最近，操作系统课程设计使用伯克利大学的Nachos做为实验平台，老师也照搬伯克利的Project要求，开始我们的操作系统课程设计。

结合自己的学习过程和课设要求，我觉得对Nachos的学习首先应该从KThread.java入手,首先应该看明白这个类的所有函数的意思。

这个主要是为第一个proj，实现join()函数做准备。Join()方法的含义：当前线程a在运行，执行b.join(),则a阻塞，直到线程b结束，a继续执行。

具体的要求有：Join函数的作用即为等待某线程运行完毕。当前线程 (唯一一个正在运行的线程) A调用另一个线程 (处于就绪状态) B的join函数时 (A 和 B 在Nachos中均为KThread类型对象)，A被挂起，直到B运行结束后， join函数返回，A才能继续运行。注意在一个KThread对象上只能调用一次join，且当前线程不能对自身调用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 //等待另外一个线程结束的这个线程不能是线程自己

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 }

为此，我设置了一个KThread的队列joinQueue，当B调用join函数时，将正在执行的A线程放入joinQueue，只有在B执行完后，在finish()里面会将队列中的A线程状态变为ready，从而继续执行A线程，具体finish()实现的代码如下：

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 }

后续测试程序如下：

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 就绪");

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

this.joinee.join();//启动a线程，阻塞b线程

System.out.println("B_thread 执行结束");

}privateKThread joinee;

}public static class A_thread implementsRunnable {

A_thread(intnum) {this.num =num;

}public voidrun() {

System.out.println("A_thread 就绪");

System.out.println("A_thread开始执行");//This should just kill some cycles

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

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

KThread.currentThread().yield();

}

System.out.println("A_thread 执行结束");

}private intnum;

}

}