spring 面試題

1.事務何時回滾

默認spring事務只在發生未被捕獲的 runtimeexcetpion時纔回滾。  
   spring aop  異常捕獲原理：被攔截的方法需顯式拋出異常，並不能經任何處理，這樣aop代理才能捕獲到方法的異常，才能進行回滾，默認狀況下aop只捕獲runtimeexception的異常，但能夠經過  
    
配置來捕獲特定的異常並回滾  
  換句話說在service的方法中不使用try catch 或者在catch中最後加上throw new runtimeexcetpion（），這樣程序異常時才能被aop捕獲進而回滾
  解決方案： 
  方案1.

    例如service層處理事務，那麼service中的方法中不作異常捕獲，或者在catch語句中最後增長throw new RuntimeException()語句，以便讓aop捕獲異常再去回滾，而且在service上層（webservice客戶端，view層action）要繼續捕獲這個異常並處理
  方案2.

    在service層方法的catch語句中增長：

    TransactionAspectSupport.currentTransactionStatus().setRollbackOnly();語句，手動回滾，這樣上層就無需去處理異常（如今項目的作法）

RuleBasedTransactionAttribute
/**
	 * Winning rule is the shallowest rule (that is, the closest in the
	 * inheritance hierarchy to the exception). If no rule applies (-1),
	 * return false.
	 * @see TransactionAttribute#rollbackOn(java.lang.Throwable)
	 */
	@Override
	public boolean rollbackOn(Throwable ex) {
		if (logger.isTraceEnabled()) {
			logger.trace("Applying rules to determine whether transaction should rollback on " + ex);
		}

		RollbackRuleAttribute winner = null;
		int deepest = Integer.MAX_VALUE;

		if (this.rollbackRules != null) {
			for (RollbackRuleAttribute rule : this.rollbackRules) {
				int depth = rule.getDepth(ex);
				if (depth >= 0 && depth < deepest) {
					deepest = depth;
					winner = rule;
				}
			}
		}

		if (logger.isTraceEnabled()) {
			logger.trace("Winning rollback rule is: " + winner);
		}

		// User superclass behavior (rollback on unchecked) if no rule matches.
		if (winner == null) {
			logger.trace("No relevant rollback rule found: applying default rules");
			return super.rollbackOn(ex);
		}

		return !(winner instanceof NoRollbackRuleAttribute);
	}

//經過源碼能夠看出，spring默認只對，RuntimeException和Error作處理了
DefaultTransactionAttribute
@Override
public boolean rollbackOn(Throwable ex) {
		return (ex instanceof RuntimeException || ex instanceof Error);
}