Unsafe的用法

我们写程序，一般不直接使用Unsafe类。 JDK的类特别是并发框架中大量使用到Unsafe的功能，比如：AtomicInteger, LockSupport.所以，了解Unsafe提供的功能与机制是非常必要的。

如何获取Unsafe对象？

		Field theUnsafeInstance = Unsafe.class.getDeclaredField("theUnsafe");
		theUnsafeInstance.setAccessible(true);
		return (Unsafe) theUnsafeInstance.get(Unsafe.class);

功能1：操作对象的属性， 可以绕过private关键字。

		TestUnsafe a = (TestUnsafe) unsafe.allocateInstance(aClass);
		Field f = TestUnsafe.class.getDeclaredField("num");
		long offset = unsafe.objectFieldOffset(f);
		unsafe.compareAndSwapInt(a, offset, 0, 2);
		System.out.println(a);

功能2：直接操作DirectBuffer，高效。

File counters = new File(System.getProperty("java.io.tmpdir"), "counters.deleteme");
		System.out.println(counters.getAbsolutePath());
        counters.deleteOnExit();
        FileChannel fc = new RandomAccessFile(counters, "rw").getChannel();
        MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, 1024);
        long address = ((DirectBuffer) mbb).address();        
        int value = unsafe.getIntVolatile(null, address);
        System.out.println("---" + value);
        System.out.println(unsafe.compareAndSwapInt(null, address, value, 2));

功能3：线程的阻塞与唤醒：

		Thread t = new Thread(new Runnable() {
			@Override
			public void run() {
				System.out.println("线程t阻塞");
				unsafe.park(false, 0l);
				System.out.println("线程t唤醒");
			}
		});
		t.start();
		
		Thread.sleep(10000);
        //线程挂起和恢复
//		unsafe.park(false,1000*1000*1000*100); //nanosecond
		System.out.println("唤醒t！");
		unsafe.unpark(t);

功能4：CAS操作：

		TestUnsafe a = (TestUnsafe) unsafe.allocateInstance(aClass);
		Field f = TestUnsafe.class.getDeclaredField("num");
		long offset = unsafe.objectFieldOffset(f);
		unsafe.compareAndSwapInt(a, offset, 0, 2);
		System.out.println(a);