Re: Question on jars

On 03/23/2011 06:43 PM, Anonymous wrote:


Yes, they are, for both. But no doubt you missed some tiny detail of how to
use them.

There are a lot of classpath interactions with file systems and JAR structures.

Think of package dots (".") as cognate to directory slashes ("/").

  com.lewscanon.someapplication/module
  com/lewscanon/someapplication/module/

Presto!

That's the directory structure. What goes into directories? Files! What
goes into packages? Types! (Classes and interfaces)

  com.lewscanon.someapplication/module.Foo
  com/lewscanon/someapplication/module/Foo.class

Notice that I give you relative paths here. "com/" relative to what? On the
package side, it's a meaningless question - it corresponds to the root of all
packages, the "default" (unnamed) package.

So really, we have:

  com.lewscanon.someapplication/module.Foo
  /com/lewscanon/someapplication/module/Foo.class

But that just begs the same question - what is the root?

The root is some part of the classpath. The classpath specifies a bunch of
roots, and the JVM searches each one in turn trying to satisfy a "load" command.

So suppose you have directories in a file system, and your classpath starts at:

  /opt/applications/lewscanon

so really, 'Foo' is loaded from

  /opt/applications/lewscanon/com/lewscanon/someapplication/module/Foo.class

Simple?

This part drove me nucking futs when I was first learning Java.

Now, what's a classpath for this example?

$ java -cp /opt/applications/lewscanon:. \
   com.lewscanon.someapplication/module.Foo

The classpath is the cited "lewscanon" directory first, followed by "."
(current directory).

Clearly, for this example 'com.lewscanon.someapplication/module.Foo' is a main
class. It is found relative to the first root of the classpath.

JARs? You want to know about JARs? Well, a JAR can root a classpath, too.

$ java -cp someapplication.jar \
   com.lewscanon.someapplication/module.Foo

This time I left out the ".", because there's no sense in finding the main
class in the JAR if I'm looking in my current directory. YMMV.

Now the JAR is the root of a classpath. But what's inside the JAR? Look with
your favorite unzip utility (for example, "jar"):

  $ jar -tf someapplication.jar
  META-INF/
  META-INF/MANIFEST.MF
  com/
  com/lewscanon/
  com/lewscanon/someapplication/
  com/lewscanon/someapplication/module/
  META-INF/persistence.xml
  com/lewscanon/someapplication/module/Foo.class
  ...

The internal directory structure corresponds to what should be at the root of
a classpath.

But, wait! There's more! If you set up that magic "META-INF/MANIFEST.MF"
correctly (RTFM for details), then you can indicate the "Main-Class:" so that
the JAR system finds it automatically, and all "Class-Path:" specifications
come from within the JAR (ignoring the command line).

$ java -jar someapplication.jar

<http://download.oracle.com/javase/6/docs/technotes/guides/jar/index.html>

  $ cat /opt/applications/lewscanon/someapplication/src/conf/MANIFEST.MF
  Manifest-Version: 1.0
  Main-Class: com.lewscanon.someapplication/module.Foo
  Class-Path: log4j.jar postgresql-9.0-801.jdbc4.jar eclipselink.jar

  $ ls
  eclipselink.jar log4j.jar postgresql-9.0-801.jdbc4.jar someapplication.jar
  $

So yes, a JAR can provide a library or package an executable.

--
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg