英文:
How does polling with thread.sleep() work?
问题
以下是翻译好的部分：
我不明白为什么输出仅打印一次“Not reached yet”，而不是每次计数器小于100时都打印一次。我期望输出会打印“Not reached yet”100次，然后打印“Reached”。
英文:
I am new to threads in Java, I am trying to understand how they work; considering the following code:
public class ZooInfo {
private static int counter = 0;
public static void main(String args[]) throws InterruptedException {
new Thread(() -> {
for(int i=0; i<500; i++) ZooInfo.counter++;
}).start();
while(ZooInfo.counter<100) {
System.out.println("Not reached yet");
Thread.sleep(1000);
}
System.out.println("Reached!");
}
}
The output of the above code:
Not reached yet
Reached!
I don't understand why the output prints "Not reached yet" just once, and not for every time counter is less than a 100. I was expecting the output to print "Not reached yet" a 100 then print "Reached"?
答案1
得分: 3
"我不明白为什么输出仅在计数器小于100时打印一次"尚未到达"，而不是每次计数器小于100时都打印一次。 " - 实际上是这样的！但是，ZooInfo.counter的值仅在每秒钟检查一次。启动的线程（new Thread(() -> { for(int i=0; i<500; i++) ZooInfo.counter++; }).start();）在前100次迭代中所需的时间明显少于一秒。删除Thread.sleep(...)并不一定会改变行为（对我来说，它会导致Not yet reached!被打印1-3次）。这是由于新创建的线程仍然领先。此外，这两个线程不会"同步运行"，即一个线程可能比另一个线程更快或更慢。
此外，正如@Amongalen和@Polygnome所指出的，向System.out打印速度较慢。我们可以通过使用StringBuilder来改进：
final String lineSeparator = System.lineSeparator();
new Thread(() -> {
for (int i = 0; i < 500; i++) ZooInfo.counter++;
}).start();
final StringBuilder builder = new StringBuilder();
while (ZooInfo.counter < 100) {
builder.append("Not reached yet").append(lineSeparator);
}
builder.append("Reached!");
System.out.println(builder.toString());
这将增加打印"Not reached yet!"的次数。
备注：字段private static int counter = 0;还应该声明为volatile，以确保线程之间有适当的可见性。
英文:
"I don't understand why the output prints "Not reached yet" just once, and not for every time counter is less than a 100." - It does! The value of ZooInfo.counter, however, is only checked once a second. The thread started (new Thread(() -> { for(int i=0; i<500; i++) ZooInfo.counter++; }).start(); will take significantly less than a second for the first 100 iterations. Removing the Thread.sleep(...) does not necessarily change the behaviour (for me, it leads to Not yet reached! being printend 1-3 times). This is due to the fact that the newly created thread still has a head start. Also, the two threads do not run "in unison", i.e. one thread may proceed faster or slower than the other.
Furthermore, as was pointed out by @Amongalen and @Polygnome, printing to System.out is slow. We can improve this by using a StringBuilder:
final String lineSeparator = System.lineSeparator();
new Thread(() -> {
for (int i = 0; i < 500; i++) ZooInfo.counter++;
}).start();
final StringBuilder builder = new StringBuilder();
while (ZooInfo.counter < 100) {
builder.append("Not reached yet").append(lineSeparator);
}
builder.append("Reached!");
System.out.println(builder.toString());
This will increase the number of times Not reached yet! is printed.
A remark: the field private static int counter = 0; should also be declared volatile to guarantee proper visibility between threads.
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