MyBatis框架源码解析

朱雀 2022-04-25 10:14 354阅读 0赞

MyBatis是一个优秀的持久层框架，通过简洁的XML配置方式就能消除以前传统  
使用JDBC出现大量重复的代码，以及参数的设置和结果集的映射. 如果能够懂的底层代码和原理，在使用上就会更得心应手。而且出现错误，也能更快的找出错误的原因。解析源码  
还有个好处，就是能够看一下大神的设计思想。这些设计思想也可以运用到实际开发中，而且如果自己明白框架怎么写之后，自己也可以尝试去写一个开源框架出来; 同时，阅读解析源码，也是高级程序员必备的技能之一;

下面我们就从一个简单的示例开始源码的分析:
    
    public static void main(String[] args) {
        String resource = "Configuration.xml";
        Reader reader;
        try {
            //解析配置文件得到Reader
            reader = Resources.getResourceAsReader(resource);
            //通过配置文件得到SqlSessionFactory
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
            //得到SqlSession
            SqlSession session = sqlSessionFactory.openSession();
            try {
                //调用查询方法 完成封装
                List<User> users =  session.selectList("cn.itsource.mapper.UserMapper.getUser");
                users.forEach(user->{
                     System.out.println(user);
                 });
            } finally {
                session.close();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

(1) 第一步 得到SqlSessionFactory对象 ,关键方法 SqlSessionFactoryBuilder().build(reader).我们跟进源码SqlSessionFactoryBuilder类的build方法：

public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
        SqlSessionFactory var5;
        try {
    	// XMLConfigBuilder XML配置解析类
            XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
    	//parser.parse()解析结果放入Configuration里面
    	//build方法得到DefaultSqlSessionFactory
            var5 = this.build(parser.parse());
        } catch (Exception var14) {
            throw ExceptionFactory.wrapException("Error building SqlSession.", var14);
        } finally {
            ErrorContext.instance().reset();
    
            try {
                reader.close();
            } catch (IOException var13) {
                ;
            }
    
        }
    
        return var5;
    }

通过build方法 最终得到DefaultSqlSessionFactory

public SqlSessionFactory build(Configuration config) {
        return new DefaultSqlSessionFactory(config);
    }

已经得到SqlSessionFactory对象了，而且把配置文件信息存入了DefaultSqlSessionFactory 这个类的config变量上;

（2）通过 sqlSessionFactory.openSession()得到SqlSession对象  
源码如下:

public SqlSession openSession() {
    	通过该方法得到SqlSession
        return this.openSessionFromDataSource(this.configuration.getDefaultExecutorType(), (TransactionIsolationLevel)null, false);
    }

openSessionFromDataSource方法如下:

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;
    
        DefaultSqlSession var8;
        try {
    	//从配置类里面得到环境变量
            Environment environment = this.configuration.getEnvironment();
    	//得到事务配置
            TransactionFactory transactionFactory = this.getTransactionFactoryFromEnvironment(environment);
    	//得到事务对象
            tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    	//通过事务得到执行器
            Executor executor = this.configuration.newExecutor(tx, execType, autoCommit);
           //通过执行器得到DefaultSqlSession对象-关键代码
    	 var8 = new DefaultSqlSession(this.configuration, executor);
        } catch (Exception var12) {
            this.closeTransaction(tx);
            throw ExceptionFactory.wrapException("Error opening session.  Cause: " + var12, var12);
        } finally {
            ErrorContext.instance().reset();
        }
    
        return var8;
    }

最终得到一个SqlSession对象(DefaultSqlSession是SqlSession实现类)

(3)通过sqlSession对象调用SelectList方法返回数据

List<User> users =  session.selectList("cn.itsource.mapper.UserMapper.getUser");

我们进入核心的selectList方法里面去:

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
        List var6;
        try {
    	//从配置得到Mapper语句对象 –解析mapper.xml文件
            MappedStatement ms = this.configuration.getMappedStatement(statement);
    	//通过执行器调用query方法 封装数据 返回结果
            List<E> result = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
            var6 = result;
        } catch (Exception var10) {
            throw ExceptionFactory.wrapException("Error querying database.  Cause: " + var10, var10);
        } finally {
            ErrorContext.instance().reset();
        }
    
        return var6;
    }

Query方法最终调用BaseExecutor里面query方法：

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //动态解析sql
        BoundSql boundSql = ms.getBoundSql(parameter);
        //缓存
    CacheKey key = this.createCacheKey(ms, parameter, rowBounds, boundSql);
    //查询
        return this.query(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }

在这个方法里面做了下面一些事情：  
(1) 解析动态的sql  
(2) 加入缓存里面  
(3) 进行查询

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 (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            if (this.queryStack == 0 && ms.isFlushCacheRequired()) {
                this.clearLocalCache();
            }
    
            List list;
            try {
                ++this.queryStack;
                list = resultHandler == null ? (List)this.localCache.getObject(key) : null;
                if (list != null) {
                    this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
                } else {
                    list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
                }
            } finally {
                --this.queryStack;
            }
    
            if (this.queryStack == 0) {
                Iterator i$ = this.deferredLoads.iterator();
    
                while(i$.hasNext()) {
                    BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)i$.next();
                    deferredLoad.load();
                }
    
                this.deferredLoads.clear();
                if (this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
                    this.clearLocalCache();
                }
            }
    
            return list;
        }
    }

从上面的源码中 可以看出最终调用：queryFromDatabase方法  
这个方法最终会调用SimpleExecutor的doQuery方法

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
        Statement stmt = null;
    
        List var9;
        try {
            Configuration configuration = ms.getConfiguration();
            StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, rowBounds, resultHandler, boundSql);
            stmt = this.prepareStatement(handler, ms.getStatementLog());
    	//通过handler调用query查询方法
            var9 = handler.query(stmt, resultHandler);
        } finally {
            this.closeStatement(stmt);
        }
    
        return var9;
    }

最终调用PreparedStatementHandler类的query方法

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
        PreparedStatement ps = (PreparedStatement)statement;
        ps.execute();
    	//处理结果的封装
        return this.resultSetHandler.handleResultSets(ps);
    }

大致的流程图：  
得到SqlSession的过程  
![源码时代][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpbGxhaW55MTM1Nzk_size_16_color_FFFFFF_t_70]  
调用方法的过程  
![源码时代][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpbGxhaW55MTM1Nzk_size_16_color_FFFFFF_t_70 1]  
封装数据过程：  
![源码时代][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpbGxhaW55MTM1Nzk_size_16_color_FFFFFF_t_70 2]

好了,这次源码分析就到这个地方; 通过这次源码剖析,我们看到了mybatis从加载配置文件到调用方法以及返回数据进行封装的整个过程; 从源码里面我们看出，框架底层分别使用很多类和接口来完成开发，而且具备单一职责的原则，一个类就做一个事情，通过层层组合，就完成整套流程，当然 这次分析源码，就是一部分内容。下次在继续.

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