1 package com.atlassian.plugin.util;
2
3 import java.util.Deque;
4 import java.util.LinkedList;
5
6 /**
7 * This utility provides a thread local stack of {@link ClassLoader}s.
8 * The current "top" of the stack is the thread's current context class loader.
9 * This can be used when implementing delegating plugin {@link java.util.logging.Filter}s or {@link javax.servlet.Servlet}s
10 * that need to set the {@link ClassLoader} to the {@link com.atlassian.plugin.classloader.PluginClassLoader} the filter
11 * or servlet is declared in.
12 *
13 * @since 2.5.0
14 */
15 public class ClassLoaderStack {
16 // We don't override initialValue as tomcat logs warnings if there is an object
17 // left in the thread local (even if it is an empty list)
18 private static final ThreadLocal<Deque<ClassLoader>> classLoaderStack = new ThreadLocal<Deque<ClassLoader>>();
19
20 /**
21 * Makes the given classLoader the new ContextClassLoader for this thread, and pushes the current ContextClassLoader
22 * onto a ThreadLocal stack so that we can do a {@link #pop} operation later to return to that ContextClassLoader.
23 *
24 * <p>
25 * Passing null is allowed and will act as a no-op. This means that you can safely {@link #pop} a ClassLoader and {@link #push} it back in
26 * and it will work safely whether the stack was empty at time of {@link #pop} or not.
27 *
28 * @param loader The new ClassLoader to set as ContextClassLoader.
29 */
30 public static void push(ClassLoader loader) {
31 if (loader == null) {
32 return;
33 }
34
35 Deque<ClassLoader> stack = classLoaderStack.get();
36 if (stack == null) {
37 stack = new LinkedList<ClassLoader>();
38 classLoaderStack.set(stack);
39 }
40 stack.push(Thread.currentThread().getContextClassLoader());
41 Thread.currentThread().setContextClassLoader(loader);
42 }
43
44 /**
45 * Pops the current ContextClassLoader off the stack, setting the new ContextClassLoader to the previous one on the stack.
46 * <ul>
47 * <li>If the stack is not empty, then the current ClassLoader is replaced by the previous one on the stack, and then returned.</li>
48 * <li>If the stack is empty, then null is returned and the current ContextClassLoader is not changed.</li>
49 * </ul>
50 *
51 * @return the previous ContextClassLoader that was just replaced, or null if the stack is empty.
52 */
53 public static ClassLoader pop() {
54 Deque<ClassLoader> stack = classLoaderStack.get();
55 if (stack == null || stack.isEmpty()) {
56 return null;
57 }
58 ClassLoader currentClassLoader = Thread.currentThread().getContextClassLoader();
59 Thread.currentThread().setContextClassLoader(stack.pop());
60 if (stack.isEmpty()) {
61 classLoaderStack.remove();
62 }
63
64 return currentClassLoader;
65 }
66 }