如何避免死锁

死锁发生的条件
互斥，共享资源只能被一个线程占用
占有且等待，线程 t1 已经取得共享资源 s1，尝试获取共享资源 s2 的时候，不释放共享资源 s1
不可抢占，其他线程不能强行抢占线程 t1 占有的资源 s1
循环等待，线程 t1 等待线程 t2 占有的资源，线程 t2 等待线程 t1 占有的资源
避免死锁的方法
对于以上 4 个条件，只要破坏其中一个条件，就可以避免死锁的发生。
对于第一个条件 "互斥" 是不能破坏的，因为加锁就是为了保证互斥。
其他三个条件，我们可以尝试
一次性申请所有的资源，破坏 "占有且等待" 条件
占有部分资源的线程进一步申请其他资源时，如果申请不到，主动释放它占有的资源，破坏 "不可抢占" 条件
按序申请资源，破坏 "循环等待" 条件
使用管理类一次性申请所有的资源，破坏 "占有且等待" 条件示例

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package constxiong.concurrency.a023; import java.util.HashSet; import java.util.Set; /**  * 测试 一次性申请所有的资源，破坏 "占有且等待" 条件示例  * @author ConstXiong  * @date 2019-09-24 14:04:12  */ public class TestBreakLockAndWait {            //单例的资源管理类         private final static Manger manager = new Manger();                   //资源1         private static Object res1 = new Object();                   //资源2         private static Object res2 = new Object();                   public static void main(String[] args) {                 new Thread(() -> {                         boolean applySuccess = false;                         while (!applySuccess) {                                 //向管理类，申请res1和res2,申请失败，重试                                 applySuccess = manager.applyResources(res1, res2);                                 if (applySuccess) {                                         try {                                                 System.out.println("线程：" + Thread.currentThread().getName() + " 申请 res1、res2 资源成功");                                                 synchronized (res1) {                                                         System.out.println("线程：" + Thread.currentThread().getName() + " 获取到 res1 资源的锁");                                                         //休眠 1秒                                                         try {                                                                 Thread.sleep(1000);                                                         } catch (Exception e) {                                                                 e.printStackTrace();                                                         }                                                         synchronized (res2) {                                                                 System.out.println("线程：" + Thread.currentThread().getName() + " 获取到 res2 资源的锁");                                                         }                                                 }                                         } finally {                                                 manager.returnResources(res1, res2);//归还资源                                         }                                 } else {                                         System.out.println("线程：" + Thread.currentThread().getName() + " 申请 res1、res2 资源失败");                                         //申请失败休眠 200 毫秒后重试                                         try {                                                 Thread.sleep(200);                                         } catch (Exception e) {                                                 e.printStackTrace();                                         }                                 }                         }                 }).start();                                   new Thread(() -> {                         boolean applySuccess = false;                         while (!applySuccess) {                                 //向管理类，申请res1和res2,申请失败，重试                                 applySuccess = manager.applyResources(res1, res2);                                 if (applySuccess) {                                         try {                                                 System.out.println("线程：" + Thread.currentThread().getName() + " 申请 res1、res2 资源成功");                                                 synchronized (res2) {                                                         System.out.println("线程：" + Thread.currentThread().getName() + " 获取到 res1 资源的锁");                                                         //休眠 1秒                                                         try {                                                                 Thread.sleep(1000);                                                         } catch (Exception e) {                                                                 e.printStackTrace();                                                         }                                                         synchronized (res1) {                                                                 System.out.println("线程：" + Thread.currentThread().getName() + " 获取到 res2 资源的锁");                                                         }                                                 }                                         } finally {                                                 manager.returnResources(res1, res2);//归还资源                                         }                                 } else {                                         System.out.println("线程：" + Thread.currentThread().getName() + " 申请 res1、res2 资源失败");                                         //申请失败休眠 200 毫秒后重试                                         try {                                                 Thread.sleep(200);                                         } catch (Exception e) {                                                 e.printStackTrace();                                         }                                 }                         }                 }).start();                           }           }    /**  * 资源申请、归还管理类  * @author ConstXiong  * @date 2019-09-24 14:10:57  */ class Manger {                   //资源存放集合         private Set<Object> resources = new HashSet<Object>();                   /**          * 申请资源          * @param res1          * @param res2          * @return          */         synchronized boolean applyResources(Object res1, Object res2) {                 if (resources.contains(res1) || resources.contains(res1)) {                         return false;                 } else {                         resources.add(res1);                         resources.add(res2);                         return true;                 }         }                   /**          * 归还资源          * @param res1          * @param res2          */         synchronized void returnResources(Object res1, Object res2) {                 resources.remove(res1);                 resources.remove(res2);         }           }

打印结果如下，线程-1 在线程-0 释放完资源后才能成功申请 res1 和 res2 的锁

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线程：Thread-0 申请 res1、res2 资源成功 线程：Thread-0 获取到 res1 资源的锁 线程：Thread-1 申请 res1、res2 资源失败 线程：Thread-1 申请 res1、res2 资源失败 线程：Thread-1 申请 res1、res2 资源失败 线程：Thread-1 申请 res1、res2 资源失败 线程：Thread-1 申请 res1、res2 资源失败 线程：Thread-0 获取到 res2 资源的锁 线程：Thread-1 申请 res1、res2 资源失败 线程：Thread-1 申请 res1、res2 资源成功 线程：Thread-1 获取到 res1 资源的锁 线程：Thread-1 获取到 res2 资源的锁

使用 Lock 的 tryLock() 方法，获取锁失败释放所有资源，破坏 "不可抢占" 条件示例

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package constxiong.concurrency.a023;    import java.util.Random; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;    /**  * 测试 占有部分资源的线程进一步申请其他资源时，如果申请不到，主动释放它占有的资源，破坏 "不可抢占" 条件  * @author ConstXiong  * @date 2019-09-24 14:50:51  */ public class TestBreakLockOccupation {                   private static Random r = new Random();            private static Lock lock1 = new ReentrantLock();                   private static Lock lock2 = new ReentrantLock();                   public static void main(String[] args) {                 new Thread(() -> {                         //标识任务是否完成                         boolean taskComplete = false;                         while (!taskComplete) {                                 lock1.lock();                                 System.out.println("线程：" + Thread.currentThread().getName() + " 获取锁 lock1 成功");                                 try {                                         //随机休眠，帮助造成死锁环境                                         try {                                                 Thread.sleep(r.nextInt(30));                                         } catch (Exception e) {                                                 e.printStackTrace();                                         }                                                                                   //线程 0 尝试获取 lock2                                         if (lock2.tryLock()) {                                                 System.out.println("线程：" + Thread.currentThread().getName() + " 获取锁 lock2 成功");                                                 try {                                                         taskComplete = true;                                                 } finally {                                                         lock2.unlock();                                                 }                                         } else {                                                 System.out.println("线程：" + Thread.currentThread().getName() + " 获取锁 lock2 失败");                                         }                                 } finally {                                         lock1.unlock();                                 }                                                                   //随机休眠，避免出现活锁                                 try {                                         Thread.sleep(r.nextInt(10));                                 } catch (Exception e) {                                         e.printStackTrace();                                 }                         }                 }).start();                                   new Thread(() -> {                         //标识任务是否完成                         boolean taskComplete = false;                         while (!taskComplete) {                                 lock2.lock();                                 System.out.println("线程：" + Thread.currentThread().getName() + " 获取锁 lock2 成功");                                 try {                                         //随机休眠，帮助造成死锁环境                                         try {                                                 Thread.sleep(r.nextInt(30));                                         } catch (Exception e) {                                                 e.printStackTrace();                                         }                                                                                   //线程2 尝试获取锁 lock1                                         if (lock1.tryLock()) {                                                 System.out.println("线程：" + Thread.currentThread().getName() + " 获取锁 lock1 成功");                                                 try {                                                         taskComplete = true;                                                 } finally {                                                         lock1.unlock();                                                 }                                         } else {                                                 System.out.println("线程：" + Thread.currentThread().getName() + " 获取锁 lock1 失败");                                         }                                 } finally {                                         lock2.unlock();                                 }                                                                   //随机休眠，避免出现活锁                                 try {                                         Thread.sleep(r.nextInt(10));                                 } catch (Exception e) {                                         e.printStackTrace();                                 }                         }                 }).start();         }           }

打印结果如下

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线程：Thread-0 获取锁 lock1 成功 线程：Thread-1 获取锁 lock2 成功 线程：Thread-1 获取锁 lock1 失败 线程：Thread-1 获取锁 lock2 成功 线程：Thread-0 获取锁 lock2 失败 线程：Thread-1 获取锁 lock1 成功 线程：Thread-0 获取锁 lock1 成功 线程：Thread-0 获取锁 lock2 成功

按照一定的顺序加锁，破坏 "循环等待" 条件示例

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package constxiong.concurrency.a023;    /**  * 测试 按序申请资源，破坏 "循环等待" 条件  * @author ConstXiong  * @date 2019-09-24 15:26:23  */ public class TestBreakLockCircleWait {            private static Object res1 = new Object();                   private static Object res2 = new Object();                             public static void main(String[] args) {                 new Thread(() -> {                         Object first = res1;                         Object second = res2;                         //比较 res1 和 res2 的 hashCode,如果 res1 的 hashcode > res2，交换 first 和 second。保证 hashCode 小的对象先加锁                         if (res1.hashCode() > res2.hashCode()) {                                 first = res2;                                 second = res1;                         }                         synchronized (first) {                                 System.out.println("线程：" + Thread.currentThread().getName() + "获取资源 " + first + " 锁成功");                                 try {                                         Thread.sleep(100);                                 } catch (Exception e) {                                         e.printStackTrace();                                 }                                 synchronized(second) {                                         System.out.println("线程：" + Thread.currentThread().getName() + "获取资源 " + second + " 锁成功");                                 }                         }                 }).start();                                   new Thread(() -> {                         Object first = res1;                         Object second = res2;                         //比较 res1 和 res2 的 hashCode,如果 res1 的 hashcode > res2，交换 first 和 second。保证 hashCode 小的对象先加锁                         if (res1.hashCode() > res2.hashCode()) {                                 first = res2;                                 second = res1;                         }                         synchronized (first) {                                 System.out.println("线程：" + Thread.currentThread().getName() + "获取资源 " + first + " 锁成功");                                 try {                                         Thread.sleep(100);                                 } catch (Exception e) {                                         e.printStackTrace();                                 }                                 synchronized(second) {                                         System.out.println("线程：" + Thread.currentThread().getName() + "获取资源 " + second + " 锁成功");                                 }                         }                 }).start();         }           }

打印结果如下

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线程：Thread-0获取资源 java.lang.Object@7447157c 锁成功 线程：Thread-0获取资源 java.lang.Object@7a80f45c 锁成功 线程：Thread-1获取资源 java.lang.Object@7447157c 锁成功 线程：Thread-1获取资源 java.lang.Object@7a80f45c 锁成功