sun.misc.Unsafe


实例化一个类，不调用构造函数，不会初始化
修改私有字段 
抛出checked异常，方法签名上不需要声明
使用堆外内存  分配释放操作堆外内存
CAS操作 ， AtomicInteger  使用 unsafe compareAndSwapInt 方法
阻塞/唤醒线程   park/unpark



Unsafe park() 阻塞线程
Unsafe unpark() 唤醒线程


使用 unsafe 分配释放堆外内存
long address = unsafe.allocateMemory(1024);
unsafe.freeMemory(address+2048);
越界释放会发生错误

java(63084,0x70000021a000) malloc: *** error for object 0x7fdcab002400: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
        



例子代码

import java.lang.reflect.Field;

import sun.misc.Unsafe;

public class UnsafeTest {
		
	private static class Pet {
		private int age = 9;
		
		public int getAge(){
			return age;
		}
	}
	
	public static void main(String[] args) throws Exception {
		Unsafe unsafe = null;
		try{
			unsafe = Unsafe.getUnsafe();
		}catch(Throwable e){
			System.out.println("getUnsafe error,"+e);
		}
		// 反射获取
		Field f = Unsafe.class.getDeclaredField("theUnsafe");
        f.setAccessible(true);
        unsafe = (Unsafe) f.get(null);
        System.out.println("unsafe="+unsafe);
        
        Pet pet = new Pet();
        System.out.println("age="+pet.getAge());
        
        Pet pet2 = (Pet) unsafe.allocateInstance(Pet.class);
        // 0 , 只给对象分配内存 , 不会初始化
        System.out.println("age2="+pet2.getAge());

        // 直接设置私有变量值
        Field ageField = Pet.class.getDeclaredField("age");
        unsafe.putInt(pet2, unsafe.objectFieldOffset(ageField), 3);
        System.out.println("age3="+pet2.getAge());

 
        try{
        	// Unsafe抛出checked异常不需要方法签名上声明
        	unsafeThrowCheckedException(unsafe);
        }catch(Exception e){
            System.out.println(e);
        }
        
        long address = unsafe.allocateMemory(1024);
        unsafe.putInt(address, 7);
        unsafe.putInt(address+4, 8);

        System.out.println(unsafe.getInt(address));
        System.out.println(unsafe.getInt(address+1));
        System.out.println(unsafe.getInt(address+2));
        System.out.println(unsafe.getInt(address+3));
        System.out.println(unsafe.getInt(address+4));
        System.out.println(unsafe.getInt(address+5));
        //System.out.println(unsafe.getInt(address+2048));
        
        unsafe.freeMemory(address);
        // java(63084,0x70000021a000) malloc: *** error for object 0x7fdcab002400: pointer being freed was not allocated
        // *** set a breakpoint in malloc_error_break to debug
        // unsafe.freeMemory(address+2048);

	}
	
	private static void unsafeThrowCheckedException(Unsafe unsafe){
		unsafe.throwException(new Exception("unsafeThrowCheckedException test"));
	}

}