A pre-shutdown JVM hook

PreShutdown.java

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;

/**
 * Sequentially executes hooks on shutdown, before any shutdown hook registered
 * with Runtime#addShutdownHook(Thread).
 * Not portable, tested with Sun JVM 1.6u22
 */
public class PreShutdown {
	static PreShutdown INSTANCE;
	Runnable originalHook;
	List<Thread> preShutdownHooks = new ArrayList<Thread>();

	public static synchronized PreShutdown get() {
		try {
			if (INSTANCE == null) {
				INSTANCE = new PreShutdown();
			}
			return INSTANCE;
		} catch (Exception ex) {
			throw new Error(ex);
		}
	}

	private PreShutdown() throws Exception {
		Thread warmUp = new Thread();
		Runtime.getRuntime().addShutdownHook(warmUp);
		Runtime.getRuntime().removeShutdownHook(warmUp);
		Class<?> c = Class.forName("java.lang.Shutdown");
		Field hf = c.getDeclaredField("hooks");
		boolean hfa = hf.isAccessible();
		try {
			hf.setAccessible(true);
			// TODO synchronize on Shutdown.lock
			// TODO handle shutdown in progress
			Runnable[] hooks = (Runnable[]) hf.get(null);
			originalHook = hooks[1];
			hooks[1] = new Runnable() {
				@Override
				public void run() {
					runHooks();
				}
			};
		} finally {
			hf.setAccessible(hfa);
		}
	}

	public synchronized void addHook(Thread hook) {
		preShutdownHooks.add(hook);
	}

	public synchronized void removeHook(Thread hook) {
		preShutdownHooks.remove(hook);
	}

	private synchronized void runHooks() {
		// run all PreShutdown Hooks seqentially in reverse order of registration
		for (int i = preShutdownHooks.size() - 1; i >= 0; i--) {
			Thread hook = preShutdownHooks.get(i);
			hook.start();
			try {
				hook.join();
			} catch (InterruptedException ex) {
			}
		}
		originalHook.run();
	}
}