在写一个MVC框架，需要从包中扫描出组件并注册到容器中，而JDK没有提供现成的从方法，只能自己实现。

功能：
给定一个包名，编程得到该包(和其所有子包)下所有的类文件。如，输入包名com.myapp.util, 输出该包下类的全限定名com.myapp.util.StringUtils, com.app.util.ImageUtils等。

思路：
有的web server在部署运行时会解压jar包，因此class文件会在普通的文件目录下。如果web server不解压jar包，则class文件会直接存在于Jar包中。对于前者，只需定位到class文件所在目录，然后将class文件名读取出即可；对于后者，则需先定位到jar包所在目录，然后使用JarInputStream读取Jar包，得到class类名。

实现：
这是从写好的项目代码中直接copy出来的，如果要运行这段代码，需要把所有的Logger.debug改成System.out.println()

/**
 * This scanner is used to find out all classes in a package.
 * Created by whf on 15-2-26.
 */
public class ClasspathPackageScanner implements PackageScanner {
    private Logger logger = LoggerFactory.getLogger(ClasspathPackageScanner.class);

    private String basePackage;
    private ClassLoader cl;

    /**
     * Construct an instance and specify the base package it should scan.
     * @param basePackage The base package to scan.
     */
    public ClasspathPackageScanner(String basePackage) {
        this.basePackage = basePackage;
        this.cl = getClass().getClassLoader();

    }

    /**
     * Construct an instance with base package and class loader.
     * @param basePackage The base package to scan.
     * @param cl Use this class load to locate the package.
     */
    public ClasspathPackageScanner(String basePackage, ClassLoader cl) {
        this.basePackage = basePackage;
        this.cl = cl;
    }

    /**
     * Get all fully qualified names located in the specified package
     * and its sub-package.
     *
     * @return A list of fully qualified names.
     * @throws IOException
     */
    @Override
    public List<String> getFullyQualifiedClassNameList() throws IOException {
        logger.info("开始扫描包{}下的所有类", basePackage);

        return doScan(basePackage, new ArrayList<>());
    }

    /**
     * Actually perform the scanning procedure.
     *
     * @param basePackage
     * @param nameList A list to contain the result.
     * @return A list of fully qualified names.
     *
     * @throws IOException
     */
    private List<String> doScan(String basePackage, List<String> nameList) throws IOException {
        // replace dots with splashes
        String splashPath = StringUtil.dotToSplash(basePackage);

        // get file path
        URL url = cl.getResource(splashPath);
        String filePath = StringUtil.getRootPath(url);

        // Get classes in that package.
        // If the web server unzips the jar file, then the classes will exist in the form of
        // normal file in the directory.
        // If the web server does not unzip the jar file, then classes will exist in jar file.
        List<String> names = null; // contains the name of the class file. e.g., Apple.class will be stored as "Apple"
        if (isJarFile(filePath)) {
            // jar file
            if (logger.isDebugEnabled()) {
                logger.debug("{} 是一个JAR包", filePath);
            }

            names = readFromJarFile(filePath, splashPath);
        } else {
            // directory
            if (logger.isDebugEnabled()) {
                logger.debug("{} 是一个目录", filePath);
            }

            names = readFromDirectory(filePath);
        }

        for (String name : names) {
            if (isClassFile(name)) {
                //nameList.add(basePackage + "." + StringUtil.trimExtension(name));
                nameList.add(toFullyQualifiedName(name, basePackage));
            } else {
                // this is a directory
                // check this directory for more classes
                // do recursive invocation
                doScan(basePackage + "." + name, nameList);
            }
        }

        if (logger.isDebugEnabled()) {
            for (String n : nameList) {
                logger.debug("找到{}", n);
            }
        }

        return nameList;
    }

    /**
     * Convert short class name to fully qualified name.
     * e.g., String -> java.lang.String
     */
    private String toFullyQualifiedName(String shortName, String basePackage) {
        StringBuilder sb = new StringBuilder(basePackage);
        sb.append('.');
        sb.append(StringUtil.trimExtension(shortName));

        return sb.toString();
    }

    private List<String> readFromJarFile(String jarPath, String splashedPackageName) throws IOException {
        if (logger.isDebugEnabled()) {
            logger.debug("从JAR包中读取类: {}", jarPath);
        }

        JarInputStream jarIn = new JarInputStream(new FileInputStream(jarPath));
        JarEntry entry = jarIn.getNextJarEntry();

        List<String> nameList = new ArrayList<>();
        while (null != entry) {
            String name = entry.getName();
            if (name.startsWith(splashedPackageName) && isClassFile(name)) {
                nameList.add(name);
            }

            entry = jarIn.getNextJarEntry();
        }

        return nameList;
    }

    private List<String> readFromDirectory(String path) {
        File file = new File(path);
        String[] names = file.list();

        if (null == names) {
            return null;
        }

        return Arrays.asList(names);
    }

    private boolean isClassFile(String name) {
        return name.endsWith(".class");
    }

    private boolean isJarFile(String name) {
        return name.endsWith(".jar");
    }

    /**
     * For test purpose.
     */
    public static void main(String[] args) throws Exception {
        PackageScanner scan = new ClasspathPackageScanner("cn.fh.lightning.bean");
        scan.getFullyQualifiedClassNameList();
    }

}

上面的代码中用到了StringUtils类，如下：

public class StringUtil {
    private StringUtil() {

    }

    /**
     * "file:/home/whf/cn/fh" -> "/home/whf/cn/fh"
     * "jar:file:/home/whf/foo.jar!cn/fh" -> "/home/whf/foo.jar"
     */
    public static String getRootPath(URL url) {
        String fileUrl = url.getFile();
        int pos = fileUrl.indexOf('!');

        if (-1 == pos) {
            return fileUrl;
        }

        return fileUrl.substring(5, pos);
    }

    /**
     * "cn.fh.lightning" -> "cn/fh/lightning"
     * @param name
     * @return
     */
    public static String dotToSplash(String name) {
        return name.replaceAll("\\.", "/");
    }

    /**
     * "Apple.class" -> "Apple"
     */
    public static String trimExtension(String name) {
        int pos = name.indexOf('.');
        if (-1 != pos) {
            return name.substring(0, pos);
        }

        return name;
    }

    /**
     * /application/home -> /home
     * @param uri
     * @return
     */
    public static String trimURI(String uri) {
        String trimmed = uri.substring(1);
        int splashIndex = trimmed.indexOf('/');

        return trimmed.substring(splashIndex);
    }
}

执行结果：