持久層Mybatis3底層原始碼分析，原理解析

Mybatis-持久層的框架，功能是非常強大的，對於移動網際網路的高併發 和 高效能是非常有利的，相對於Hibernate全自動的ORM框架，Mybatis簡單，易於學習，sql編寫在xml檔案中，和程式碼分離，易於維護，屬於半ORM框架，對於面向使用者層面的網際網路業務效能和併發，可以通過sql優化解決一些問題。

現如今大部分公司都在使用Mybatis，所以我們要理解框架底層的原理。閒話不多說。

Mybatis框架的核心入口 是SqlSessionFactory介面，我們先看一下它的程式碼

public interface SqlSessionFactory {

  SqlSession openSession();

  SqlSession openSession(boolean autoCommit);
  SqlSession openSession(Connection connection);
  SqlSession openSession(TransactionIsolationLevel level);

  SqlSession openSession(ExecutorType execType);
  SqlSession openSession(ExecutorType execType, boolean autoCommit);
  SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);
  SqlSession openSession(ExecutorType execType, Connection connection);

  Configuration getConfiguration();

}

SqlSessionFactory介面很多過載的openSession方法，返回sqlSession型別 物件， 還有Configuration類（這個類非常強大，下面會梳理），我們先看一下SqlSession的程式碼

public interface SqlSession extends Closeable {

 
  <T> T selectOne(String statement);


  <T> T selectOne(String statement, Object parameter);

  <E> List<E> selectList(String statement);

  <E> List<E> selectList(String statement, Object parameter);

  <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds);

  <K, V> Map<K, V> selectMap(String statement, String mapKey);
  <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey);

  <K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds);

  <T> Cursor<T> selectCursor(String statement);
  <T> Cursor<T> selectCursor(String statement, Object parameter);

  <T> Cursor<T> selectCursor(String statement, Object parameter, RowBounds rowBounds);


  void select(String statement, Object parameter, ResultHandler handler);


  void select(String statement, ResultHandler handler);


  List<BatchResult> flushStatements();

  /**
   * Closes the session
   */
  @Override
  void close();

  void clearCache();


  Configuration getConfiguration();


  <T> T getMapper(Class<T> type);

  Connection getConnection();
}

只是展示了部分程式碼，但我們可以看到，sqlSeesion裡面 大多數方法是 增刪改查的執行方法，包括查詢返回不同的資料結構，比較注意的是clearCache()和getConnection()方法，一個是清楚快取，一個是獲取連線，獲取資料庫連線在這不在描述， 為什麼要注意清楚快取那，因為mybatis框架是實現了 快取的，分為一級快取，二級快取，當增刪改的時候就會呼叫此方法，刪除快取（後續會專門寫一篇文章來分析Mybatis快取），先在這給大家熟悉一下。

上面的SqlSessionFactory和SqlSeesion都是介面，我們在看一下實現類DefaultSqlSessionFactory和DefaultSqlSession,下面展示DefaultSqlSessionFactory的比較核心的程式碼

  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);
      final Executor executor = configuration.newExecutor(tx, execType);
      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();
    }
  }

其實SqlSessionFactory中的多個過載openSeesion方法最終都是執行的這個方法，我們可以看到這個方法中 通過 configuration屬性 獲取到Executor 執行器物件，DefaultSqlSession構造器把這configuration和executor當成構造引數，初始化建立一個 DefaultSqlSession物件，然後我們在展示一下DefaultSqlSession程式碼中的大家一看就理解的程式碼

  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      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();
    }
  }

看到這個方法大家估計就會看明白了，底層執行的就是 通過Executor 物件執行的 查詢， 通過configuration獲取到 要執行的sql,獲取到我們需要的結果。

從上面程式碼可以看出 Configuration 類無處不在，那我們就去看一下原始碼

public class Configuration {

  protected Environment environment;

  protected boolean safeRowBoundsEnabled;
  protected boolean safeResultHandlerEnabled = true;
  protected boolean mapUnderscoreToCamelCase;
  protected boolean aggressiveLazyLoading;
  protected boolean multipleResultSetsEnabled = true;
  protected boolean useGeneratedKeys;
  protected boolean useColumnLabel = true;
  protected boolean cacheEnabled = true;
  protected boolean callSettersOnNulls;
  protected boolean useActualParamName = true;
  protected boolean returnInstanceForEmptyRow;

  protected String logPrefix;
  protected Class <? extends Log> logImpl;
  protected Class <? extends VFS> vfsImpl;
  protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
  protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
  protected Set<String> lazyLoadTriggerMethods = new HashSet<String>(Arrays.asList(new String[] { "equals", "clone", "hashCode", "toString" }));
  protected Integer defaultStatementTimeout;
  protected Integer defaultFetchSize;
  protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
  protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;
  protected AutoMappingUnknownColumnBehavior autoMappingUnknownColumnBehavior = AutoMappingUnknownColumnBehavior.NONE;

  protected Properties variables = new Properties();
  protected ReflectorFactory reflectorFactory = new DefaultReflectorFactory();
  protected ObjectFactory objectFactory = new DefaultObjectFactory();
  protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();

  protected boolean lazyLoadingEnabled = false;
  protected ProxyFactory proxyFactory = new JavassistProxyFactory(); // #224 Using internal Javassist instead of OGNL

  protected String databaseId;
  /**
   * Configuration factory class.
   * Used to create Configuration for loading deserialized unread properties.
   *
   * @see <a href='https://code.google.com/p/mybatis/issues/detail?id=300'>Issue 300 (google code)</a>
   */
  protected Class<?> configurationFactory;

  protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
  protected final InterceptorChain interceptorChain = new InterceptorChain();
  protected final TypeHandlerRegistry typeHandlerRegistry = new TypeHandlerRegistry();
  protected final TypeAliasRegistry typeAliasRegistry = new TypeAliasRegistry();
  protected final LanguageDriverRegistry languageRegistry = new LanguageDriverRegistry();

  protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection");
  protected final Map<String, Cache> caches = new StrictMap<Cache>("Caches collection");
  protected final Map<String, ResultMap> resultMaps = new StrictMap<ResultMap>("Result Maps collection");
  protected final Map<String, ParameterMap> parameterMaps = new StrictMap<ParameterMap>("Parameter Maps collection");
  protected final Map<String, KeyGenerator> keyGenerators = new StrictMap<KeyGenerator>("Key Generators collection");

  protected final Set<String> loadedResources = new HashSet<String>();
  protected final Map<String, XNode> sqlFragments = new StrictMap<XNode>("XML fragments parsed from previous mappers");

  protected final Collection<XMLStatementBuilder> incompleteStatements = new LinkedList<XMLStatementBuilder>();
  protected final Collection<CacheRefResolver> incompleteCacheRefs = new LinkedList<CacheRefResolver>();
  protected final Collection<ResultMapResolver> incompleteResultMaps = new LinkedList<ResultMapResolver>();
  protected final Collection<MethodResolver> incompleteMethods = new LinkedList<MethodResolver>();

  /*
   * A map holds cache-ref relationship. The key is the namespace that
   * references a cache bound to another namespace and the value is the
   * namespace which the actual cache is bound to.
   */
  protected final Map<String, String> cacheRefMap = new HashMap<String, String>();}

上面的程式碼都是 Configuration類中的屬性值，上面的boolean 型別的屬性 都是一些配置的屬性，比如useGeneratedKeys是否開啟使用返回主鍵，cacheEnabled是否開啟快取等等，下面的Map型別的 就是儲存一些我們專案中需要編寫的sql.xml檔案，我們可以通過變數名大致推測出來儲存的結果，比如typeAliasRegistry 儲存的別名，mappedStatements 儲存的sql，resultMaps儲存的結果等，當然這些map的key對應的就是 sql.xml中的唯一的id，分析到現在，我們大致知道Mybatis框架底層的執行原理了。

但是，這時候就有個疑問了，入口類是SqlSessionFactory，那是怎麼載入資源的那，我們通過名稱尋找原始碼，可以找到一個SqlSessionFactoryBuilder（這些開發開源框架的牛人們不管技術NB，對類的命名也是很值的大家效仿的），builder--載入， 說明這個類就是載入 SqlSessionFactory,我們看一下程式碼

public class SqlSessionFactoryBuilder {

  public SqlSessionFactory build(Reader reader) {
    return build(reader, null, null);
  }

  public SqlSessionFactory build(Reader reader, String environment) {
    return build(reader, environment, null);
  }

  public SqlSessionFactory build(Reader reader, Properties properties) {
    return build(reader, null, properties);
  }

  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

  public SqlSessionFactory build(InputStream inputStream) {
    return build(inputStream, null, null);
  }

  public SqlSessionFactory build(InputStream inputStream, String environment) {
    return build(inputStream, environment, null);
  }

  public SqlSessionFactory build(InputStream inputStream, Properties properties) {
    return build(inputStream, null, properties);
  }

  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }
    
  public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

}

檢視程式碼中的build方法，可以看出是 通過流來載入xml檔案 ，包括mybatis的配置檔案和 sql.xml檔案，返回一個DefaultSqlSessionFactory 物件。

本篇檔案只是介紹了mybatis的底層執行原理，喜歡深入瞭解的可以自己去深入瞭解一下。

以上是個人理解，歡迎大家來討論，不喜勿噴！謝謝！！

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