临界资源问题的原因：某一个线程在对临界资源进行访问时，还没来得及完全修改临界资源的值，临界资源就被其他线程拿去访问，导致多个线程访问同一资源。直观表现为打印结果顺序混乱。

解决方法：加锁

静态方法中用类锁，非静态方法中用对象锁。

1.同步代码段：synchronized（）{...}

2.同步方法：使用关键字synchronized修饰的方法

3.使用显式同步锁ReentrantLock

锁池描述的即为锁外等待的状态

方法一：同步代码段：synchronized（）{...}

public class SourceConflict {  public static void main(String[] args) {    //实例化4个售票员，用4个线程模拟4个售票员        Runnable r = () -> {      while (TicketCenter.restCount > 0) {        synchronized(" ") {          if (TicketCenter.restCount <= 0) {            return;          }          System.out.println(Thread.currentThread().getName() + "卖出一张票，剩余" + --TicketCenter.restCount + "张票");        }      }    };        //用4个线程模拟4个售票员    Thread thread1 = new Thread(r, "thread-1");    Thread thread2 = new Thread(r, "thread-2");    Thread thread3 = new Thread(r, "thread-3");    Thread thread4 = new Thread(r, "thread-4");        //开启线程    thread1.start();    thread2.start();    thread3.start();    thread4.start();      }  }//实现四名售票员共同售票，资源共享，非独立//Lambda表达式或匿名内部类内部捕获的局部变量必须显式的声明为 final 或实际效果的的 final 类型，而捕获实例或静态变量是没有限制的class TicketCenter{  public static int restCount = 100; }

方法二：同步方法，即使用关键字synchronized修饰的方法

public class SourceConflict2 {  public static void main(String[] args) {    //实例化4个售票员，用4个线程模拟4个售票员        Runnable r = () -> {      while (TicketCenter.restCount > 0) {        sellTicket();      }    };        //用4个线程模拟4个售票员    Thread thread1 = new Thread(r, "thread-1");    Thread thread2 = new Thread(r, "thread-2");    Thread thread3 = new Thread(r, "thread-3");    Thread thread4 = new Thread(r, "thread-4");        //开启线程    thread1.start();    thread2.start();    thread3.start();    thread4.start();      }    private synchronized static void sellTicket() {      if (TicketCenter.restCount <= 0) {      return;    }    System.out.println(Thread.currentThread().getName() + "卖出一张票，剩余" + --TicketCenter.restCount + "张票");  }}class TicketCenter{  public static int restCount = 100; }

方法三：使用显式同步锁ReentrantLock

import java.util.concurrent.locks.ReentrantLock;public class SourceConflict3 {  public static void main(String[] args) {    //实例化4个售票员，用4个线程模拟4个售票员        //显式锁    ReentrantLock lock = new ReentrantLock();    Runnable r = () -> {      while (TicketCenter.restCount > 0) {        lock.lock();        if (TicketCenter.restCount <= 0) {          return;        }        System.out.println(Thread.currentThread().getName() + "卖出一张票，剩余" + --TicketCenter.restCount + "张票");        lock.unlock();      }    };        //用4个线程模拟4个售票员    Thread thread1 = new Thread(r, "thread-1");    Thread thread2 = new Thread(r, "thread-2");    Thread thread3 = new Thread(r, "thread-3");    Thread thread4 = new Thread(r, "thread-4");        //开启线程    thread1.start();    thread2.start();    thread3.start();    thread4.start();      }  }class TicketCenter{  public static int restCount = 100; }