RRWL-broken.java

import java.util.concurrent.locks.ReentrantReadWriteLock;

class CachedData {
   Object data;
   volatile boolean cacheValid;
   final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

   Object loadData() throws Exception {
      throw new Exception("Load Failed");
   }
   void use(Object data) {
   }
   void processCachedData() {
     Object data = null;
     rwl.readLock().lock();
     if (!cacheValid) {
       // Must release read lock before acquiring write lock
       rwl.readLock().unlock();
       rwl.writeLock().lock();
       try {
         // Recheck state because another thread might have
         // acquired write lock and changed state before we did.
         if (!cacheValid) {
           data = loadData();
           cacheValid = true;
         }
         // Downgrade by acquiring read lock before releasing write lock
         rwl.readLock().lock();
       } catch (Exception e) {
          System.out.println("load failed");
       } finally {
         rwl.writeLock().unlock(); // Unlock write, still hold read
       }
     }

     try {
       use(data);
     } finally {
       rwl.readLock().unlock();
     }
   }

   public static void main(String[] args) {
     CachedData d = new CachedData();
     d.processCachedData(); 
   }
}

RRWL-fixed.java

// Note the rwl.readLock().lock(); moved to finally block
import java.util.concurrent.locks.ReentrantReadWriteLock;

class CachedData {
   Object data;
   volatile boolean cacheValid;
   final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

   Object loadData() throws Exception {
      throw new Exception("Load Failed");
   }
   void use(Object data) {
   }
   void processCachedData() {
     Object data = null;
     rwl.readLock().lock();
     if (!cacheValid) {
       // Must release read lock before acquiring write lock
       rwl.readLock().unlock();
       rwl.writeLock().lock();
       try {
         // Recheck state because another thread might have
         // acquired write lock and changed state before we did.
         if (!cacheValid) {
           data = loadData();
           cacheValid = true;
         }
       } catch (Exception e) {
          System.out.println("load failed");
       } finally {
         // Downgrade by acquiring read lock before releasing write lock
         rwl.readLock().lock();
         rwl.writeLock().unlock(); // Unlock write, still hold read
       }
     }

     try {
       use(data);
     } finally {
       rwl.readLock().unlock();
     }
   }

   public static void main(String[] args) {
     CachedData d = new CachedData();
     d.processCachedData(); 
   }
}