SecondHand C0D1NG

Update: The typo has been fixed if you are using Dynamic web module version 5.0

When you are using the Servlet Template in Eclipse, you may encounter the error saying “The import javax.servlet cannot be resolved.”

For example, I am using Eclipse for Mac (Version 4.19.0). First, create a Dynamic Web Project, target runtime: Apache Tomcat v10.0, Dynamic web module version 4.0. And inside this project, create a new servlet file using the Create Servlet Wizard. Soon you will see the errors pop up in the servlet file you just created.

1
2
3
4
5
6
7

// BEFORE
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

How to fix it? All you have to do is changing the superclass name from javax into jakarta. Why does this happen? I think it is because, as mentioned on Wikipedia page, that from 12 Jun 2020 and on, API moved from package javax.servlet to jakarta.servlet. Let’s hope Eclipse will update the template someday in future. For the exising codes out there, I guess they are doing well.

1
2
3
4
5
6
7

// AFTER
import java.io.IOException;
import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

Of course, you can create an empty class file to build servlet from scratch, to avoid such issues. Just in case you do face the similar errors in Eclipse when coding Java, I hope the simple fix would help you.

Wikipedia page of Jakarta Servlet:
https://en.wikipedia.org/wiki/Jakarta_Servlet

Finally, I got hexo upgraded.

Simple & powerful as it is advertised.

It worked fine, I guess.

I have been thinking about to code myself a blog from scratch because the hexo blog seems out of date. The interactions between Github and Hexo are sometimes failing and I got warnings of it might not be working in some future. Maybe it’s good time to practice some front-end coding with html/css and javascript.

The coding experience on Leetcode is fantastic because you can always get instant feedback that your code is not working. Then you try again and get the feedback again that your codes fail. Back and forth serveral times until you get it.

To be honest, I like Project Euler more, because of its simplicity and the sense of art comes from the simplicity. Still, I think you need the Leetcode to practice to get a job. In this sense, it’s a wonderful website that gives you hope of the employment.

The book of Code Complete has been sitting on my book shelf for a long time. I always feel that it was a big book and the content is too much for my level. But these feelings were purely generated from the imagination because I never ever actually tried to read the book.

Until now. I began the reading. And it turns out the content makes sense to me in a lot of ways. I think it is considered as a classic book for a good reason.

It might take some time before I can finish the book, but I can say it now to anyone hesitate to read it, today is the day, just read it.

I saw something in the shell code as follows.

1

cat /dev/random > /dev/null 2>&1

Or something similar.

1

export lang="en_us.utf-8" >/dev/null 2>&1

They all have this part.

1

>/dev/null 2>&1

I did some research to find out what it means. Basically, the number 1 is standard output, the number 2 is the standard error, the char > is to redirect the flow of information, and 2>&1 means to treat the 2 the same way as the 1, and /dev/null, you know, is in the middle of nowhere.

So, to put them all together, this piece of code means that throw the standard output and standard error away as garbage. You will never see anything stdout or stderr on the screen or anywhere on disk. No, just say goodbye to them. They are gone for good.

CRUD operations in MongoDB, i.e. Create, Read, Update, Delete.

The code is for Pymongo.

Preparation

1
2
3
4
5
6
7
8
9
10
11

import datetime
from pymongo import MongoClient

client = MongoClient()
db = client.test_database
collection = db.test_collection

post = {'author': 'Mike',
        'text': 'My first blog post!',
        'tags': ['mongodb', 'python', 'pymongo'],
        'date': datetime.datetime.utcnow()}

CREATE:

1
2
3
4

def create_lines():
    collection.insert_one(post)
    # if we have multiple posts (an array of dictionaries)
    collection.insert_many(posts)

READ:

1
2
3
4

def read_lines():
    collection.find_one({'author': 'Mike'})
    # if we want to find multiple records at the same time
    collection.find_many({'author': 'Mike'})

UPDATE:

1
2
3
4
5
6

def update_lines():
    collection.update_one({'author': 'Mike'},
                          {'$set': {'text': 'My second blog post!'}})
    # if we want to update all the relevant records at the same time
    collection.update_many({'author': 'Mike'},
                          {'$set': {'text': 'My second blog post!'}})

DELETE:

1
2
3
4

def delete_lines():
    collection.delete_one({'author': 'Mike'})
    # if we want to delete the matched records all at once
    collection.delete_many({'author': 'Mike'})

I would say MongoDB is designed in a way that is very user-friendly.

Check out the pymongo website for more detailed instructions.
https://pymongo.readthedocs.io/en/stable/tutorial.html

Make the database in a simpler way and make it fast instead of designing complicated architecture of tables. MongoDB can do this with its document model to store things more naturally which can be later fit into your applications. Remember, it’s fast. I think it can make your life easier.

Sharding is basically to break up your data across servers to improve the performance and/or infrastructure.

A replica set of MongoDB for example is replicating your data on any of your servers while sharding is breaking up the data across your servers. The process is called MongoS using Shard Key. When you connect and send queries to MongoDB, MongoS will look up the data and get it from the right place and also merge data if necessary because they can be on different servers.

You can google it for more details, maybe you can find the wikipedia page if you are curious enough. Anyway, put it in a simple way, the paradox is talking about the contradiction between the reality that we have not met any aliens and the high probability there should be so many aliens in the cosmos because the universe is so big.

It occurs to me that when a society becomes more civilized, it kind of loses the impulse to invade as one way of exploration. You need to treat all those living things equally because you can see the tendency that we treat with each other more fairly no matter who you are, where you come from, or what are you. In the end, everyone lives happily in their chambers and forgets the big universe exists.

Or maybe, there could be a pandemic out there in the universe, everyone during this period of time needs to stay low and keep quiet. It is equivalent to the dark forest theory I know. But the twist here is that the pandemic can end, and it will.