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What is a Java ClassLoader?
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I'm still fairly new to java and currently working on a text-based adventure game as a practice project.
The engine loads scenes to play that are all child-classes from a "Scene" superclass and appear as for eg. "dungeon.java". Now I want the game to be expandable.
The ideas is that a user can drop new scenes as .java-files into a "Scenes" folder and everytime the game is launched it reads all files in that folder and safes them into a "Scene"-class array.
My problem is that I don't know how to code this. I googled a lot and tried various phrasings but all I can find are tutorials for reading lines from txt-files or similar.
Is it even possible to read a complete file into a variable without serialzation?
I already tried the following code, but didn't get around to test it yet.
private static void buildScenePool() {
File scenesFolder = new File("/scenes");
\\ setup filter for .java files
FilenameFilter javafilter = (dir, name) -> name.endsWith(".java");
File[] sceneList = scenesFolder.listFiles(javafilter);
\\ create new arry large enough for all scenes
allScenes = new Scene[sceneList.length];
try{
FileInputStream fileIn = new FileInputStream(scenesFolder);
ObjectInputStream objectIn = new ObjectInputStream(fileIn);
\\ iterate trough the list array and safe files to array
for (int x = 0; x < allScenes.length; x++) {
allScenes[x] = objectIn.readObject( (Scene)sceneList[x] );
}
objectIn.close;
} catch (IOException e) {
System.err.println(e.toString());
}
}
This is rather very complicated. java code is normally compiled, and thus, that means you'd have to scan for a new java file, compile it (which is its own complicated ordeal, as that also means setting up the classpath and the like properly so that the compiler knows what to do), then load in the class file the compiler produced, and then call the appropriate methods in there.
You can do that. It's just, quite complicated. Your standard JVM doesn't necessarily even ship with a compiler; this is solvable too (either demand that this runs only on one that does, and the modern deployment rules for java involve you getting a JVM on your user's machines, so you thus pick one that does include a compiler – or you can just ship the compiler as dependency with your app, javac is itself a java app and runs on any JVM).
However, the more usual approach is to not actually use java for this. Instead, use something java-like, but not java: Scripting languages, like groovy, or javascript (okay, that is not particularly java-like perhaps).
That is its own sort of complication. There are no easy answers to any of this.
I think it's time to first think broad strokes and determine how you want the user's experience (that is, a user that wants to add a scene) should be, and then ask a new SO question about your specific choice.
You write em, they install em
In this model, users simply download or pick a 'scene' impl that someone else wrote (a 'real' programmer with a full fork of the entire source code, an IDE, a build tool, the works). Don't knock it - programming is hard, and saying: "Oh, look, anybody can customize a scene, it's easy, just open notepad.exe, write something like this (example java file here), and dump it in the Scene folder and off you go!", but this is not at all easy. To us programmers that seems familiar at least, but to your average user you're asking them to just rattle off a poem in ancient sumerian - normal people don't program. And if they do program, they're programmers. They would much rather get instructions about how to fork a project and set it up in an IDE than some bizarreness about tossing raw java files someplace.
Scripting
This is what Processing (for programming arduinos) does, more or less what webbrowsers did (which explains why javascript is so weird), and most 'plugin systems' for pseudo-smart editors like TextMate and Emacs do: You script them. As in, you fully buy into the idea that the custom stuff written by the user is extremely simple and highly targeted (really only makes sense to run within the confines of your app, not as standalone stuff - and dependencies don't generally come up), and pick a language that fits that model, and java certainly is not that.
Obvious options are javascript and groovy. This is certainly possible but not at all easy. Still, if you're interested, search the web for tutorials on how to run javascript or groovy inside a JVM, and you'll get plenty of hits.
Java code, and you compile it
That's what your question is positing as only option. I don't recommend it, but you can do this if you must. Be aware that it seems to me, based on the way you worded your question and your example code which makes various newbie mistakes (such as print-and-continue exception handling, which is always wrong, using obsolete APIs, and messing with built-in serialization) that this is a few ballparks beyond your current skillset. A challenge is always cool, so, if you want to go for it, you should! Just be aware it'll be the most difficult thing you've ever written and it'll take a few fully dedicated weeks, with a lot of reading and experimenting.
Just definitions, really
The central tenet so far has been that you can actually program. Instructions that make the machine act in certain ways. Possibly you don't need any of that. If a Scene is really just a background colour and a few widgets displayed here and there, should it even be code at all? Maybe you just want a declarative setup: The plugin/scene writer just declares certain properties and that's all they get to do, you just 'run' such a declarative definition. In which case the 'language' of the declaration can be in JSON, XML, YAML, TOML, or any other format designed for configuration files and such, and you can forego the hairy business of attempting to compile/run user-provided code in the first place.
In order to load the Java classes into your application, you need to compile them. You could do this from Java by invoking the javac executable. See Starting a process in Java? for instructions on how you could do that. Once compiled, you'd then need to load the classes into the JVM using a class loader, e.g. by invoking ClassLoader.defineClass. You probably want to configure a protection domain as well, to prevent user provided classes from misbehaving.
However, Java might not be the best approach for extending your application. You could consider using a scripting language instead, like JavaScript. See Nashorn (an open source script engine that was included in previous versions of Java, and that can now be downloaded separately) and the Java Scripting Programmer's Guide for more information.
I've been working on a Maven project consisting entirely of Java, and lately started to mix Scala code into it.
I'm amazed by the great expressiveness Scala offers, the easy use of scala-maven-plugin, and especially the incredible interoperability between Java and Scala.
However, I hit one inconvenience; according to the Maven's convention, Java's source code goes into src/main/java, whereas Scala's into src/main/scala. I found it quite cumbersome because I have to frequently go back and forth Java and Scala source files and every time I have to traverse the deep hierarchy of package directories (I often close tabs to keep my editor from cluttered).
So the question is: Is it recommended to maintain separate directories src/main/java and src/main/scala? If so, why?
To add more background, I've been working on the web application framework Wicket, whose convention is to put the HTML files alongside with their corresponding Java files. If we keep the directories separated, naturally the HTML files are separated as well (I don't think putting Scala files and corresponding HTML files in different directories makes sense). And then it goes "why I can't find Foo.html? Oh, I was looking for the wrong directory."
The source files themselves are very easy to distinguish both by humans and by machines by inspecting their extensions. I configured pom.xml to handle both Java and Scala put together in src/main/java and it worked (compiles and runs). On the other hand, separating directories poses a risk of defining conflicting classes in Java and in Scala, a careless mistake.
Well, I don't want to name a directory java if it contained not only Java's but also Scala's. But this is the only point I can come up with for separating directories.
(Edit: I've come up with a workaround-interpretation; let us think java stands for Java Virtual Machine. In this way, having src/main/c doesn't contradict if we ever decided to use JNI because C doesn't run on JVM.)
Oh, and one more point; my project is not intended as an open-source project; development convenience is preferred than trying hard to follow conventions.
This question is inspired by this answer.
I'd say yes, re-use code as much as possible. Maybe in future you can use this Java piece somewhere else...
As you probably know, you can use Java in Scala projects but not Scala in Java projects. So in this specific example it will help you with (future?) Java projects. If you want to re-use a piece of your Java code you can do that in either Java projects as well as Scala projects.
So i.m.h.o. it doesn't stop at the src/main/... but you should really put them even in different components.
Btw, little side note: if I'm correct, Wicket allows you to put the html somewhere else too, even in a different project... I saw it being handy (only) once, where we had to create different frontend for different clients of us. The java code stayed the same, the wicket-id's as well, but the html changed everywhere. Though it did give us some problems as well using the Qwicky plugin, as it could not find the html files in our IDE anymore.
So I'm battling an endless chaotic system.
There's a glassfish server which provides several reports from a sql server. I need to find which pages are accessing what tables.
The system is not homogenous, meaning that some pages might have their SQL query directly in the jsp page while others might have a separate DAO.Java file.
I do have the Java sources inside each directory.
My only idea would be to do individual searches for each table I'm looking for, but this will take loads of time... and in the future I'll have to do this again.
Maybe there's a better way? Maybe I could index all lines of code in the server? Any suggestions?
Wow, this is an old question. Maybe it should be closed. But if anyone finds themselves in a similar situation:
Group all source code you have. Got code without source? Decompile it.
Get a *nix system.
Use cat and grep to quickly run through everything.
So I have written a few Java classes that take input via the main method as passed arguments and then print to the screen. Is there any way I can 'port' this to something I can include in the HTML of a web document? There are multiple classes that work together so I would need some way of including all of these and forcing one specific file to run. For example, if I have Class1.java and Class2.java I would want to run Class1.java but somehow the second class must be included as the first relies on it.
I understand that this is probably not as simple as I'm making it, for example there may be complications with changing stdout to use the application screen etc, but anything that can get me started would be helpful.
Thanks!
The closest fit to what you are asking for is something called CGI, which is a technology that was popular 15 years ago, but is rarely used now. The next best fit would be to wrap your classes in something that can run in a java servlet container.
Simply put, instead of taking arguments in on the main method, you would accept them as URL parameters. Instead of printing to stdout, you would print to a special stream that sends the content over a socket to a web browser. There are many fine tutorials on the web for creating servlets. I like this one: http://helloworldprograms.blogspot.com/2010/08/servlet-hello-world.html
I want to find a library that I can use from my Java application that will allow me to access specific Javadoc in the scope of my project (I specify where Javadocs are located). Just like in Netbeans, I want to potentially access the Javadoc from html files locally and remotely, and from source.
I expect that I could use code from Netbeans to achieve this, but I don't know how, and I can't easily digest their documentation.
Today I started thinking about the same thing.
From CI point of view, I could use #author annotation to send e-mail to someone, who wrote a test that is failing with error, not with a failure.
Google didn't help me (or I didn't google deep enough), so I started wondering how to do it on my own.
First thing that came to my mind is writing a little tool that will check all *.java files specified in a directory, bound file name to annotations and allow user to perform some actions on them.
Is that reasonable?