Develop Reference

Difference between a jar file and a library in Java

NetBeans allows the programmer to add a library and a jar file.
What is the difference between a jar file and a library? Is library similar to GAC assembly as in Windows.
There are similar questions, but they are way too specific and I was not able to understand the difference.

to put things very simple : library is a collection of jars
You could like create a global library java-ee which contains all Java EE related jar files. Then you could use this global library in your different projects. It will be simpler to manage them; and adding in new projects.

A JAR serves the same function an an Assembly in the C#/.net world. It's a collection of java classes, a manifest, and optionally other resources, such as properties files.
A library is a more abstract concept, in java, a library is usually packaged as a JAR (Java ARchive), or a collection of JARs.

A jar file is zip archive containing among other files, the java class files. A Netbeans library contains resources required by the project, including jar files.

If well understood: A library is simply a folder that groups classes. For example in JDK, a library present there is a group of classes stored together.
If not mistaken a .jar file is a group of compiled classes in .class format and was created by Java creators so a program will be OS independent; which means within a JVM you will run your app in .jar format on a Linux, Windows, etc without re-coding tour app for various OSs.

This article explains it all..
It states
Java's libraries are commonly known as class libraries. However, Java
refers to class libraries as packages.

Questions about .jar files and running a java program through PHP

I'm building a website which will use files in order to render social graphs. These files are created by a backend in JAVA. This JAVA program consists of 4 classes and 4 libraries. I know that in order to run a JAVA program through PHP, I need to call it with the "exec" command but all the examples I saw have .jar executables and not .java files in the syntax. So, I'd like to ask two questions:
Is it possible to call just the .java main class from PHP without creating a .jar file? And if so, what's the syntax of the "exec" command?
If I have to create the .jar executable, would I just have to place it in the website folder and just call it? I mean, does the .jar file contain all the classes and libraries that the program needs? I don't talk about JAVA libraries, I have 4 specific libraries (MongoDB, Neo4j, GEXF parser and Lucene DB) which are not part of Java defaults. Thanks in advance and please forgive my syntax and spelling mistakes.

You have to at a minimum compile the .java files into .class files. It would be easier if they are in a jar file, but that is not required.
1)
a) java -cp /location/to/.class/files
b) java -jar /location/to/the.jar
2) the jar file does not have to be executable, though if that makes it easier, set the manifest up correctly and there you go.
When you say libraries, you mean other .jar files or native .dll or .so libraries? If the later, you have to have your library path setup to find them.

There is a PHP-Java bridge project on SourceForge. Also, you could expose your back end process by running it as a servlet, which is probably the proper way of doing it. If all else fails you can just "exec" it.

Converting a Scala library to a DLL (.NET)

I'm trying to create a Dll out of a scala-class. I'm using IntelliJ together with SBT. I've already found a way to convert .jar files into a Dll, using the ikvm-converter. Now the problem: When I use "package" under SBT to create a .jar file out of my .scala file and try to convert it afterwards with ikvmc into a Dll the resulting library is empty when integrated in C#...
For example converting the Jama-Library (which is written in Java) works fine, where converting Scama (written in Scala) does not work.
Is there a way to do this conversion of scala code into a dll? Is there a "Scala to Java"-conversion tool?
Best Regards,
Christoph

I have no knowledge of .NET, but judging from SK-logic's and your comments to the questions: sbt package does not include the Scala runtime library, because it assumes you are going to export your project as a library to be used within other Scala projects.
Therefore, you will need to create a "fat" jar that contains the runtime. For example, in this blog you can see how the author creates a fully self-contained executable, by converting both the project jar and the runtime jar.
There are different tools to do that with sbt. The easiest would be sbt-assembly, but you will end up with a very large file, because it just adds the whole runtime. If that is a problem, you may want to filter the runtime instead, using the proguard plugin. More on this topic in another StackOverflow entry.

Linking Java with C++ statically using JNI

Is it possible to link a C++ library to a Java program statically, in a way that will make them into a single file ,just like linking 2 C++ libraries?
(I read that java programs can also be compiled to EXE).

Theoretically this should be possible to create one EXE that already includes the required JNI functions used by the JVM.
This EXE would have to load the Java part by starting a JVM instance in the same process (by loading jvm.dll and executing it as shown in question JNI Java in c++).
The Java-EXE-wrapper I know do not support something like this as they come with a pre-compiled EXE that gets the used JAR attached as resource. Therefore I assume you would have to build you own C/C++ executable and implement all the functionality you need.

When I use JNI I include the dll with JNI support into my jar file. Then access it by classpath. You will have single jar file.

It isn't possible unless you have access to a static version of the jvm.lib library. It is distributed as a dynamic-link library referring to jvm.dll. You can't do this.

Call Scala code from Java?

I'm using Netbeans to write Scala and Java.
Netbeans generated a .jar file for the Scala project. I tried importing that file in the Java project. But I couldn't import any class from that .jar file into my Java project.
I also tried importing scala-library.jar to the java project, and could import classes from that jar.
I want to write my library in Scala, then expose a small interface that only involves Java stuff, then write a Java wrapper so that people can use it as a Java package. Is that possible? How do I do it?
Thank you very much.

There should be no problem in doing this.
Have you verified (e.g. using WinZip or the jar utility) that your .jar file actually contains the relevant .class files? (use jar tvf mylib.jar to check)
If you have verified that the correct .class files exist in your jar file, what is the runtime error you are seeing? Is it a NoClassDefFoundError? If so, which class cannot be found? How are you referring to the (Scala) class in your (Java) code?