Guava Cache特性：refreshAfterWrite只阻塞回源線程，其他線程返回舊值

上一篇文章"Guava Cache特性：對於同一個key，只讓一個請求回源load數據，其他線程阻塞等待結果"提到：如果緩存過期，恰好有多個線程讀取同一個key的值，那麼guava只允許一個線程去加載數據，其餘線程阻塞。這雖然可以防止大量請求穿透緩存，但是效率低下。使用refreshAfterWrite可以做到：只阻塞加載數據的線程，其餘線程返回舊數據。

package net.aty.guava;

import com.google.common.base.Stopwatch;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;

import java.util.UUID;
import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;


public class Main {

    // 模擬一個需要耗時2s的數據庫查詢任務
    private static Callable<String> callable = new Callable<String>() {
        @Override
        public String call() throws Exception {
            System.out.println("begin to mock query db...");
            Thread.sleep(2000);
            System.out.println("success to mock query db...");
            return UUID.randomUUID().toString();
        }
    };


    // 1s後刷新緩存
    private static LoadingCache<String, String> cache = CacheBuilder.newBuilder().refreshAfterWrite(1, TimeUnit.SECONDS)
            .build(new CacheLoader<String, String>() {
                @Override
                public String load(String key) throws Exception {
                    return callable.call();
                }
            });

    private static CountDownLatch latch = new CountDownLatch(1);


    public static void main(String[] args) throws Exception {

        // 手動添加一條緩存數據,睡眠1.5s讓其過期
        cache.put("name", "aty");
        Thread.sleep(1500);

        for (int i = 0; i < 8; i++) {
            startThread(i);
        }

        // 讓線程運行
        latch.countDown();

    }

    private static void startThread(int id) {
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    System.out.println(Thread.currentThread().getName() + "...begin");
                    latch.await();
                    Stopwatch watch = Stopwatch.createStarted();
                    System.out.println(Thread.currentThread().getName() + "...value..." + cache.get("name"));
                    watch.stop();
                    System.out.println(Thread.currentThread().getName() + "...finish,cost time=" + watch.elapsed(TimeUnit.SECONDS));
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });

        t.setName("Thread-" + id);
        t.start();
    }


}

通過輸出結果可以看出：當緩存數據過期的時候，真正去加載數據的線程會阻塞一段時間，其餘線程立馬返回過期的值，顯然這種處理方式更符合實際的使用場景。

有一點需要注意：我們手動向緩存中添加了一條數據，並讓其過期。如果沒有這行代碼，程序執行結果如下。

由於緩存沒有數據，導致一個線程去加載數據的時候，別的線程都阻塞了(因爲沒有舊值可以返回)。所以一般系統啓動的時候，我們需要將數據預先加載到緩存，不然就會出現這種情況。

還有一個問題不爽：真正加載數據的那個線程一定會阻塞，我們希望這個加載過程是異步的。這樣就可以讓所有線程立馬返回舊值，在後臺刷新緩存數據。refreshAfterWrite默認的刷新是同步的，會在調用者的線程中執行。我們可以改造成異步的，實現CacheLoader.reload()。

package net.aty.guava;

import com.google.common.base.Stopwatch;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.util.concurrent.ListenableFuture;
import com.google.common.util.concurrent.ListeningExecutorService;
import com.google.common.util.concurrent.MoreExecutors;

import java.util.UUID;
import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;


public class Main {

    // 模擬一個需要耗時2s的數據庫查詢任務
    private static Callable<String> callable = new Callable<String>() {
        @Override
        public String call() throws Exception {
            System.out.println("begin to mock query db...");
            Thread.sleep(2000);
            System.out.println("success to mock query db...");
            return UUID.randomUUID().toString();
        }
    };

    // guava線程池,用來產生ListenableFuture
    private static ListeningExecutorService service = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(10));

    private static LoadingCache<String, String> cache = CacheBuilder.newBuilder().refreshAfterWrite(1, TimeUnit.SECONDS)
            .build(new CacheLoader<String, String>() {
                @Override
                public String load(String key) throws Exception {
                    return callable.call();
                }

                @Override
                public ListenableFuture<String> reload(String key, String oldValue) throws Exception {
                    System.out.println("......後臺線程池異步刷新:" + key);
                    return service.submit(callable);
                }
            });

    private static CountDownLatch latch = new CountDownLatch(1);


    public static void main(String[] args) throws Exception {
        cache.put("name", "aty");
        Thread.sleep(1500);

        for (int i = 0; i < 8; i++) {
            startThread(i);
        }

        // 讓線程運行
        latch.countDown();

    }

    private static void startThread(int id) {
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    System.out.println(Thread.currentThread().getName() + "...begin");
                    latch.await();
                    Stopwatch watch = Stopwatch.createStarted();
                    System.out.println(Thread.currentThread().getName() + "...value..." + cache.get("name"));
                    watch.stop();
                    System.out.println(Thread.currentThread().getName() + "...finish,cost time=" + watch.elapsed(TimeUnit.SECONDS));
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });

        t.setName("Thread-" + id);
        t.start();
    }


}