类加载在Java

Question

我怎么可以扫描特定的jar文件，而不是扫描完整的Java类路径中同时使用一个类加载器？

例如： - 我有Jar1，Jar2，Jar3，Jar4类路径，但我想同时使用类加载器来扫描某一类只扫描Jar4。

Answer 1

如果你不模块拆分或使用在不同的类装载器就像OSGI拆分组件技术，你不能没有某种形式的结构，或者一个类加载器的一个新实例做到这一点。

让我们说你从你在上下文中使用的类加载器：

        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()]);

然而，这是太复杂了，有时你不具有这样的结构可用。如果你得到一个新的类加载器，你可以告诉它来扫描最后例如父类加载器和这样做：

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东西：一类是确定了与类加载器的因素。因此，人们不能有：

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

作为第一com.acme.Car和第二从不同的类加载器加载，因此是不同的类。

这个例子是从目前接受的答案为：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);
        }
    }
}