在Java裏體驗一把協程Quasar

在還沒有協程的時代

操作系統產生一個進程，進程再產生若干個線程並行的處理邏輯，線程的切換由操作系統負責調度。每個線程都有自己的棧

• 棧是否有一些默認值？
  的確有默認值，不同的虛擬機不同的平臺是不同的，想了解的可以看這裏 https://docs.oracle.com/cd/E13150_01/jrockit_jvm/jrockit/jrdocs/refman/optionX.html#wp1024112

• 棧可以改變默認值嗎？
  可以，比如您可以使用java -Xss1M將堆棧大小的最大值設置爲1M。

所以指望一個進程開啓上萬個線程是不現實的。但是實際上我們也不會這麼幹，因爲起這麼多線程並不能充分的利用CPU，大部分線程處於等待狀態，CPU也沒有這麼核讓線程使用。所以一般線程數目都是CPU的核數。

傳統的J2EE系統都是基於每個請求佔用一個線程去完成完整的業務邏輯（包括事務）。所以系統的吞吐能力取決於每個線程的操作耗時。如果遇到很耗時的I/O行爲，則整個系統的吞吐立刻下降，比如JDBC是同步阻塞的，這也是爲什麼很多人都說數據庫是瓶頸的原因。這裏的耗時其實是讓CPU一直在等待I/O返回，說白了線程根本沒有利用CPU去做運算，而是處於空轉狀態。暴殄天物啊。另外過多的線程，也會帶來更多的ContextSwitch開銷。

點評：JDK中有了線程，允許一定的併發處理，而且有封裝很好的ThreadPool，可以用來管理大量的線程生命週期，但是本質上還是不能很好的解決線程數量的問題，以及線程空轉佔用CPU資源的問題。

爲了解決同步阻塞時線程空轉佔用CPU資源的問題，行業早期流行的解決方案之一就是單線程加上異步回調。遇到需要進行I/O操作的地方，就直接讓出CPU資源，然後註冊一個回調函數，其他邏輯則繼續往下走，I/O結束後帶着結果向事件隊列裏插入執行結果，然後由事件調度器調度回調函數，傳入結果。

點評：爲了保存之前的棧上數據，你要麼選擇帶着一起放入回調函數裏，要麼就不停的嵌套，從而引起反人類的Callback hell。而且業務邏輯上也有一定的割裂

協程應運而生

目前JDK原生是不支持協程的，一直被go工程師鄙視，Java工程師苦協程久已，大師puniverse看不下去了，基於字節碼技術搞了一個Quasar來支持協程，目前一直在更新維護

• Quasar項目作者
  puniverse

• Quasar用戶指南
  https://docs.paralleluniverse.co/quasar/

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

基於Quasar的第一個協程示例

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

2.測試代碼
public static void main(String[] args) throws Exception {
//使用阻塞隊列來獲取結果。
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;
//這裏的Fiber有點像Callable,可以返回數據
Fiber fiber = new Fiber<>((SuspendableCallable) () -> {
//這裏用於測試內存佔用量
Fiber.sleep(1000);
System.out.println(“in-” + finalI + “-” + LocalDateTime.now().format(formatter));
return finalI;
});
//開始執行
fiber.start();
//加入隊列
fiberQueue.add(fiber);
}
while (true) {
//阻塞
Fiber fiber = fiberQueue.take();
System.out.println(“out-” + fiber.get() + “-” + LocalDateTime.now().format(formatter));
}
}

3.測試運行
啓動時需要指定javaagent -javaagent:${ $path/quasar-core.jar} ,其它的跟普通java程序沒啥區別

運行日誌
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Quasar原理剖析

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