我创建了一个程序,将线性搜索(搜索-1)划分为4个单独的线程。
public class main {
static boolean found = false;
public static void main(String[] args) {
// TODO Auto-generated method stub
int threadCount = 4; //amount of threads to use
Random rand = new Random();
Searcher[] s_arr = new Searcher[threadCount]; //array of threads
int[] arr = new int[10000]; //array to search through
for (int i = 0; i < arr.length; i++) //randomizing #'s in array
arr[i] = (int) (rand.nextFloat() * 1000);
int randIndex = rand.nextInt(arr.length); //choose random index
arr[randIndex] = -1; //set random index to = -1
for (int i = 0; i < threadCount; i++) { //
s_arr[i] = new Searcher(Arrays.copyOfRange(arr, i * (arr.length/threadCount), (i+1) * (arr.length/threadCount)),
(int) (i), i); //assign subarray for this thread to search through
System.out.println(s_arr[i].wait);
s_arr[i].start();
}
//CODE IN QUESTION HERE ----------------------------
//while (!found) ;
while (!found) //wait until value is found
{
try {
Thread.sleep(1);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
//-----------------------------------------------------------
System.out.println("found!");
for (int i = 0; i < threadCount; i++) {
try {
s_arr[i].join(); //wait for the threads in order before continuing
System.out.println("Thread ["+i+"] completed");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println("All threads stopped, program complete.");
}
}
public class Searcher extends Thread {
int[] arr;
int wait;
int index;
public Searcher(int[] arr, int wait, int i) {
this.arr = arr;
this.wait = wait;
this.index = i;
}
@Override
public void run() {
for (int i = 0; i < arr.length; i++) {
if (arr[i] == -1) {
System.out.println("["+index+"] -1 Found at index: "+i);
main.found = true;
break;
}
if (main.found) break;
//purposely slow down this thread
try {
Thread.sleep(wait);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println("["+index+"] has stopped");
}
}
我已经标记了有问题的代码,在使用第一个(注释掉)的while循环时,程序不会超出该点,但是如果我切换并在其下面使用另一个while循环(强制每个迭代器等待1毫秒),程序运行正常。
为什么这样做,有没有更有效/实用的方法来完成此任务?
[在空循环语句的情况下反复读取非易失性字段可能会导致无限循环,因为编译器优化可能会将对该字段的访问移出了循环。
来源:help.semmle.com
如果将static boolean found = false;
替换为volatile static boolean found = false;
,则第一个循环将起作用,但我不建议这样做,因为这会浪费您的CPU时间。您应该考虑使用wait
和notify
。
static boolean found
下方,添加static final Object lock = new Object();
并用]替换两个while
循环>
try { synchronized (lock) { // we will wait here until we get notified lock.wait(); } } catch (InterruptedException e) { e.printStackTrace(); }
也在
main.found = true
添加之后
synchronized (main.lock) { main.lock.notify(); }
最后,您的代码应该看起来像这样
public class main {
static boolean found;
static final Object lock = new Object();
public static void main(String[] args) {
// TODO Auto-generated method stub
int threadCount = 4; //amount of threads to use
Random rand = new Random();
Searcher[] s_arr = new Searcher[threadCount]; //array of threads
int[] arr = new int[10000]; //array to search through
for (int i = 0; i < arr.length; i++) //randomizing #'s in array
arr[i] = (int) (rand.nextFloat() * 1000);
int randIndex = rand.nextInt(arr.length); //choose random index
arr[randIndex] = -1; //set random index to = -1
for (int i = 0; i < threadCount; i++) { //
s_arr[i] = new Searcher(Arrays.copyOfRange(arr, i * (arr.length/threadCount), (i+1) * (arr.length/threadCount)),
(int) (i), i); //assign subarray for this thread to search through
System.out.println(s_arr[i].wait);
s_arr[i].start();
}
try {
synchronized (lock) {
lock.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("found!");
for (int i = 0; i < threadCount; i++) {
try {
s_arr[i].join(); //wait for the threads in order before continuing
System.out.println("Thread ["+i+"] completed");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println("All threads stopped, program complete.");
}
}
class Searcher extends Thread {
int[] arr;
int wait;
int index;
public Searcher(int[] arr, int wait, int i) {
this.arr = arr;
this.wait = wait;
this.index = i;
}
@Override
public void run() {
for (int i = 0; i < arr.length; i++) {
if (arr[i] == -1) {
System.out.println("["+index+"] -1 Found at index: "+i);
main.found = true;
synchronized (main.lock) {
main.lock.notify();
}
break;
}
if (main.found) break;
//purposely slow down this thread
try {
Thread.sleep(wait);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println("["+index+"] has stopped");
}
}