synchronized线程同步问题

//多线程不同步的解决办法-锁机制
/*第一个线程第2个线程交替对共享数据进行操作，线程不同步导致错误，运用锁机制实现线程的同步。*/
public class ThreadNoSynchronized {

	public static void main(String[] args) {
		ShareData shareData = new ShareData();
		ThreadDemo td1 = new ThreadDemo("Thread1", shareData);
		ThreadDemo td2 = new ThreadDemo("Thread2", shareData);
		td1.start();
		td2.start();
	}

}

class ShareData {
	public static String shareData = "";
}

class ThreadDemo extends Thread {
	//声明ShareData数据域
	private ShareData demoShare;

	public ThreadDemo() {
		//声明，实现无参数构造方法
	}

	ThreadDemo(String szName, ShareData demoShare) {
		//调用父类构造方法
		super(szName);
		//初始化demoShare域
		this.demoShare = demoShare;
	}

	public void run() {
		System.out.println(this.getName() + ":" + ShareData.shareData);
		
		//同步快，并指出同步数据 demoShare
		synchronized (demoShare) {//key :synchronized 指定同步代码块，给demoShare加锁
			
			for (int i = 0; i < 5; i++) {
				if (this.getName().equals("Thread1")) {  //当前线程是Thread1
					ShareData.shareData = "这是第1个线程";
					try {
						// 线程休眠
						Thread.sleep((int) Math.random() * 50); 

					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					System.out.println(this.getName() + ":"
							+ ShareData.shareData);
				} else if (this.getName().equals("Thread2")) {//当前线程是Thread2
					ShareData.shareData = "这是第2个线程";
					try {
						// 线程休眠
						Thread.sleep((int) Math.random() * 50);

					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					System.out.println(this.getName() + ":"
							+ ShareData.shareData);
				}

			}
		}
	}

}