死锁

【1】Java 中的死锁

  public static void main(String[] arg) {

        DeathLock deathLock = new DeathLock();
        ThreadA threadA = new ThreadA(deathLock);
        threadA.start();
        ThreadB threadB = new ThreadB(deathLock);
        threadB.start();
    }


    static class ThreadA extends Thread {

        private DeathLock deathLock;

        public ThreadA(DeathLock deathLock) {
            this.deathLock = deathLock;
        }

        @Override
        public void run() {
            try {
                deathLock.read();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    static class ThreadB extends Thread {
        private DeathLock deathLock;

        public ThreadB(DeathLock deathLock) {
            this.deathLock = deathLock;
        }

        @Override
        public void run() {
            try {
                deathLock.write();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    static class DeathLock {
        private Object obj1 = new Object();
        private Object obj2 = new Object();

        public void read() throws InterruptedException {
            synchronized (obj1) {
                Thread.sleep(5000);;
                synchronized (obj2) {
                    System.out.println("complete read");
                }
            }
        }

        public void write() throws InterruptedException {
            synchronized (obj2) {
                Thread.sleep(5000);
                synchronized (obj1) {
                    System.out.println("complete write");
                }
            }
        }
    }


结果截图：
jps查看相关信息

结论：由于对资源的相互等待，你等待我的释放，我等待你的释放，最终形成了一个循环闭环。


【2】数据库中的死锁表现

     事务A：

begin tran A

update [Student] set Age=10 where Id=1

waitfor delay '00:00:05'

update [Student] set Age=12 where Id=2

commit tran  A

    事务B：

    

begin tran B

update [Student] set Age=13 where Id=2

waitfor delay '00:00:05'

update [Student] set Age=11 where Id=1

commit tran  B

    结果截图：

           

         

         

   结论：可能会造成数据表的【锁死】，后续的查询陷入无止境的转圈状态，查看表死锁的信息：

SELECT request_session_id spid ,
OBJECT_NAME(resource_associated_entity_id) [Student]
FROM sys.dm_tran_locks
WHERE resource_type = 'OBJECT'

kill 51（spid）  --去除死锁状态

总结：只要不满足死锁的任一条件，可避免死锁。