编写一个程序，开启 3 个线程 A,B,C，这三个线程的输出分别为 A、B、C，每个线程将自己的 输出在屏幕上打印 10 遍，要求输出的结果必须按顺序显示。如：ABCABCABC....
核心在于多线程同步
完整代码：https://github.com/mightofcode/javacc
方法1，轮询AtomicInteger
缺点是轮询白耗CPU，性能很差
package com.mocyx.javacc.printer;
import org.springframework.stereotype.Component;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
/**
* 轮询AtomicInteger实现交替输出ABC
* @author Administrator
*/
@Component
public class PollingPrinter implements Printer {
private final AtomicInteger atomicInteger = new AtomicInteger(0);
class Worker implements Runnable {
private String pstr;
private int index;
private int gap;
private int max;
public Worker(String pstr, int index, int gap, int max) {
this.pstr = pstr;
this.index = index;
this.gap = gap;
this.max = max;
}
@Override
public void run() {
while (true) {
int v = atomicInteger.get();
if (v == max) {
return;
} else {
if (v % gap == index) {
System.out.print(pstr);
atomicInteger.set(v + 1);
}
}
}
}
}
@Override
public void print() {
List threads = new ArrayList<>();
threads.add(new Thread(new Worker("A", 0, 3, 30)));
threads.add(new Thread(new Worker("B", 1, 3, 30)));
threads.add(new Thread(new Worker("C", 2, 3, 30)));
for (Thread t : threads) {
t.start();
}
try {
for (Thread t : threads) {
t.join();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
方法2，使用Lock & Condition同步
使用Lock & Condition要注意：
1 检查条件谓词，避免信号丢失和过早唤醒
2 注意在finally中进行unlock，否则出现异常会hang住
package com.mocyx.javacc.printer;
import org.springframework.stereotype.Component;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* 使用Lock and Condition进行同步
*
* @author Administrator
*/
@Component
public class SignalPrinter implements Printer {
private final Lock lock = new ReentrantLock();
private volatile int counter = 0;
class Worker implements Runnable {
Condition curCondition;
Condition nextCondition;
String pstr;
int max;
int index;
int gap;
public Worker(String pstr, int index, int gap, int max, Condition curCondition, Condition nextCondition) {
this.pstr = pstr;
this.max = max;
this.curCondition = curCondition;
this.nextCondition = nextCondition;
this.index = index;
this.gap = gap;
}
private boolean isMyTurn() {
return counter % gap == index;
}
@Override
public void run() {
while (true) {
lock.lock();
try {
while (!isMyTurn()) {
try {
curCondition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
if (counter < max) {
System.out.print(pstr);
}
counter += 1;
nextCondition.signalAll();
} finally {
lock.unlock();
}
if (counter >= max) {
return;
}
}
}
}
@Override
public void print() {
List threads = new ArrayList<>();
List conditions = new ArrayList<>();
conditions.add(lock.newCondition());
conditions.add(lock.newCondition());
conditions.add(lock.newCondition());
threads.add(new Thread(new Worker("A", 0, 3, 30, conditions.get(0), conditions.get(1))));
threads.add(new Thread(new Worker("B", 1, 3, 30, conditions.get(1), conditions.get(2))));
threads.add(new Thread(new Worker("C", 2, 3, 30, conditions.get(2), conditions.get(0))));
for (Thread t : threads) {
t.start();
}
try {
for (Thread t : threads) {
t.join();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
方法3，使用Semphore进行同步
相比Lock & Condition，使用Semphore代码比较简洁，不容易出错
package com.mocyx.javacc.printer;
import org.springframework.stereotype.Component;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Semaphore;
/**
* @author Administrator
*/
@Component
public class SemaphorePrinter implements Printer {
class Worker implements Runnable {
private String pstr;
private Semaphore curSemphore;
private Semaphore nextSemphore;
private int count = 0;
Worker(String pstr, int count, Semaphore curSemphore, Semaphore nextSemphore) {
this.pstr = pstr;
this.count = count;
this.curSemphore = curSemphore;
this.nextSemphore = nextSemphore;
}
@Override
public void run() {
for (int i = 0; i < count; i++) {
try {
curSemphore.acquire(1);
System.out.print(pstr);
nextSemphore.release(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
@Override
public void print() {
List threads = new ArrayList<>();
List semaphores = new ArrayList<>();
semaphores.add(new Semaphore(0));
semaphores.add(new Semaphore(0));
semaphores.add(new Semaphore(0));
threads.add(new Thread(new Worker("A", 10, semaphores.get(0), semaphores.get(1))));
threads.add(new Thread(new Worker("B", 10, semaphores.get(1), semaphores.get(2))));
threads.add(new Thread(new Worker("C", 10, semaphores.get(2), semaphores.get(0))));
for (Thread t : threads) {
t.start();
}
semaphores.get(0).release(1);
try {
for (Thread t : threads) {
t.join();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
方法4，使用BlockingQueue进行同步
思路跟go channel类似，通过BlockingQueue传递信息
package com.mocyx.javacc.printer;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
/**
* 使用阻塞队列进行同步
*
* @author Administrator
*/
public class QueuePrinter implements Printer {
static class Msg {
public static final Msg PRINT_SUCCESS = new Msg();
public static final Msg PRINT = new Msg();
public static final Msg QUIT = new Msg();
}
class Channel {
BlockingQueue inQueue = new ArrayBlockingQueue(100);
BlockingQueue outQueue = new ArrayBlockingQueue(100);
}
class Worker implements Runnable {
Channel inChannel;
String pstr;
Worker(String pstr, Channel inChannel) {
this.inChannel = inChannel;
this.pstr = pstr;
}
@Override
public void run() {
while (true) {
try {
Msg msg = inChannel.inQueue.take();
if (msg == Msg.PRINT) {
System.out.print(pstr);
inChannel.outQueue.put(Msg.PRINT_SUCCESS);
} else if (msg == Msg.QUIT) {
return;
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
@Override
public void print() {
List threads = new ArrayList<>();
List channels = new ArrayList<>();
channels.add(new Channel());
channels.add(new Channel());
channels.add(new Channel());
threads.add(new Thread(new Worker("A", channels.get(0))));
threads.add(new Thread(new Worker("B", channels.get(1))));
threads.add(new Thread(new Worker("C", channels.get(2))));
for (Thread t : threads) {
t.start();
}
for (int i = 0; i < 30; i++) {
try {
channels.get(i % channels.size()).inQueue.put(Msg.PRINT);
channels.get(i % channels.size()).outQueue.take();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
for (Channel c : channels) {
c.inQueue.add(Msg.QUIT);
}
try {
for (Thread t : threads) {
t.join();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
方法0，Sleep同步法
这个方法确实能工作，但是可能会被面试官打
package com.mocyx.javacc.printer;
import java.util.ArrayList;
import java.util.List;
/**
* sleep and sleep
*
* @author Administrator
*/
public class SleepPrinter implements Printer {
static class Worker implements Runnable {
private String printStr;
private int sleepGap;
private int delay;
private int count;
public Worker(String printStr, int delay, int sleepGap, int count) {
this.printStr = printStr;
this.sleepGap = sleepGap;
this.delay = delay;
this.count = count;
}
@Override
public void run() {
try {
Thread.sleep(delay);
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int i = 0; i < count; i++) {
try {
Thread.sleep(sleepGap);
System.out.print(printStr);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
@Override
public void print() {
List threads = new ArrayList<>();
threads.add(new Thread(new Worker("A", 00, 30, 10)));
threads.add(new Thread(new Worker("B", 10, 30, 10)));
threads.add(new Thread(new Worker("C", 20, 30, 10)));
for (Thread t : threads) {
t.start();
}
try {
for (Thread t : threads) {
t.join();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}