实现并发线程按顺序输出1234 java基础

【实现并发线程按顺序输出1234】问题:
有first,second,third,forth四个线程,first输出1，second输出2，third输出3，forth输出4。要求, 同时启动四个线程, 按顺序输出1234, 且未无限循环输出。
这是一个多线程协同的问题，本身多线程是没有执行顺序的, 顺序不一定。
但是可以采取一定方式使多线程按一定顺序执行。
ReentrantLock ReentrantLock来解决, 还有个state整数用来判断轮到谁执行了。

public class PrintTwo { private static Lock lock = new ReentrantLock(); //通过JDK5中的锁来保证线程的访问的互斥 private static int state = 0; static class First extends Thread { @Override public void run() { while (true) { lock.lock(); if (state % 4 == 0) { System.out.println("1"); state++; } lock.unlock(); } } }static class Second extends Thread { @Override public void run() { while (true) { lock.lock(); if (state % 4 == 1) { System.out.println("2"); state++; } lock.unlock(); } } }static class Third extends Thread { @Override public void run() { while (true) { lock.lock(); if (state % 4 == 2) { System.out.println("3"); state++; } lock.unlock(); } } }static class Forth extends Thread { @Override public void run() { while (true) { lock.lock(); if (state % 4 == 3) { System.out.println("4"); state++; } lock.unlock(); } } }public static void main(String[] args) { First first = new First(); Second second = new Second(); Third third = new Third(); Forth forth = new Forth(); first.start(); second.start(); third.start(); forth.start(); } }

Semphore Semphore,完成对信号量的控制，可以控制某个资源可以被同时访问的个数，通过acquire()获取一个许可，如果没有就等待，而release()释放一个许可。

package Thread; import java.util.concurrent.Semaphore; public class Print1234 { public static Semaphore sem1; public static Semaphore sem2; public static Semaphore sem3; public static Semaphore sem4; static class FirstThread extends Thread { public void run() { try { while (true) { sem1.acquire(); System.out.println("1"); sem2.release(); } } catch (InterruptedException e) { e.printStackTrace(); } } }static class SecondThread extends Thread { public void run() { try { while (true) { sem2.acquire(); System.out.println("2"); sem3.release(); } } catch (InterruptedException e) { e.printStackTrace(); } } }static class ThirdThread extends Thread { public void run() { try { while (true) { sem3.acquire(); System.out.println("3"); sem4.release(); } } catch (InterruptedException e) { e.printStackTrace(); } } }static class ForthThread extends Thread { public void run() { try { while (true) { sem4.acquire(); System.out.println("4"); sem1.release(); } } catch (InterruptedException e) { e.printStackTrace(); } } }public static void main(String[] args) { sem1 = new Semaphore(1); sem2 = new Semaphore(1); sem3 = new Semaphore(1); sem4 = new Semaphore(1); try { // 不要有sem1.acquire() sem2.acquire(); sem3.acquire(); sem4.acquire(); } catch (InterruptedException e) { e.printStackTrace(); }new FirstThread().start(); new SecondThread().start(); new ThirdThread().start(); new ForthThread().start(); } }

volatile volatile，保证被修饰的变量在读写前都会与主存交互更新。一个变量被volatile修饰后，则不同线程对这个变量进行操作时，总是从内存中读取最新值，即每次更新对其他线程都是立即可见的。

public class PrintThree { volatile static int state = 0; static class First extends Thread { @Override public void run() { while (true) { if (state % 4 == 0) { System.out.println("1"); state++; } } } }static class Second extends Thread { @Override public void run() { while (true) { if (state % 4 == 1) { System.out.println("2"); state++; } } } }static class Third extends Thread { @Override public void run() { while (true) { if (state % 4 == 2) { System.out.println("3"); state++; } } } }static class Forth extends Thread { @Override public void run() { while (true) { if (state % 4 == 3) { System.out.println("4"); state++; } } } }public static void main(String[] args) { First first = new First(); Second second = new Second(); Third third = new Third(); Forth forth = new Forth(); first.start(); second.start(); third.start(); forth.start(); } }