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JAVA死锁

　　死锁是这样一种情形：多个线程同时被阻塞，它们中的一个或者全部都在等待某个资源被释放。由于线程被无限期地阻塞，因此程序不能正常运行。

　　简单的说就是:线程死锁时，第一个线程等待第二个线程释放资源，而同时第二个线程又在等待第一个线程释放资源。假设这种情况一直持续下去，这样就会发生死锁现象。

　导致死锁的根源在于不适当地运用“synchronized”关键词来管理线程对特定对象的访问。“synchronized”关键词的作用是，确保在某个时刻只有一个线程被允许执行特定的代码块，因此，被允许执行的线程首先必须拥有对变量或对象的排他性访问权。当线程访问对象时，线程会给对象加锁，而这个锁导致其它也想访问同一对象的线程被阻塞，直至第一个线程释放它加在对象上的锁。

demo:

public class SyncDeadLock {
    private static Object locka = new Object();
    private static Object lockb = new Object();
 
    public static void main(String[] args) {
        new SyncDeadLock().deadLock();
    }
 
    private void deadLock() {
        Thread thread1 = new Thread(new Runnable() {
            public void run() {
                synchronized (locka) {
                    try {
                        System.out.println(Thread.currentThread().getName()+ " get locka ing!");
                        Thread.sleep(500);
                        System.out.println(Thread.currentThread().getName()+ " after sleep 500ms!");
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName()+ " need lockb!Just waiting!");
                    synchronized (lockb) {
                        System.out.println(Thread.currentThread().getName()+ " get lockb ing!");
                    }
                }
            }
        }, "thread1");
 
        Thread thread2 = new Thread(new Runnable() {
            public void run() {
                synchronized (lockb) {
                    try {
                        System.out.println(Thread.currentThread().getName()+ " get lockb ing!");
                        Thread.sleep(500);
                        System.out.println(Thread.currentThread().getName()+ " after sleep 500ms!");
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName()+ " need locka! Just waiting!");
                    synchronized (locka) {
                        System.out.println(Thread.currentThread().getName()+ " get locking!");
                    }
                }
            }
        }, "thread2");
 
        thread1.start();
        thread2.start();
    }
}