自定义URLClassLoader 加载指定class

1、java本身的三个ClassLoader

BootClassLoader

(baidu：Bootstrp加载器是用C++语言写的，它是在Java虚拟机启动后初始化的，它主要负责加载%JAVA_HOME%/jre/lib,-Xbootclasspath参数指定的路径以及%JAVA_HOME%/jre/classes中的类)

ExtClassLoader

(baidu：Bootstrp loader加载ExtClassLoader,并且将ExtClassLoader的父加载器设置为Bootstrp loader.ExtClassLoader是用Java写的，具体来说就是 sun.misc.Launcher$ExtClassLoader，ExtClassLoader主要加载%JAVA_HOME%/jre/lib/ext，此路径下的所有classes目录以及java.ext.dirs系统变量指定的路径中类库)

AppClassLoader

(baidu：Bootstrp loader加载完ExtClassLoader后，就会加载AppClassLoader,并且将AppClassLoader的父加载器指定为 ExtClassLoader。AppClassLoader也是用Java写成的，它的实现类是 sun.misc.Launcher$AppClassLoader，主要负责加载classpath所指定的位置的类或者是jar文档，它也是Java程序默认的类加载器, Thread.currentThread().getContextClassLoader() 就是它，

ClassLoader.getSystemClassLoader() 也是它)

2、自定义个ClassLoader ，用于其它目的

public class BSClassLoader extends URLClassLoader {
   private final String JAR_SUFFIX = ".jar";
   private final String FILE_PROTOCOL = "file";
   private final String SLASH = "/";
   private final String ESCAPE_SEPARATOR = "\\";

   public BSClassLoader () {
       this(new URL[] {},Thread.currentThread().getContextClassLoader());
   }

public BSClassLoader (ClassLoader parent) {
this(new URL[] {}, parent);
}

   public BSClassLoader (URL[] urls) {
       super(urls);
   }

   public BSClassLoader (URL[] urls, ClassLoader parent) {
       super(urls, parent);
   }

   public Class<?> findInternalClass(String name) {
       return findLoadedClass(name);
   }

   public void addJarsDir(String path) {
       File pathFile = new File(path);
       if (!pathFile.exists()) {
           return;
       }
       if (!pathFile.isDirectory()) {
           return;
       }
       File[] jarFiles = pathFile.listFiles();
       for (File jarFile : jarFiles) {
           String fileName = jarFile.getName();
           if (fileName.endsWith(JAR_SUFFIX)) {
               addJar(jarFile);
           } else {
               continue;
           }
       }
   }

   public void addJar(File file) {
       String repositoryPath = file.getAbsolutePath();
       repositoryPath = PathUtils.transformToCanonical(repositoryPath);
       addRepository(repositoryPath, false);
   }

   public void addClassDir(String path) {
       File pathFile = new File(path);
       if (!pathFile.exists()) {
           return;
       }
       if (!pathFile.isDirectory()) {
           return;
       }
       path = PathUtils.transformToCanonical(path);
       addRepository(path, true);
   }

   private void addRepository(String path, boolean slash) {
       try {
           path = path.replace(ESCAPE_SEPARATOR, SLASH);
           if (slash) {
               if (!path.endsWith(SLASH)) {
                   path = path.concat(SLASH);
               }
           }
           URL url = new URL(FILE_PROTOCOL, null, path);
           addURL(url);
       } catch (Exception e) {
           throw new IllegalArgumentException(e);
       }
   }

}

public abstract class PathUtils {
private final static String ESCAPE_SEPARATOR = "\\";
private final static String CANONICAL_SEPARATOR = "/";

   public static String transformToCanonical(String path) {
       if (StringUtils.isBlank(path)) {
           return path;
       } else {
           return path.replace(ESCAPE_SEPARATOR, CANONICAL_SEPARATOR);
       }
   }
}

3、使用系统AppClassLoader ：

//自定义包

public class AppClassLoader {
   private final String JAR_SUFFIX = ".jar";
   private final String FILE_PROTOCOL = "file";
   private final String METHOD = "addURL";
   private final String SLASH = "/";
   private final String ESCAPE_SEPARATOR = "\\";
   private URLClassLoader loader;
   private Method method;

   public AppClassLoader() {
       try {
           loader = (URLClassLoader) Thread.currentThread().getContextClassLoader();
           method = URLClassLoader.class.getDeclaredMethod(METHOD, new Class[] { URL.class });
           method.setAccessible(true);
       } catch (Exception e) {
           throw new RuntimeException(e);
       }
   }

   public Class<?> loadClass(String name) throws ClassNotFoundException {
       return loader.loadClass(name);
   }

   private void addRepository(String path, boolean slash) {
       try {
           path = path.replace(ESCAPE_SEPARATOR, SLASH);
           if (slash) {
               if (!path.endsWith(SLASH)) {
                   path = path.concat(SLASH);
               }
           }
           URL url = new URL(FILE_PROTOCOL, null, path);
           method.invoke(loader, new Object[] { url });
       } catch (Exception e) {
           throw new RuntimeException(e);
       }
   }
}

4、使用

如果BootClassLoader 使用不好，可以试试AppClassLoader

BootClassLoader 用不好，一定是你对jvm 的ClassLoader双亲委派机制没理解好，那就用这个万能的AppClassLoader

BootClassLoader loader = new BootClassLoader();

//AppClassLoader loader = new AppClassLoader ();

String appHome = "f://a/b/server"

String repertory1 = appHome.concat("/webapp/WEB-INF/classes");
String repertory2 = appHome.concat("/webapp/WEB-INF/lib");
String repertory3 = appHome.concat("/lib");
loader.addClassDir(repertory1);
loader.addJarsDir(repertory2);
loader.addJarsDir(repertory3);

try {
           Class<?> server = loader.loadClass("com.bs.Server");
           Method method = server.getMethod("start");
           method.setAccessible(true);
           method.invoke(server.newInstance());
       } catch (Exception e) {
           throw new RuntimeException(e);
       }

关键代码，自定义启停脚本，你可自行先琢磨

自定义URLClassLoader 加载指定class

猜你喜欢