The other thing to understand is that java.exe and javac.exe were written by two different teams of people who never had coffee together. That’s why the command line switches are so infuriatingly inconsistent.

All applications that you run with java.exe (such as HelloWorld.java ) must have a

public static void main ( String[] args )

Long Command Lines

• Specify classpath as SET CLASSPATH= rather than a command line option -cp .
• Write a little kicker program in C or Java that exec’s java.exe with the command line. That way it can be arbitrarily long.
• Use 4NT or 4DOS. It lets you have lines 1023 characters long, and provides for line continuation.

CLASSPATH And File Naming Recipes

1. Use JDK 1.1 or later. Configure your SET CLASSPATH= to clear it out. Avoid JDK 1.0 if you can because its CLASSPATH is more complicated since it has to help java find the standard classes.
2. In all that follows, everything is strictly case sensitive.
3. To run a HelloWorld.class app, in the default package in C:\MyDir, use

   CD \MyDir
   java.exe -classpath . HelloWorld

   Note that the -classpath must come before the HelloWorld classname. To remember the order, you can think of it like this. java.exe needs to know the classpath before it can search for the class. If you get them reversed you will just get a mysterious NoClassDefFoundError.
4. The following will not work to run a HelloWorld.class app, in the default package in a jar in C:\MyDir, use
5. To run a HelloWorld.class app in C:\com\mindprod\mypackage, in package com.mindprod.mypackage, use

   CD \
   java.exe -classpath . com.mindprod.mypackage.HelloWorld

6. The following will not work to run a HelloWorld.class app in C:\com\mindprod\mypackage, in package com.mindprod.mypackage:

   CD E:\com\mindprod\mypackage
   java.exe -classpath . com.mindprod.mypackage.HelloWorld

   For that to work, you must use smartj instead of java.exe.
7. The following will not work to run a HelloWorld.class app in C:\com\mindprod\mypackage, in a jar in package com.mindprod.mypackage, use
8. The following will work to run a HelloWorld.class app in C:\com\mindprod\mypackage, in a jar in package com.mindprod.mypackage, use

   REM For this to workYou must specify the class to execute,
   REM com.mindprod.mypackage.HelloWorld in the manifest Main-Class entry.
   CD \AnyDir
   java.exe -jar HelloWorld.jar

9. If for any reason the examples shown do not work with your version of java.exe, try various combinations of com.mindprod.mypackage.HelloWorld, com/mindprod/mypackage/HelloWorld and com\mindprod\mypackage\HelloWorld.

The Java.exe Command Line

• Note, unlike with javac.exe or <APPLET CODE, you don’t specify the extension (.class).
• Note, unlike all other command line utilities in the universe, you cannot specify the fully qualified filename of the class file. You must however specify the full package name.
• The CLASSPATH must provide access to all the needed *.class, *.zip and *.jar files.
• If you want to run an Applet you need to use AppletViewer or a browser instead of java.exe, and you need to compose some HTML with an <APPLET tag in it.
• The switches are case sensitive.
• Make sure you specify the -options before the name of the class you want to execute, otherwise they will be interpreted as parameters to your main method.
• If you use the -jar option, the java.exe ignores the classpath. It will only look in that jar. You can’t specify a class. You must use the Main-Class specified in the manifest. If there is no Main-Class, then nothing will work. To execute some other class besides the Main-Class, don’t use -jar ; put the jar on the classpath instead.
• With JDK 1.4.2 the command line switches had a major overhaul. Many of the old switches no longer work.

set jopts=-Xms4m
...
java.exe %jopts% MyClass

Parameters with Embedded Blanks

On windows you can enclose your blank-containing parameters in "s and they will be stripped off before your main program sees them.

Awkward Characters in Command Line Parameters

• Parameters cannot begin with - since those are for java.exe itself. So you must enclose such parameters inside quotes, e.g.

  java.exe -jar myjar.jar "-something"

• Since spaces separate parameters, you can’t have spaces inside parameters unless you surround them in quotes, e.g.

  java.exe -jar myjar.jar "new improved blue raspberry flavour"

• Since quotes surround parameters, you can’t have quotes inside parameters unless you precede them with \, e.g.

  java.exe -jar myjar.jar "She said, \"I'm leaving\"."

  Backslash in any other context just means plain backslash. Unlike in Java String literals, you don’t double each backslash.
• Somebody was asleep at the switch when they devised this convention.

  java.exe -jar myjar.jar "F:\Program Files\"

  That is illegal! and you can’t specify it another way.
• You can’t have newline characters inside your parameters. If you need them, you will need to cook up some encoding scheme of your own, perhaps URLEncoder, and explicitly decode that parameter yourself.
• You can’t have any character you cannot key in the native encoding in which you write your bat stream. If you need them, you will gain have to roll your own scheme to pass them through.
• I’m told you can use \\" to stand for ", but I have not tried it. This is so Mickey Mouse I can’t believe grown companies made it up.

Jars

Multiple java.exes

To complicate things further the java.exe in system32 is just a dummy. It looks in the registry and then decides which real java.exe to use. The last JVM installed gets to be the one used, even if it is older. To switch JVMs, you must normally reinstall the one you want.

How can you make order of this chaos and manage your choice of JVM with some finesse?

You could fiddle with the registry to switch JVMs. See the JVM Manager student project. However, there is a simpler, cruder way.

1. Delete the copy of java.exe and javaw.exe in C:\WINDOWS\system32 or C:\WINNT\system32.
2. Define a SET JAVA_HOME=C:\Program Files\java\jre1.6.0_06 (no quotes!) environment variable to point to the where the \bin directory is containing the java.exe you want to use.
3. Make all references in your bat files to your JVM use %JAVA_HOME%\bin\java.exe, ditto for javaw.exe and javac.exe.
4. Change JAVA_HOME to switch JVMs. Unfortunately, IE, Opera and Netscape won’t pay any attention to you.

Aborting

Under the Hood

Learning More