Unsafe类compareAndSwapInt

package com.wenc.keywords;
 
import java.lang.reflect.Field;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
 
import sun.misc.Unsafe;
 
public class CompareAndSwapTest {
    
    
	
	static class Target{
    
    
		public int value = 10;
	}
	
	public static void main(String[] args) throws SecurityException, NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
    
    
		//通过反射获得Unsafe实例，仅BootstrapClassLoader加载的类
		//（$JAVA_HOME/lib目录下jar包包含的类，如java.util.concurrent.atomic.AtomicInteger）
		//才能通过Unsafe.getUnsafe静态方法获取
		Field field = Unsafe.class.getDeclaredField("theUnsafe");
		field.setAccessible(true);
		Unsafe unsafe = (Unsafe) field.get(null);
		
		//获得Target实例域value
		Field valueField = Target.class.getDeclaredField("value");
		//实例化Target
		Target t = new Target();
		System.out.println("原始value值:" + valueField.get(t));
		
		//获得实例域在class文件里的偏移量
		final long valueOffset = unsafe.objectFieldOffset(valueField);
		
		//第一次swap
		System.out.println("第一次swap(10,20)函数返回值:" + unsafe.compareAndSwapInt(t, valueOffset, 10, 20));
		System.out.println("第一次swap(10,20)后value值:" + valueField.get(t));
				
		//第二次swap
		System.out.println("第一次swap(10,20)函数返回值:" + unsafe.compareAndSwapInt(t, valueOffset, 10, 20));
		System.out.println("第一次swap(10,20)后value值:" + valueField.get(t));		
	}
	
 
}

原始value值:10
第一次swap(10,20)函数返回值:true
第一次swap(10,20)后value值:20
第一次swap(10,20)函数返回值:false
第一次swap(10,20)后value值:20
更新值成功时输出true，反之输出false。感觉说了句废话... sign