classloader.java

代码是一个分类器的实现,其中使用了部分weka的源代码。可以将项目导入eclipse运行

package de.fub.bytecode.util;

import java.util.*;
import java.io.*;
import java.util.zip.*;
import de.fub.bytecode.*;
import de.fub.bytecode.classfile.*;
import de.fub.bytecode.generic.*;

/**
 * <p>Drop in replacement for the standard class loader of the JVM. You can use it
 * in conjunction with the JavaWrapper to dynamically modify/create classes
 * as they're requested.</p>
 *
 * <p>This class loader recognizes special requests in a distinct
 * format, i.e., when the name of the requested class contains with
 * "$$BCEL$$" it calls the createClass() method with that name
 * (everything bevor the $$BCEL$$ is considered to be the package
 * name. You can subclass the class loader and override that
 * method. "Normal" classes class can be modified by overriding the
 * modifyClass() method which is called just before defineClass().</p>
 *
 * <p>There may be a number of packages where you have to use the default
 * class loader (which may also be faster). You can define the set of packages
 * where to use the system class loader in the constructor. The default value contains
 * "java.", "sun.", "javax."</p>
 *
 * @version $Id: ClassLoader.java,v 1.6 2001/09/03 09:30:25 dahm Exp $
 * @author  <A HREF="http://www.berlin.de/~markus.dahm/">M. Dahm</A>
 * @see JavaWrapper
 * @see ClassPath
 */
public class ClassLoader extends java.lang.ClassLoader {
  private Hashtable classes = new Hashtable(); // Hashtable is synchronized luckily
  private String[] ignored_packages = {
    "java.", "javax.", "sun."
  };

  public ClassLoader() {
  }

  /** @param ignored_packages classes contained in these packages will be loaded
   * with the system class loader
   */
  public ClassLoader(String[] ignored_packages) {
    String[] new_p = new String[ignored_packages.length + this.ignored_packages.length];

    System.arraycopy(this.ignored_packages, 0, new_p, 0, this.ignored_packages.length);
    System.arraycopy(ignored_packages, 0, new_p, this.ignored_packages.length,
		     ignored_packages.length);

    this.ignored_packages = new_p;
  }
  
  protected Class loadClass(String class_name, boolean resolve) 
    throws ClassNotFoundException 
  {
    Class cl = null;

    /* First try: lookup hash table.
     */
    if((cl=(Class)classes.get(class_name)) == null) {
      /* Second try: Load system class using system class loader. You better
       * don't mess around with them.
       */
      for(int i=0; i < ignored_packages.length; i++) {
	if(class_name.startsWith(ignored_packages[i])) {
	  cl = Class.forName(class_name);
	  break;
	}
      }

      if(cl == null) {
	JavaClass clazz = null;

	/* Third try: Special request?
	 */
	if(class_name.indexOf("$$BCEL$$") >= 0)
	  clazz = createClass(class_name);
	else // Fourth try: Load classes via repository
	  clazz = modifyClass(Repository.lookupClass(class_name));

	if(clazz != null) {
	  byte[] bytes  = clazz.getBytes();
	  cl = defineClass(class_name, bytes, 0, bytes.length);
	} else // Fourth try: Use default class loader
	  cl = Class.forName(class_name);
      }
      
      if(resolve)
	resolveClass(cl);
    }

    classes.put(class_name, cl);

    return cl;
  }

  /** Override this method if you want to alter a class before it gets actually
   * loaded. Does nothing by default.
   */
  protected JavaClass modifyClass(JavaClass clazz) {
    return clazz;
  }
  
  /** 
   * Override this method to create you own classes on the fly. The
   * name contains the special token $$BCEL$$. Everything before that
   * token is consddered to be a package name. You can encode you own
   * arguments into the subsequent string. You must regard however not
   * to use any "illegal" characters, i.e., characters that may not
   * appear in a Java class name too<br>
   *
   * The default implementation interprets the string as a encoded compressed
   * Java class, unpacks and decodes it with the Utility.decode() method, and
   * parses thee resulting byte array and returns the resulting JavaClass object.
   *
   * @param class_name compressed byte code with "$$BCEL$$" in it
   */
  protected JavaClass createClass(String class_name) {
    int    index     = class_name.indexOf("$$BCEL$$");
    String real_name = class_name.substring(index + 8);

    JavaClass clazz = null;
    try {
      byte[]      bytes  = Utility.decode(real_name, true);
      ClassParser parser = new ClassParser(new ByteArrayInputStream(bytes), "foo");

      clazz = parser.parse();
    } catch(Throwable e) {
      e.printStackTrace();
      return null;
    }

    // Adapt the class name to the passed value
    ConstantPool cp = clazz.getConstantPool();

    ConstantClass cl = (ConstantClass)cp.getConstant(clazz.getClassNameIndex(),
						     Constants.CONSTANT_Class);
    ConstantUtf8 name = (ConstantUtf8)cp.getConstant(cl.getNameIndex(),
						     Constants.CONSTANT_Utf8);
    name.setBytes(class_name.replace('.', '/'));

    return clazz;
  }
}