A，B两个线程使用同一个Condition对象时，A线程唤醒等待中的B之后必须进入等待吗？
0
悬赏园豆：50
[待解决问题]
我是一个JAVA新手，最近在做一道多线程的题，题目：
使用A线程打印1-52，B线程打印A-Z，要求按照12A34B56C....5152Z的顺序进行交替打印。
我在编程的时候遇到两个问题。
问题1：
当A线程打印完52后，唤醒B线程的语句 condition.signal(); 没有发挥作用，导致Z打印不出来。然后我在 condition.signal(); 这个语句后边跟了一个 condition.await(); 就正常打印出Z了。 因此我想问下，是不是执行中的线程唤醒同一个Condition对象下等待的线程之后，自己必须进入等待状态呀？这是为什么呢？
问题2：
我的这段代码在Eclipse执行完毕之后，Terminate按钮还是处于红色状态的，说明程序还在等待中，这是为什么啊？
问题补充：
源代码如下：
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
class NumThread extends Thread {
PrinterClass pc;
NumThread (PrinterClass pc){
this.pc = pc;
}
public void run() {
while(PrinterClass.number < 53) {
pc.printMethod(1);
}
}}
class ChapThread extends Thread {
PrinterClass pc;
ChapThread (PrinterClass pc){
this.pc = pc;
}
public void run() {
while (PrinterClass.chapter < 26) {
pc.printMethod('p');
}
}}
class PrinterClass {
public static int number = 1;
public static int chapter = 0;
private static int flag = 0;
private final Lock printLock = new ReentrantLock();
private Condition condition = printLock.newCondition();
public void printMethod (Object printContents) {
printLock.lock();
if (flag ==0 || flag == 1) {
if (printContents instanceof Integer) {
System.out.println(number);
flag ++;
number++;
if (number > 52) {
try {
condition.signal();
condition.await();
}
catch (Exception e) {
}
finally{
printLock.unlock();
}
}
printLock.unlock();
}
if (printContents instanceof Character) {
try {
condition.signal();
condition.await();
}
catch (Exception e) {
printLock.unlock();
}
}
}
else if (flag == 2) {
if (printContents instanceof Character) {
System.out.println((char)(65+chapter));
flag = 0;
chapter++;
printLock.unlock();
}
if (printContents instanceof Integer) {
try {
condition.signal();
condition.await();
}
catch (Exception e) {
printLock.unlock();
}
}
}
}}
public class MultiThread_ReentrantLock {
public static void main(String[] args) {
// TODO Auto-generated method stub
PrinterClass pc = new PrinterClass();
NumThread nt = new NumThread(pc);
nt.start();
new ChapThread(pc).start();
}}
所有回答(1)
0
感觉你的实现思路就有问题，可以参考我写的demo
附上我的代码
#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
namespace its {
std::mutex mtx;
std::condition_variable cv;
int index = 1;
void print_number() {
std::unique_lock<std::mutex> lck(mtx);
for (int i = 1; i <= 52; i++) {
if (index % 3 == 0) {
cv.wait(lck);
}
std::cout << i << " ";
index++;
cv.notify_all();
}
}
void print_alphabet() {
std::unique_lock<std::mutex> lck(mtx);
for (char i = 'A'; i <= 'Z'; i++) {
if (index % 3 != 0) {
cv.wait(lck);
}
std::cout << i << std::endl;
index++;
cv.notify_all();
}
}
}
int main() {
std::thread threads[2];
threads[0] = std::thread(its::print_number);
threads[1] = std::thread(its::print_alphabet);
std::cout << "2 threads ready to race...\n";
for (auto &th : threads)
th.join();
return 0;
}