JAVA CAS技术运用于多线程(Unsafe)

咱们来测试以下代码，注意代码的写法：

一、普通的写法：

if (unsafe_var == 23) {

// System.out.println("我判断出来了，unsafe_var ==23，我设置为46..");

// try {

// //模拟业务代码

// Thread.sleep(1000);

// } catch (InterruptedException e) {

// e.printStackTrace();

// }

// unsafe_var = 46;

// }

能够看到打印出多行：

我判断出来了，unsafe_var ==23，我设置为46..

我判断出来了，unsafe_var ==23，我设置为46..

二、用Unsafe提供的CAS技术，能够这样写：

if (unsafe.compareAndSwapInt(this, intParamOffset, 23, 46)) {

try {

				////模拟业务代码

				Thread.sleep(1000);

			} catch (InterruptedException e) {

				e.printStackTrace();

			}

			System.out.println("我判断出来了，unsafe_var == 23，我设置为46..");

		}

因为原子性，多个线程只会打印一行：

我判断出来了，unsafe_var == 23，我设置为46.

测试代码以下：

package com.xue.gang.volatiler.unsafe;

import java.lang.reflect.Field;

import java.util.concurrent.CountDownLatch;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import sun.misc.Unsafe;

public class UnsafeRunner {

public static void main(String args[]) throws InterruptedException {

	int size = 1000;

	CountDownLatch countDownLatch = new CountDownLatch(size);

	TomUnsafeRunner tomRunner = new TomUnsafeRunner(false, countDownLatch,
			"runner");

	ExecutorService executorService = Executors.newCachedThreadPool();

	for (int i = 1; i <= size; i++) {
		executorService.execute(new Thread2RunUnsafe(countDownLatch,
				tomRunner, i + "_号"));
	}
	countDownLatch.await();
	executorService.shutdown();

	// new Thread(volatileRunner).start();
}

static class Thread2RunUnsafe implements Runnable {
	private CountDownLatch countDownLatch;
	private TomUnsafeRunner tomRunner;
	private String name;

	public Thread2RunUnsafe(CountDownLatch countDownLatch,
			TomUnsafeRunner tomRunner, String name) {
		super();
		this.countDownLatch = countDownLatch;
		this.tomRunner = tomRunner;
		this.name = name;
	}

	public void run() {
		System.out.println(this.name + ":running...");
		this.tomRunner.doWork();
		System.out.println(this.name + ":结束...");
		this.countDownLatch.countDown();

	}
}

static class TomUnsafeRunner {

	volatile boolean shutdownRequested = false;
	// boolean shutdownRequested = false;
	String name;
	int unsafe_var = 23;

	public TomUnsafeRunner(boolean shutdownRequested,
			CountDownLatch countDownLatch, String name) {
		super();
		this.shutdownRequested = shutdownRequested;

		this.name = name;
	}

	public void shutdown() {
		this.shutdownRequested = true;
	}

	public void doWork() {


		////////////////////通常写法///////////////

// if (unsafe_var == 23) {

// System.out.println("我判断出来了，unsafe_var ==23，我设置为46..");

// try {

// //模拟业务代码

// Thread.sleep(1000);

// } catch (InterruptedException e) {

// e.printStackTrace();

// }

// unsafe_var = 46;

// }

/////////////////用JAVA CAS技术
		Unsafe unsafe = UnsafeSupport.getInstance();
		Class clazz = TomUnsafeRunner.class;
		Field[] fields = clazz.getDeclaredFields();
		System.out.println("fieldName:fieldOffset");
		// 获取属性偏移量，能够经过这个偏移量给属性设置
		for (Field f : fields) {
			System.out.println(f.getName() + ":"
					+ unsafe.objectFieldOffset(f));
		}
		// arg0, arg1, arg2, arg3 分别是目标对象实例，目标对象属性偏移量，当前预期值，要设的值
		// unsafe.compareAndSwapInt(arg0, arg1, arg2, arg3)
		// 偏移量编译后通常不会变的,intParam这个属性的偏移量
		// unsafe_var:8
		long intParamOffset = 8;

		if (unsafe.compareAndSwapInt(this, intParamOffset, 23, 46)) {
			try {
				////模拟业务代码
				Thread.sleep(1000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println("我判断出来了，unsafe_var == 23，我设置为46..");
		}
	}
}
static class UnsafeSupport {
	//private static Logger log = Logger.getLogger(UnsafeSupport.class);

	private static Unsafe unsafe;

	static {
		Field field;
		try {
			// 由反编译Unsafe类得到的信息
			field = Unsafe.class.getDeclaredField("theUnsafe");
			field.setAccessible(true);
			// 获取静态属性,Unsafe在启动JVM时随rt.jar装载
			unsafe = (Unsafe) field.get(null);
		} catch (Exception e) {
			//log.error("Get Unsafe instance occur error", e);
		}
	}

	/**
	 * 获取{@link Unsafe }
	 */
	public static Unsafe getInstance() {
		return unsafe;
	}
}

}

运行以上代码，设置Eclipse：

将Windows->Preferences->Java-Complicer->Errors/Warnings->Deprecated and restricted API，中的Forbidden references(access rules)设置为Warning，便可以编译经过。

参考blog:

http://zeige.iteye.com/blog/1182571

http://blog.csdn.net/hengyunabc/article/details/7657934 (做者有个测试，unsafe写入1百万条log用时0.234秒，用java自带的logger，用时7.347秒)