同步的本质在于：一个线程等待另一个线程执行完毕后才可以继续执行。但是，现在相关的几个线程彼此之间都在等待着，那么就会造成死锁。

范例：死锁

package hhh.Test;
class Pen {
    private String pen = "笔" ;
    public String getPen() {
        return pen;
    }
}
class Book {
    private String book = "本" ;
    public String getBook() {
        return book;
    }
}
public class test {
    private static Pen pen=new Pen();
    private static Book book=new Book();
    public static void main(String[] args){

        Thread threadA=new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (pen){
                    System.out.println("我有笔...");
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    synchronized (book){
                        System.out.println("但是我还想要本子...");
                    }
                }
            }
        });
        threadA.setName("threadA");
        threadA.start();

        Thread threadB=new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (book){
                    System.out.println("我有本子...");
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    synchronized (pen){
                        System.out.println("但是我还想要书...");
                    }
                }
            }
        });
        threadB.setName("threadB");
        threadB.start();
    }
}

死锁一旦出现之后，整个程序就将中断执行，所以死锁属于严重性问题。过多的同步会造成死锁，对于资源的上锁一定要注意不要成"环"。

避免死锁：银行家算法