java中的类加载器

Question

如何在使用类加载器时扫描特定的 jar 文件而不是扫描完整的 java 类路径？

例如： 我在类路径中有 Jar1、Jar2、Jar3、Jar4，但我想在使用类加载器扫描特定类时只扫描 Jar4。

Answer 1

如果您不拆分模块或使用在 OSGI 等不同类加载器中拆分组件的技术，那么如果没有某种结构或类加载器的新实例，您将无法做到这一点。

假设您使用当前上下文中的类加载器：

        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        String path = packageName.replace('.', '/');
        Enumeration resources = classLoader.getResources(path);
        List dirs = new ArrayList();
        while (resources.hasMoreElements()) {
            URL resource = resources.nextElement();
            dirs.add(new File(resource.getFile()));
        }
        ArrayList classes = new ArrayList();
        for (File directory : dirs) {
            classes.addAll(findClasses(directory, packageName));
        }
        return classes.toArray(new Class[classes.size()]);

但是，这太复杂了，有时您没有这样的结构可用。如果你得到一个新的类加载器，你可以告诉它最后扫描父类加载器并执行以下操作：

ParentLastURLClassLoader classLoader = new ParentLastURLClassLoader(
        Arrays.asList(new File("a.jar").toURI().toURL()));
Class<?> carClass = classLoader.loadClass("com.acme.Car");
Vehicle someCar = (Vehicle) carClass.newInstance();

在这个例子中，我们可以注意到一些重要的事情：一个类是用类加载器作为因子来识别的。所以不可能有：

com.acme.Car carClass = classLoader.loadClass("com.acme.Car");

因为第一个 com.acme.Car 和第二个是从不同的类加载器加载的，因此是不同的类。

此示例来自当前接受的答案：How do I create a parent-last / child-first ClassLoader in Java, or How to override an old Xerces version that was already loaded in the parent CL?

/**
 * A parent-last classloader that will try the child classloader first and then the parent.
 * This takes a fair bit of doing because java really prefers parent-first.
 * 
 * For those not familiar with class loading trickery, be wary
 */
private static class ParentLastURLClassLoader extends ClassLoader 
{
    private ChildURLClassLoader childClassLoader;

    /**
     * This class allows me to call findClass on a classloader
     */
    private static class FindClassClassLoader extends ClassLoader
    {
        public FindClassClassLoader(ClassLoader parent)
        {
            super(parent);
        }

        @Override
        public Class<?> findClass(String name) throws ClassNotFoundException
        {
            return super.findClass(name);
        }
    }

    /**
     * This class delegates (child then parent) for the findClass method for a URLClassLoader.
     * We need this because findClass is protected in URLClassLoader
     */
    private static class ChildURLClassLoader extends URLClassLoader
    {
        private FindClassClassLoader realParent;

        public ChildURLClassLoader( URL[] urls, FindClassClassLoader realParent )
        {
            super(urls, null);

            this.realParent = realParent;
        }

        @Override
        public Class<?> findClass(String name) throws ClassNotFoundException
        {
            try
            {
                // first try to use the URLClassLoader findClass
                return super.findClass(name);
            }
            catch( ClassNotFoundException e )
            {
                // if that fails, we ask our real parent classloader to load the class (we give up)
                return realParent.loadClass(name);
            }
        }
    }

    public ParentLastURLClassLoader(List<URL> classpath)
    {
        super(Thread.currentThread().getContextClassLoader());

        URL[] urls = classpath.toArray(new URL[classpath.size()]);

        childClassLoader = new ChildURLClassLoader( urls, new FindClassClassLoader(this.getParent()) );
    }

    @Override
    protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
    {
        try
        {
            // first we try to find a class inside the child classloader
            return childClassLoader.findClass(name);
        }
        catch( ClassNotFoundException e )
        {
            // didn't find it, try the parent
            return super.loadClass(name, resolve);
        }
    }
}