你好,我是猿java。
今天分享的内容是 Java 的一个重量级功用:虚拟线程。
布景
2022-09-20,JDK 19 发布了GA版别,备受瞩目的协程功用也算尘土落地,不过,此次 GA版别并不是以协程来命名,而是运用了 Virtual
Thread(虚拟线程),并且是 preview预览版别。小编最早关注到协程功用是在 2020年,那时孵化项目叫做 Java project Loom,
运用的是 Fiber(直译为:纤维,意译为:轻量级线程,即协程),可是 GA版别为何终究被界说为 Virtual Thread(虚拟线程),原因不得而知。
GA: General Availability,正式发布的版别,在国外都是用 GA来指代 release版别;
JEP: JDK Enhancement Proposal, JDK增强主张,JEP是一个JDK中心技术相关的增强主张文档;
为什么需求虚拟线程
已然 Java官方推出一个和线程这么附近的概念,必定是要处理线程的某些问题,因而,咱们先回忆下线程的一些特点:
- Java中的线程是对操作体系线程的一个简略包装,线程的创立,调度和毁掉等都是由操作体系完结;
- 线程切换需求耗费CPU时刻,这部分时刻是与业务无关的;
- 线程的功用直承受操作体系处理才能的影响;
因而,线程是一种重量级的资源,作为一名 Java程序员应该深有体会。所以,为了更好的办理线程,Java选用了池化(线程池)的方法进行办理线程,防止线程频频创立和毁掉带来的开支。可是,虽然线程池防止线程大部分创立和毁掉的开支,可是线程的调度仍是直承受操作体系的影响,那么有没有更好的方法来打破这种约束,因而,虚拟线程就孕育而生。
在 JDK 19源码中,官方直接在 java.lang包下新增一个 VirtualThread类来表示虚拟线程,为了更好的区别虚拟线程和原有的 Thread线程,官方给 Thread类赋予了一个巨大上的姓名:渠道线程。
下面给出了 JDK 19中虚拟线程的 Diagram截图以及渠道线程和体系线程的联系图:
想了解更多联系线程的常识,能够参阅往期的文章:深度剖析:Java线程运转机制,程序员必看的常识点!
怎么创立虚拟线程
1.经过 Thread.startVirtualThread()创立
如下示例代码,经过 Thread.startVirtualThread()能够创立一个新的并且已发动的虚拟线程,该方法等价于 Thread.ofVirtual().start(task):
public class VirtualThreadTest {
public static void main(String[] args) {
CustomThread customThread = new CustomThread();
// 创立并且发动虚拟线程
Thread.startVirtualThread(customThread);
}
}
class CustomThread implements Runnable {
@Override
public void run() {
System.out.println("CustomThread run");
}
}
2.经过 Thread.ofVirtual()创立
如下示例代码,经过 Thread.ofVirtual().unstarted()方法能够创立一个新的未发动的虚拟线程,然后经过 Thread.start()来发动线程,也能够经过 Thread.ofVirtual().start()直接创立一个新的并已发动的虚拟线程:
public class VirtualThreadTest {
public static void main(String[] args) {
CustomThread customThread = new CustomThread();
// 创立并且不发动虚拟线程,然后 unStarted.start()方法发动虚拟线程
Thread unStarted = Thread.ofVirtual().unstarted(customThread);
unStarted.start();
// 等同于
Thread.ofVirtual().start(customThread);
}
}
class CustomThread implements Runnable {
@Override
public void run() {
System.out.println("CustomThread run");
}
}
3.经过 ThreadFactory创立
如下示例代码,经过 ThreadFactory.newThread()方法就能创立一个虚拟线程,然后经过 Thread.start()来发动线程:
public class VirtualThreadTest {
public static void main(String[] args) {
CustomThread customThread = new CustomThread();
// 获取线程工厂类
ThreadFactory factory = Thread.ofVirtual().factory();
// 创立虚拟线程
Thread thread = factory.newThread(customThread);
// 发动线程
thread.start();
}
}
class CustomThread implements Runnable {
@Override
public void run() {
System.out.println("CustomThread run");
}
}
4.经过 Executors.newVirtualThreadPerTaskExecutor()创立
如下示例代码,经过 JDK自带的Executors东西类方法创立一个虚拟线程,然后经过 executor.submit()来发动线程:
public class VirtualThreadTest {
public static void main(String[] args) {
CustomThread customThread = new CustomThread();
ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
executor.submit(customThread);
}
}
class CustomThread implements Runnable {
@Override
public void run() {
System.out.println("CustomThread run");
}
}
经过上述列举的 4种创立虚拟线程的方法能够看出,官方为了降低虚拟线程的门槛,极力复用原有的Thread线程类,这样能够滑润的过渡到虚拟线程的运用。不过,在
Java 19中,虚拟线程仍是一个预览功用,默认封闭,需求运用参数 –enable-preview 来启用该功用,预览功用源码和发动虚拟线程指令如下:
// Thread 源码,经过 @PreviewFeature 注解来标示 虚拟线程为 预览功用
public class Thread implements Runnable {
/**
* Creates a virtual thread to execute a task and schedules it to execute.
This method is equivalent to: Thread.ofVirtual().start(task);
Params: task – the object to run when the thread executes
Returns: a new, and started, virtual thread
Throws: UnsupportedOperationException – if preview features are not enabled
Since: 19
See Also: Inheritance when creating threads
* @param task
* @return
*/
@PreviewFeature(feature = PreviewFeature.Feature.VIRTUAL_THREADS)
public static Thread startVirtualThread(Runnable task) {
Objects.requireNonNull(task);
// 判别是否敞开虚拟线程功用
PreviewFeatures.ensureEnabled();
var thread = ThreadBuilders.newVirtualThread(null, null, 0, task);
thread.start();
return thread;
}
// 异常信息提示 能够经过 --enable-preview 敞开虚拟线程功用
public static void ensureEnabled() {
if (!isEnabled()) {
throw new UnsupportedOperationException(
"Preview Features not enabled, need to run with --enable-preview");
}
}
}
# 敞开虚拟线程功用
java --source 19 --enable-preview XXX.java
IDEA 中配置 –enable-preview 如下图:
为了更好的感受虚拟线程的功用,咱们模仿一个对比测验用例:别离运用虚拟线程和线程池履行10w个使命,每个线程使命睡觉10ms,统计各自的总耗时和创立的最大渠道线程总数,示例代码如下:
// 虚拟线程
public class VirtualThreadTest {
static List<Integer> list = new ArrayList<>();
public static void main(String[] args) {
// 敞开一个线程来监控当时的渠道线程(体系线程)总数
ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1);
scheduledExecutorService.scheduleAtFixedRate(() -> {
ThreadMXBean threadBean = ManagementFactory.getThreadMXBean();
ThreadInfo[] threadInfo = threadBean.dumpAllThreads(false, false);
saveMaxThreadNum(threadInfo.length);
}, 10, 10, TimeUnit.MILLISECONDS);
long start = System.currentTimeMillis();
ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
for (int i = 0; i < 10000; i++) {
executor.submit(() -> {
// 线程睡觉 10ms,能够等同于模仿业务耗时10ms
try {
TimeUnit.MILLISECONDS.sleep(10);
} catch (InterruptedException e) {
}
});
}
executor.close();
System.out.println("max:" + list.get(0) + " platform thread/os thread");
System.out.printf("totalMillis:%dms\n", System.currentTimeMillis() - start);
}
}
public class ThreadTest {
static List<Integer> list = new ArrayList<>();
public static void main(String[] args) {
// 敞开一个线程来监控当时的渠道线程(体系线程)总数
ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1);
scheduledExecutorService.scheduleAtFixedRate(() -> {
ThreadMXBean threadBean = ManagementFactory.getThreadMXBean();
ThreadInfo[] threadInfo = threadBean.dumpAllThreads(false, false);
saveMaxThreadNum(threadInfo.length);
}, 1, 1, TimeUnit.SECONDS);
long start = System.currentTimeMillis();
ExecutorService executor = Executors.newFixedThreadPool(200);
for (int i = 0; i < 100000; i++) {
executor.submit(() -> {
try {
// 线程睡觉 10ms,能够等同于模仿业务耗时10ms
TimeUnit.MILLISECONDS.sleep(10);
} catch (InterruptedException e) {
}
});
}
executor.close();
System.out.println("max:" + list.get(0) + " platform thread/os thread");
System.out.printf("totalMillis:%dms\n", System.currentTimeMillis() - start);
}
}
// 保存渠道线程的创立的最大总数
public static List<Integer> saveMaxThreadNum(int num) {
if (list.isEmpty()) {
list.add(num);
} else {
Integer integer = list.get(0);
if (num > integer) {
list.add(0, num);
}
}
return list;
}
两个示例的运转结果:
经过运转结果能够发现:
-
运用虚拟线程履行 10w个使命总耗时为:129ms,最大创立了 18个渠道线程;
-
运用线程池履行 10w个使命总耗时为:6103 ms,最大创立了 207个渠道线程;
-
两者总耗时差50倍,最大创立的渠道线程总数差 10倍,因而功用差可想而知;
中心源码解析
首先从 VirtualThread类开始,源码如下:
/**
* A thread that is scheduled by the Java virtual machine rather than the operating system.
*/
final class VirtualThread extends BaseVirtualThread {
/**
* Creates a new {@code VirtualThread} to run the given task with the given
* scheduler. If the given scheduler is {@code null} and the current thread
* is a platform thread then the newly created virtual thread will use the
* default scheduler. If given scheduler is {@code null} and the current
* thread is a virtual thread then the current thread's scheduler is used.
*
* @param scheduler the scheduler or null
* @param name thread name
* @param characteristics characteristics
* @param task the task to execute
*/
VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
super(name, characteristics, /*bound*/ false);
Objects.requireNonNull(task);
// choose scheduler if not specified
if (scheduler == null) {
Thread parent = Thread.currentThread();
if (parent instanceof VirtualThread vparent) {
scheduler = vparent.scheduler;
} else {
scheduler = DEFAULT_SCHEDULER;
}
}
this.scheduler = scheduler;
this.cont = new VThreadContinuation(this, task);
this.runContinuation = this::runContinuation;
}
/**
* 创立默认的调度器
* Creates the default scheduler.
*/
@SuppressWarnings("removal")
private static ForkJoinPool createDefaultScheduler() {
ForkJoinWorkerThreadFactory factory = pool -> {
PrivilegedAction<ForkJoinWorkerThread> pa = () -> new CarrierThread(pool);
return AccessController.doPrivileged(pa);
};
PrivilegedAction<ForkJoinPool> pa = () -> {
int parallelism, maxPoolSize, minRunnable;
String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
if (parallelismValue != null) {
parallelism = Integer.parseInt(parallelismValue);
} else {
parallelism = Runtime.getRuntime().availableProcessors();
}
if (maxPoolSizeValue != null) {
maxPoolSize = Integer.parseInt(maxPoolSizeValue);
parallelism = Integer.min(parallelism, maxPoolSize);
} else {
maxPoolSize = Integer.max(parallelism, 256);
}
if (minRunnableValue != null) {
minRunnable = Integer.parseInt(minRunnableValue);
} else {
minRunnable = Integer.max(parallelism / 2, 1);
}
Thread.UncaughtExceptionHandler handler = (t, e) -> { };
boolean asyncMode = true; // FIFO
return new ForkJoinPool(parallelism, factory, handler, asyncMode,
0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
};
return AccessController.doPrivileged(pa);
}
}
经过 VirtualThread类的源码能够总结出:
- VirtualThread承继 BaseVirtualThread类,BaseVirtualThread类承继 Thread类;
- 虚拟线程是 JVM进行调度的,而不是操作体系;
- VirtualThread类是一个终态类,因而该类无法被承继,无法被扩展;
VirtualThread类,只提供了一个结构器,接纳 4个参数:
- Executor scheduler:假如给定的调度器为空并且当时线程是渠道线程,那么新创立的虚拟线程将运用默认调度程序(底层选用 ForkJoinPool),假如给定的调度器为空并且当时线程是虚拟线程,则运用当时线程的调度程序
- String name:自界说线程名
- int characteristics:线程特征值
- Runnable task:需求履行的使命
然后咱们看下 JDK中创立虚拟线程的源码:
public class Thread implements Runnable {
/**
* Creates a virtual thread to execute a task and schedules it to execute.
This method is equivalent to: Thread.ofVirtual().start(task);
Params: task – the object to run when the thread executes
Returns: a new, and started, virtual thread
Throws: UnsupportedOperationException – if preview features are not enabled
Since: 19
See Also: Inheritance when creating threads
* @param task
* @return
*/
@PreviewFeature(feature = PreviewFeature.Feature.VIRTUAL_THREADS)
public static Thread startVirtualThread(Runnable task) {
Objects.requireNonNull(task);
// 判别是否敞开虚拟线程功用
PreviewFeatures.ensureEnabled();
var thread = ThreadBuilders.newVirtualThread(null, null, 0, task);
thread.start();
return thread;
}
// 异常信息提示 能够经过 --enable-preview 敞开虚拟线程功用
public static void ensureEnabled() {
if (!isEnabled()) {
throw new UnsupportedOperationException(
"Preview Features not enabled, need to run with --enable-preview");
}
}
}
class ThreadBuilders {
static Thread newVirtualThread(Executor scheduler,
String name,
int characteristics,
Runnable task) {
if (ContinuationSupport.isSupported()) {
return new VirtualThread(scheduler, name, characteristics, task);
} else {
if (scheduler != null)
throw new UnsupportedOperationException();
return new BoundVirtualThread(name, characteristics, task);
}
}
/**
* Returns a builder for creating a virtual {@code Thread} or {@code ThreadFactory}
* that creates virtual threads.
*
* @apiNote The following are examples using the builder:
* {@snippet :
* // Start a virtual thread to run a task.
* Thread thread = Thread.ofVirtual().start(runnable);
*
* // A ThreadFactory that creates virtual threads
* ThreadFactory factory = Thread.ofVirtual().factory();
* }
*
* @return A builder for creating {@code Thread} or {@code ThreadFactory} objects.
* @throws UnsupportedOperationException if preview features are not enabled
* @since 19
*/
@PreviewFeature(feature = PreviewFeature.Feature.VIRTUAL_THREADS)
public static Builder.OfVirtual ofVirtual() {
PreviewFeatures.ensureEnabled();
return new ThreadBuilders.VirtualThreadBuilder();
}
}
Thread.startVirtualThread()创立虚拟线程,会调用ThreadBuilders.newVirtualThread(),终究调用 new VirtualThread()结构器来创立虚拟线程。
从上文咱们在介绍虚拟线程创立的 4种方法也能够看出,虚拟线程创立的入口在 Thread 或许 Executors 类中,和曾经运用线程或许线程池的习气保持一致。
final class VirtualThread extends BaseVirtualThread {
/**
* Mounts this virtual thread onto the current platform thread. On
* return, the current thread is the virtual thread.
*/
@ChangesCurrentThread
private void mount() {
// sets the carrier thread
Thread carrier = Thread.currentCarrierThread();
setCarrierThread(carrier);
// sync up carrier thread interrupt status if needed
if (interrupted) {
carrier.setInterrupt();
} else if (carrier.isInterrupted()) {
synchronized (interruptLock) {
// need to recheck interrupt status
if (!interrupted) {
carrier.clearInterrupt();
}
}
}
// set Thread.currentThread() to return this virtual thread
carrier.setCurrentThread(this);
}
/**
* Unmounts this virtual thread from the carrier. On return, the
* current thread is the current platform thread.
*/
@ChangesCurrentThread
private void unmount() {
// set Thread.currentThread() to return the platform thread
Thread carrier = this.carrierThread;
carrier.setCurrentThread(carrier);
// break connection to carrier thread, synchronized with interrupt
synchronized (interruptLock) {
setCarrierThread(null);
}
carrier.clearInterrupt();
}
}
mount() 和 unmount() 是虚拟线程两个中心方法:
- mount(),能够将此虚拟线程挂载到当时渠道线程上,回来时,当时线程是虚拟线程;
- unmount(),从载体线程卸载此虚拟线程,回来时,当时线程是渠道线程
经过这两个方法能够看出虚拟线程是搭载在渠道线程上运转,运转完毕后,从渠道线程上卸载。
虚拟线程的状况和转换
下表总结了虚拟线程中的一切线程状况以及状况之间转化的条件:
状况 | 转换条件 |
---|---|
NEW -> STARTED | Thread.start |
STARTED -> TERMINATED | failed to start |
STARTED -> RUNNING | first run |
RUNNING -> PARKING | Thread attempts to park |
PARKING -> PARKED | cont.yield successful, thread is parked |
PARKING -> PINNED | cont.yield failed, thread is pinned |
PARKED -> RUNNABLE | unpark or interrupted |
PINNED -> RUNNABLE | unpark or interrupted |
RUNNABLE -> RUNNING | continue execution |
RUNNING -> YIELDING | Thread.yield |
YIELDING -> RUNNABLE | yield successful |
YIELDING -> RUNNING | yield failed |
RUNNING -> TERMINATED | done |
3种线程的联系
VirtualThread,Platform Thread,OS Thread 三者的联系如下图:
阐明:
在现有的线程模型下,一个 Java线程相当于一个操作体系线程,多个虚拟线程需求挂载在一个渠道线程(载体线程)上,每个渠道线程和体系线程一一对应。因而,VirtualThread是属于 JVM等级的线程,由JVM调度,它是十分轻量级的资源,运用完后立即被毁掉,因而就不需求像渠道线程相同运用池化(线程池)。
虚拟线程在履行到 IO 操作或 Blocking操作时,会主动切换到其他虚拟线程履行,从而防止当时线程等待,能够高效经过少量线程去调度很多虚拟线程,最大化提高线程的履行功率。
总结
- Virtual Thread将会在功用上带来的巨大进步,不过,现在业界80~90%的代码还跑在 Java 8上,等 JDK
19投入实际出产环境,可能需求一个漫长的过程; - 虚拟线程高度复用了现有的 Thread线程的功用,方便现有方法滑润迁移到虚拟线程;
- 虚拟线程是将 Thread作为载体线程,它并没有改变原来的线程模型;
- 虚拟线程是 JVM调度的,而不是操作体系调度;
- 运用虚拟线程能够显著进步程序吞吐量;
- 虚拟线程合适 并发使命数量很高 或许 IO密集型的场景,关于 核算密集型使命还需经过过增加CPU中心处理,或许利用分布式核算资源来来处理;
- 虚拟线程现在仅仅一个预览功用,只能从源码和简略的测验来分析,并无实在出产环境的验证;
曾一段时刻内,JDK一直致力于 Reactor呼应式编程,企图从这条路子来提高 Java的功用,可是终究发现:呼应式编程难了解,难调试,难运用,
因而又把焦点转向了同步编程,为了改善功用,虚拟线程诞生了。或许虚拟线程很难在短时刻内运用到实际出产中,可是经过官方的JDK版别发布,咱们能够看到:虽然是 Oracle这样的科技型巨子也会走弯路,了解 JDK的动态,能够协助咱们更好的把握学习 Java的重心以及后面的发展趋势。
参阅
Virtual Thread JEP
java-virtual-threads