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copied!<p>Here's an idea - on the principle that <code>synchronized</code> is an archaic and inefficient mechanism.</p> <p>Here I use an <code>AtomicReference<Phaser></code> to indicate the cache is being updated. The <code>Phaser</code> can be used to await completion of the update.</p> <pre><code>public class Test { public static class Cache { // Cache timeout. private static final long Timeout = 10000; // Last time we updated. private volatile long lastupdateTime = 0L; // The cached data. private volatile String cachedData = null; // Cache is in the progress of updating. private AtomicReference<Phaser> updating = new AtomicReference<>(); // The next Phaser to use - avoids unnecessary Phaser creation. private Phaser nextPhaser = new Phaser(1); public String getData() { // Do this just once. long now = System.currentTimeMillis(); // Watch for expiry. if (now - lastupdateTime > Timeout) { // Make sure only one thread updates. if (updating.compareAndSet(null, nextPhaser)) { // We are the unique thread that gets to do the updating. System.out.println(Thread.currentThread().getName() + " - Get ..."); // Get my new cache data. cachedData = getDataFromDatabase(); lastupdateTime = now; // Make the Phaser to use next time - avoids unnecessary creations. nextPhaser = new Phaser(1); // Get the queue and clear it - there cannot be any new joiners after this. Phaser queue = updating.getAndSet(null); // Inform everyone who is waiting that they can go. queue.arriveAndDeregister(); } else { // Wait in the queue. Phaser queue = updating.get(); if (queue != null) { // Wait for it. queue.register(); System.out.println(Thread.currentThread().getName() + " - Waiting ..."); queue.arriveAndAwaitAdvance(); System.out.println(Thread.currentThread().getName() + " - Back"); } } } // Let them have the correct data. return cachedData; } private String getDataFromDatabase() { try { // Deliberately wait for a bit. Thread.sleep(5000); } catch (InterruptedException ex) { // Ignore. } System.out.println(Thread.currentThread().getName() + " - Hello"); return "Hello"; } } public void test() throws InterruptedException { System.out.println("Hello"); // Start time. final long start = System.currentTimeMillis(); // Make a Cache. final Cache c = new Cache(); // Make some threads. for (int i = 0; i < 20; i++) { Thread t = new Thread(new Runnable() { @Override public void run() { while (System.currentTimeMillis() - start < 60000) { c.getData(); try { Thread.sleep(500); } catch (InterruptedException ex) { // Ignore. } } } }); t.setName("Thread - " + i); t.start(); // Stagger the threads. Thread.sleep(300); } } public static void main(String args[]) throws InterruptedException { new Test().test(); } } </code></pre> <p>Please note that this is the first time I have used a <code>Phaser</code> - I hope I have it right.</p> <p>Note that this algorithm may break down if the timeout is longer than the time it takes to get the data from the database. </p>

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