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Understanding the classpath

From: Java Essential Training

Video: Understanding the classpath

Once you've packaged up an application as a JAR File you're ready to run it from the command line, but in order to run it from the command line, you need to understand how the concept of the class path works. I'll described how to use the class path and how to set it on both Mac and Windows in this exercise. I'm working in the class path project, a version of my olivepress application that I've already compressed into a JAR File. If you have access to the Exercise Files you can use this file or you can use any other JAR File that you want to create.

Understanding the classpath

Once you've packaged up an application as a JAR File you're ready to run it from the command line, but in order to run it from the command line, you need to understand how the concept of the class path works. I'll described how to use the class path and how to set it on both Mac and Windows in this exercise. I'm working in the class path project, a version of my olivepress application that I've already compressed into a JAR File. If you have access to the Exercise Files you can use this file or you can use any other JAR File that you want to create.

I'm going to copy this file to my desktop, so I don't have to worry about the specific folder in which it's located, once I go to the command line. I'll right-click on the file and copy, then I'll go to the desktop and paste. Now, I'll open up a command window, on Windows, I'll run the cmd application and on Mac, open terminal, and then switch to your desktop folder, do a directory listing, type dir on Windows or LS on Mac, and you'll see the OlivePressApp.jar file there.

Now, to run the application you need to know the complete path of the Main class. I'll go back to Eclipse briefly, and show you within Main.java that the package is com.lynda.olivepress, and the class is main. In order to run the class from the command line you need to know both of those bits of information, you'll put the package and the class name together separated with a dot, and that's how you launch the application using the Java tool. I'll go back to the command window and I'll try running the application the first time.

I'll type java com.lynda.olivepress.Main, and I get the error could not find or load Main class and then the full path of the class that I typed. Now I typed that correctly, but the problem is that Java doesn't know where to find the class, here's how you tell it. You tell Java where the class is that you want to run by setting the class path. The class path is a list of file locations. Typically it starts with a single dot, meaning the current directory, and then lists all of the folders and JAR Files that contain compiled classes.

You can set the class path either as part of the Java command or as an environment variable and I'll show you both. Here's how you incorporate the class path into the command that starts the application, start with Java and then - classpath, you can shorten that to -cp if you prefer, then type in a dot for the current directory. On Windows, add a semicolon on Mac or Linux add a colon, and then type in the name of your JAR File.

I'll start by typing olive and then I'll tab, and on either Mac or Windows, that should auto complete the class path. Now type in the full path of your starting class com.lynda.olivepress.Main, make sure you spell it exactly correctly, it is case-sensitive, regardless of whether you're working on Windows or Mac press Enter or Return and the application should run. Now you can also set the class path as an environment variable, this works pretty much the same way on either Windows or Mac, but again the difference will be how you separate the values.

You use a semicolon to separate the values on Windows and colon on Mac. This time I'll type SETCLASSPATH put it in all uppercase, it doesn't matter on Windows, but it does on Mac, and then an equals operator, and then just as I did before I'll type in a dot, then a ; on Windows or a : on Mac, and then the name of the file, and once again, I can type the beginning of the file name and press tab. I'll press Enter or Return and then this time I won't need to add the -class path argument to the command, because the value was already set in the environment variable.

I'll type in java com.lynda.olivepress. Main, and now my application works again. When you set an environment variable this way, you're setting it only for the current command window session. For command on Windows or terminal on Mac, and when you close the command window or you close terminal, that setting will be lost. You could set this permanently in what Windows calls the system environment variables, or through a special file on Mac that I don't recommend doing it.

It's better to create a batch file or a bash script to set these values if it's something you need to do over and over again, but again, you can set the class path either as part of the Java command, or as an environment variable, whichever works best for you. Once you know how the class path works, you can get started on your own Java applications, running them either from within Eclipse as we've been doing throughout the entire course, or in the command window, if that works better for your debugging purposes.

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This video is part of

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Java Essential Training

71 video lessons · 71662 viewers

David Gassner
Author

 
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  1. 10m 8s
    1. Welcome
      1m 3s
    2. Is this course for you?
      5m 35s
    3. Using the exercise files
      3m 30s
  2. 31m 24s
    1. The history of Java
      5m 19s
    2. Java compilation and syntax
      8m 54s
    3. Understanding the principles of Java
      8m 28s
    4. Choosing a development environment
      8m 43s
  3. 19m 5s
    1. Installing Java on Windows
      6m 42s
    2. Installing Eclipse on Windows
      3m 19s
    3. Exploring Java on Mac OS X Leopard and Snow Leopard
      2m 27s
    4. Installing Java on Mac OS X Lion
      3m 27s
    5. Installing Eclipse on Mac OS X
      3m 10s
  4. 46m 10s
    1. Creating a Hello World application
      11m 7s
    2. Exploring the Eclipse IDE
      8m 55s
    3. Compiling and running from the command line
      8m 2s
    4. Passing arguments to the application
      8m 17s
    5. Using the Java API documentation
      4m 5s
    6. Memory management and garbage collection
      5m 44s
  5. 58m 57s
    1. Everything is an object
      5m 59s
    2. Declaring and initializing variables
      9m 15s
    3. Working with numbers
      8m 32s
    4. Converting numeric values
      6m 40s
    5. Understanding operators
      7m 58s
    6. Working with character values
      5m 14s
    7. Working with boolean values
      5m 13s
    8. Outputting primitive values as strings
      5m 33s
    9. Creating a simple calculator application
      4m 33s
  6. 53m 40s
    1. Writing conditional code
      5m 35s
    2. Using the switch statement
      8m 50s
    3. Repeating code blocks with loops
      7m 35s
    4. Creating reusable code with methods
      6m 31s
    5. Declaring methods with arguments
      5m 41s
    6. Overloading method names with different signatures
      5m 53s
    7. Passing arguments by reference or by value
      5m 35s
    8. Creating a more complex calculator application
      8m 0s
  7. 20m 30s
    1. Using the String class
      5m 44s
    2. Building strings with StringBuilder
      3m 34s
    3. Parsing string values
      3m 19s
    4. Working with date values
      7m 53s
  8. 20m 44s
    1. Understanding compile-time vs. runtime errors
      4m 5s
    2. Handling exceptions with try/catch
      4m 55s
    3. Throwing exceptions in methods
      2m 50s
    4. Using the debugger
      8m 54s
  9. 32m 22s
    1. Using simple arrays
      4m 47s
    2. Using two-dimensional arrays
      6m 17s
    3. Managing resizable arrays with ArrayList
      7m 14s
    4. Managing unordered data with HashMap
      6m 5s
    5. Looping through collections with iterators
      7m 59s
  10. 52m 2s
    1. Understanding encapsulation
      5m 59s
    2. Creating and instantiating custom classes
      8m 8s
    3. Organizing classes with packages
      6m 47s
    4. Creating and using instance methods
      6m 52s
    5. Storing data in instance variables
      6m 56s
    6. Using constructor methods
      5m 40s
    7. Managing instance data with getter and setter methods
      8m 26s
    8. Using class variables and Enum classes
      3m 14s
  11. 41m 15s
    1. Understanding inheritance and polymorphism
      9m 12s
    2. Extending custom classes
      9m 1s
    3. Overriding superclass methods
      3m 8s
    4. Casting subclass objects
      5m 3s
    5. Understanding interfaces and implementing classes
      4m 2s
    6. Creating your own interfaces
      4m 14s
    7. Using abstract classes and methods
      6m 35s
  12. 32m 17s
    1. Managing files with the core class library
      7m 46s
    2. Managing files with Apache Commons FileUtils
      7m 32s
    3. Reading a text file from a networked resource
      7m 52s
    4. Parsing an XML file with DOM
      9m 7s
  13. 17m 39s
    1. Creating your own JAR files
      4m 54s
    2. Understanding the classpath
      5m 2s
    3. Documenting code with Javadoc
      7m 43s
  14. 47s
    1. Goodbye
      47s

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