Thread-wait/notify

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java 线程通过对象的Wait和Notify进行同步，但是存在一定的危险性

其重点是：wait 和 notify 的 时间点

举例：

package com.horizon.thread.aqs;

public class NotSafeWaitNotify {

	private static StringBuffer lock = new StringBuffer();

	public static void main(String[] args) throws Exception {
		// testWaitAfterNotify();
		//testWaitBeforeNotify();
		testWaitAfterNotifyV2();
	}

	/**
	 * 先notify 后wait,不能正常工作
	 */
	private static void testWaitAfterNotify() throws Exception {
		Thread modifyThread = new Thread() {
			@Override
			public void run() {
				try {
					Thread.sleep(1000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}

				synchronized (lock) {
					lock.append("1000");
					lock.notify();
				}
			}
		};

		modifyThread.start();

		Thread.sleep(2000);
		synchronized (lock) {
			lock.wait();
		}

		System.out.println(lock);

	}

	/**
	 * 先notify 后wait,不能正常工作 
	 * 改进思路:先判断是否已经有结果
	 */
	private static void testWaitAfterNotifyV2() throws Exception {
		Thread modifyThread = new Thread() {
			@Override
			public void run() {
				try {
					Thread.sleep(1000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}

				synchronized (lock) {
					lock.append("1000");
					lock.notify();
				}
			}
		};

		modifyThread.start();

		Thread.sleep(2000);
		synchronized (lock) {
			//先判断是否已经有结果
			if (lock.length() < 0) {
				lock.wait();
			}
		}

		System.out.println(lock);

	}

	/**
	 * 先wait 后notify,正常工作
	 */
	private static void testWaitBeforeNotify() throws Exception {
		Thread modifyThread = new Thread() {
			@Override
			public void run() {
				try {
					Thread.sleep(1000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}

				synchronized (lock) {
					lock.append("1000");
					lock.notify();
				}
			}
		};

		modifyThread.start();

		synchronized (lock) {
			lock.wait();
		}

		System.out.println(lock);

	}
}