Java classpath tutorial (ClassLoader, NoClassDefFoundError, JVM, javac, javap -c, java -cp, jar cvf)

today I want to talk about an interesting detail of the java virtual machine of the so-called class path I have prepared a little examples we have three Java files in Krita Java in Java and to toggle Ostrava you can see them over here so main from Java is the entry point you can see the classic public static void main method here we call a static method of the Qataris class and here we instantiate an object of the greeter class and call a method on you can see the twirls class here you can see the critter class the details of these classes aren't really that interesting the interesting part is that main from Java uses the tour's Java at greater trouble okay um so first of all if you want to execute this example we have to compile it first so let's invoke the Java compiler and let's say that started the bottom and compile Rita Java and when we do that we can see that we get on your final Corita dot class this is the compiled version of Greater La Jolla and if we wanted to look into it we can do that with the Java P command - sea creature that should show us the implementation at the bytecode level right so for example here you can see the constructor bytecode and here you can see the grid method light quote again the details aren't really that interesting you okay but the interesting point is that this line only compiled Crito dot java and if I do the same for the trommel's Java then you will see that again we get a classifier we can peek into it and you can see the implementation issues maybe a little simpler to understand here you can see the default constructor which was defined automatically for since we didn't provide any constructor and here you can see the distance method okay interesting sidenote addy return apparently stands for double returns okay um let me remove most classifiers can I do it with our windows yeah okay cool so um what happens if we can't hire named or Java okay and here you can see that compiling main of Java also compiled photographers pull Java and greeter from Trevor because the compiler can't really know whether this is velvet Java code without having compiled for Travis class and the compiler can't really know if this is a valid constructor current if this is a valid method called without having compiler to the greeter class so that's pretty cool so if you just pass the main entry point to a little Java compiler then basically your project will be compiled okay and can we also run this name yeah here you can see the output of line 3 what we call photographers distance and here you can see the output from line and 6 while record greater good ok so now what happens if we delete some of those classifiers so for example let's get rid of the greeter class file like this ok and if I now try to run main something interesting happens you can see that line 3 with a pataga distance still worked but then in line 5 we got an exception traveling nor cluster Fonterra so I'd run time every time the java virtual machine sees the usage of a class it hasn't encountered yet it will try to a lot of the class of water would try to note that class file and in this case the class column was unsuccessful class not fault right and here you can see class one arm entry okay so that's pretty interesting so does that mean that you know I'm sorry I wanted to call her again does that mean that all the classifiers have to reside in the same directory the answer to that is no so for example I can move the greeter class one level up and the proton was plus maybe also I can see those class files no longer reside in the example directory instead they reside in the directory above oh no if you do instead of it right that's okay okay so now again if I run main you can see the class loader can't find those class files but maybe we can change the class table where to look for those classifieds that's exactly where the class path comes in so I can say the class path should be the directory of Android mobile solution main and now we can see that not even the main class could be found because of course it's not stored in the directory path so we have to tell Java the class path is not only the directory path but also the current directory all right and this is the the Linux Convention for separating class paths and other windows it's the semicolon instead of called there we go and then it works again okay so you can configure directories as class cross paths of the java virtual machine or the class loader will peek into those director is trying to find those classifieds then usually you don't have those classifiers directly in a directory but rather in a jar file so we could also say let's create a jar file create what was fire within its college my library and then let's put Rita last and into it okay let's see yeah and now I should be able to remove false class files okay and if I not go back to the example it shouldn't be possible if for X actual because in the directory above the classifieds are no longer there but I can replace this node with my daughter jar right and then it works again so the class path is if you were a list of directories and jar files where the class loader will try to look for classify that rotten okay and that's not only for the Java Runtime for normal execution of your program we can also do that during compilation so for example these Java files if I'm with greet her Java to the directory path I could have also deleted them but I don't want to delete them I just want to get them out of the way now if we compile the main Java file we will get a similar error at compile time where it says I have no idea what photographers are supposed to mean I have no idea what Greta's supposed to mean so even for the travel confident we can configure the class path let's say look at my library table jar then of the compilation short succeed but then of course if you want to run it again this won't work it would find the appropriate classes if required configure it to go for my if the current directory then it should work again right and basically this configuration is what I des and it was like maven operator will do for you automatically but now you should have a better grasp what classifiers are or you put them into a jar files and how you configure the class pass to look inside those charts