关于多线程的基础知识，可点击下面链接进行学习。
题目1
启动3个线程A、B、C，使A打印0，然后B打印1，然后C打印2，A打印3，B打印4，C打印5，依次类推。
public class PrintSequenceThread implements Runnable {
private static final Object LOCK = new Object();
/**
* 当前即将打印的数字
*/
private static int current = 0;
/**
* 当前线程编号，从0开始
*/
private int threadNo;
/**
* 线程数量
*/
private int threadCount;
/**
* 打印的最大数值
*/
private int max;
public PrintSequenceThread(int threadNo, int threadCount, int max) {
this.threadNo = threadNo;
this.threadCount = threadCount;
this.max = max;
}
@Override
public void run() {
while(true) {
synchronized (LOCK) {
// 判断是否轮到当前线程执行
while (current % threadCount != threadNo) {
if (current > max) {
break;
}
try {
// 如果不是，则当前线程进入wait
LOCK.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// 最大值跳出循环
if (current > max) {
break;
}
System.out.println("thread-" + threadNo + " : " + current);
current++;
// 唤醒其他wait线程
LOCK.notifyAll();
}
}
}
public static void main(String[] args) {
int threadCount = 3;
int max = 10;
for(int i=0;i<threadCount;i++) {
new Thread(new PrintSequenceThread(i,threadCount, max)).start();
}
}
}
题目2
编写一个程序，开启 3 个线程，这三个线程的分别为 A、B、C，每个线程打印对应的“A”、“B”、“C” 10 遍，要求输出的结果必须按顺序显示。如：ABCABCABC……
*tate初始化为1，如果state为1执行A线程，A线程修改state为2执行B线程；如果state为2执行B线程，B线程修改state为3企图执行C线程；如果state为3执行C线程，C线程修改state为1，企图执行A线程。
解法1：采用volatile修饰的全局变量state作为条件控制，用for循环加if条件判断的忙等模式。
package com.yangliu.lock;
import java.io.File;
public class ABC1 {
private static int n = 100000; //控制线程执行次数
private volatile static int state = 0; //控制线程执行条件
static class ThreadA extends Thread {
public void run() {
for(int i=0;i<n;){
if(state%3==0){
System.out.println("A,loopNum="+i);
state++;
i++;
}
}
}
}
static class ThreadB extends Thread {
public void run() {
for(int i=0;i<n;){
if(state%3==1){
System.out.println("B,loopNum="+i);
state++;
i++;
}
}
}
}
static class ThreadC extends Thread {
public void run() {
for(int i=0;i<n;){
if(state%3==2){
System.out.println("C,loopNum="+i);
state++;
i++;
}
}
}
}
public static void main(String[] args) throws InterruptedException {
long startTime = System.currentTimeMillis();
new ThreadA().start();
new ThreadB().start();
ThreadC c =new ThreadC();
c.start();
c.join();
long endTime = System.currentTimeMillis();
File f = new File("f.txt");
FileUtil.writeToFile(f, "n="+n);
FileUtil.writeToFile(f, "RunTime is "+(double)(endTime-startTime)/1000+"s");
}
}
问题：为什么共享变量要加volatile?加volatile就足够了吗？
分析：因为加volatile可以保证变量state的可见性，上一个线程对state的修改对下一个线程是可见的。另外由于有if条件做判断，所以可以确保只有单一的线程修改变量state的值，这里用volatile就足够了。
解法2:采用原子类AtomicInteger来控制变量state，其他不变
package com.yangliu.lock;
import java.io.File;
import java.util.concurrent.atomic.AtomicInteger;
public class ABC2 {
private static int n = 100000; //控制线程执行次数
private static AtomicInteger state = new AtomicInteger(0); //控制线程执行条件
static class ThreadA extends Thread {
public void run() {
for(int i=0;i<n;){
if(state.get()%3==0){
System.out.println("A,loopNum="+i);
state.getAndIncrement();
i++;
}
}
}
}
static class ThreadB extends Thread {
public void run() {
for(int i=0;i<n;){
if(state.get()%3==1){
System.out.println("B,loopNum="+i);
state.getAndIncrement();
i++;
}
}
}
}
static class ThreadC extends Thread {
public void run() {
for(int i=0;i<n;){
if(state.get()%3==2){
System.out.println("C,loopNum="+i);
state.getAndIncrement();
i++;
}
}
}
}
public static void main(String[] args) throws InterruptedException {
long startTime = System.currentTimeMillis();
new ThreadA().start();
new ThreadB().start();
ThreadC c =new ThreadC();
c.start();
c.join();
long endTime = System.currentTimeMillis();
File f = new File("f.txt");
FileUtil.writeToFile(f, "RunTime is "+(double)(endTime-startTime)/1000+"s");
}
}
解法3：采用ReentrantLock锁住整段代码
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package com.yangliu.lock;
import java.io.File;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class ABC3 {
private static int n = 100000; // 控制线程执行次数
private static int state = 0; // 控制线程执行条件
private static Lock lock = new ReentrantLock();
static class ThreadA extends Thread {
public void run() {
for (int i = 0; i < n;) {
lock.lock();
if (state % 3 == 0) {
System.out.println("A,loopNum=" + i);
state++;
i++;
}
lock.unlock();
}
}
}
static class ThreadB extends Thread {
public void run() {
for (int i = 0; i < n;) {
lock.lock();
if (state % 3 == 1) {
System.out.println("B,loopNum=" + i);
state++;
i++;
}
lock.unlock();
}
}
}
static class ThreadC extends Thread {
public void run() {
for (int i = 0; i < n;) {
lock.lock();
if (state % 3 == 2) {
System.out.println("C,loopNum=" + i);
state++;
i++;
}
lock.unlock();
}
}
}
public static void main(String[] args) throws InterruptedException {
long startTime = System.currentTimeMillis();
new ThreadA().start();
new ThreadB().start();
ThreadC c =new ThreadC();
c.start();
c.join();
long endTime = System.currentTimeMillis();
File f = new File("f.txt");
FileUtil.writeToFile(f, "RunTime is "+(double)(endTime-startTime)/1000+"s");
}
}
解法4：采用ReentrantLock，三个Condition进行await和signal操作
public class ABC4 {
private static int n = 100000; // 控制线程执行次数
private static Lock lock = new ReentrantLock();
private static Condition A = lock.newCondition();
private static Condition B = lock.newCondition();
private static Condition C = lock.newCondition();
private static int state = 0;
private static class ThreadA extends Thread {
public void run() {
lock.lock();
try {
for (int i = 0; i < n;i++) {
if (state % 3 != 0)
A.await();
System.out.println("A,loopNum=" + i);
state++;
B.signal();
}
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
static class ThreadB extends Thread {
public void run() {
lock.lock();
try {
for (int i = 0; i < n;i++) {
if (state % 3 != 1)
B.await();
System.out.println("B,loopNum=" + i);
state++;
C.signal();
}
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
static class ThreadC extends Thread {
public void run() {
lock.lock();
try {
for (int i = 0; i < n;i++) {
if (state % 3 != 2)
C.await();
System.out.println("C,loopNum=" + i);
state++;
A.signal();
}
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
public static void main(String[] args) throws InterruptedException {
long startTime = System.currentTimeMillis();
new ThreadA().start();
new ThreadB().start();
ThreadC c =new ThreadC();
c.start();
c.join();
long endTime = System.currentTimeMillis();
File f = new File("f.txt");
FileUtil.writeToFile(f, "RunTime is "+(double)(endTime-startTime)/1000+"s");
}
}
解法5：使用信号量机制，完全不用state变量进行条件控制
public class ABC5 {
private static int n = 100000;
private static Semaphore AB = new Semaphore(0);
private static Semaphore BC = new Semaphore(0);
private static Semaphore CA = new Semaphore(0);
static class ThreadA extends Thread {
@Override
public void run() {
try {
for (int i = 0; i < n; i++) {
CA.acquire();
System.out.println("A,loopNum=" + i);
AB.release();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
static class ThreadB extends Thread {
@Override
public void run() {
try {
for (int i = 0; i < n; i++) {
AB.acquire();
System.out.println("B,loopNum=" + i);
BC.release();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
static class ThreadC extends Thread {
@Override
public void run() {
try {
for (int i = 0; i < n; i++) {
BC.acquire();
System.out.println("C,loopNum=" + i);
CA.release();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) throws InterruptedException {
long startTime = System.currentTimeMillis();
CA.release(); //释放CA，让A线程先执行
new ThreadA().start();
new ThreadB().start();
ThreadC c =new ThreadC();
c.start();
c.join();
long endTime = System.currentTimeMillis();
File f = new File("f.txt");
FileUtil.writeToFile(f, "RunTime is "+(double)(endTime-startTime)/1000+"s");
}
}
当n取10万时的运行时间比较：
n=100000
RunTime is 5.623s
RunTime is 5.322s
RunTime is 4.482s
RunTime is 4.279s
RunTime is 3.842s
结果可知，
第5种信号量机制运行时间最少，最佳。
