classpath

Classpath is a parameter used by the Java platform to locate class files and resources at both compile time and runtime. It tells the Java Virtual Machine and the Java compiler where to search for user-defined classes and packages. The classpath can be specified with the -cp or -classpath option, through the CLASSPATH environment variable, or implicitly defaulting to the current working directory when none is provided. In modern usage, the default often includes the current directory, but behavior can vary by implementation.

Classpath entries may be directories or archive files such as JAR or ZIP files. When a directory is on the classpath, the loader searches for class files within that directory hierarchy. When a JAR is on the classpath, the loader reads the class files contained inside the JAR. If a requested class is not found on the entire classpath, a ClassNotFoundException is thrown. The order of entries matters: the search proceeds from the first to the last entry, and earlier entries can take precedence over later ones.

Path entries are separated differently across operating systems: a semicolon separates entries on Windows, while a colon is used on Unix-like systems. Examples include: java -cp lib/foo.jar:lib/bar.jar:. com.example.Main on Unix, and java -cp "lib/foo.jar;lib/bar.jar;." com.example.Main on Windows. The -cp and -classpath options are equivalent.

In Java 9 and later, a module system introduces the modulepath, providing a separate mechanism for modular applications, though classpath remains supported for non-modular code. Build tools like Maven and Gradle manage runtime classpaths automatically, reducing manual configuration.