java - 如何使用 Guava 的 CacheBuiler 作为 ConcurrentLRUCache

Question

所以我使用 Guava 的 CacheBuilder 作为 ConcurrentLRUCache，这意味着该缓存是线程安全的并且具有 LRU 属性，请参阅 CacheBuilder Guava Docs 。

我的假设是，当同时启动具有相同 key 的多个线程时，将使用 CyclicBarrier，然后一个线程将 put() 放入缓存，而其他线程则等待。之后，其余线程将看到一个值已在缓存中，并且不会 put() 到缓存中。

这不适用于下面的代码，因为每个线程都会创建一个 new Object() 并将其放入缓存中。通过运行测试并查看控制台来验证每次是否创建了不同的对象。

• 我使用的方式有什么本质上的错误吗
  缓存构建器？
• 我可以使用更好的方法吗？
• 有我可以使用的库吗？

请并谢谢!

import java.util.concurrent.CyclicBarrier;

import org.junit.Test;

import com.google.common.cache.Cache;
import com.google.common.cache.CacheBuilder;

public class GuavaLRUCacheTest {
    private Cache<String, Object> concurrentLRUCache = CacheBuilder.newBuilder().maximumSize(100).concurrencyLevel(1).build();

    @Test
    public void test() throws Exception {
        // The gate size is set based on the (number of threads to run) + (1 for the current thread).
        CyclicBarrier gate = new CyclicBarrier(4);

        // Same key is used for all threads
        ConcurrentLRUTestThread t1 = new ConcurrentLRUTestThread(gate, "key1");
        ConcurrentLRUTestThread t2 = new ConcurrentLRUTestThread(gate, "key1");
        ConcurrentLRUTestThread t3 = new ConcurrentLRUTestThread(gate, "key1");

        t1.start();
        t2.start();
        t3.start();

        // Open the gate on all threads.
        gate.await();

        t1.join();
        t2.join();
        t3.join();
    }

    class ConcurrentLRUTestThread extends Thread {
        private CyclicBarrier gate;
        private String key;
        public ConcurrentLRUTestThread(CyclicBarrier gate, String key) {
            this.gate = gate;
            this.key = key;
        }
        @Override
        public void run() {
            try {
                gate.await();
                if (concurrentLRUCache.getIfPresent(key) == null) {
                    System.out.println(">>>>> "+ System.nanoTime() +" - "+Thread.currentThread().getId() + " before put " + concurrentLRUCache.getIfPresent(key));
                    concurrentLRUCache.put(key, new Object());
                    System.out.println(">>>>> "+ System.nanoTime() +" - "+Thread.currentThread().getId() + " after put " + concurrentLRUCache.getIfPresent(key));
                } else{
                    System.out.println(">>>>> "+ System.nanoTime() +" - "+Thread.currentThread().getId() + " else " + concurrentLRUCache.getIfPresent(key));
                }  
            } catch (Throwable x) {
                System.out.println(">>>>> "+ System.currentTimeMillis() +" - "+Thread.currentThread().getId() + " ConcurrentLRUTestThread exception");
            }
        }
    }
}

Answer 1

您首先调用cache.getIfPresent，然后尝试调用cache.put。这将不起作用，因为这不是作为单个原子操作完成的。多个线程可以看到缓存中缺少值，这将导致多次 cache.put 调用。解决这个问题的直接方法是创建一个包含检查和操作的关键部分。那么只有一个线程会发现缓存中没有值。幸运的是，Cache 已经包含执行此操作的方法:Cache.get(key, valueLoader)。只需使用它来检索值即可:

    public class GuavaLRUCacheTest {

    private Cache concurrentLRUCache = CacheBuilder.newBuilder().maximumSize(100).concurrencyLevel(1).build();

    @Test
    public void test() throws Exception {
        // The gate size is set based on the (number of threads to run) + (1 for the current thread).
        CyclicBarrier gate = new CyclicBarrier(4);

        // Same key is used for all threads
        ConcurrentLRUTestThread t1 = new ConcurrentLRUTestThread(gate, "key1");
        ConcurrentLRUTestThread t2 = new ConcurrentLRUTestThread(gate, "key1");
        ConcurrentLRUTestThread t3 = new ConcurrentLRUTestThread(gate, "key1");

        t1.start();
        t2.start();
        t3.start();

        // Open the gate on all threads.
        gate.await();

        t1.join();
        t2.join();
        t3.join();
    }

    class ConcurrentLRUTestThread extends Thread {
        private CyclicBarrier gate;
        private String key;
        public ConcurrentLRUTestThread(CyclicBarrier gate, String key) {
            this.gate = gate;
            this.key = key;
        }
        @Override
        public void run() {
            try {
                gate.await();
                concurrentLRUCache.get(key, Object::new);
            } catch (Throwable x) {
                System.out.println(">>>>> "+ System.currentTimeMillis() +" - "+Thread.currentThread().getId() + " ConcurrentLRUTestThread exception");
            }
        }
    }
}