如何在命令行中为多个包设置类路径？

Question

我正在尝试制作一个 java 程序来导入 2 个包，但不知道如何在单个命令行中为多个包设置类路径。

Answer 1

Just call, java -cp <CLASSPATH> <MainClass> . Read here PATH and CLASSPATH to find the right syntax for the CLASSPATH according your operating system.

Answer 2

You might want to read the Oracle tech note on setting the classpath , but the general idea is that you have a single classpath variable / command line argument that is composed of multiple paths separated using the standard path separator for your platform - on Windows this is generally ";" and on Unix ":" (I say generally because if you use bash via cygwin or similar on Windows you will use the unix separator).

Regardless, the classpath is made up of paths to directories that contain class files, to a specific jar file, or (in recent java versions) a wildcard that will match jars (but not classes).

If you have classes in a jar or directory, they must be in directory-package format, eg the following class is expected to be in a directory foo/bar/Baz.class :

package foo.bar;

public class Baz {
    public static void main(final String[] args) {
    }
}

So, given a Unix directory /home/user/project/classes (which contains a directory foo/bar containing Baz.class , a jar file /home/user/project/lib/dependency.jar , and a current working directory of /home/user/project we have:

• The classpath using absolute paths: /home/user/project/classes:/home/user/project/lib/dependency.jar .

• The classpath using relative paths classes:lib/dependency.jar .

One can pass this to java using either the -classpath or -cp command line arguments:

• java -classpath classes:lib/dependency.jar foo.bar.Baz
• java -cp classes:lib/dependency.jar foo.bar.Baz

Alternatively, the CLASSPATH environment variable can be set (it is usually more obvious to set it on the command line and less prone to causing unexpected side effects). In bash this might be:

• CLASSPATH=classes:lib/dependency.jar java foo.bar.Baz
• export CLASSPATH=classes:lib/dependency.jar; java foo.bar.Baz

Note that when using an executable jar (one with a Main-Class attribute in its manifest) it is necessary for the classpath to also be defined in the manifest and the command line classpath is ignored (see the java -jar documentation ).