java read也需要加锁

今天被问到read需不需要加锁，结果没答上来。自己写了一个程序试了一下，答案是肯定的，read加锁是为了保证执行的顺序，让线程不会读到脏数据。

public class TestThread {
	private int a = 1;
	
	public synchronized void add(){
		this.a = this.a + 1;
		System.out.println(Thread.currentThread().getName() + "current a is " + a);
	}
	
	public int get(){
		System.out.println("current a is " + a);
		return a;
	}
	
	public static void main(String[] args) throws Exception{
		final TestThread testThread = new TestThread();
		Thread t1 = new Thread(new Runnable(){
			public void run(){
				testThread.add();
				//make sure the second thread to be executed at this moment
				try {
					Thread.sleep(10000);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				
				testThread.get();
			}
		}
		);
		
		Thread t2 = new Thread(new Runnable(){
			public void run(){
				//make sure the first thread to be started firstly at beginning
				try {
					Thread.sleep(5000);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				testThread.add();
				testThread.get();
			}
		}
		);
		
		t1.start();
		t2.start();
	}
}

output:
Thread-0current a is 2
Thread-1current a is 3
current a is 3
current a is 3

如果对testThread.add();testThread.get();加锁就不会出现刚才读到脏数据的问题。