多线程打印宝典
Posted zyxhangzhou
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典中典多线程打印题，我来归纳一下：
- 三个线程分别打印 A，B，C，要求这三个线程一起运行，打印 n 次，输出形如“ABCABCABC....”的字符串
使用Semaphore
这一招也是我最喜欢的，简单无脑
public class Main {
private static final Semaphore[] s = {new Semaphore(1), new Semaphore(0), new Semaphore(0)};
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int n = input.nextInt();
input.close();
new Thread(() -> {
try {
for (int i = 0; i < n; i++) {
s[0].acquire();
System.out.print(\'A\');
s[1].release();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}).start();
new Thread(() -> {
try {
for (int i = 0; i < n; i++) {
s[1].acquire();
System.out.print(\'B\');
s[2].release();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}).start();
new Thread(() -> {
try {
for (int i = 0; i < n; i++) {
s[2].acquire();
System.out.print(\'C\');
s[0].release();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}).start();
}
}
使用ReentrantLock
public class Main {
private final int times;
private int status;
private static final Lock lock = new ReentrantLock();
private static final Condition c1 = lock.newCondition();
private static final Condition c2 = lock.newCondition();
private static final Condition c3 = lock.newCondition();
public Main(int times) {
this.times = times;
}
public static void main(String[] args) {
Main main = new Main(10);
new Thread(() -> {
main.printLetter(0, c1, c2);
}, "A").start();
new Thread(() -> {
main.printLetter(1, c2, c3);
}, "B").start();
new Thread(() -> {
main.printLetter(2, c3, c1);
}, "C").start();
}
public void printLetter(int targetStatus, Condition current, Condition next) {
for (int i = 0; i < times; ) {
lock.lock();
try {
while (status % 3 != targetStatus) {
current.await();
}
++status;
++i;
System.out.print(Thread.currentThread().getName());
next.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
}
使用Lock/Condition的话虽说代码比Semaphore复杂，但是可以做到精准控制，比如，AA 打印 5 次，BB 打印10 次，CC 打印 15 次，重复 10 次
public class Main {
private int status;
private static final Lock lock = new ReentrantLock();
private static final Condition c1 = lock.newCondition();
private static final Condition c2 = lock.newCondition();
private static final Condition c3 = lock.newCondition();
public static void main(String[] args) {
Main main = new Main();
new Thread(() -> {
for (int i = 0; i < 5; i++)
main.printLetter(0, c1, c2, 5);
}, "A").start();
new Thread(() -> {
for (int i = 0; i < 5; i++)
main.printLetter(1, c2, c3, 10);
}, "B").start();
new Thread(() -> {
for (int i = 0; i < 5; i++) {
main.printLetter(2, c3, c1, 15);
System.out.print(" | ");
}
}, "C").start();
}
public void printLetter(int targetStatus, Condition current, Condition next, int times) {
lock.lock();
try {
while (status % 3 != targetStatus) {
current.await();
}
for (int i = 0; i < times; i++) {
System.out.print(Thread.currentThread().getName());
}
++status;
next.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
使用 LockSupport
LockSupport 是 JDK 底层的基于 sun.misc.Unsafe 来实现的类，用来创建锁和其他同步工具类的基本线程阻塞原语。它的静态方法unpark()和park()可以分别实现阻塞当前线程和唤醒指定线程的效果，所以用它解决这样的问题会更容易一些。
public class Main {
private static final Thread[] threads = new Thread[3];
public static void main(String[] args) {
threads[0] = new Thread(() -> {
for (int i = 0; i < 50; i++) {
System.out.print(Thread.currentThread().getName());
LockSupport.unpark(threads[1]);
LockSupport.park();
}
}, "A");
threads[1] = new Thread(() -> {
for (int i = 0; i < 50; i++) {
LockSupport.park();
System.out.print(Thread.currentThread().getName());
LockSupport.unpark(threads[2]);
}
}, "B");
threads[2] = new Thread(() -> {
for (int i = 0; i < 50; i++) {
LockSupport.park();
System.out.print(Thread.currentThread().getName());
LockSupport.unpark(threads[0]);
}
}, "C");
for (Thread t : threads) {
t.start();
}
}
}
这种方法与Semaphore有异曲同工之妙，也是十分简单的一种方式！
- 交替打印奇偶数字
public class Main {
private int status;
private static final Lock lock = new ReentrantLock();
private static final Condition c1 = lock.newCondition();
private static final Condition c2 = lock.newCondition();
public static void main(String[] args) {
Main main = new Main();
new Thread(() -> {
main.printNum(100, 0, c1, c2);
}, "even:").start();
new Thread(() -> {
main.printNum(100, 1, c2, c1);
}, "odd:").start();
}
public void printNum(int times, int targetStatus, Condition current, Condition next) {
while (status < times) {
lock.lock();
try {
while (status % 2 != targetStatus) {
current.await();
}
System.out.print(Thread.currentThread().getName() + status + " ");
++status;
next.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
}
使用LockSupport一如既往的简单易懂。
public class Main {
private static final Thread[] threads = new Thread[2];
public static void main(String[] args) {
threads[0] = new Thread(() -> {
// 注意上下的循环steps一定要是一样的，否则程序不会运行结束
for (int i = 0; i < 100; i += 2) {
System.out.print(Thread.currentThread().getName() + i + " ");
LockSupport.unpark(threads[1]);
LockSupport.park();
}
}, "even:");
threads[1] = new Thread(() -> {
for (int i = 1; i <= 100; i += 2) {
LockSupport.park();
System.out.print(Thread.currentThread().getName() + i + " ");
LockSupport.unpark(threads[0]);
}
}, "odd:");
for (Thread t : threads) {
t.start();
}
}
}
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