What is a classpath and how do I set it

Question

I was just reading this line      The first thing the format   method does is load a Velocity template from the classpath named output vm   Please explain what was meant by classpath in this context  and how I should set the classpath

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Setting the CLASSPATH System Variable

To display the current CLASSPATH variable, use these commands in Windows and UNIX (Bourne shell): In Windows: C:\> set CLASSPATH In UNIX: % echo $CLASSPATH

To delete the current contents of the CLASSPATH variable, use these commands: In Windows: C:\> set CLASSPATH= In UNIX: % unset CLASSPATH; export CLASSPATH

To set the CLASSPATH variable, use these commands (for example): In Windows: C:\> set CLASSPATH=C:\users\george\java\classes In UNIX: % CLASSPATH=/home/george/java/classes; export CLASSPATH

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For linux users  and to sum up and add to what others have said here  you should know the following     CLASSPATH is what Java uses to look through multiple directories to find all the different classes it needs for your script  unless you explicitly tell it otherwise with the -cp override    Using -cp requires that you keep track of all the directories manually and copy-paste that line every time you run the program  not preferable IMO   The colon       character separates the different directories  There is only one  CLASSPATH and it has all the directories in it  So  when you run  export CLASSPATH       you want to include the current value   CLASSPATH  in order to append to it  For example   export CLASSPATH   export CLASSPATH  CLASSPATH  usr share java mysql-connector-java-5 1 12 jar   In the first line above  you start CLASSPATH out with just a simple  dot  which is the path to your current working directory  With that  whenever you run java it will look in the current working directory  the one you re in  for classes  In the second line above   CLASSPATH grabs the value that you previously entered     and appends the path to a mysql dirver  Now  java will look for the driver AND for your classes  echo  CLASSPATH   is super handy  and what it returns should read like a colon-separated list of all the directories  and  jar files  you want java looking in for the classes it needs  Tomcat does not use CLASSPATH   Read what to do about that here   https   tomcat apache org tomcat-8 0-doc class-loader-howto html

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When programming in Java  you make other classes available to the class you are writing by putting something like this at the top of your source file  import org javaguy coolframework MyClass   Or sometimes you  bulk import  stuff by saying  import org javaguy coolframework     So later in your program when you say  MyClass mine   new MyClass     The Java Virtual Machine will know where to find your compiled class  It would be impractical to have the VM look through every folder on your machine  so you have to provide the VM a list of places to look  This is done by putting folder and jar files on your classpath  Before we talk about how the classpath is set  let s talk about  class files  packages  and  jar files  First  let s suppose that MyClass is something you built as part of your project  and it is in a directory in your project called output  The  class file would be at output org javaguy coolframework MyClass class  along with every other file in that package   In order to get to that file  your path would simply need to contain the folder  output   not the whole package structure  since your import statement provides all that information to the VM  Now let s suppose that you bundle CoolFramework up into a  jar file  and put that CoolFramework jar into a lib directory in your project  You would now need to put lib CoolFramework jar into your classpath  The VM will look inside the jar file for the org javaguy coolframework part  and find your class  So  classpaths contain   JAR files  and Paths to the top of package hierarchies   How do you set your classpath  The first way everyone seems to learn is with environment variables  On a unix machine  you can say something like  export CLASSPATH  home myaccount myproject lib CoolFramework jar  home myaccount myproject output   On a Windows machine you have to go to your environment settings and either add or modify the value that is already there  The second way is to use the -cp parameter when starting Java  like this  java -cp  quot  home myaccount myproject lib CoolFramework jar  home myaccount myproject output  quot   MyMainClass  A variant of this is the third way which is often done with a  sh or  bat file that calculates the classpath and passes it to Java via the -cp parameter  There is a  quot gotcha quot  with all of the above   On most systems  Linux  Mac OS  UNIX  etc  the colon character       is the classpath separator   In windowsm the separator is the semicolon       So what s the best way to do it  Setting stuff globally via environment variables is bad  generally for the same kinds of reasons that global variables are bad  You change the CLASSPATH environment variable so one program works  and you end up breaking another program  The -cp is the way to go  I generally make sure my CLASSPATH environment variable is an empty string where I develop  whenever possible  so that I avoid global classpath issues  some tools aren t happy when the global classpath is empty though - I know of two common  mega-thousand dollar licensed J2EE and Java servers that have this kind of issue with their command-line tools

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Static member of a class can be called directly without creating object instance  Since the main method is static Java virtual Machine can call it without creating any instance of a class which contains the main method  which is start point of program

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The classpath is the path where the Java Virtual Machine look for user-defined classes  packages and resources in Java programs   In this context  the format   method load a template file from this path

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Classpath is an environment variable of system  The setting of this variable is used to provide the root of any package hierarchy to java compiler

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Think of it as Java s answer to the PATH environment variable - OSes search for EXEs on the PATH  Java searches for classes and packages on the classpath

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The classpath in this context is exactly what it is in the general context  anywhere the VM knows it can find classes to be loaded  and resources as well  such as output vm in your case    I d understand Velocity expects to find a file named output vm anywhere in  no package   This can be a JAR  regular folder      The root of any of the locations in the application s classpath

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CLASSPATH is an environment variable  i e   global variables of the operating system available to all the processes  needed for the Java compiler and runtime to locate the Java packages used in a Java program   Why not call PACKAGEPATH   This is similar to another environment variable PATH  which is used by the CMD shell to find the executable programs   CLASSPATH can be set in one of the following ways   CLASSPATH can be set permanently in the environment  In Windows  choose control panel   System   Advanced   Environment Variables   choose  System Variables   for all the users  or  User Variables   only the currently login user    choose  Edit   if CLASSPATH already exists  or  New    Enter  CLASSPATH  as the variable name   Enter the required directories and JAR files  separated by semicolons  as the value  e g      c  javaproject classes d  tomcat lib servlet-api jar    Take note that you need to include the current working directory  denoted by      in the CLASSPATH   To check the current setting of the CLASSPATH  issue the following command    gt  SET CLASSPATH  CLASSPATH can be set temporarily for that particular CMD shell session by issuing the following command    gt  SET CLASSPATH   c  javaproject classes d  tomcat lib servlet-api jar  Instead of using the CLASSPATH environment variable  you can also use the command-line option -classpath or -cp of the javac and java commands  for example    gt  java    classpath c  javaproject classes com abc project1 subproject2 MyClass3

[java] What is a classpath and how do I set it?

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