ThreadクラスのgetContextClassLoader()がnullを返す条件

ThreadクラスのgetContextClassLoader()がnullを返す条件を知りたいです。

Java
1/**
2     * Returns the context {@code ClassLoader} for this thread. The context
3     * {@code ClassLoader} is provided by the creator of the thread for use
4     * by code running in this thread when loading classes and resources.
5     * If not {@linkplain #setContextClassLoader set}, the default is the
6     * {@code ClassLoader} context of the parent thread. The context
7     * {@code ClassLoader} of the
8     * primordial thread is typically set to the class loader used to load the
9     * application.
10     *
11     *
12     * @return  the context {@code ClassLoader} for this thread, or {@code null}
13     *          indicating the system class loader (or, failing that, the
14     *          bootstrap class loader)
15     *
16     * @throws  SecurityException
17     *          if a security manager is present, and the caller's class loader
18     *          is not {@code null} and is not the same as or an ancestor of the
19     *          context class loader, and the caller does not have the
20     *          {@link RuntimePermission}{@code ("getClassLoader")}
21     *
22     * @since 1.2
23     */
24    @CallerSensitive
25    public ClassLoader getContextClassLoader() {
26        if (contextClassLoader == null)
27            return null;
28        SecurityManager sm = System.getSecurityManager();
29        if (sm != null) {
30            ClassLoader.checkClassLoaderPermission(contextClassLoader,
31                                                   Reflection.getCallerClass());
32        }
33        return contextClassLoader;
34    }