mybatis是一款优秀的持久层框架，它支持定制化 SQL、存储过程以及高级映射。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生类型、接口和 Java 的 POJO（Plain Old Java Objects，普通老式 Java 对象）为数据库中的记录。

mybatis关键类

通过源码了解mybatis

 1 @Slf4j
 2 public class MybatisTest {
 3 
 4   //一级缓存
 5   @Test
 6   public void test() throws IOException {
 7 
 8     String resource = "mybatis-config.xml";
 9     InputStream inputStream = Resources.getResourceAsStream(resource);
10     SqlSessionFactory sqlSessionFactory =new SqlSessionFactoryBuilder().build(inputStream);
11     SqlSession sqlSession = sqlSessionFactory.openSession();
12     sqlSession.selectOne("com.jiagouedu.mybatis.UserMapper.selectUser",3);
13     //log.info("user1：{}", sqlSession.selectOne("com.jiagouedu.mybatis.UserMapper.selectUser", 3));
14     //log.info("user2：{}", sqlSession.selectOne("com.jiagouedu.mybatis.UserMapper.selectUser", 3));
15     //sqlSession.commit();
16    // log.info("user3：{}", sqlSession.selectOne("com.jiagouedu.mybatis.UserMapper.selectUser", 3));
17    // log.info("user4：{}", sqlSession.selectOne("com.jiagouedu.mybatis.UserMapper.selectUser", 3));
18   }
19 }

我们使用的是常用的xml形式进行配置mybatis相关属性及SQL编写。

mybatis-config.xml

 1 <?xml version="1.0" encoding="UTF-8" ?>
 2 <!DOCTYPE configuration
 3         PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
 4         "http://mybatis.org/dtd/mybatis-3-config.dtd">
 5 <configuration>
 6    <!-- <settings>
 7         <setting name="cacheEnabled" value="true" />
 8     </settings>-->
 9     <typeAliases>
10         <typeAlias type="com.jiagouedu.pojo.User" alias="user"></typeAlias>
11     </typeAliases>
12 
13     <environments default="development">
14         <environment id="development">
15             <transactionManager type="JDBC"/>
16             <dataSource type="POOLED">
17                 <property name="driver" value="com.mysql.jdbc.Driver"/>
18                 <property name="url" value="jdbc:mysql://localhost:3306/tl-vip"/>
19                 <property name="username" value="root"/>
20                 <property name="password" value="20152974"/>
21             </dataSource>
22         </environment>
23     </environments>
24 
25     <mappers>
26       <mapper resource="mybatis/UserMapper.xml"/>
27     </mappers>
28 </configuration>

View Code

 1 <?xml version="1.0" encoding="UTF-8" ?>
 2 <!DOCTYPE mapper
 3         PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
 4         "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
 5 <mapper namespace="com.jiagouedu.mybatis.UserMapper">
 6     <!--<cache eviction="LRU" type="com.jiagouedu.cache.MybatisRedisCache"/>-->
 7     <select id="selectUser" parameterType="integer" resultType="user">
 8         select * from user where id = #{id}
 9     </select>
10 
11 </mapper>

UserMapper.xml

我们先看一下build方法，主要进行构建xml并进行解析parse()方法。

 1 public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
 2         SqlSessionFactory var5;
 3         try {
 4             XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
 5             var5 = this.build(parser.parse());
 6         } catch (Exception var14) {
 7             throw ExceptionFactory.wrapException("Error building SqlSession.", var14);
 8         } finally {
 9             ErrorContext.instance().reset();
10 
11             try {
12                 inputStream.close();
13             } catch (IOException var13) {
14                 ;
15             }
16 
17         }
18 
19         return var5;
20     }

我们再来看一下parser.parse()方法

1 public Configuration parse() {
2         if (this.parsed) {
3             throw new BuilderException("Each XMLConfigBuilder can only be used once.");
4         } else {
5             this.parsed = true;
6             this.parseConfiguration(this.parser.evalNode("/configuration"));
7             return this.configuration;
8         }
9     }

this.parser.evalNode("/configuration")大家应该猜到它是在找xml文件中的configuration节点，如果不确认大家可以进到this.parseConfiguration方法看一下

 1 private void parseConfiguration(XNode root) {
 2         try {
 3             this.propertiesElement(root.evalNode("properties"));
 4             Properties settings = this.settingsAsProperties(root.evalNode("settings"));
 5             this.loadCustomVfs(settings);
 6             this.typeAliasesElement(root.evalNode("typeAliases"));
 7             this.pluginElement(root.evalNode("plugins"));
 8             this.objectFactoryElement(root.evalNode("objectFactory"));
 9             this.objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
10             this.reflectorFactoryElement(root.evalNode("reflectorFactory"));
11             this.settingsElement(settings);
12             this.environmentsElement(root.evalNode("environments"));
13             this.databaseIdProviderElement(root.evalNode("databaseIdProvider"));
14             this.typeHandlerElement(root.evalNode("typeHandlers"));
15             this.mapperElement(root.evalNode("mappers"));
16         } catch (Exception var3) {
17             throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + var3, var3);
18         }
19     }

properties、settings、typeAliases。。。。这些属性大家都知道怎么去配置吧，所以受builder方法主要就是进行xml文件的读取并加载到内存当中。

解析完第一步的源码后，然后进行我们的第二步sqlSessionFactory.openSession()的源码，看看它做了哪些工作？

 1  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
 2         Transaction tx = null;
 3 
 4         DefaultSqlSession var8;
 5         try {
 6             Environment environment = this.configuration.getEnvironment();
 7             TransactionFactory transactionFactory = this.getTransactionFactoryFromEnvironment(environment);
 8             tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
 9             Executor executor = this.configuration.newExecutor(tx, execType);
10             var8 = new DefaultSqlSession(this.configuration, executor, autoCommit);
11         } catch (Exception var12) {
12             this.closeTransaction(tx);
13             throw ExceptionFactory.wrapException("Error opening session.  Cause: " + var12, var12);
14         } finally {
15             ErrorContext.instance().reset();
16         }
17 
18         return var8;
19     }

new DefaultSqlSession(this.configuration, executor, autoCommit);这是关键的一步，前面就是取我们xml中的配置并开启数据库事务。最后返回我们的sqlsession。
我们再看一下第三步，就是我们sqlSession.selectOne("com.jiagouedu.mybatis.UserMapper.selectUser",3);取数据的那句，

 1  public <T> T selectOne(String statement, Object parameter) {
 2         List<T> list = this.selectList(statement, parameter);
 3         if (list.size() == 1) {
 4             return list.get(0);
 5         } else if (list.size() > 1) {
 6             throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
 7         } else {
 8             return null;
 9         }
10     }

 1 public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
 2         List var5;
 3         try {
 4             MappedStatement ms = this.configuration.getMappedStatement(statement);
 5             var5 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
 6         } catch (Exception var9) {
 7             throw ExceptionFactory.wrapException("Error querying database.  Cause: " + var9, var9);
 8         } finally {
 9             ErrorContext.instance().reset();
10         }
11 
12         return var5;
13     }

this.configuration.getMappedStatement(statement);主要就是取出我们写的mapper。xml对象，以便后续进行sql拼写等操作。

this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);这个操作比较多，我们还是直接看源码

1 public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
2         BoundSql boundSql = ms.getBoundSql(parameterObject);
3         CacheKey key = this.createCacheKey(ms, parameterObject, rowBounds, boundSql);
4         return this.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
5     }

boundSql 就是下列对象，大家肯定不陌生就是我们自己写的SQL，并且参数它也拿到了。

this.createCacheKey方法比较厉害，我们还是看源码

 1 public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
 2         if (this.closed) {
 3             throw new ExecutorException("Executor was closed.");
 4         } else {
 5             CacheKey cacheKey = new CacheKey();
 6             cacheKey.update(ms.getId());
 7             cacheKey.update(rowBounds.getOffset());
 8             cacheKey.update(rowBounds.getLimit());
 9             cacheKey.update(boundSql.getSql());
10             List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
11             TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
12             Iterator var8 = parameterMappings.iterator();
13 
14             while(var8.hasNext()) {
15                 ParameterMapping parameterMapping = (ParameterMapping)var8.next();
16                 if (parameterMapping.getMode() != ParameterMode.OUT) {
17                     String propertyName = parameterMapping.getProperty();
18                     Object value;
19                     if (boundSql.hasAdditionalParameter(propertyName)) {
20                         value = boundSql.getAdditionalParameter(propertyName);
21                     } else if (parameterObject == null) {
22                         value = null;
23                     } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
24                         value = parameterObject;
25                     } else {
26                         MetaObject metaObject = this.configuration.newMetaObject(parameterObject);
27                         value = metaObject.getValue(propertyName);
28                     }
29 
30                     cacheKey.update(value);
31                 }
32             }
33 
34             if (this.configuration.getEnvironment() != null) {
35                 cacheKey.update(this.configuration.getEnvironment().getId());
36             }
37 
38             return cacheKey;
39         }
40     }

cacheKey.update(ms.getId());cacheKey.update(rowBounds.getOffset());cacheKey.update(rowBounds.getLimit());cacheKey.update(boundSql.getSql());
这就是大家所说的mybatis的一级缓存，用的是id+offset+limit+sql组合的key，然后我们在看this.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);方法

 1 public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
 2         Cache cache = ms.getCache();
 3         if (cache != null) {
 4             this.flushCacheIfRequired(ms);
 5             if (ms.isUseCache() && resultHandler == null) {
 6                 this.ensureNoOutParams(ms, parameterObject, boundSql);
 7                 List<E> list = (List)this.tcm.getObject(cache, key);
 8                 if (list == null) {
 9                     list = this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
10                     this.tcm.putObject(cache, key, list);
11                 }
12 
13                 return list;
14             }
15         }
16 
17         return this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
18     }

我们看到，执行sql之前他会先查看缓存中是否有数据，如果有数据将会直接将缓存中的数据返回，如果没有将执行SQL，执行完SQL之后将会再次放入以及缓存当中，我们看一下源码是不是这样做的

 1 public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
 2         ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
 3         if (this.closed) {
 4             throw new ExecutorException("Executor was closed.");
 5         } else {
 6             if (this.queryStack == 0 && ms.isFlushCacheRequired()) {
 7                 this.clearLocalCache();
 8             }
 9 
10             List list;
11             try {
12                 ++this.queryStack;
13                 list = resultHandler == null ? (List)this.localCache.getObject(key) : null;
14                 if (list != null) {
15                     this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
16                 } else {
17                     list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
18                 }
19             } finally {
20                 --this.queryStack;
21             }
22 
23             if (this.queryStack == 0) {
24                 Iterator var8 = this.deferredLoads.iterator();
25 
26                 while(var8.hasNext()) {
27                     BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)var8.next();
28                     deferredLoad.load();
29                 }
30 
31                 this.deferredLoads.clear();
32                 if (this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
33                     this.clearLocalCache();
34                 }
35             }
36 
37             return list;
38         }
39     }

我们再看看this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);

 1 private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
 2         this.localCache.putObject(key, ExecutionPlaceholder.EXECUTION_PLACEHOLDER);
 3 
 4         List list;
 5         try {
 6             list = this.doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
 7         } finally {
 8             this.localCache.removeObject(key);
 9         }
10 
11         this.localCache.putObject(key, list);
12         if (ms.getStatementType() == StatementType.CALLABLE) {
13             this.localOutputParameterCache.putObject(key, parameter);
14         }
15 
16         return list;
17     }

正如我们想的那样，他就是在查询完之后走的缓存存放，以便下次重新查询的时候提高效率，就不用再次查询数据库，来减少数据库压力。

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