多线程之：死锁

死锁指两个或者多个线程持有锁的同时并等待对方持有的锁，导致无限期等待的情况，通常发生于以不同顺序请求同一组锁。

两个线程以不同顺序获取一组锁会导致死锁，如：

public class DeadLockTest {

    public static class LockGroup{
        private  Object objA=new Object();
        private  Object objB=new Object();
        public  Object getObjA() {
            return objA;
        }
        public Object getObjB() {
            return objB;
        }
        
    }
    
    public static void main(String[] args) throws Exception {
        final LockGroup group=new LockGroup();
        Thread threadA=new Thread(new  Runnable() {
            public void run() {
                synchronized (group.getObjA()) {
                    System.out.println("try to get lockb......");
                    synchronized (group.getObjB()) {
                        System.out.println("get all locks now......");
                    }
                }
            }
        });
        Thread threadB=new Thread(new  Runnable() {
            public void run() {
                synchronized (group.getObjB()) {
                    System.out.println("try to get locka......");
                    synchronized (group.getObjA()) {
                        System.out.println("get all locks now......");
                    }
                }
            }
        });
        threadA.start();
        threadB.start();
        threadA.join();
        threadB.join();//将会发生死锁
    }
}