ClassLoader xref

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    1   /*
    2    * Copyright  2000-2004 The Apache Software Foundation
    3    *
    4    *  Licensed under the Apache License, Version 2.0 (the "License");
    5    *  you may not use this file except in compliance with the License.
    6    *  You may obtain a copy of the License at
    7    *
    8    *      http://www.apache.org/licenses/LICENSE-2.0
    9    *
    10   *  Unless required by applicable law or agreed to in writing, software
    11   *  distributed under the License is distributed on an "AS IS" BASIS,
    12   *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13   *  See the License for the specific language governing permissions and
    14   *  limitations under the License.
    15   *
    16   */
    17  package org.apache.bcel.util;
    18
    19  import java.io.ByteArrayInputStream;
    20  import java.util.Hashtable;
    21  import org.apache.bcel.Constants;
    22  import org.apache.bcel.classfile.ClassParser;
    23  import org.apache.bcel.classfile.ConstantClass;
    24  import org.apache.bcel.classfile.ConstantPool;
    25  import org.apache.bcel.classfile.ConstantUtf8;
    26  import org.apache.bcel.classfile.JavaClass;
    27  import org.apache.bcel.classfile.Utility;
    28
    29  /***
    30   * <p>Drop in replacement for the standard class loader of the JVM. You can use it
    31   * in conjunction with the JavaWrapper to dynamically modify/create classes
    32   * as they're requested.</p>
    33   *
    34   * <p>This class loader recognizes special requests in a distinct
    35   * format, i.e., when the name of the requested class contains with
    36   * "$$BCEL$$" it calls the createClass() method with that name
    37   * (everything bevor the $$BCEL$$ is considered to be the package
    38   * name. You can subclass the class loader and override that
    39   * method. "Normal" classes class can be modified by overriding the
    40   * modifyClass() method which is called just before defineClass().</p>
    41   *
    42   * <p>There may be a number of packages where you have to use the
    43   * default class loader (which may also be faster). You can define the
    44   * set of packages where to use the system class loader in the
    45   * constructor. The default value contains "java.", "sun.",
    46   * "javax."</p>
    47   *
    48   * @version $Id: ClassLoader.java 386056 2006-03-15 11:31:56Z tcurdt $
    49   * @author  <A HREF="mailto:m.dahm@gmx.de">M. Dahm</A>
    50   * @see JavaWrapper
    51   * @see ClassPath
    52   */
    53  public class ClassLoader extends java.lang.ClassLoader {
    54
    55      public static final String[] DEFAULT_IGNORED_PACKAGES = {
    56              "java.", "javax.", "sun."
    57      };
    58      private Hashtable classes = new Hashtable(); // Hashtable is synchronized thus thread-safe
    59      privateong> String[] ignored_packages;
    60      private Repository repository = SyntheticRepository.getInstance();
    61
    62
    63      /*** Ignored packages are by default ( "java.", "sun.",
    64       * "javax."), i.e. loaded by system class loader
    65       */
    66      public ClassLoader() {
    67          this(DEFAULT_IGNORED_PACKAGES);
    68      }
    69
    70
    71      /*** @param deferTo delegate class loader to use for ignored packages
    72       */
    73      public ClassLoader(java.lang.ClassLoader deferTo) {
    74          super(deferTo);
    75          this.ignored_packages = DEFAULT_IGNORED_PACKAGES;
    76          this.repository = new ClassLoaderRepository(deferTo);
    77      }
    78
    79
    80      /*** @param ignored_packages classes contained in these packages will be loaded
    81       * with the system class loader
    82       */
    83      publicClassLoader(String[] ignored_packages) {/package-summary.html">ong> ClassLoader(String[] ignored_packages) {
    84          this.ignored_packages = ignored_packages;
    85      }
    86
    87
    88      /*** @param ignored_packages classes contained in these packages will be loaded
    89       * with the system class loader
    90       * @param deferTo delegate class loader to use for ignored packages
    91       */
    92      publicClassLoader(java/lang/ClassLoader deferTo, String[] ignored_packages) {/package-summary.html">ong> ClassLoader(java.lang.ClassLoader deferTo, String[] ignored_packages) {
    93          this(ignored_packages);
    94          this.repository = new ClassLoaderRepository(deferTo);
    95      }
    96
    97
    98      protected Class loadClass( String class_name, boolean resolve ) throws ClassNotFoundException {
    99          Class cl = null;
    100         /* First try: lookup hash table.
    101          */
    102         if ((cl = (Class) classes.get(class_name)) == null) {
    103             /* Second try: Load system class using system class loader. You better
    104              * don't mess around with them.
    105              */
    106             for (int i = 0; i < ignored_packages.length; i++) {
    107                 rong>if (class_name.startsWith(ignored_packages[i])) {
    108                     cl = getParent().loadClass(class_name);
    109                     break;
    110                 }
    111             }
    112             if (cl == null) {
    113                 JavaClass clazz = null;
    114                 /* Third try: Special request?
    115                  */
    116                 if (class_name.indexOf("$$BCEL$$") >= 0) {
    117                     clazz = createClass(class_name);
    118                 } else { // Fourth try: Load classes via repository
    119                     if ((clazz = repository.loadClass(class_name)) != null) {
    120                         clazz = modifyClass(clazz);
    121                     } else {
    122                         throw new ClassNotFoundException(class_name);
    123                     }
    124                 }
    125                 if (clazz != null) {
    126                     byte[] bytes = clazz.getBytes();
    127                     cl = defineClass(class_name, bytes, 0, bytes.length);
    128                 } else {
    129                     cl = Class.forName(class_name);
    130                 }
    131             }
    132             if (resolve) {
    133                 resolveClass(cl);
    134             }
    135         }
    136         classes.put(class_name, cl);
    137         return cl;
    138     }
    139
    140
    141     /*** Override this method if you want to alter a class before it gets actually
    142      * loaded. Does nothing by default.
    143      */
    144     protected JavaClass modifyClass( JavaClass clazz ) {
    145         return clazz;
    146     }
    147
    148
    149     /***
    150      * Override this method to create you own classes on the fly. The
    151      * name contains the special token $$BCEL$$. Everything before that
    152      * token is consddered to be a package name. You can encode you own
    153      * arguments into the subsequent string. You must regard however not
    154      * to use any "illegal" characters, i.e., characters that may not
    155      * appear in a Java class name too<br>
    156      *
    157      * The default implementation interprets the string as a encoded compressed
    158      * Java class, unpacks and decodes it with the Utility.decode() method, and
    159      * parses the resulting byte array and returns the resulting JavaClass object.
    160      *
    161      * @param class_name compressed byte code with "$$BCEL$$" in it
    162      */
    163     protected JavaClass createClass( String class_name ) {
    164         int index = class_name.indexOf("$$BCEL$$");
    165         String real_name = class_name.substring(index + 8);
    166         JavaClass clazz = null;
    167         try {
    168             byte[] bytes = Utility.decode(real_name, true);
    169             ClassParser parser = new ClassParser(new ByteArrayInputStream(bytes), "foo");
    170             clazz = parser.parse();
    171         } catch (Throwable e) {
    172             e.printStackTrace();
    173             return null;
    174         }
    175         // Adapt the class name to the passed value
    176         ConstantPool cp = clazz.getConstantPool();
    177         ConstantClass cl = (ConstantClass) cp.getConstant(clazz.getClassNameIndex(),
    178                 Constants.CONSTANT_Class);
    179         ConstantUtf8 name = (ConstantUtf8) cp.getConstant(cl.getNameIndex(),
    180                 Constants.CONSTANT_Utf8);
    181         name.setBytes(class_name.replace('.', '/'));
    182         return clazz;
    183     }
    184 }