在Java里体验一把协程Quasar

在还没有协程的时代

操作系统产生一个进程，进程再产生若干个线程并行的处理逻辑，线程的切换由操作系统负责调度。每个线程都有自己的栈

• 栈是否有一些默认值？
  的确有默认值，不同的虚拟机不同的平台是不同的，想了解的可以看这里 https://docs.oracle.com/cd/E13150_01/jrockit_jvm/jrockit/jrdocs/refman/optionX.html#wp1024112

• 栈可以改变默认值吗？
  可以，比如您可以使用java -Xss1M将堆栈大小的最大值设置为1M。

所以指望一个进程开启上万个线程是不现实的。但是实际上我们也不会这么干，因为起这么多线程并不能充分的利用CPU，大部分线程处于等待状态，CPU也没有这么核让线程使用。所以一般线程数目都是CPU的核数。

传统的J2EE系统都是基于每个请求占用一个线程去完成完整的业务逻辑（包括事务）。所以系统的吞吐能力取决于每个线程的操作耗时。如果遇到很耗时的I/O行为，则整个系统的吞吐立刻下降，比如JDBC是同步阻塞的，这也是为什么很多人都说数据库是瓶颈的原因。这里的耗时其实是让CPU一直在等待I/O返回，说白了线程根本没有利用CPU去做运算，而是处于空转状态。暴殄天物啊。另外过多的线程，也会带来更多的ContextSwitch开销。

点评：JDK中有了线程，允许一定的并发处理，而且有封装很好的ThreadPool，可以用来管理大量的线程生命周期，但是本质上还是不能很好的解决线程数量的问题，以及线程空转占用CPU资源的问题。

为了解决同步阻塞时线程空转占用CPU资源的问题，行业早期流行的解决方案之一就是单线程加上异步回调。遇到需要进行I/O操作的地方，就直接让出CPU资源，然后注册一个回调函数，其他逻辑则继续往下走，I/O结束后带着结果向事件队列里插入执行结果，然后由事件调度器调度回调函数，传入结果。

点评：为了保存之前的栈上数据，你要么选择带着一起放入回调函数里，要么就不停的嵌套，从而引起反人类的Callback hell。而且业务逻辑上也有一定的割裂

协程应运而生

目前JDK原生是不支持协程的，一直被go工程师鄙视，Java工程师苦协程久已，大师puniverse看不下去了，基于字节码技术搞了一个Quasar来支持协程，目前一直在更新维护

• Quasar项目作者
  puniverse

• Quasar用户指南
  https://docs.paralleluniverse.co/quasar/

• Quasar源码
  https://github.com/puniverse/quasar

基于Quasar的第一个协程示例

1.添加pom依赖
　　　　
co.paralleluniverse
quasar-core
0.7.9
jdk8

2.测试代码
public static void main(String[] args) throws Exception {
//使用阻塞队列来获取结果。
LinkedBlockingQueue<Fiber> fiberQueue = new LinkedBlockingQueue<>();
DateTimeFormatter formatter = DateTimeFormatter.ofPattern(“yyyy-MM-dd HH:mm:ss”);
for (int i = 0; i < 100; i++) {
int finalI = i;
//这里的Fiber有点像Callable,可以返回数据
Fiber fiber = new Fiber<>((SuspendableCallable) () -> {
//这里用于测试内存占用量
Fiber.sleep(1000);
System.out.println(“in-” + finalI + “-” + LocalDateTime.now().format(formatter));
return finalI;
});
//开始执行
fiber.start();
//加入队列
fiberQueue.add(fiber);
}
while (true) {
//阻塞
Fiber fiber = fiberQueue.take();
System.out.println(“out-” + fiber.get() + “-” + LocalDateTime.now().format(formatter));
}
}

3.测试运行
启动时需要指定javaagent -javaagent:${ $path/quasar-core.jar} ,其它的跟普通java程序没啥区别

运行日志
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Quasar原理剖析

https://blog.csdn.net/guzhangyu12345/article/details/84666423
https://segmentfault.com/a/1190000006079389