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Java Invoker


This essay does not describe an existing computer program, just one that should exist. This essay is about a suggested student project in Java programming. This essay gives a rough overview of how it might work. I have no source, object, specifications, file layouts or anything else useful to implementing this project. Everything I have prepared to help you is right here.

This project outline is not like the artificial, tidy little problems you are spoon-fed in school, when all the facts you need are included, nothing extraneous is mentioned, the answer is fully specified, along with hints to nudge you toward a single expected canonical solution. This project is much more like the real world of messy problems where it is up to you to fully the define the end point, or a series of ever more difficult versions of this project and research the information yourself to solve them.

Everything I have to say to help you with this project is written below. I am not prepared to help you implement it; or give you any additional materials. I have too many other projects of my own.

Though I am a programmer by profession, I don’t do people’s homework for them. That just robs them of an education.

You have my full permission to implement this project in any way you please and to keep all the profits from your endeavour.

Please do not email me about this project without reading the disclaimer above.

This is a very simple project for a C programmer to make Java more useful in Windows. In Windows 95/98/ME/NT/W2K/XP/W2K3, you can’t just click on a jar or class file to execute it. Oracle made a deliberate and stubborn error in the design of java.exe that made it impossible to set up .class as a standard executable file extension. To run a class file, from the command line you must write:
java.exe X … -cp … etc.
without the .class, not:
java.exe X.class … -cp … etc.
Windows has a mechanism for adding extra junk to the command line for extension associations, but not for stripping the extension. To invoke a jar file from the command line, happily you don’t need to strip the *.jar extension, like this.
java.exe -jar X.jar -cp … etc.
This means you can directly set up *.jar association like this:
java.exe -jar %1 -cp… etc.
How then to make *.class files similarly clickably executable? You need to write a little C program called invoke, that simply strips the .class and passes the command line parameters on to java.exe with the exec function.

Then to set up your .class association you write:

C:\someplace\invoke.exe %1 -cp… etc.
There is a second problem. The parameter you put on the java command line is not a file name. It is the name of a class. Java.exe won’t accept things like this:
java.exe C:\mdir\X … -cp … etc.
What were these turkeys at Oracle thinking? So one other thing invoke has to do is change the current directory to where the class file lives, so that the generated unqualified class name will work on the java.exe command line.

Now what happens when the class in a package? The class name is something like com.mindprod.mypackage.X. The relationship between this and the filename is tenuous. You might run screaming from the room.

Alternatively you could disassemble java.exe and fix it. If you want to get fancy, you should be able to configure the directory where java.exe lives and have invoke automatically add some standard gobbledegook to the command line for you, either by hard coding it in, or by putting it in an INI file, or by taking it off a special invoke command line switch.

SmartJ took this idea considerably further. Unfortunately it does not support Java version 1.2 or 1.3. Perhaps you might eventually write a SmartJ capable of dealing with all JVMs (Java Virtual Machines).

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