利用URLClassLoader读取Jar包并反射类（利用Tomcat源码）

第一次写博客，写的不好请见谅。

背景：在公司内部项目中碰到了一个需求，我需要读取本地jar包然后反射其中的实体类来获取其类名、属性等字段并保存。

分析：显然，我们首先需要用到反射，但是反射需要有类加载器来加载类。比如classLoader.load("com.test.Test");对加载器不了解的朋友可以自行百度。因为是读的本地的jar包，我们当前web用的appClassLoader显然不够用，也无法load本地的类，因此我们可以自己使用URLClassLoader创建一个加载器。（在这里我是用了tomcat生成自定义classloader的方法，很简单）。

方法：将jar存在项目根路径下，创建ClassLoader之前获取jar包的所有路径即可。

实施：

/** * 创建classloader（调用了tomcat的api） * @return * @throws Exception */private ClassLoader createClassLoader() throws Exception {    //在这里拿到存在本地的jar包的路径    //模仿tomcat的处理方式    String value = JarProperties.getProperty("jar.path");    if (value == null) {        return null;    }    //处理字符串路径    value = replace(value);    List<ClassLoaderFactory.Repository> repositories = new ArrayList<>();    //分割成路径数组    String[] repositoryPaths = getPaths(value);    for (String repository : repositoryPaths) {        // Local repository        if (repository.endsWith("*.jar")) {            repository = repository.substring                    (0, repository.length() - "*.jar".length());            repositories.add(                    new ClassLoaderFactory.Repository(repository, ClassLoaderFactory.RepositoryType.GLOB));        } else if (repository.endsWith(".jar")) {            repositories.add(                    new ClassLoaderFactory.Repository(repository, ClassLoaderFactory.RepositoryType.JAR));        } else {            repositories.add(                    new ClassLoaderFactory.Repository(repository, ClassLoaderFactory.RepositoryType.DIR));        }    }    return ClassLoaderFactory.createClassLoader(repositories, null);}

其中JarProperties如下：

public class JarProperties {    private static final Logger logger = LoggerFactory.getLogger(JarProperties.class);    private static Properties properties = null;    static {        loadProperties();    }    /**     * @param name The property name     * @return specified property value     */    public static String getProperty(String name) {        return properties.getProperty(name);    }    private static void loadProperties() {        InputStream is = null;        try {            ClassLoader cl = JarProperties.class.getClassLoader();            URL url = cl.getResource("jar.properties");            if (url != null) {                is = url.openStream();            }        } catch (Exception e) {            e.printStackTrace();            logger.error("url打开stream错误");//            logger.error(e.getMessage() + "没有找到.properties文件");        }        if (is != null) {            try {                properties = new Properties();                properties.load(is);            } catch (Exception e) {                e.printStackTrace();                logger.error("加载jar.properties文件出错");            } finally {                try {                    is.close();                } catch (IOException e) {                    e.printStackTrace();                    logger.error("无法关闭jar.properties" + e);                }            }        }        if (is == null) {            // Do something            logger.error("没有找到jar.properties文件");            // That's fine - we have reasonable defaults.            properties = new Properties();        }    }}
jar.properties:
jar.path = "${authority.home}/jar","${authority.home}/jar/*.jar"
然后在ClassLoaderFactory.createClassLoader(repositories, null)中：
/** * Create and return a new class loader, based on the configuration * defaults and the specified directory paths: * * @param repositories List of class directories, jar files, jar directories *                     or URLS that should be added to the repositories of *                     the class loader. * @param parent Parent class loader for the new class loader, or *  <code>null</code> for the system class loader. * @return the new class loader * * @exception Exception if an error occurs constructing the class loader */public static ClassLoader createClassLoader(List<Repository> repositories,                                            final ClassLoader parent)    throws Exception {    if (log.isDebugEnabled())        log.debug("Creating new class loader");    // Construct the "class path" for this class loader    Set<URL> set = new LinkedHashSet<>();    if (repositories != null) {        for (Repository repository : repositories)  {            if (repository.getType() == RepositoryType.URL) {                URL url = buildClassLoaderUrl(repository.getLocation());                if (log.isDebugEnabled())                    log.debug("  Including URL " + url);                set.add(url);            } else if (repository.getType() == RepositoryType.DIR) {                File directory = new File(repository.getLocation());                directory = directory.getCanonicalFile();                if (!validateFile(directory, RepositoryType.DIR)) {                    continue;                }                URL url = buildClassLoaderUrl(directory);                if (log.isDebugEnabled())                    log.debug("  Including directory " + url);                set.add(url);            } else if (repository.getType() == RepositoryType.JAR) {                File file=new File(repository.getLocation());                file = file.getCanonicalFile();                if (!validateFile(file, RepositoryType.JAR)) {                    continue;                }                URL url = buildClassLoaderUrl(file);                if (log.isDebugEnabled())                    log.debug("  Including jar file " + url);                set.add(url);            } else if (repository.getType() == RepositoryType.GLOB) {                File directory=new File(repository.getLocation());                directory = directory.getCanonicalFile();                if (!validateFile(directory, RepositoryType.GLOB)) {                    continue;                }                if (log.isDebugEnabled())                    log.debug("  Including directory glob "                        + directory.getAbsolutePath());                String filenames[] = directory.list();                if (filenames == null) {                    continue;                }                for (int j = 0; j < filenames.length; j++) {                    String filename = filenames[j].toLowerCase(Locale.ENGLISH);                    if (!filename.endsWith(".jar"))                        continue;                    File file = new File(directory, filenames[j]);                    file = file.getCanonicalFile();                    if (!validateFile(file, RepositoryType.JAR)) {                        continue;                    }                    if (log.isDebugEnabled())                        log.debug("    Including glob jar file "                            + file.getAbsolutePath());                    URL url = buildClassLoaderUrl(file);                    set.add(url);                }            }        }    }    // Construct the class loader itself    final URL[] array = set.toArray(new URL[set.size()]);    if (log.isDebugEnabled())        for (int i = 0; i < array.length; i++) {            log.debug("  location " + i + " is " + array[i]);        }    return AccessController.doPrivileged(            new PrivilegedAction<URLClassLoader>() {                @Override                public URLClassLoader run() {                    if (parent == null)                        return new URLClassLoader(array);                    else                        return new URLClassLoader(array, parent);                }            });}
具体注释我就不写了，就是把路径处理一下然后放在数组中然后new URLClassLoader(array)就可以。
大部分代码都是tomcat源码中的，我是直接用的tomcat的类完成的这个接口。所以说，多看源码还是很有帮助滴。
人生第一篇博客结束！！！！！