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

時間 2019-11-13

標籤 guava cache 特性 refreshafterwrite 阻塞線程其餘返回欄目 Java 简体版

原文原文鏈接

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，其餘線程阻塞等待結果"提到：若是緩存過時，剛好有多個線程讀取同一個key的值，那麼guava只容許一個線程去加載數據，其他線程阻塞。這雖然能夠防止大量請求穿透緩存，可是效率低下。使用refreshAfterWrite能夠作到：只阻塞加載數據的線程，其他線程返回舊數據。java

[java] view plain copy數據庫

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 {
// 模擬一個須要耗時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讓其過時
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();
}
}

經過輸出結果能夠看出：當緩存數據過時的時候，真正去加載數據的線程會阻塞一段時間，其他線程立馬返回過時的值，顯然這種處理方式更符合實際的使用場景。緩存

有一點須要注意：咱們手動向緩存中添加了一條數據，並讓其過時。若是沒有這行代碼，程序執行結果以下。app

因爲緩存沒有數據，致使一個線程去加載數據的時候，別的線程都阻塞了(由於沒有舊值能夠返回)。因此通常系統啓動的時候，咱們須要將數據預先加載到緩存，否則就會出現這種狀況。dom

還有一個問題不爽：真正加載數據的那個線程必定會阻塞，咱們但願這個加載過程是異步的。這樣就可讓全部線程立馬返回舊值，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。咱們能夠改形成異步的，實現CacheLoader.reload()。異步

[java] view plain copyide

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 {
// 模擬一個須要耗時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();
}
}

相關標籤/搜索

每日一句

每一个你不满意的现在，都有一个你没有努力的曾经。