引入mapper.xml文件或者配置mapper接口的目录。
<select id="getUser" resultType="user" >
   select * from user where id=#{id}
 </select>

这样的一个select标签会在初始化配置文件时被解析封装成一个MappedStatement对象,然后存储在Configuration对象的mappedStatements属性中,mappedStatements 是一个HashMap,存储时key = 全限定类名 + 方法名,value = 对应的MappedStatement对象

  • 在configuration中对应的属性为
Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")
  • 在XMLConfigBuilder中的处理:
private void parseConfiguration(XNode root) {
  try {
    // 省略其他标签的处理
    mapperElement(root.evalNode("mappers"));
  } catch (Exception e) {
    throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
  }
}

继续源码中的步骤,进入 executor.query()

//此方法在SimpleExecutor的父类BaseExecutor中实现
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
	//根据传入的参数动态获得SQL语句,最后返回用BoundSql对象表示
    BoundSql boundSql = ms.getBoundSql(parameter);
    //为本次查询创建缓存的Key
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
 }
 
//进入query的重载方法中
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());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
      	// 如果缓存中没有本次查找的值,那么从数据库中查询
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

//从数据库查询
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      // 查询的方法
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    // 将查询结果放入缓存
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

// SimpleExecutor中实现父类的doQuery抽象方法
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();
      // 传入参数创建StatementHanlder对象来执行查询
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      // 创建jdbc中的statement对象
      stmt = prepareStatement(handler, ms.getStatementLog());
      // StatementHandler进行处理
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

// 创建Statement的方法
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    //条代码中的getConnection方法经过重重调用最后会调用openConnection方法,从连接池中获得连接。
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
  }
//从连接池获得连接的方法
protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    //从连接池获得连接
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommit);
  }


//进入StatementHandler进行处理的query,StatementHandler中默认的是PreparedStatementHandler
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    //原生jdbc的执行
    ps.execute();
    //处理结果返回。
    return resultSetHandler.handleResultSets(ps);
  }

接口方式

回顾一下写法:

public static void main(String[] args) {
		//前三步都相同
		InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
		SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
		SqlSession sqlSession = factory.openSession();
		
		//这里不再调用SqlSession 的api,而是获得了接口对象,调用接口中的方法。
		UserMapper mapper = sqlSession.getMapper(UserMapper.class);
		List<User> list = mapper.getUserByName("tom");
}

思考一个问题,通常的Mapper接口我们都没有实现的方法却可以使用,是为什么呢?答案很简单 动态代理


开始之前介绍一下MyBatis初始化时对接口的处理:MapperRegistry是Configuration中的一个属性,它内部维护一个HashMap用于存放mapper接口的工厂类,每个接口对应一个工厂类。mappers中可以配置接口的包路径,或者某个具体的接口类。

<!-- 将包内的映射器接口实现全部注册为映射器 -->
<mappers>
  <mapper class="com.demo.mapper.UserMapper"/>
  <package name="com.demo.mapper"/>
</mappers>
  • 当解析mappers标签时,它会判断解析到的是mapper配置文件时,会再将对应配置文件中的增删改查标签一 一封装成MappedStatement对象,存入mappedStatements中。(上文介绍了)
  • 当判断解析到接口时,会创建此接口对应的MapperProxyFactory对象,存入HashMap中,key = 接口的字节码对象,value = 此接口对应的MapperProxyFactory对象。
//MapperRegistry类
public class MapperRegistry {
  private final Configuration config;
  //这个类中维护一个HashMap存放MapperProxyFactory
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();

  //解析到接口时添加接口工厂类的方法
  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 {
        //重点在这行,以接口类的class对象为key,value为其对应的工厂对象,构造方法中指定了接口对象
        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);
        }
      }
    }
  }
}

正文:
进入sqlSession.getMapper(UserMapper.class)中

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

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

//MapperRegistry中的getMapper
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
	//从MapperRegistry中的HashMap中拿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 {
      // 通过动态代理工厂生成示例。
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
}

//MapperProxyFactory类中的newInstance方法
 public T newInstance(SqlSession sqlSession) {
 	// 创建了JDK动态代理的Handler类
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    // 调用了重载方法
    return newInstance(mapperProxy);
  }

//MapperProxy类,实现了InvocationHandler接口
public class MapperProxy<T> implements InvocationHandler, Serializable {
  
  //省略部分源码	

  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;
  
  // 构造,传入了SqlSession,说明每个session中的代理对象的不同的!
  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }
  
  //省略部分源码
}

//重载的方法,由动态代理创建新示例返回。
protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
 }

在动态代理返回了示例后,我们就可以直接调用mapper类中的方法了,说明在MapperProxy中的invoke方法中已经为我们实现了方法。

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      //判断调用是是不是Object中定义的方法,toString,hashCode这类非。是的话直接放行。
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    } 
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    // 重点在这:MapperMethod最终调用了执行的方法
    return mapperMethod.execute(sqlSession, args);
  }


public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //判断mapper中的方法类型,最终调用的还是SqlSession中的方法
    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()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
          if (method.returnsOptional() &&
              (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = Optional.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;
  }

mybatis原理
https://lunasaw.github.io/2020/11/04/mybatis-princpal/
著者
luna
作成日
2020年11月4日
著作権