mybatis-spring.jar對SQLErrorCodes的擴展點 原

1、mybatis-spring.jar作用

mybatis-spring.jar可以讓mybatis代碼無縫地整合到Spring中。使用這個類庫中的類，Spring將會加載必要的MyBatis工廠類和session 類，同時也提供了一個簡單的方式來注入MyBatis數據映射器和SqlSession到業務層的bean中，而且它也會處理事務。並將MyBatis拋出的PersistenceException異常包裝成Spring 的DataAccessException異常(抽象類，數據訪問異常)拋出，下面會詳細說明。

1.1、配置文件

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean" lazy-init="false">
	<property name="dataSource" ref="dataSource" />
	<property name="mapperLocations" value="classpath:sqlmapper/*Mapper.xml" />
	<property name="plugins">
		<list>
			<bean class="com.pinganfu.common.pagination.PaginationInterceptor">
				<property name="dialect">
					<bean class="com.pinganfu.common.pagination.OracleDialect" />
				</property>
			</bean>
		</list>
	</property>
/bean>

<bean id="sqlSession" class="org.mybatis.spring.SqlSessionTemplate">
  <constructor-arg index="0" ref="sqlSessionFactory" />
</bean>

2、SqlSessionTemplate源碼解讀

2.1、處理流程：SqlSessionTemplate內部持有一個mybatis的DefaultSqlSession（使用JDK動態代理生成的類，sqlSessionProxy），通過此代理類可以對SqlSession執行的異常進行包裝之後再拋出，SqlSessionTemplate在創建時會傳入一個exceptionTranslator（MyBatisExceptionTranslator）。MyBatisExceptionTranslator內部引用了SQLExceptionTranslator（SQLErrorCodeSQLExceptionTranslator），SQLExceptionTranslator在初始化時會根據DataSource選擇對應的SQLErrorCodes。SQLErrorCodes由SQLErrorCodesFactory在類加載時初始化，SQLErrorCodesFactory先去加載org/springframework/jdbc/support/sql-error-codes.xml文件中定義的SQLErrorCodes，再去加載classpath，/WEB-INF/classes/sql-error-codes.xml下用戶自定的SQLErrorCodes。

2.2、SqlSessionTemplate初始化

  // SqlSessionTemplate.java
  public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
      PersistenceExceptionTranslator exceptionTranslator) {

    notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
    notNull(executorType, "Property 'executorType' is required");

    this.sqlSessionFactory = sqlSessionFactory;
    this.executorType = executorType;
    this.exceptionTranslator = exceptionTranslator;
    // sqlSessionProxy初始化的地方
    this.sqlSessionProxy = (SqlSession) newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[] { SqlSession.class },
        new SqlSessionInterceptor());
  } 
  // SqlSessionTemplate.java
  public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType) {
    // exceptionTranslator初始化的地址，爲MyBatisExceptionTranslator
    this(sqlSessionFactory, executorType,
        new MyBatisExceptionTranslator(
            sqlSessionFactory.getConfiguration().getEnvironment().getDataSource(), true));
  } 

2.3、SqlSessionInterceptor攔截器

  // SqlSessionInterceptor.java
  private class SqlSessionInterceptor implements InvocationHandler {
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      // 獲取目標SqlSession
      final SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try {
        // 執行SqlSession
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          // force commit even on non-dirty sessions because some databases require
          // a commit/rollback before calling close()
          sqlSession.commit(true);
        }
        return result;
      } catch (Throwable t) {
        Throwable unwrapped = unwrapThrowable(t);
        // 對mybatis拋出的PersistenceException進行轉換
        if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
          Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
          if (translated != null) {
            unwrapped = translated;
          }
        }
        throw unwrapped;
      } finally {
        closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
      }
    }
  }

2.4、 MyBatisExceptionTranslator對異常的翻譯

  // MyBatisExceptionTranslator.java
  public DataAccessException translateExceptionIfPossible(RuntimeException e) {
    if (e instanceof PersistenceException) {
      // Batch exceptions come inside another PersistenceException
      // recursion has a risk of infinite loop so better make another if
      if (e.getCause() instanceof PersistenceException) {
        e = (PersistenceException) e.getCause();
      }
      if (e.getCause() instanceof SQLException) {
        // 初始化異常翻譯器
        this.initExceptionTranslator();
        // 對異常進行翻譯
        return this.exceptionTranslator.translate(e.getMessage() + "\n", null, (SQLException) e.getCause());
      }
      return new MyBatisSystemException(e);
    } 
    return null;
  }
 
  private synchronized void initExceptionTranslator() {
    if (this.exceptionTranslator == null) {
      this.exceptionTranslator = new SQLErrorCodeSQLExceptionTranslator(this.dataSource);
    }
  }

2.5、SQLErrorCodesFactory加載默認和自定義的SqlErrorCodes

  protected SQLErrorCodesFactory() {
    Map<String, SQLErrorCodes> errorCodes;

	try {
	  DefaultListableBeanFactory lbf = new DefaultListableBeanFactory();
	  lbf.setBeanClassLoader(getClass().getClassLoader());
	  XmlBeanDefinitionReader bdr = new XmlBeanDefinitionReader(lbf);

	  // Load default SQL error codes.
	  Resource resource = loadResource(SQL_ERROR_CODE_DEFAULT_PATH);
	  if (resource != null && resource.exists()) {
	 	bdr.loadBeanDefinitions(resource);
	  } else {
		logger.warn("Default sql-error-codes.xml not found (should be included in spring.jar)");
	  }
          // Load custom SQL error codes, overriding defaults.
	  resource = loadResource(SQL_ERROR_CODE_OVERRIDE_PATH);
	  if (resource != null && resource.exists()) {
	    bdr.loadBeanDefinitions(resource);
		logger.info("Found custom sql-error-codes.xml file at the root of the classpath");
	  }

	  // Check all beans of type SQLErrorCodes.
	  errorCodes = lbf.getBeansOfType(SQLErrorCodes.class, true, false);
	  if (logger.isInfoEnabled()) {
	    logger.info("SQLErrorCodes loaded: " + errorCodes.keySet());
		}
	  }
        catch (BeansException ex) {
  	   logger.warn("Error loading SQL error codes from config file", ex);
	   errorCodes = Collections.emptyMap();
        }
  	   this.errorCodesMap = errorCodes;
  }

 

Ref：

http://www.mybatis.org/spring/zh/index.html#