最近，操作系統(tǒng)課程設(shè)計(jì)使用伯克利大學(xué)的Nachos做為實(shí)驗(yàn)平臺(tái)，老師也照搬伯克利的Project要求，開(kāi)始我們的操作系統(tǒng)課程設(shè)計(jì)。

結(jié)合自己的學(xué)習(xí)過(guò)程和課設(shè)要求，我覺(jué)得對(duì)Nachos的學(xué)習(xí)首先應(yīng)該從KThread.java入手,首先應(yīng)該看明白這個(gè)類的所有函數(shù)的意思。

這個(gè)主要是為第一個(gè)proj，實(shí)現(xiàn)join()函數(shù)做準(zhǔn)備。Join()方法的含義：當(dāng)前線程a在運(yùn)行，執(zhí)行b.join(),則a阻塞，直到線程b結(jié)束，a繼續(xù)執(zhí)行。

具體的要求有：Join函數(shù)的作用即為等待某線程運(yùn)行完畢。當(dāng)前線程 (唯一一個(gè)正在運(yùn)行的線程) A調(diào)用另一個(gè)線程 (處于就緒狀態(tài)) B的join函數(shù)時(shí) (A 和 B 在Nachos中均為KThread類型對(duì)象)，A被掛起，直到B運(yùn)行結(jié)束后， join函數(shù)返回，A才能繼續(xù)運(yùn)行。注意在一個(gè)KThread對(duì)象上只能調(diào)用一次join，且當(dāng)前線程不能對(duì)自身調(diào)用join。Waits for this thread to finish. If this thread is already finished, return immediately. This method must only be called once; the second call is not guaranteed to return. This thread must not be the current thread.

1 public voidjoin() {2

3 Lib.debug(dbgThread, "Joining to thread: " +toString());4 //等待另外一個(gè)線程結(jié)束的這個(gè)線程不能是線程自己

5 Lib.assertTrue(this !=currentThread);6

7 if (this.status ==statusFinished)8 return;9

10 boolean intStatus =Machine.interrupt().disable();11

12 if (joinQueue == null) {13 joinQueue = ThreadedKernel.scheduler.newThreadQueue(true);14 //Notify this thread queue that a thread has received access,without going through request() and nextThread()15 joinQueue.acquire(this);17 }18 //Notify this thread queue that the specified thread is waiting for19 //access

20 joinQueue.waitForAccess(currentThread);21 KThread.sleep();22 Machine.interrupt().restore(intStatus);23 }

為此，我設(shè)置了一個(gè)KThread的隊(duì)列joinQueue，當(dāng)B調(diào)用join函數(shù)時(shí)，將正在執(zhí)行的A線程放入joinQueue，只有在B執(zhí)行完后，在finish()里面會(huì)將隊(duì)列中的A線程狀態(tài)變?yōu)閞eady，從而繼續(xù)執(zhí)行A線程，具體finish()實(shí)現(xiàn)的代碼如下：

1 public static voidfinish() {2 Lib.debug(dbgThread, "Finishing thread: " +currentThread.toString());3

4 Machine.interrupt().disable();5

6 Machine.autoGrader().finishingCurrentThread();7

8 Lib.assertTrue(toBeDestroyed == null);9 toBeDestroyed =currentThread;10

11 currentThread.status =statusFinished;12

13 KThread joinedKThread;14 if (currentThread.joinQueue != null)15 while ((joinedKThread = currentThread.joinQueue.nextThread()) != null)16 joinedKThread.ready();17 sleep();18 }

后續(xù)測(cè)試程序如下：

packagenachos.threads;import nachos.machine.*;public classKThreadTest {publicKThreadTest() {

}public static voidsimpleJoinTest() {

KThread A_thread= new KThread(new KThreadTest.A_thread(5));

KThread B_thread= new KThread(newKThreadTest.B_thread(A_thread));

B_thread.fork();

B_thread.join();

}public static class B_thread implementsRunnable {

B_thread(KThread joinee) {this.joinee =joinee;

}public voidrun() {

System.out.println("B_thread 就緒");

System.out.println("Forking and joining A_thread...");this.joinee.fork();//Causes this thread to begin execution.

this.joinee.join();//啟動(dòng)a線程，阻塞b線程

System.out.println("B_thread 執(zhí)行結(jié)束");

}privateKThread joinee;

}public static class A_thread implementsRunnable {

A_thread(intnum) {this.num =num;

}public voidrun() {

System.out.println("A_thread 就緒");

System.out.println("A_thread開(kāi)始執(zhí)行");//This should just kill some cycles

for (int i = 0; i < this.num; ++i) {

System.out.println("A_thread 循環(huán) 第" + i + " 次");

KThread.currentThread().yield();

}

System.out.println("A_thread 執(zhí)行結(jié)束");

}private intnum;

}

}

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