mybatis-spring源码分析（一）：默认MyBatisExceptionTranslator的实现方式

说明：对官网里面的一些描述有一些神秘，就来看了一下源码，究竟是如何实现默认使用MyBatisExceptionTranslator进行异常转换的？

public class SqlSessionTemplate implements SqlSession {

  private final SqlSessionFactory sqlSessionFactory;

  private final ExecutorType executorType;

  private final SqlSession sqlSessionProxy;

  private final PersistenceExceptionTranslator exceptionTranslator;

  /**
   * Constructs a Spring managed SqlSession with the {@code SqlSessionFactory}
   * provided as an argument.
   *
   * @param sqlSessionFactory
   */
  public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
    this(sqlSessionFactory, sqlSessionFactory.getConfiguration().getDefaultExecutorType());
  }

  /**
   * Constructs a Spring managed SqlSession with the {@code SqlSessionFactory}
   * provided as an argument and the given {@code ExecutorType}
   * {@code ExecutorType} cannot be changed once the {@code SqlSessionTemplate}
   * is constructed.
   *
   * @param sqlSessionFactory
   * @param executorType
   */
  public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType) {
    this(sqlSessionFactory, executorType,
        new MyBatisExceptionTranslator(
            sqlSessionFactory.getConfiguration().getEnvironment().getDataSource(), true));
  }

  /**
   * Constructs a Spring managed {@code SqlSession} with the given
   * {@code SqlSessionFactory} and {@code ExecutorType}.
   * A custom {@code SQLExceptionTranslator} can be provided as an
   * argument so any {@code PersistenceException} thrown by MyBatis
   * can be custom translated to a {@code RuntimeException}
   * The {@code SQLExceptionTranslator} can also be null and thus no
   * exception translation will be done and MyBatis exceptions will be
   * thrown
   *
   * @param sqlSessionFactory
   * @param executorType
   * @param exceptionTranslator
   */
  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;
    this.sqlSessionProxy = (SqlSession) newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[] { SqlSession.class },
        new SqlSessionInterceptor());
  }

　　上面是SqlSessionTemplate的源码，可以看出其三个构造函数是一层调用一层，即我在配置SqlSessionTemplate bean时，如下配置就可以达到默认使用MyBatisExceptionTranslator的效果。因为在第一个构造函数中调用了第二个构造函数，第二个构造函数又调用了第三个构造函数，并在此时new  MyBatisExceptionTranslator。

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