如何在运行时知道JAR文件是否已存在于类路径中？

Question

What is the best way to know at runtime if a particular JAR file is already in my classpath ? (if that is not the case I should add it at runtime).

I do not know in advance the name of the jar nor the classes in it. A user can select it. The jar represents a runtime plugable component (a driver in my problem).

Answer 1

A pragmatic way: Class.forName("com.myclass") where com.myclass is a class that is inside (and only inside) your target jar; if that throws a ClassNotFoundException , then the jar is not on you current classpath.

Bear in mind, though, that loading a jar at runtime is not very simple, you need to mess with classloaders. As a rule (there are exceptions) that's not the way, you should be able to add explicitly the jar to the classpath before running.

Update: the updated question states that we don't know in advance "the name of the jar nor the classes in it"; if so, this answer obviously does not apply. And the answer will depend on your specific classloader. In the usual scenario, AlexAndas' answer should work.

Answer 2

String classpath = System.getProperty("java.class.path")

this will give you what is on your classpath. you can then parse that for the jar file you want

Answer 3

try using this method:

public static void isJarExist(String jarName)
    {
        ClassLoader classLoader = ClassLoader.getSystemClassLoader();
        if (classLoader instanceof URLClassLoader)
        {
            URLClassLoader classLoader2 = (URLClassLoader) classLoader;
            URL [] urls = classLoader2.getURLs();
            for (URL url : urls)
            {
                File file = new File(url.getFile());
                if (file.getPath().endsWith(jarName))
                {
                    System.out.println(jarName + " exist");
                    return;
                }
            }
            System.out.println(jarName + " not exist");
        }
    }

just pass your jar like isJarExist("myjar.jar), you can also modify it to return boolean as you wish

Answer 4

You can't do that in general since there might be classloaders in your application that don't offer information of the jar-files. In the case of URLClassLoaders, you can use the solution of Alex Adas