java - 在运行时从 Java 类路径中删除文件夹

Question

有没有一种方法可以从类路径中删除文件夹，类似于在运行时添加文件夹(Can a directory be added to the class path at runtime?)

Answer 1

请在下面找到一个片段作为技术示例来演示添加/删除路径。

在任意目录下创建以下源文件

import java.io.File;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.Stack;
import sun.misc.URLClassPath;

public class EvilPathDemo {

    public static void addPath(String path) throws Exception {
        URL u = new File(path).toURI().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});
    }

    public static void removePath(String path) throws Exception {
        URL url = new File(path).toURI().toURL();
        URLClassLoader urlClassLoader = (URLClassLoader) 
            ClassLoader.getSystemClassLoader();
        Class<?> urlClass = URLClassLoader.class;
        Field ucpField = urlClass.getDeclaredField("ucp");
        ucpField.setAccessible(true);
        URLClassPath ucp = (URLClassPath) ucpField.get(urlClassLoader);
        Class<?> ucpClass = URLClassPath.class;
        Field urlsField = ucpClass.getDeclaredField("urls");
        urlsField.setAccessible(true);
        Stack urls = (Stack) urlsField.get(ucp);
        urls.remove(url);
    }

    public static void main(String[] args) throws Exception {
        String parm = args.length == 1 ? args[0] : "";
        String evilPath = "/tmp";

        String classpath = System.getProperty("java.class.path");
        boolean isEvilPathSet = false;
        for (String path : classpath.split(File.pathSeparator)) {
            if (path.equalsIgnoreCase(evilPath)) {
                System.out.printf("evil path '%s' in classpath%n", evilPath);
                isEvilPathSet = true;
                break;
            }
        }
        if (isEvilPathSet && parm.equalsIgnoreCase("REMOVE")) {
            System.out.printf("evil path '%s' will be removed%n", evilPath);
            removePath(evilPath);
        }
        tryToLoad("Foo");
        if (parm.equalsIgnoreCase("ADD")) {
            System.out.printf("evil path '%s' will be added%n", evilPath);
            addPath(evilPath);
        }
        tryToLoad("Bar");
    }

    private static void tryToLoad(String className) {
        try {
            Class<?> foo = Class.forName(className);
            System.out.printf("class loaded: %s%n", foo.getName());
        } catch (ClassNotFoundException ex) {
            System.out.println(ex);
        }
    }
}

.

public class Foo {
    static {
        System.out.println("I'm foo...");
    }
}

.

public class Bar {
    static {
        System.out.println("I'm bar...");
    }
}

编译如下

javac EvilPathDemo.java
javac -d /tmp Foo.java Bar.java

在测试期间，我们将尝试加载类 Foo 和 Bar。

类路径中没有/tmp

java -cp . EvilPathDemo
java.lang.ClassNotFoundException: Foo
java.lang.ClassNotFoundException: Bar

将/tmp 添加到类路径

java -cp . EvilPathDemo add
java.lang.ClassNotFoundException: Foo
evil path '/tmp' will be added
I'm bar...
class loaded: Bar

在类路径中使用/tmp

java -cp .:/tmp EvilPathDemo
evil path '/tmp' in the classpath
I'm foo...
class loaded: Foo
I'm bar...
class loaded: Bar

从类路径中删除/tmp

java -cp .:/tmp EvilPathDemo remove
evil path '/tmp' in the classpath
evil path '/tmp' will be removed
java.lang.ClassNotFoundException: Foo
java.lang.ClassNotFoundException: Bar

在测试过程中，我发现以下情况不起作用。

• 添加路径(邪恶路径)；
  tryToLoad("Foo");
  移除路径(邪恶路径)；//没有效果
  tryToLoad("栏");
• 移除路径(邪恶路径)；
  tryToLoad("Foo");
  添加路径(邪恶路径)；//没有效果
  tryToLoad("栏");
• tryToLoad("Foo");
  移除路径(邪恶路径)；//没有效果
  tryToLoad("栏");

我没有花时间找出原因。因为我没有看到它有任何实际用途。如果您真的需要/希望使用类路径，请查看类加载器的工作原理。