Java类路径（-cp）并将执行传递给该类

Question

I am relatively new to Java but have a fair understanding about how the class path works with respect to providing a list of folder and jars that make classes available to other classes.

I have compiled a JAR (lets say example.jar) that has a main function where execution normally begins. Sometimes I want execution to begin in a different class (lets say myAlternateClass.java), with its own main. I can achieve this by doing using the -cp argument when executing the jar, for example;

java -cp example.jar myAlternateClass

This works as I require but I am unsure of what exactly is happening here.

Answer 1

I'm not 100% sure on exactly what you're looking for, but I'll give it a shot.

There are two ways to use a jar file. If the jar file has a Main-Class specified in its META-INF/MANIFEST.MF file, then you can load java with the jar file and execution will start in the main method of that class.

java -jar example.jar

On the other hand, a jar file can simply be loaded onto the classpath, which makes all of the classes within it available for use. This is the example you are giving:

java -cp example.jar org.somewhere.MySecondClass

The -cp example.jar puts all of the classes within the jar on the class path and the second argument org.somewhere.MySecondClass gives the class at which execution should begin. This second argument would have to be within the jar since specifying a classpath overrides the default (which is just the current directory). In this case, java ignores any Main-Class specified in the MANIFEST.MF file of the jar (if one even is specified).

Multiple jar files as well as directories of java files not in a jar can be specified by putting colons between them. So,

java -jar example.jar:. MyClass

could launch MyClass from the current directory, but place example.jar on the classpath so that MyClass could create instances of whatever classes are available within example.jar.