defensive-coding-guide/defensive-coding/en-US/Java/snippets/SecurityManager-Callback.xml

<?xml version='1.0' encoding='utf-8' ?>
<!DOCTYPE programlisting PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
]>
<!-- Automatically generated file.  Do not edit. -->
<programlisting language="Java">
interface Callback&#60;T&#62; {
	T call(boolean flag);
}

class CallbackInvoker&#60;T&#62; {
	private final AccessControlContext context;
	Callback&#60;T&#62; callback;

	CallbackInvoker(Callback&#60;T&#62; callback) {
	    context = AccessController.getContext();
	    this.callback = callback;
	}

	public T invoke() {
	    // Obtain increased privileges.
	    return AccessController.doPrivileged(new PrivilegedAction&#60;T&#62;() {
		    @Override
		    public T run() {
			// This operation would fail without
			// additional privileges.
			final boolean flag = Boolean.getBoolean("some.property");

			// Restore the original privileges.
			return AccessController.doPrivileged(
                        new PrivilegedAction&#60;T&#62;() {
				@Override
				public T run() {
				    return callback.call(flag);
				}
			    }, context);
		    }
		});
	}
}
</programlisting>