Concurrency programming in Java

Volatile

The Java volatile keyword is used to mark a Java variable as 'being stored in main memory'. More precisely that means, that every read of a volatile variable will be read from the computer's main memory, and not from the CPU cache, and that every write to a volatile variable will be written to main memory, and not just to the CPU cache.

Refs: Jenkov.com: Java Volatile Keyword - http://tutorials.jenkov.com/java-concurrency/volatile.html,
Java67: How to stop a thread in Java? Example - http://www.java67.com/2015/07/how-to-stop-thread-in-java-example.html

AtomicInteger

There are two main uses of AtomicInteger:

1. As an atomic counter that can be used by many threads concurrently - incrementAndGet()
2. As a primitive that supports compare-and-swap instruction to implement non-blocking algorithms - compareAndSet()

private volatile int counter;
public int getNextUniqueIndex() {
    return counter++; // Not atomic operation
}

Performing an operation 'counter++' will cause huge problems in concurrency programming. It is not an atomic operation and can have different values in different threads. Therefore always use AtomicInteger to share variables:

private AtomicInteger counter;
public int getNextUniqueIndex() {
    return counter.getAndIncrement();
}

Ref: StackOverflow: Practical uses for AtomicInteger - http://stackoverflow.com/questions/4818699/practical-uses-for-atomicinteger

JSP (JavaServer Pages)

JSTL - Core <c:if> Tag

The <c:if> tag evaluates an expression and displays its body content only if the expression evaluates to true. The <c:if> tag has the following attributes −

Example

<c:if test="${condition}">
</c:if>

<c:choose>
    <c:when test="${condition}">
    </c:when>
    <c:otherwise>
    </c:otherwise>
</c:choose>

Ref: TutorialsPoint: JSP - https://www.tutorialspoint.com/jsp/jstl_core_if_tag.htm

Intellij IDEA

If we want to import a module, but it is not a library, you can associate the project with the main application as follows in IntelliJ IDEA:

1. Project Structure -> Modules -> Add new Module (+)
2. Project Structure -> Modules -> Dependencies -> Add Module Dependency

Linux & Server Administration

Finding files

$ grep -rnw '/path/to/somewhere/' -e 'pattern'

• -r or -R is recursive,
• -n adds line numbers,
• -w stands for match the whole word,
• -l (lower-case L) can be added to just give the file name of matching files

Ref: How do I find all files containing specific text on Linux? - https://stackoverflow.com/questions/16956810/how-do-i-find-all-files-containing-specific-text-on-linux

Network

Show ports based on PID of a process

$ sudo netstat -tulpn | grep ${PID}

Show information about CPU

$ cat /proc/cpuinfo

Check the connection to the server and open port on a specific remote computer

$ telnet SERVER_ADDRESS PORT

Connect to a remote server, the best way through the command 'ssh'

$ ssh USER@SERVER_ADDRESS

Less

Less is a program similar to more (1), but which allows backward movement in the file as well as forward movement. Useful commands when browsing logs:

• Waiting for latest lines at EOF - SHIFT + F
• Finish of waiting at EOF - CTRL + C
• Search for a specific text - /${TEXT}
• Leave or finish - Q

Nano

GNU nano is a small and friendly text editor. Besides basic text editing, nano offers many extra features like an interactive search and replace, go to line and column number, auto-indentation, feature toggles, internationalization support, and filename tab completion.

• Search for a specific text - CTRL + W

Grep

grep is a command-line utility for searching plain-text data sets for lines that match a regular expression. Its name comes from the ed command g/re/p (globally search a regular expression and print), which has the same effect: doing a global search with the regular expression and printing all matching lines. Grep was originally developed for the Unix operating system, but later available for all Unix-like systems.

grep -E option is for extended regexp. If you use the grep command with -E option, you just need to use | to separate multiple patterns for the or condition.

$ ps aux | grep -E 'ssh|PID'

It is useful for the showing the title of listing processes when filter for a specific command.

Copying on Mac OS X

To copy files on Mac OS X on a remote server can be used FileZilla program, because it uses a secure FTPS protocol. Another good alternative to browsing the file system is e.g. Double Commander (similar to Total Commander) and Midnight Commander.

• https://filezilla-project.org/
• https://doublecmd.sourceforge.io/
• https://midnight-commander.org/