ClassLoaderBuilder.java revision 649e3c0004d965fe388ad454eeb20e307a12edd2 
1/*
2 * Copyright (C) 2010 The Android Open Source Project
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
17package tests.util;
18
19import java.io.File;
20import java.net.MalformedURLException;
21import java.net.URL;
22import java.net.URLClassLoader;
23import java.util.ArrayList;
24import java.util.HashSet;
25import java.util.List;
26import java.util.Set;
27
28/**
29 * Builds a configured class loader. The constructed class loader can load a
30 * private copy of a class in addition to the copy in the application class
31 * loader. It can also refuse to load a class altogether, even if that class
32 * exists on the classpath.
33 */
34public final class ClassLoaderBuilder {
35    private ClassLoader parent = ClassLoaderBuilder.class.getClassLoader();
36    private final Set<String> prefixesToNotInherit = new HashSet<String>();
37    private final Set<String> prefixesToLoad = new HashSet<String>();
38
39    public ClassLoaderBuilder parent(ClassLoader parent) {
40        this.parent = parent;
41        return this;
42    }
43
44    /**
45     * @param classNamePrefix the prefix of classes that can be loaded by both
46     *     the constructed class loader and the application class loader.
47     */
48    public ClassLoaderBuilder withPrivateCopy(String classNamePrefix) {
49        prefixesToLoad.add(classNamePrefix);
50        return this;
51    }
52
53    /**
54     * @param classNamePrefix the prefix of classes that will not be loaded by
55     *     this class loader. Attempts to load such classes will fail at runtime
56     *     with a NoClassDefFoundError.
57     */
58    public ClassLoaderBuilder without(String classNamePrefix) {
59        prefixesToNotInherit.add(classNamePrefix);
60        return this;
61    }
62
63    public ClassLoader build() {
64        // make a defensive copy in case this builder is reused!
65        final Set<String> prefixesToNotInherit = new HashSet<String>(this.prefixesToNotInherit);
66        final Set<String> prefixesToLoad = new HashSet<String>(this.prefixesToLoad);
67
68        /*
69         * To load two copies of a given class in the VM, we end up creating two
70         * new class loaders: a bridge class loader and a leaf class loader.
71         *
72         * The bridge class loader is a child of the application class loader.
73         * It never loads any classes. All it does is decide when to delegate to
74         * the application class loader (which has a copy of everything) and
75         * when to fail.
76         *
77         * The leaf class loader is a child of the bridge class loader. It
78         * uses the same classpath as the application class loader. It loads
79         * anything that its parent failed on.
80         */
81
82        ClassLoader bridge = new ClassLoader(parent) {
83            @Override protected Class<?> loadClass(String className, boolean resolve)
84                    throws ClassNotFoundException {
85                for (String prefix : prefixesToLoad) {
86                    if (className.startsWith(prefix)) {
87                        /* ClassNotFoundException causes the child loader to load the class. */
88                        throw new ClassNotFoundException();
89                    }
90                }
91
92                for (String prefix : prefixesToNotInherit) {
93                    if (className.startsWith(prefix)) {
94                        /* NoClassDefFoundError prevents the class from being loaded at all. */
95                        throw new NoClassDefFoundError();
96                    }
97                }
98
99                return super.loadClass(className, resolve);
100            }
101        };
102
103        try {
104            // first try to create a PathClassLoader for a dalvik VM...
105            String classPath = System.getProperty("java.class.path");
106            return (ClassLoader) Class.forName("dalvik.system.PathClassLoader")
107                    .getConstructor(String.class, ClassLoader.class)
108                    .newInstance(classPath, bridge);
109        } catch (Exception ignored) {
110        }
111
112        // fall back to a URLClassLoader on a JVM
113        List<URL> classpath = new ArrayList<URL>();
114        classpath.addAll(classpathToUrls("java.class.path"));
115        classpath.addAll(classpathToUrls("sun.boot.class.path"));
116        return new URLClassLoader(classpath.toArray(new URL[classpath.size()]), bridge);
117    }
118
119    private List<URL> classpathToUrls(String propertyName) {
120        try {
121            String classpath = System.getProperty(propertyName);
122            List<URL> result = new ArrayList<URL>();
123            for (String pathElement : classpath.split(File.pathSeparator)) {
124                result.add(new File(pathElement).toURI().toURL());
125            }
126            return result;
127        } catch (MalformedURLException e) {
128            throw new RuntimeException(e);
129        }
130    }
131}
132