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Quick tips

Always use (forward)slash for paths

On *NIX systems, path separator is /, so you shouldn't have any problems.
On Windows, oh boy... You need to escape backslash in strings, e.g. for path C:\foo\uvw you write "C:\\foo\\uvw".
What also makes a problem are properties files. When you write to it you get C:\foo\uvw but when you read you get an exception telling you it can't figure out \uvw escape character... :D
So, easiest way is when you write to a file, replace all \ with /, myStr.replace("\\", "/");

Running a JAR

A runnable JAR must have a MANIFEST.MF file in it.
The manifest file must have Main-Class: com.example.MainClass field in order to know which class is app's entry point.

Classpath

Classpath is a list of URLs (can be a directory, a JAR, a .class file) where JVM should search for .class files.
Default value (if not specified) is current folder, equivalent to: java -cp . .

https://docs.oracle.com/javase/8/docs/technotes/tools/windows/classpath.html

You can print it easily:

String classpath = System.getProperty("java.class.path");
String[] classpathEntries = classpath.split(File.pathSeparator);
System.out.println("CLASSPATH: " + Arrays.asList(classpathEntries));

Classpath ROOT (can be more of them) is the root of a classpath entry.
E.g. if your classpath is .:/var/whatever, classpath roots are . (current folder) and /var/whatever. I think... :D https://stackoverflow.com/a/49876435/4496364

When you require something from classpath, it searches locations in order they are defined.
E.g. if you have two classes with same name, the first one found will be loaded. Same is applied for files etc.
See https://stackoverflow.com/a/6935725/4496364

Recursively add all JARs from a folder?

Lets say you have this structure:

  • lib/
    • example1.jar
    • example2.jar
    • a/
      • a1.jar
    • b/
      • b1.jar

This is what -cp does:

  • -cp lib/* adds example1.jar and example2.jar
  • -cp lib/*:lib/a/* adds example1.jar, example2.jar and a1.jar

Windows command line too long..

  1. use the trick above with -cp myjars/*, or
  2. make a "path jar", where you specify all the dependencies:
    http://todayguesswhat.blogspot.com/2011/03/jar-manifestmf-class-path-referencing.html

https://stackoverflow.com/a/9053165/4496364

classLoader.getResource vs getClass.getResource

https://stackoverflow.com/questions/14739550/difference-between-getclass-getclassloader-getresource-and-getclass-getres

IO streams

IO *{Input,Ouptut}Streams (like InputStream, BufferedOutputStream) are used for R/W binary data.

IO *{Input,Ouptut}{Reader,Writer}s (like FileWriter, BufferedReader) are used for R/W character data.