可以在运行时将目录添加到类路径吗？

Question

为了更好地理解Java中的工作原理,我想知道我是否可以在运行时动态地将目录添加到类路径中.

例如,如果我使用"java -jar mycp.jar"启动.jar并输出java.class.path属性,我可能会得到:

java.class.path: '.:/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java'

现在可以在运行时修改此类路径以添加另一个目录吗？(例如在使用位于我想添加的目录中的.jar对类进行第一次调用之前).

Answer 1

您可以使用以下方法:

URLClassLoader.addURL(URL url)

但是你需要用反射做这个,因为方法是protected:

public static void addPath(String s) throws Exception {
    File f = new File(s);
    URL u = f.toURL();
    URLClassLoader urlClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
    Class urlClass = URLClassLoader.class;
    Method method = urlClass.getDeclaredMethod("addURL", new Class[]{URL.class});
    method.setAccessible(true);
    method.invoke(urlClassLoader, new Object[]{u});
}

请参阅反射 Java Trail .特别是反射的缺点

Answer 2

更新2014:这是来自2011年Jonathan Spooner接受的答案中的代码,稍作重写,让Eclipse的验证器不再创建警告(弃用,rawtypes)

//need to do add path to Classpath with reflection since the URLClassLoader.addURL(URL url) method is protected:
public static void addPath(String s) throws Exception {
    File f = new File(s);
    URI u = f.toURI();
    URLClassLoader urlClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
    Class<URLClassLoader> urlClass = URLClassLoader.class;
    Method method = urlClass.getDeclaredMethod("addURL", new Class[]{URL.class});
    method.setAccessible(true);
    method.invoke(urlClassLoader, new Object[]{u.toURL()});
}

Answer 3

是的,你可以使用URLClassLoader..见这里的例子.不使用反射.

- 编辑 -

根据建议从链接复制示例.

import javax.naming.*;
import java.util.Hashtable;
import java.net.URLClassLoader;
import java.net.URL;
import java.net.MalformedURLException;

public class ChangeLoader {

    public static void main(String[] args) throws MalformedURLException {
    if (args.length != 1) {
        System.err.println("usage: java ChangeLoader codebase_url");
        System.exit(-1);
    }

    String url = args[0];
    ClassLoader prevCl = Thread.currentThread().getContextClassLoader();

    // Create class loader using given codebase
    // Use prevCl as parent to maintain current visibility
    ClassLoader urlCl = URLClassLoader.newInstance(new URL[]{new URL(url)}, prevCl);

        try {
        // Save class loader so that we can restore later
            Thread.currentThread().setContextClassLoader(urlCl);

        // Expect that environment properties are in
        // application resource file found at "url"
        Context ctx = new InitialContext();

        System.out.println(ctx.lookup("tutorial/report.txt"));

        // Close context when no longer needed
        ctx.close();
    } catch (NamingException e) {
        e.printStackTrace();
        } finally {
            // Restore
            Thread.currentThread().setContextClassLoader(prevCl);
        }
    }
}