线程之“死锁”问题

导致线程死锁原因是：线程同步嵌套，即线程1调用线程2，线程2调用线程1 。

所以要避免同步嵌套。

死锁例子：

/**
 * 死锁出现情况是：一般是嵌套同步造成的。
 * 死锁是线程a访问线程b里面的代码，而线程b则要访问线程a里面的代码，
 * 线程a要访问b则要获得b的锁，b要访问a则要获得a的锁，这就造成了两个
 * 线程互相等待，导致程序不动的结果。
 */
public class DeadLock4 {

	public static void main(String[] args) {
		ThreadDemo t1 = new ThreadDemo(true);
		ThreadDemo t2 = new ThreadDemo(false);
		
		t1.start();
		t2.start();
	}
	
}


class ThreadDemo extends Thread {
	private boolean flag;
	private static final Object OBJ1 = new Object();
	private static final Object OBJ2 = new Object();
	
	public ThreadDemo() {
		
	}
	
	public ThreadDemo(boolean flag) {
		this.flag = flag;
	}
	
	@Override
	public void run() {
		if (flag) {
			synchronized(ThreadDemo.OBJ1) {
				System.out.println("if obj1");
				synchronized(ThreadDemo.OBJ2) {
					System.out.println("if obj2");
				}
			}
		} else {
			synchronized(ThreadDemo.OBJ2) {
				System.out.println("else obj2");
				synchronized(ThreadDemo.OBJ1) {
					System.out.println("else obj1");
				}
			}
		}
	}
}