"java -classpath" - Specifying Class Path

Java Tools Tutorials - Herong's Tutorial Examples

∟"java -classpath" - Specifying Class Path

This section provides a tutorial example on how to use the '-classpath' option to specify the class path for the 'java' tool to load any classes required during the execution.

The most commonly used "java" option is "-classpath", which specifies a list of path names where the launcher will search for compiled type definitions. If "-classpath" is not specified, the current directory will be used as the class path.

To experiment how the Java program launcher uses "-classpath", I wrote the following simple source file, Echoer.java:

/* Echoer.java
 * Copyright (c) 2005 HerongYang.com. All Rights Reserved.
 */
public class Echoer {
   private String req = null;
   private String res = null;
   public void setReq(String r) {
      req = new String(r);
      char[] a = r.toCharArray();
      int n = a.length;
      for (int i=0; i<n/2; i++) {
         char t = a[i];
         a[i] = a[n-1-i];
         a[n-i-1] = t;
      }
      res = new String(a);
   }
   public String getRes() {
      return res;
   }
}

I also wrote the following testing class, EchoerTest.java:

/* EchoerTest.java
 * Copyright (c) 2005 HerongYang.com. All Rights Reserved.
 */
public class EchoerTest {
   public static void main(String[] a) {
      Echoer e = new Echoer();
      e.setReq("Hello world!");
      System.out.println(e.getRes());
   }
}

Then I stored them in two different directories, and tried to launch EchoerTest:

herong> mkdir .\echo
herong> copy Echoer.java .\echo

herong> mkdir .\test
herong> copy EchoerTest.java .\test

herong> cd test

herong\test> javac -classpath ..\echo EchoerTest.java

herong\test> java EchoerTest
Exception in thread "main" java.lang.NoClassDefFoundError: Echoer
        at EchoerTest.main(EchoerTest.java:6)
Caused by: java.lang.ClassNotFoundException: Echoer
        at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass...
        at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader....
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java...
        ... 1 more

herong\test> java -classpath ..\echo EchoerTest
Error: Could not find or load main class EchoerTest
Caused by: java.lang.ClassNotFoundException: EchoerTest

herong\test> java -classpath .;..\echo EchoerTest
!dlrow olleH

Note that:

The first time I tried to launch "EchoerTest" without "-classpath", "java" failed to find "Echoer" class, because "java" uses the current directory as the default class path.
The second time I tried to launch "EchoerTest" with "-classpath ..\echo", "java" failed to find "EchoerTest" class, because the specified class path didn't include the current directory.
The third time I tried to launch "EchoerTest" with "-classpath .;..\echo", "java" was able to find both "EchoerTest" and "Echoer".