本文來自于HeapDump性能社區(qū)！ ！有性能問題，上HeapDump性能社區(qū)！
1.永遠(yuǎn)不要忽略InterruptedException
讓我們檢查以下代碼片段：
public class Task implements Runnable {
private final BlockingQueue<String> queue = ...;
@Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
String result = getOrDefault(() -> queue.poll(1L, TimeUnit.MINUTES), "default");
//do smth with the result
}
}
<T> T getOrDefault(Callable<T> supplier, T defaultValue) {
try {
return supplier.call();
} catch (Exception e) {
logger.error("Got exception while retrieving value.", e);
return defaultValue;
}
}
}
代碼的問題是不可能終止線程，因?yàn)樗诘却?duì)列中的新元素，所以中斷標(biāo)志永遠(yuǎn)不會(huì)恢復(fù)：
正在運(yùn)行代碼的線程被中斷。
BlockingQueue#poll() 拋出 InterruptedException 并清除中斷標(biāo)志。
當(dāng)標(biāo)志被清除時(shí)， while 循環(huán)條件 ( !Thread.currentThread().isInterrupted()) 為ture。
為防止這種行為，請(qǐng)始終讀取 InterruptedException 并在方法顯式（通過聲明 throwing InterruptedException）或隱式（通過聲明/拋出原始 Exception）拋出時(shí)恢復(fù)中斷標(biāo)志：
<T> T getOrDefault(Callable<T> supplier, T defaultValue) {
try {
return supplier.call();
} catch (InterruptedException e) {
logger.error("Got interrupted while retrieving value.", e);
Thread.currentThread().interrupt();
return defaultValue;
} catch (Exception e) {
logger.error("Got exception while retrieving value.", e);
return defaultValue;
}
}
2. 注意使用專用的執(zhí)行器進(jìn)行阻塞操作
開發(fā)人員通常不希望因?yàn)橐粋€(gè)“慢動(dòng)作”而使整個(gè)服務(wù)器無響應(yīng)。不幸的是，對(duì)于 RPC，響應(yīng)時(shí)間通常是不可預(yù)測(cè)的。
假設(shè)一臺(tái)服務(wù)器有 100 個(gè)工作線程，并且有一個(gè)端點(diǎn)，它以 100 RPS 調(diào)用。它在內(nèi)部進(jìn)行 RPC 調(diào)用，通常需要 10 毫秒。在某個(gè)時(shí)間點(diǎn)，這個(gè) RPC 的響應(yīng)時(shí)間變成了 2 秒，而服務(wù)器在尖峰期間唯一能做的就是等待這些調(diào)用，而其他端點(diǎn)根本無法訪問。
@GET
@Path("/genre/{name}")
@Produces(MediaType.APPLICATION_JSON)
public Response getGenre(@PathParam("name") String genreName) {
Genre genre = potentiallyVerySlowSynchronousCall(genreName);
return Response.ok(genre).build();
}
解決問題的最簡單方法是將進(jìn)行阻塞調(diào)用的代碼提交到線程池：
@GET
@Path("/genre/{name}")
@Produces(MediaType.APPLICATION_JSON)
public void getGenre(@PathParam("name") String genreName, @Suspended AsyncResponse response) {
response.setTimeout(1L, TimeUnit.SECONDS);
executorService.submit(() -> {
Genre genre = potentiallyVerySlowSynchronousCall(genreName);
return response.resume(Response.ok(genre).build());
});
}
3. 注意MDC值的傳播
MDC（映射診斷上下文）通常用于存儲(chǔ)單個(gè)任務(wù)的特定值。例如，在 Web 應(yīng)用程序中，它可能為每個(gè)請(qǐng)求存儲(chǔ)一個(gè)請(qǐng)求 ID 和一個(gè)用戶 ID，因此 MDC 使查找與單個(gè)請(qǐng)求或整個(gè)用戶活動(dòng)相關(guān)的日志條目變得更加容易。
2017-08-27 14:38:30,893 INFO [server-thread-0] [requestId=060d8c7f, userId=2928ea66] c.g.s.web.Controller - Message.
不幸的是，如果代碼的某些部分在專用線程池中執(zhí)行，則來自提交任務(wù)的線程的 MDC 值不會(huì)傳播。在以下示例中，第 7 行的日志條目包含“requestId”，而第 9 行的日志條目不包含：
@GET
@Path("/genre/{name}")
@Produces(MediaType.APPLICATION_JSON)
public void getGenre(@PathParam("name") String genreName, @Suspended AsyncResponse response) {
try (MDC.MDCCloseable ignored = MDC.putCloseable("requestId", UUID.randomUUID().toString())) {
String genreId = getGenreIdbyName(genreName); //Sync call
logger.trace("Submitting task to find genre with id '{}'.", genreId); //'requestId' is logged
executorService.submit(() -> {
logger.trace("Starting task to find genre with id '{}'.", genreId); //'requestId' is not logged
Response result = getGenre(genreId) //Async call
.map(artist -> Response.ok(artist).build())
.orElseGet(() -> Response.status(Response.Status.NOT_FOUND).build());
response.resume(result);
});
}
}
這可以通過使用 MDC#getCopyOfContextMap() 來解決：
public void getGenre(@PathParam("name") String genreName, @Suspended AsyncResponse response) {
try (MDC.MDCCloseable ignored = MDC.putCloseable("requestId", UUID.randomUUID().toString())) {
...
logger.trace("Submitting task to find genre with id '{}'.", genreId); //'requestId' is logged
withCopyingMdc(executorService, () -> {
logger.trace("Starting task to find genre with id '{}'.", genreId); //'requestId' is logged
...
});
}
}
private void withCopyingMdc(ExecutorService executorService, Runnable function) {
Map<String, String> mdcCopy = MDC.getCopyOfContextMap();
executorService.submit(() -> {
MDC.setContextMap(mdcCopy);
try {
function.run();
} finally {
MDC.clear();
}
});
}
4.關(guān)于線程的重命名
自定義線程名稱以簡化讀取日志和線程轉(zhuǎn)儲(chǔ)。這可以通過 在創(chuàng)建ExecutorService期間傳遞ThreadFactory來完成。流行的實(shí)用程序庫中有很多 ThreadFactory接口的實(shí)現(xiàn)：
com.google.common.util.concurrent.ThreadFactoryBuilder in Guava。
Spring 中的org.springframework.scheduling.concurrent.CustomizableThreadFactory。
Apache Commons Lang 3 中的org.apache.commons.lang3.concurrent.BasicThreadFactory。
ThreadFactory threadFactory = new BasicThreadFactory.Builder()
.namingPattern("computation-thread-%d")
.build();
ExecutorService executorService = Executors.newFixedThreadPool(numberOfThreads, threadFactory);
雖然ForkJoinPool沒有使用ThreadFactory接口，但也支持線程的重命名：
ForkJoinPool.ForkJoinWorkerThreadFactory forkJoinThreadFactory = pool -> {
ForkJoinWorkerThread thread = ForkJoinPool.defaultForkJoinWorkerThreadFactory.newThread(pool);
thread.setName("computation-thread-" + thread.getPoolIndex());
return thread;
};
ForkJoinPool forkJoinPool = new ForkJoinPool(numberOfThreads, forkJoinThreadFactory, null, false);
只需將線程轉(zhuǎn)儲(chǔ)與默認(rèn)名稱進(jìn)行比較：
"pool-1-thread-3" #14 prio=5 os_prio=31 tid=0x00007fc06b19f000 nid=0x5703 runnable [0x0000700001ff9000]
java.lang.Thread.State: RUNNABLE
at com.github.sorokinigor.article.tipsaboutconcurrency.setthreadsname.TaskHandler.compute(TaskHandler.java:16)
...
"pool-2-thread-3" #15 prio=5 os_prio=31 tid=0x00007fc06aa10800 nid=0x5903 runnable [0x00007000020fc000]
java.lang.Thread.State: RUNNABLE
at com.github.sorokinigor.article.tipsaboutconcurrency.setthreadsname.HealthCheckCallback.recordFailure(HealthChecker.java:21)
at com.github.sorokinigor.article.tipsaboutconcurrency.setthreadsname.HealthChecker.check(HealthChecker.java:9)
...
"pool-1-thread-2" #12 prio=5 os_prio=31 tid=0x00007fc06aa10000 nid=0x5303 runnable [0x0000700001df3000]
java.lang.Thread.State: RUNNABLE
at com.github.sorokinigor.article.tipsaboutconcurrency.setthreadsname.TaskHandler.compute(TaskHandler.java:16)
...
對(duì)于名稱有特指的線程：
"task-handler-thread-1" #14 prio=5 os_prio=31 tid=0x00007fb49c9df000 nid=0x5703 runnable [0x000070000334a000]
java.lang.Thread.State: RUNNABLE
at com.github.sorokinigor.article.tipsaboutconcurrency.setthreadsname.TaskHandler.compute(TaskHandler.java:16)
...
"authentication-service-ping-thread-0" #15 prio=5 os_prio=31 tid=0x00007fb49c9de000 nid=0x5903 runnable [0x0000700003247000]
java.lang.Thread.State: RUNNABLE
at com.github.sorokinigor.article.tipsaboutconcurrency.setthreadsname.HealthCheckCallback.recordFailure(HealthChecker.java:21)
at com.github.sorokinigor.article.tipsaboutconcurrency.setthreadsname.HealthChecker.check(HealthChecker.java:9)
...
"task-handler-thread-0" #12 prio=5 os_prio=31 tid=0x00007fb49b9b5000 nid=0x5303 runnable [0x0000700003144000]
java.lang.Thread.State: RUNNABLE
at com.github.sorokinigor.article.tipsaboutconcurrency.setthreadsname.TaskHandler.compute(TaskHandler.java:16)
...
并想象可能有超過 3 個(gè)線程。
5. 使用 LongAdder 作為計(jì)數(shù)器
考慮使用 java.util.concurrent.atomic.LongAdder而不是 AtomicLong / AtomicInteger 用于高爭用的計(jì)數(shù)器。LongAdder維護(hù)多個(gè)單元的值并在需要時(shí)增加它們的數(shù)量，這會(huì)導(dǎo)致更高的吞吐量，但與 AtomicXX 系列類相比，也會(huì)導(dǎo)致更高的內(nèi)存消耗。
ongAdder counter = new LongAdder();
counter.increment();
...
long currentValue = counter.sum();
更多高并發(fā)案例：
高并發(fā)服務(wù)優(yōu)化篇：JVM--工程師進(jìn)階的必經(jīng)之路
淺談JDK并發(fā)包下面的分治思想及分治思想在高并發(fā)場(chǎng)景的運(yùn)用
