Mybatis源碼解析(二) 創建SqlSession

前言

上篇文章講到了SqlSessionFactory的創建,這篇文章繼續對Mybatis的會話進行解析,那什麼是會話?

會話就是客戶端與數據庫建立的對話,一個Client和一個Database必須有一個會話,才能相互之間進行數據交互,它在Mybatis裏面對應的是SqlSession。

那麼哪些事情是和會話相關的?

數據庫的增刪查改
數據庫鏈接
數據庫事務
數據緩存

Mybatis是怎麼去實現上這些功能的?那麼接下來我們,就從代碼的角度去分析SqlSession

創建SqlSession

sqlSession操作數據庫流程

DefaultSqlSessionFactory通過openSession()方法創建DefaultSqlSession實例

public class DefaultSqlSessionFactory implements SqlSessionFactory {

 @Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }
  
 private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      //1.獲取Environment配置
      final Environment environment = configuration.getEnvironment();
      //2.獲取TransactionFactory
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //3.從configuration裏面拿到Executor
      final Executor executor = configuration.newExecutor(tx, execType);
      //4.實例化SqlSession
      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();
    }
  }
    private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
    if (environment == null || environment.getTransactionFactory() == null) {
      return new ManagedTransactionFactory();
    }
    return environment.getTransactionFactory();
  }
}

上面的代碼做了4個動作:

獲取Environment配置
獲取TransactionFactory
從configuration裏面拿到Executor
實例化SqlSession

1.獲取Environment配置
通過configuration.getEnvironment()是從配置文件裏拿到Environment對象

public final class Environment {
  private final String id;
  private final TransactionFactory transactionFactory;
  private final DataSource dataSource;
 }

Environment對象裏面有三個字段成員,分別爲id,transactionFactory,dataSource。

id是environment節點的名字,transactionFactory是用來創建事務的工廠,dataSource是數據庫的鏈接,這3個成員在下圖中都能找到對應的配置我們再回想一下,SqlSessionFactoryBuilder中的有個build()方法的重載,其中有個需要傳入environment的參數的,它的作用就是指定這個environment的id(Mybatis是可以配置多個environment,如果有配置多個則需要指定使用的是哪個)

2.獲取TransactionFactory
我們再看getTransactionFactoryFromEnvironment(environment)方法

  private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
    if (environment == null || environment.getTransactionFactory() == null) {
      return new ManagedTransactionFactory();
    }
    return environment.getTransactionFactory();
  }

“environment==null || environment.getTransactionFactory() == null” 用來判斷用戶配置TransactionFactory是否爲空,如果空的話返回默認的

public class ManagedTransactionFactory implements TransactionFactory {

  @Override
  public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
    return new ManagedTransaction(ds, level, closeConnection);
  }
}

ManagedTransactionFactory.ManagedTransactionFactory對象創建的是ManagedTransaction事務,這個事務其實就是沒有事務,裏面並沒有commit和rolback實現(所以如果)

3.從configuration裏面拿到Executor
從configuration中拿到Executor對象(configuration其實也算是個工廠)

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

Executor是執行器,是Mybatis真正用來執行操作的類,executorType參數是Executor的類型,從外面傳進來,如果用戶沒有指定是哪類執行器,SqlSessionFactory會從configuration.getDefaultExecutorType()中拿到默認的(Simple類型)

Executor有4種類型:

BatchExecutor可批量操作的
ReuseExecutor可重複使用的
SimpleExecutor默認的
CachingExecutor帶有二級緩存的

newExecutor()方法通過傳進來的executorType去實例化對應的Executor,如果配置了cacheEnabled=true的,還會將實例化後的executor再包裝一下CachingExecutor。

4. 實例化SqlSession
SqlSession的創建是通過"new DefaultSqlSession(configuration, executor, autoCommit)"去實例化出來的,這裏傳了三個參數,分別爲configuration配置,executor執行器和autoCommit是否自動提交

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);
  void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler);
  int insert(String statement);
  int insert(String statement, Object parameter);
  int update(String statement);
  int update(String statement, Object parameter);
  int delete(String statement);
  int delete(String statement, Object parameter);
  void commit();
  void commit(boolean force);
  void rollback();
  void rollback(boolean force);
  List<BatchResult> flushStatements();
  void close();
  void clearCache();
  Configuration getConfiguration();
  <T> T getMapper(Class<T> type);
  Connection getConnection();
}

從上面定義的接口上看,我們可以把它歸爲4類型:

數據庫的增刪查改操作
事務的提交和回滾
數據連接的關閉
緩存清理

接下來我們再看DefaultSqlSession的字段成員和構造函數

public class DefaultSqlSession implements SqlSession {

  //配置信息
  private final Configuration configuration;
  //執行器
  private final Executor executor;
  //是否自動提交
  private final boolean autoCommit;
  //是否變髒
  private boolean dirty;
  //遊標(作用是可以用來一條一條的查詢結果集)
  private List<Cursor<?>> cursorList;

  public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
    this.configuration = configuration;
    this.executor = executor;
    this.dirty = false;
    this.autoCommit = autoCommit;
  }
}

DefaultSqlSession 有5個成員變量,其中configuration,executor,autoCommit都是從外面傳進來的,dirty的初始值爲false,是用來記錄sqlSession是否有數據未做提交(insert和update之後會改成true,然後 comit,rollback或close之後都會重新把它改爲false),cursorList是遊標的集合,DefaultSqlSession只有在執行selectCursor時纔會進行對它初始化(遊標是用於遍歷結果集的,每次只查一條數據).

接下來我們來看selectOne方法

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

可以看到selectOne其實是特殊的selectList()方法,它會對結果集的條數作判斷,如果超過了一條會拋出異常,只有條數等於1的情況纔拿出第一條數據返回。(所以使用selectOne一定要保證查詢語句只會查出一條數據,如果有多條數據,Mybatis不會幫你做處理的)

  @Override
  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();
    }
  }
  
 @Override
  public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      executor.query(ms, wrapCollection(parameter), rowBounds, handler);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

上面的只列出了select()方法的兩個重載,一個有ResultHandler和一個沒有ResultHandler的,區別帶ResultHandler的是沒有返回參數,它需要通過ResultHandler去處理結果集, select()方法首先時獲取MappedStatement,MappedStatement是執行操作的內容的模板,Mybatis在通過SqlSessionFactoryBuider創建SqlSessionFactory時,就已經把所有的Mapper配置轉成MappedStatement並緩存到了Configuration了(我在上篇博客有介紹MappedStatement初始化的過程)

  public MappedStatement getMappedStatement(String id) {
    return this.getMappedStatement(id, true);
  }
  public MappedStatement getMappedStatement(String id, boolean validateIncompleteStatements) {
    if (validateIncompleteStatements) {
      buildAllStatements();
    }
    return mappedStatements.get(id);
  }

configuration.getMappedStatement()的方法,是通過MappedStatement的id直接去configuration裏的Map緩存裏面拿的,拿到了MappedStatement之後,再把parameter通過包裝成數組的形式,最後交給executor的query()方法去執行(下一篇再介紹executor的原理)。executor的query()也有對應的重載,帶ResultHandler和不帶的,帶ResultHandler的方法是沒有返回值的,它直接是填充到ResultHandler裏面去。

插入,刪除,修改功能:

  @Override
  public int insert(String statement) {
    return insert(statement, null);
  }
  @Override
  public int insert(String statement, Object parameter) {
    return update(statement, parameter);
  }
  @Override
  public int update(String statement) {
    return update(statement, null);
  }
  @Override
  public int update(String statement, Object parameter) {
    try {
      dirty = true;
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.update(ms, wrapCollection(parameter));
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error updating database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
  @Override
  public int delete(String statement) {
    return update(statement, null);
  }
  @Override
  public int delete(String statement, Object parameter) {
    return update(statement, parameter);
  }

不管是insert、update、delete,在sqlSession裏面其實都是執行update操作,區分開來的作用只是sqlSession的模板的一種格式,和select類似,update()方法也是把執行的細節放在executor裏面,只不過executor.update()方法返回時的是int(影響條數)

除了增刪查改之外,sqlSession的其他方法

 @Override
  public void commit() {
    commit(false);
  }
 @Override
  public void commit(boolean force) {
    try {
      //執行提交,force爲true時強制提交,false需要判斷數據是後變髒和是否設置爲自動提交
      executor.commit(isCommitOrRollbackRequired(force));
      dirty = false;
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error committing transaction.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
  @Override
  public void rollback() {
    rollback(false);
  }
  @Override
  public void rollback(boolean force) {
    try {
      //執行回滾,force爲true時強制提交,false需要判斷數據是後變髒和是否設置爲自動提交
      executor.rollback(isCommitOrRollbackRequired(force));
      dirty = false;
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error rolling back transaction.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
   //判斷是否提交,force爲true時強制執行
  private boolean isCommitOrRollbackRequired(boolean force) {
    return (!autoCommit && dirty) || force;
  }
  @Override
  public void clearCache() {
    //清除本地緩存
    executor.clearLocalCache();
  }

   @Override
  public void close() {
    try {
      //關閉連接
      executor.close(isCommitOrRollbackRequired(false));
      closeCursors();
      dirty = false;
    } finally {
      ErrorContext.instance().reset();
    }
  }
  @Override
  public List<BatchResult> flushStatements() {
    try {
      //該方法針對批量操作的,批量操作時將statement緩存到一個list上,調用這個方法時才把list中的Statement提交到數據庫,提交完後再清空list
      return executor.flushStatements();
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error flushing statements.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

executor執行commit()、rollback()和close()時,會通過上面提到的TransactionFactory類,拿到具體的Transaction,再調用Transaction的commit、rollback和close方法

結語

SqlSession其實是一個模板方法,裏面封裝了用戶一些常用的操作,它的內部並沒有太多的執行細節,而是把複雜的處理邏輯放到了Executor裏面去

這篇對SqlSession介紹的博客到此爲止,後面會繼續分析Excutor的執行過程。

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Mybatis源碼解析(二) 創建SqlSession

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