并发编程陷阱系列(七)读多写少使用synchronized导致性能下降

对并发读的情况进行测试：

public class SynchronizedDemo {
	static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

	public static void main(String[] args) throws Exception {
		Data data = new Data();
		Worker t1 = new Worker(data, true, "t1");
		Worker t2 = new Worker(data, true, "t2");
		t1.start();

		t2.start();
	}

	static class Worker extends Thread {
		Data data;
		boolean read;

		public Worker(Data data, boolean read, String threadName) {
			super(threadName);
			this.data = data;
			this.read = read;
		}

		public void run() {
			if (read)
				data.get();
			else
				data.set();
		}
	}

	/**
	 * 数据类
	 */
	static class Data {
		/**
		 * 写数据
		 */
		public synchronized void set() {
			System.out.println(Thread.currentThread().getName() + " set:begin "
					+ sdf.format(new Date()));
			try {
				Thread.sleep(5000);
			} catch (Exception e) {

			} finally {
				System.out.println(Thread.currentThread().getName()
						+ " set:end " + sdf.format(new Date()));
			}

		}

		/**
		 * 读数据
		 */
		public synchronized int get() {
			System.out.println(Thread.currentThread().getName()
					+ " get :begin " + sdf.format(new Date()));
			try {
				Thread.sleep(5000);
			} catch (Exception e) {

			} finally {
				System.out.println(Thread.currentThread().getName()
						+ " get :end " + sdf.format(new Date()));
			}
			return 1;
		}
	}
}

 synchronized执行的结果：

t1 get :begin 2013-05-06 22:57:50
t1 get :end 2013-05-06 22:57:55
t2 get :begin 2013-05-06 22:57:55
t2 get :end 2013-05-06 22:58:00
t1先执行,t1 结束后;t2再开始执行,直到结束。对象的方法中一旦加入synchronized修饰，则任何时刻只能有一个线程访问synchronized修饰的方法，所以我们看到结果中是串行执行的。

public class ReadWriteLockDemo {
	static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

	public static void main(String[] args) throws Exception {
		Data data = new Data();
		Worker t1 = new Worker(data, true, "t1");
		Worker t2 = new Worker(data, true, "t2");
		t1.start();
		t2.start();
	}

	static class Worker extends Thread {
		Data data;
		boolean read;

		public Worker(Data data, boolean read, String threadName) {
			super(threadName);
			this.data = data;
			this.read = read;
		}

		public void run() {
			if (read)
				data.get();
			else
				data.set();
		}
	}

	/**
	 * 数据类
	 */
	static class Data {
		ReadWriteLock lock = new ReentrantReadWriteLock();
		Lock read = lock.readLock();
		Lock write = lock.writeLock();

		/**
		 * 写数据
		 */
		public void set() {
			write.lock();
			System.out.println(Thread.currentThread().getName()
					+ " set:begin " + sdf.format(new Date()));
			try {
				Thread.sleep(5000);
			} catch (Exception e) {

			} finally {
				System.out.println(Thread.currentThread().getName()
						+ " set:end " + sdf.format(new Date()));
				write.unlock();
			}

		}

		/**
		 * 读数据
		 */
		public int get() {
			read.lock();
			System.out.println(Thread.currentThread().getName()
					+ " get :begin " + sdf.format(new Date()));
			try {
				Thread.sleep(5000);
			} catch (Exception e) {

			} finally {
				System.out.println(Thread.currentThread().getName()
						+ " get :end " + sdf.format(new Date()));
				read.unlock();
			}
			return 1;
		}
	}
}

使用读锁的执行结果：

t1 get :begin 2013-05-06 23:00:41
t2 get :begin 2013-05-06 23:00:41
t1 get :end 2013-05-06 23:00:46
t2 get :end 2013-05-06 23:00:46

这两个线程同时开始，同时结束。读锁允许多个线程共同读取资源（没有其他线程获取到写锁或者没有正在写数据）。