【Java多執行緒程式設計題】兩個執行緒迴圈輸出A和B
摘要：
非個人面試，這是京東面試的一道程式設計題
前言：這道題挺有意思的，剛好我剛學了基礎的Java多執行緒，那就試一試。另外以下的兩種方案都需要注意就是，wait()
需要放在while
迴圈裡以防錯失notify
訊號
一種程式碼風...
非個人面試，這是京東面試的一道程式設計題
前言：這道題挺有意思的，剛好我剛學了基礎的Java多執行緒，那就試一試。另外以下的兩種方案都需要注意就是，wait()
需要放在while
迴圈裡以防錯失notify
訊號
一種程式碼風格比較好的方案：同步方法
package thread.print;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class CyclicPrint {
private ExecutorService executorService = Executors.newFixedThreadPool(2);
private boolean printA = true;
private synchronized void printA() throws InterruptedException {
while (!printA) wait();
System.out.println("A");
printA = false;
notifyAll();
}
private synchronized void printB() throws InterruptedException {
while (printA) wait();
System.out.println("B");
printA = true;
notifyAll();
}
private Thread a = new Thread(() -> {
while (!Thread.interrupted()) {
try {
printA();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
private Thread b = new Thread(() -> {
while (!Thread.interrupted()) {
try {
printB();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
public CyclicPrint() throws InterruptedException {
executorService.execute(a);
executorService.execute(b);
Thread.sleep(1000);
executorService.shutdownNow();
}
public static void main(String[] args) throws InterruptedException {
new CyclicPrint();
}
}
另外一種與上面大同小異的方案，但是可讀性較之低一些：同步臨界區
package thread.print;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class CyclicPrint2 {
private ExecutorService executorService = Executors.newFixedThreadPool(2);
private final CyclicPrint2 p = this;
private boolean printA = true;
private Thread a = new Thread(() -> {
while (!Thread.interrupted()) {
synchronized (p) {
while (!printA) {
try {
p.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("A");
printA = false;
p.notifyAll();
}
}
});
private Thread b = new Thread(() -> {
while (!Thread.interrupted()) {
synchronized (p) {
while (printA) {
try {
p.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("B");
printA = true;
p.notifyAll();
}
}
});
public CyclicPrint2() throws InterruptedException {
executorService.execute(a);
executorService.execute(b);
Thread.sleep(1000);
executorService.shutdownNow();
}
public static void main(String[] args) throws InterruptedException {
new CyclicPrint();
}
}
Lock
+Condition
版
package thread.print;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class CyclicPrint3 {
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
private ExecutorService executorService = Executors.newFixedThreadPool(2);
private boolean printA = true;
private void printA() throws InterruptedException {
lock.lock();
while (!printA) condition.await();
System.out.println("A");
printA = false;
condition.signalAll();
lock.unlock();
}
private void printB() throws InterruptedException {
lock.lock();
while (printA) condition.await();
System.out.println("B");
printA = true;
condition.signalAll();
lock.unlock();
}
private Thread a = new Thread(() -> {
while (!Thread.interrupted()) {
try {
printA();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
private Thread b = new Thread(() -> {
while (!Thread.interrupted()) {
try {
printB();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
public CyclicPrint3() throws InterruptedException {
executorService.execute(a);
executorService.execute(b);
Thread.sleep(1000);
executorService.shutdownNow();
}
public static void main(String[] args) throws InterruptedException {
new CyclicPrint3();
}
}
結語：剛開始這道題還是難住我了的，因為剛學完基礎，最初甚至萌生用CountDownLatch
或是CyclicBarrier
等新構件解決，但應該是行不通的。然後慢慢想才有了同步+迴圈條件的解決方案，效果還不錯。