Java: JAR files

Creating and using .jar files

A .jar file is a Java Archive.

Java .jar files vs. C/C++ linking

There is one superficial aspect in which creating a .jar file in Java and linking C/C++ are similar: that is,they allow us to combine the artifacts of separately compiled units of software (.class files in Java, or .o files in C/C++) into a single unit that is executable.

We should look beyond this superficial similarity to see some important differences.

There is no linking phase in Java as there is in C/C++. In C++, part of linking is turning resolving symbolic addresses of variables and functions into offsets. These offsets become virtual addresses during loading. In Java, the analogous activities happen at run time, and are taken care of at run time by the Class Loader and the Java Virtual Machine (JVM), and are handled very differently from in C/C++.

Instead, what happens when creating a .jar file is that we combine a set of .class files into what is essentially just a glorified .zip archive plus a bit of metadata called a manifest. The manifest can specify, among other things, which of the .class files in the .jar contains the main() method that should be executed when the .jar file is “executed”.

One important difference here is that while a compiled C/C++ program must contain exactly one main (no more, and no fewer), a Java .jar file can contain 0, 1 or many classes with main() methods.

• If some jar file foo.java contains no classes with main() methods, then you can’t specify a main method in the manifest, and you can’t, therefore, write java -jar foo.java and expect a program to run.
• If some jar file foo.java contains exactly one class ‘Bar’ with a main() methods, and if you specify that class in the manifest, java -jar foo.java will run the main from class ‘Bar’.
• If some jar file foo.java contains exactly many classes ‘Bar,Fum,Baz, etc.’ with main() methods, you may specify one of them, say, Bar in the manifest as the default main, and when you run java -jar foo.java, that main will run. But, you can also run the other main methods via: java -cp foo.java Fum, or java -cp foo.java Baz, etc. In this case, you are putting foo.jar into the classpath, and then specifying the class you want to run. You can also use this method if the .jar has a single class with a main, but that class is not specified in the manifest.

References

• https://docs.oracle.com/javase/tutorial/deployment/jar/index.html
• https://docs.oracle.com/javase/tutorial/deployment/jar/manifestindex.html