一起学习Mybatis----mybatis的运行原理

不念不忘少年蓝@ 2024-02-19 14:21 37阅读 0赞

**mybatis的生命周期包含4部分：**

1. SqlSessionFactoryBuilder

2.SqlSessionFactory

3.SqlSession

4.Mapper

mybatis的运行是先根据相关的配置文件通过SqlSessionFactoryBuilder构建SqlSessionFactory，在根据SqlSessionFactory去创建响应的SqlSession，SqlSession在根据Mapper传递的sql,进行相关的动作，返回结构给Mapper.

**构建SqlSessionFactory过程：**

第一步：  
通过 org.apache. ibatis. builder.xmL.XMLConfigBuilder解析配置的XML文件,读出配置参数,并将读取的数据存入这个org.apache ibatis session. Configuration类中。注意MyBatis几乎所有的配置都是存在这里的。  
第二步：  
使用 Confinguration对象去创建 SqlSession Factory。 My Batis中的 SqlSession Factory是一个接口,而不是实现类,为此 My Batis提供了一个默认的 Sqlsession Factory实现类,我们般都会使用它 org.apache ibatis session. defaults. Defaultsqlsession Factory。注意,在大部分情况下我们都没有必要自己去创建新的 Sqlsession Factory的实现类。

Configuration 源码：

public class Configuration {
        protected Environment environment;
        protected boolean safeRowBoundsEnabled;
        protected boolean safeResultHandlerEnabled;
        protected boolean mapUnderscoreToCamelCase;
        protected boolean aggressiveLazyLoading;
        protected boolean multipleResultSetsEnabled;
        protected boolean useGeneratedKeys;
        protected boolean useColumnLabel;
        protected boolean cacheEnabled;
        protected boolean callSettersOnNulls;
        protected boolean useActualParamName;
        protected boolean returnInstanceForEmptyRow;
        protected String logPrefix;
        protected Class<? extends Log> logImpl;
        protected Class<? extends VFS> vfsImpl;
        protected LocalCacheScope localCacheScope;
        protected JdbcType jdbcTypeForNull;
        protected Set<String> lazyLoadTriggerMethods;
        protected Integer defaultStatementTimeout;
        protected Integer defaultFetchSize;
        protected ExecutorType defaultExecutorType;
        protected AutoMappingBehavior autoMappingBehavior;
        protected AutoMappingUnknownColumnBehavior autoMappingUnknownColumnBehavior;
        protected Properties variables;
        protected ReflectorFactory reflectorFactory;
        protected ObjectFactory objectFactory;
        protected ObjectWrapperFactory objectWrapperFactory;
        protected boolean lazyLoadingEnabled;
        protected ProxyFactory proxyFactory;
    。。。。。。

Configuration作用：

1. 读入配置文件,包括基础配置的XML文件和映射器的XML文件。  
2.初始化基础配置,比如 My Batis的别名等,一些重要的类对象,例如,插件、映射器、 ObjectFactory和 type Handler对象  
3.提供单例,为后续创建 SessionFactory服务并提供配置的参数执行一些重要的对象方法,初始化配置信息。

在第一步中Mybatis会读取所有的xml文件，将其初始化保存到 Configuration类的单例对象中，初始化包含了：properties全局参数，settings设置，typeAliases别名，typeHandler类型处理器，ObjectFactory对象，plugin插件，environment环境，DatabaseldProvider数据库标识，Mapper映射器。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 1][]

sqlSessionFactory=new SqlSessionFactoryBuilder(). build(inputstream).

**org.apache.ibatis.session.SqlSessionFactoryBuilder中的builder方法。**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 2][]  
 MyBatis会根据 Configuration的配置读取所配置的信息,构建 SqlsessionFactory对象。

**SqLSession运行过程：**

探究其原理的了解一下 Mapper映射原理。

Mapper映射器：构建需要执行的Sql脚本。由三部分组成：

1. MappedStatement,它保存映射器的一个节点( select|insert|delete|update)。包括许多我们配置的SQL、SQL的id、缓存信息、 resultMap、 parameterType、 resultType、languageDriver等重要配置内容。  
2.SqLSource,它是提供 BoundS对象的地方,它是 MappedStatement的一个属性。  
3. BoundSql,它是建立SQL和参数的地方。它有3个常用的属性:SQL、 parameterObject ,parameterMappings。

Mapper映射是通过动态代理实现的。

/**
     *    Copyright 2009-2015 the original author or authors.
     *
     *    Licensed under the Apache License, Version 2.0 (the "License");
     *    you may not use this file except in compliance with the License.
     *    You may obtain a copy of the License at
     *
     *       http://www.apache.org/licenses/LICENSE-2.0
     *
     *    Unless required by applicable law or agreed to in writing, software
     *    distributed under the License is distributed on an "AS IS" BASIS,
     *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     *    See the License for the specific language governing permissions and
     *    limitations under the License.
     */
    package org.apache.ibatis.binding;
    
    import java.lang.reflect.Method;
    import java.lang.reflect.Proxy;
    import java.util.Map;
    import java.util.concurrent.ConcurrentHashMap;
    
    import org.apache.ibatis.session.SqlSession;
    
    /**
     * @author Lasse Voss
     */
    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;
      }
    
      public Class<T> getMapperInterface() {
        return mapperInterface;
      }
    
      public Map<Method, MapperMethod> getMethodCache() {
        return methodCache;
      }
    
      @SuppressWarnings("unchecked")
      protected T newInstance(MapperProxy<T> mapperProxy) {
       //动态代理对Mapper接口实行了代理模式，而代理的方法都放置到了MapperProxy类中了
        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);
      }
    
    }

package org.apache.ibatis.binding;
    
    import java.io.Serializable;
    import java.lang.reflect.InvocationHandler;
    import java.lang.reflect.Method;
    import java.util.Map;
    
    import org.apache.ibatis.reflection.ExceptionUtil;
    import org.apache.ibatis.session.SqlSession;
    
    /**
      动态代理模式
     * @author Clinton Begin
     * @author Eduardo Macarron
     */
    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 {
        //判断它是否是一个类，显然这里 Mapper是一个接口不是类,所以判定失败
        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);
      }
    
      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;
      }
    
    }

上面运用了 invoke方法。一旦 mapper是一个代理对象,那么它就会运行到 invoke方法里面, invoke首先判断它是否是一个类,显然这里 Mapper是一个接口不是类,所以判定失败。那么就会生成 MapperMethod对象,它是通过 cachedMapperMethod方法对其初始化的,然后执行 execute方法,把 sqlsession和当前运行的参数传递进去。  
![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 3][]

Mapper执行的过程是通过 **Executor、 StatementHandler、Parameter Handler和 Resulthandler**来完成数据库操作和结果返回的。

Executor代表执行器,由它来调度 Statementhandler、 ParameterHandler、 Resulthandler等来执行对应的SQL。

StatementHandler的作用是使用数据库的 Statement( PreparedStatement)执行操作,它是四大对象的核心,起到承上启下的作用。

ParameterHandler用于SQL对参数的处理。

Resulthandler是进行最后数据集( Resultset)的封装返回处理的。

Executor执行器：创建执行器首先根据Configuration获取执行器类型。

在创建sqlSession就开始初始化：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 4][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 5][]

生成指定的执行器。

public class SimpleExecutor extends BaseExecutor {
    
      。。。。。。。。。。。。。。。。。。。。。。。。。。。。
    
      @Override
      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();
          StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
          stmt = prepareStatement(handler, ms.getStatementLog());
          return handler.<E>query(stmt, resultHandler);
        } finally {
          closeStatement(stmt);
        }
      }
    
     。。。。。。。。。。。。。。。。。。。。。。。。。。
    
    }

显然 MyBatis根据 Configuration来构建 Statementhandler,然后使用 prepareStatement方法,对SQL编译并对参数进行初始化,我们在看它的实现过程,它调用了 Statementhandler的prepare()进行了预编译和基础设置,然后通过 StatementHandler的 parameterize()来设置参数并执行, resulthandler再组装査询结果返回给调用者来完成一次查询。

数据库会话器 StatementHandler：专门处理数据库会话的。

创建会话器：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 6][]

![20181207201214182.png][]

RoutingStatementHandler不是我们真实的服务对象,它是通过适配模式找到对应的StatementHandler来执行的。在MyBatis中, StatementHandler和 Executor一样分为三种SimpleStatementHandler、PreparedStatementHandler、CallableStatementHandler它所对应的是三种执行器。

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

我们最常用的是PreparedStatementHandler：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 7][]

预编译

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 8][]

预编译完成就需要开始设置参数。

ParameterHandler：

![20181207202051860.png][]

![20181207202139903.png][]

![20181207202419334.png][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 9][]

设置完参数就需要开始查询了，进行结构封装。

Resulthandler：

实现：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 10][]

结果封装返回。

SqlSession运行原理图：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzU0OTU3OA_size_16_color_FFFFFF_t_70 11][]

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[20181207201214182.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/f88b4faefdaf48bb974c7e48b460d6d0.png
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[20181207202051860.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/db1e3a44cf574a43b08c8e8786837552.png
[20181207202139903.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/5aecc5b3329f4de5a1952c5e2e54dfed.png
[20181207202419334.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/d7c255af48be4a9aa852f0819a289028.png
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