事务回滚原理

        // 单测方法执行前,移除容器原来的事务管理器,然后开启一个新的事务
           @Override
       	public void beforeTestMethod(final TestContext testContext) throws Exception {
       		Method testMethod = testContext.getTestMethod();
        		Class<?> testClass = testContext.getTestClass();
        		Assert.notNull(testMethod, "Test method of supplied TestContext must not be null");
       		TransactionContext txContext = TransactionContextHolder.removeCurrentTransactionContext();
        		Assert.state(txContext == null, "Cannot start new transaction without ending existing transaction");
       		PlatformTransactionManager tm = null;
       		TransactionAttribute transactionAttribute = this.attributeSource.getTransactionAttribute(testMethod, testClass);
       		if (transactionAttribute != null) {
        			transactionAttribute = TestContextTransactionUtils.createDelegatingTransactionAttribute(testContext,
        				transactionAttribute);
       			if (logger.isDebugEnabled()) {
        				logger.debug("Explicit transaction definition [" + transactionAttribute +
       						"] found for test context " + testContext);
        			}
       			if (transactionAttribute.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
       				return;
        			}
        			tm = getTransactionManager(testContext, transactionAttribute.getQualifier());
        			Assert.state(tm != null,
        					() -> "Failed to retrieve PlatformTransactionManager for @Transactional test: " + testContext);
        		}
       		if (tm != null) {
        			txContext = new TransactionContext(testContext, tm, transactionAttribute, isRollback(testContext));
        			runBeforeTransactionMethods(testContext);
        			txContext.startTransaction();
        			TransactionContextHolder.setCurrentTransactionContext(txContext);
        		}
        	}
       	// 单测方法执行结束后,结束事务然后回滚或提交
       	@Override
       	public void afterTestMethod(TestContext testContext) throws Exception {
       		Method testMethod = testContext.getTestMethod();
        		Assert.notNull(testMethod, "The test method of the supplied TestContext must not be null");
       		TransactionContext txContext = TransactionContextHolder.removeCurrentTransactionContext();
       		// If there was (or perhaps still is) a transaction...
       		if (txContext != null) {
       			TransactionStatus transactionStatus = txContext.getTransactionStatus();
       			try {
       				// If the transaction is still active...
       				if (transactionStatus != null && !transactionStatus.isCompleted()) {
        					txContext.endTransaction();
        				}
        			}
       			finally {
        				runAfterTransactionMethods(testContext);
        			}
        		}
        	}