SQLiteDatabase事务操作为什么一定要执行endTra

只有执行了endTransaction方法，事务操作才会真正提交到数据库。
来看源码：

    /**
     * End a transaction. See beginTransaction for notes about how to use this and when transactions
     * are committed and rolled back.
     */
    public void endTransaction() {
        acquireReference();
        try {
            getThreadSession().endTransaction(null); 
        } finally {
            releaseReference(); //做一些引用回收工作
        }
    }

    public void endTransaction(CancellationSignal cancellationSignal) {
        throwIfNoTransaction();
        assert mConnection != null;

        endTransactionUnchecked(cancellationSignal, false); 
    }

    private void endTransactionUnchecked(CancellationSignal cancellationSignal, boolean yielding) {
        if (cancellationSignal != null) {
            cancellationSignal.throwIfCanceled();
        }

        final Transaction top = mTransactionStack;
        boolean successful = (top.mMarkedSuccessful || yielding) && !top.mChildFailed; 
        //mMarkedSuccessful在setTransactionSuccessful时被标记为true

        RuntimeException listenerException = null;
        final SQLiteTransactionListener listener = top.mListener;
        if (listener != null) { //默认的beginTransaction方法，listener是空，这里不再分析
            try {
                if (successful) {
                    listener.onCommit(); // might throw
                } else {
                    listener.onRollback(); // might throw
                }
            } catch (RuntimeException ex) {
                listenerException = ex;
                successful = false;
            }
        }

        mTransactionStack = top.mParent;
        recycleTransaction(top);

        if (mTransactionStack != null) {
            if (!successful) {
                mTransactionStack.mChildFailed = true;
            }
        } else {
            try {
                if (successful) { //如果mMarkedSuccessful被标记为true并且中途没有异常，则真正提交到数据库
                    mConnection.execute("COMMIT;", null, cancellationSignal); // might throw
                } else { // 否则回滚
                    mConnection.execute("ROLLBACK;", null, cancellationSignal); // might throw
                }
            } finally {
                releaseConnection(); // might throw
            }
        }

        if (listenerException != null) {
            throw listenerException;
        }
    }