Java – Runtime.getRuntime().addShutdownHook()

a useful method when program done and run last step task.

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public class AddShutdownHookTest {
 
	static class Message extends Thread {
 
		public void run() {
			System.out.println("Bye.");
		}
	}
 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		try {
 
			// register Message as shutdown hook
			Runtime.getRuntime().addShutdownHook(new Message());
 
			// print the state of the program
			System.out.println("Program is starting...");
 
			// cause thread to sleep for 3 seconds
			System.out.println("Waiting for 3 seconds...");
			Thread.sleep(3000);
 
			// print that the program is closing
			System.out.println("Program is closing...");
 
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

Nginx – reverse proxy as load-balance in ubuntu

1. install nginx in ubuntu

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$ sudo apt install ngnix

2. print ngnix conf path

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$ sudo ngnix -t

3. create node.conf, in nginx.conf, it would load all file matched *.conf under conf.d/

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$ touch /etc/nginx/conf.d/
$ vim /etc/nginx/conf.d/node.conf

4. node.conf file context.
it is best to keep X-Real-IP and X-Forwarded-For, because in node.js app, it would record client’s ip address into log

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upstream nodeserver {
   server localhost:3085;
}
 
server {
   listen 8080;
   server_name linode.emacslisp.com;
 
   location /test {
       proxy_pass http://nodeserver;   
       proxy_set_header Host $host;
       proxy_set_header X-Real-IP $remote_addr;
       proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
       proxy_redirect     off;
       proxy_next_upstream error timeout invalid_header http_500 http_502 http_503 http_504;
       proxy_max_temp_file_size 0;
       proxy_connect_timeout      90;
       proxy_send_timeout         90;
       proxy_read_timeout         90;
       proxy_buffer_size          4k;
       proxy_buffers              4 32k;
       proxy_busy_buffers_size    64k;
       proxy_temp_file_write_size 64k;       
   }
}

Emacs – lldb with gud-lldb.el

since gdb is disabled in latest version of mac,
it is not that easy to enable gdb.
although there is way to enable gdb, once mac os is updated, we have to search new ways again.

it seems like we have to use lldb.
there is a few ways to use lldb with UI,
such as eclipse for c/c++ with lldb plugin.

but for emacs, it has a plugin with lldb

https://github.com/ptrv/emacs.d/blob/master/site-lisp/gud-lldb.el

emacs-debug

Tomcat Src – tomcat logging

This is starting of tomcat source code analysis.

Tomcat Log is simple.

the following is main example on how to invoke tomcat logging

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package main;
 
import org.apache.juli.logging.Log;
import org.apache.juli.logging.LogFactory;
 
public class tomcat {
	public static void main(String[] args) {
		Log log = LogFactory.getLog(tomcat.class);
		log.info("this is a info message from tomcat");
		log.debug("this is a debug message from tomcat");
	}
}

but when debugging tomcat src, I find that org.apache.juli.ClassLoaderLogManager is invoked.

Its entry is in catalina.sh as starting up script for tomcat.

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-Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager

YaraScanService – How to remove it

after Mac OS is upgraded to High Sierra, YaraScanService which is part of MRT.app would start automatically.
YaraScanService takes about 20GB memory in my macbook pro.

so it would make whole mac os frozen.

Even it is working, it would stop a while and start to work.
User wouldn’t feel smoothly operation.

Here is step on how to remove YaraScanService
1. reboot mac os and press ‘Command + R’ to boot into recovery mode.
2. Click the “Utilities” menu and select “Terminal” to open a terminal window.
3. run command csrutil
# csrutil status
# csrutil disable
4. reboot mac os again, now command such as ‘rm -rf xxx’ will work and won’t receive any deny message.
5. after login into macos, open “Terminal” and run
$ sudo rm -rf /System/Library/CoreService/MRT.app
6. in /System/Library/LaunchDaemons , run
$ grep -nri “mrt” *

and delete all files that are displayed on result
7. in /System/Library/LaunchAgent, run
$ grep -nri “mrt” *

and delete all files that are displayed on result

8. reboot mac os, now do some test to check YaraScanService is started or not.
9. go to recovery mode to enable csrutil
$ csrutil enable

Backtrace – leetcode 17 – Letter Combinations of a Phone Number

this is a very classic backtrace problem.

Idea is simple, use ‘call stack’ to store string “a…z”

stack n – z
.
.
stack 1 – a

if stack.length == targetLenth
loop though stack to get char array to string.

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package com.dw.leetcode;
 
import java.util.*;
 
public class Letter_Combinations_of_a_Phone_Number_17 {
	static final char[][] number = { { 'a', 'b', 'c' }, { 'd', 'e', 'f' }, { 'g', 'h', 'i' }, { 'j', 'k', 'l' },
			{ 'm', 'n', 'o' }, { 'p', 'q', 'r', 's' }, { 't', 'u', 'v' }, { 'w', 'x', 'y', 'z' }
	};
 
    public List<String> letterCombinations(String digits) {
    	List<String> result = new ArrayList<String>();
    	if(digits.length() == 0){
            return new ArrayList<>(); 
        }
 
    	foo(digits, 0, 0, digits.length(), new ArrayList<Character>(), result);
    	return result;
    }
 
    public void foo(String digits, int index, int total, int len, List<Character> temp, List<String> result) {
    	if(total == len) {
    		StringBuilder stringBuilder = new StringBuilder();
    		for(Character c: temp) {
    			stringBuilder.append(c);
    		}
    		result.add(stringBuilder.toString());
    		return;
    	}
 
    	int numberIndex = Integer.parseInt(Character.toString(digits.charAt(index)));
 
    	for(char c: number[numberIndex - 2]) {
    		temp.add(c);
    		foo(digits, index+1, total+1, len, temp, result);
    		temp.remove(temp.size() - 1);
    	}
    }
 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Letter_Combinations_of_a_Phone_Number_17 s = new Letter_Combinations_of_a_Phone_Number_17();
		System.out.println(s.letterCombinations("234"));
	}
 
}

javascript – download Blob file directly

As we know that for security issue, browser would disable javascript to download file directly.

but today what I want to show is to use javascript to click on ‘a’ link to download file
using Blob

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  var blob = new Blob([csvFile], { type: 'text/csv;charset=utf-8;' });
  if (navigator.msSaveBlob) { // IE 10+
    navigator.msSaveBlob(blob, filename);
  } else {
    var link = document.createElement("a");
    if (link.download !== undefined) { 
      var url = URL.createObjectURL(blob);
      link.setAttribute("href", url);
      link.setAttribute("download", filename);
      link.style.visibility = 'hidden';
      document.body.appendChild(link);
      link.click();
      document.body.removeChild(link);
    }
  }

Anypoint Studio Frozen Problem when starting on Mac Platform

Anypoint Studio Frozen Problem when starting on Mac Platform

solution: most important is to install jdk 1.8.0 151, this is max version of anypoint studio that support.
any version older that 151 would fail to work.

Edit target AnypointStudio.ini
AnypointStudio.app/Contents/Eclipse/AnypointStudio.ini

use
-vm
/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/bin
to point to JAVA_HOME of jdk 1.8 151

use
-clean to clean up workspace

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-clean
-startup
../Eclipse/plugins/org.eclipse.equinox.launcher_1.3.100.v20150511-1540.jar
-vm
/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/bin
--launcher.library
../Eclipse/plugins/org.eclipse.equinox.launcher.cocoa.macosx.x86_64_1.1.300.v20150602-1417
-vmargs
-Xms512m
-Xmx1536m
-XX:MaxPermSize=512m
-Dosgi.instance.area.default=@user.home/AnypointStudio/workspace
-Dhttps.protocols=TLSv1.1,TLSv1.2
-Djava.awt.headless=true
-XstartOnFirstThread
-Dorg.eclipse.swt.internal.carbon.smallFonts

javascript – using await to make sleep

In old day, when using javascript to create timer,

setInterval or setTimeout are standard function to implement
A few other way to do is using third-party npm library.
or even calling c++ as native timer.

with ES2016, await and asyn keyword has been introducted into.
Here is example.

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function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}
 
async function demo() {
  console.log('Taking a break...');
  await sleep(2000);
  console.log('Two second later');
}
 
demo();