Java Unsafe compareAndSwap 并发

热度 ℃

使用Unsafe.compareAndSwapXXX方法来实现并发，它是原子的并且可以用来实现高性能的数据结构。

下面是一个例子，被很多线程使用的共享对象的值的问题。

Counter

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interface Counter {
    void increment();
    long getCounter();
}

Counter 实例

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static class CASCounter implements Counter {
	private volatile long counter = 0;
	private Unsafe unsafe;
	private long offset;
	public CASCounter() throws Exception {
		unsafe = getUnsafe();
		offset = unsafe.objectFieldOffset(CASCounter.class.getDeclaredField("counter"));
	}
	@Override
	public void increment() {
		long before = counter;
		while (!unsafe.compareAndSwapLong(this, offset, before, before + 1)) {
			before = counter;
		}
	}
	@Override
	public long getCounter() {
		return counter;
	}
}
static class AtomicCounter implements Counter {
	AtomicLong counter = new AtomicLong(0);
	@Override
	public void increment() {
		counter.incrementAndGet();
	}
	@Override
	public long getCounter() {
		return counter.get();
	}
}
static class LockCounter implements Counter {
	private volatile long counter = 0;
	private WriteLock lock = new ReentrantReadWriteLock().writeLock();
	@Override
	public void increment() {
		lock.lock();
		counter++;
		lock.unlock();
	}
	@Override
	public long getCounter() {
		return counter;
	}
}
static class SyncCounter implements Counter {
	private volatile long counter = 0;
	@Override
	public synchronized void increment() {
		counter++;
	}
	@Override
	public long getCounter() {
		return counter;
	}
}
static class StupidCounter implements Counter {
	private volatile long counter = 0;
	@Override
	public void increment() {
		counter++;
	}
	@Override
	public long getCounter() {
		return counter;
	}
}
static class CounterClient implements Runnable {
	private volatile Counter c;
	private int num;
	public CounterClient(Counter c, int num) {
		this.c = c;
		this.num = num;
	}
	@Override
	public void run() {
		for (int i = 0; i < num; i++) {
			c.increment();
		}
	}
}

Test

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public static void main(String[] args) throws Exception {
	int NUM_OF_THREADS = 200;
	int NUM_OF_INCREMENTS = 100000;
	ExecutorService service = Executors.newFixedThreadPool(NUM_OF_THREADS);
	Counter counter = null;
	counter = new StupidCounter(); // 创建counter对象
	// Counter result: 2087134
	// Time passed in ms:469
	counter = new SyncCounter(); // 创建counter对象
	// Counter result: 10000000
	// Time passed in ms:1407
	counter = new LockCounter(); // 创建counter对象
	// Counter result: 10000000
	// Time passed in ms:1259
	counter = new AtomicCounter(); // 创建counter对象
	// Counter result: 10000000
	// Time passed in ms:3361
	counter = new CASCounter(); // 创建counter对象
	// Counter result: 10000000
	// Time passed in ms:3939
	long before = System.currentTimeMillis();
	for (int i = 0; i < NUM_OF_THREADS; i++) {
		service.submit(new CounterClient(counter, NUM_OF_INCREMENTS));
	}
	service.shutdown();
	service.awaitTermination(1, TimeUnit.MINUTES);
	long after = System.currentTimeMillis();
	System.out.println("Counter result: " + counter.getCounter());
	System.out.println("Time passed in ms:" + (after - before));
}

参考

Link