java多线程死锁问题

1、什么是死锁
2、写一个死锁

public class Demo10 {

    /**
     * 写一个死锁
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Thread threada = new Thread(new DeadTest(true));
        Thread threadb = new Thread(new DeadTest(false));
        threada.start();
        threadb.start();}

}
class DeadTest implements Runnable{
    boolean flag;
    DeadTest(boolean flag){
        this.flag = flag;
    }
    public void run(){
         if(flag){
             synchronized(Mylock.locka){
                 System.out.println("if locka");
                 synchronized(Mylock.lockb){
                     System.out.println("if lockb");     
                 }
             }           
         }else{
             synchronized(Mylock.lockb){
                 System.out.println("else lockb");
                 synchronized(Mylock.locka){
                     System.out.println("else locka");   
                 }
             } 
         }  
    }
}
class Mylock{
    static Object locka = new Object();
    static Object lockb = new Object();
}