整理了几个曾经从网上记录sum.msic.Unsafe类的演示样例。供大家參考：

package com.fish.unsafe;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;

import org.junit.Before;
import org.junit.Test;

import sun.misc.Unsafe;

/**
 * @version 1.0
 * @author Fish
 */
public class UnsafeTest {
	public static Unsafe unsafe;

	@Before
	public void init() throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException {
		// 通过反射获取rt.jar下的Unsafe类
		Field field = Unsafe.class.getDeclaredField("theUnsafe");
		field.setAccessible(true);
		unsafe = (Unsafe) field.get(null);
	}

	/**
	 * allocateInstance初始化对象不调用构造方法
	 * 
	 * @Description:
	 * @throws InstantiationException
	 * @throws IllegalAccessException
	 * @author Fish
	 * @date 2015年6月24日
	 */
	@Test
	public void testAllocateInstance() throws InstantiationException, IllegalAccessException {
		UnsafeClass un = new UnsafeClass(); // constructor
		System.out.println(un.a()); // prints 1
		UnsafeClass uu2 = UnsafeClass.class.newInstance(); // reflection
		System.out.println(uu2.a()); // prints 1
		UnsafeClass uu3 = (UnsafeClass) unsafe.allocateInstance(UnsafeClass.class); // unsafe
		System.out.println(uu3.a());// prints 0
	}

	/**
	 * 改动private变量的值
	 * 
	 * @Description:
	 * @throws NoSuchFieldException
	 * @throws SecurityException
	 * @author Fish
	 * @date 2015年6月24日
	 */
	@Test
	public void testObjectFieldOffset() {
		try {
			UnsafeClass un = new UnsafeClass();
			System.out.println(un.isEqual());
			Field f = un.getClass().getDeclaredField("a");
			unsafe.putInt(un, unsafe.objectFieldOffset(f), 42); // memory
			System.out.println(un.isEqual()); // true, access granted
		} catch (NoSuchFieldException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (SecurityException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

	/**
	 * 获取对象的大小
	 * @Description: 
	 * @author Fish
	 * @date 2015年6月24日
	 */
	@Test
	public void testSizeof() {
		UnsafeClass un = new UnsafeClass();
		System.out.println(sizeOf1(un));
	}

	public static long sizeOf1(Object object) {
		HashSet<Field> fields = new HashSet<Field>();
		Class<?
> c = object.getClass();
		while (c != Object.class) {
			for (Field f : c.getDeclaredFields()) {
				if ((f.getModifiers() & Modifier.STATIC) == 0) {
					fields.add(f);
				}
			}
			c = c.getSuperclass();
		}
		// get offset
		long maxSize = 0;
		for (Field f : fields) {
			long offset = unsafe.objectFieldOffset(f);
			if (offset > maxSize) {
				maxSize = offset;
			}
		}
		return ((maxSize / 8) + 1) * 8; // padding
	}

	/**
	 * 浅拷贝：该方法能够用来拷贝不论什么类型的对象。动态计算它的大小。
	 * 
	 * @Description:
	 * @author Fish
	 * @date 2015年6月24日
	 */
	@Test
	public void testShallowCopy() {
		List<String> list = new ArrayList<String>();
		list.add("123A");
		list.add("123B");
		list.add("123C");
		long size = sizeOf1(list);
		long start = toAddress(list);
		long address = unsafe.allocateMemory(size);
		unsafe.copyMemory(start, address, size);
		System.out.println(fromAddress(address));
	}

	private static long normalize(int value) {
		if (value >= 0)
			return value;
		return (~0L >>> 32) & value;
	}

	public static long toAddress(Object obj) {
		Object[] array = new Object[] { obj };
		long baseOffset = unsafe.arrayBaseOffset(Object[].class);
		return normalize(unsafe.getInt(array, baseOffset));
	}

	static Object fromAddress(long address) {
		Object[] array = new Object[] { null };
		long baseOffset = unsafe.arrayBaseOffset(Object[].class);
		unsafe.putLong(array, baseOffset, address);
		return array[0];
	}

	/**
	 * 密码隐藏：通常我们会将用完的密码设置为null，可是在设置为null到GC垃圾回收该对象，是有一定的时间。在这段时间内，该对象还存在于内存中。
	 * 就在这段时间内。非常可能被黑客利用
	 * 
	 * @Description:
	 * @author Fish
	 * @date 2015年6月24日
	 */
	@Test
	public void testPassword() {
		String password = new String("l00k@myHor$e");
		String fake = new String(password.replaceAll(".", "?"));
		System.out.println(password); // l00k@myHor$e
		System.out.println(fake); // ??
??????
?
?
??
		unsafe.copyMemory(fake, 0L, null, toAddress(password), sizeOf1(password));
		System.out.println(password); // ??
?????
???
??
		System.out.println(fake); // ?????
?
??
????
	}

	 /**
	 * 动态创建类
	 * @Description:
	 * @author Fish
	 * @date 2015年6月24日
	 */
	 @Test
	 public void testDynamicClasses() {
		 try {
			 File f = new File("D:\\A.class");
			 FileInputStream input = new FileInputStream(f);
			 byte[] content = new byte[(int) f.length()];
			 input.read(content);
			 input.close();
                         <span style="color:#FF0000;">// unsafe.defineClass(null, content, 0, content.length);该方法在JDK8找不到</span>
                         Class<?
> c = unsafe.defineClass(null, content, 0, content.length);
			 c.getMethod("a").invoke(c.newInstance(), null); // 1
		 } catch (FileNotFoundException e) {
			 e.printStackTrace();
		 } catch (IOException e) {
			 e.printStackTrace();
		 }
	 }

	/**
	 * 大数组：这样的方式的内存分配不在堆上。且不受GC管理，所以必须小心Unsafe.freeMemory()的使用
	 * @Description: 
	 * @author Fish
	 * @date 2015年6月24日
	 */
	@Test
	public void testBigArray() {
		long SUPER_SIZE = (long) Integer.MAX_VALUE * 2;
		SuperArray array = new SuperArray(SUPER_SIZE);
		System.out.println("Array size:" + array.size()); // 4294967294
		int sum = 0;
		for (int i = 0; i < 100; i++) {
			array.set((long) Integer.MAX_VALUE + i, (byte) 3);
			sum += array.get((long) Integer.MAX_VALUE + i);
		}
		System.out.println("Sum of 100 elements:" + sum); // 300
	}

}

class SuperArray {
	private final static int BYTE = 1;

	private long size;

	private long address;

	public SuperArray(long size) {
		this.size = size;
		address = UnsafeTest.unsafe.allocateMemory(size * BYTE);
	}

	public void set(long i, byte value) {
		UnsafeTest.unsafe.putByte(address + i * BYTE, value);
	}

	public int get(long idx) {
		return UnsafeTest.unsafe.getByte(address + idx * BYTE);
	}

	public long size() {
		return size;
	}
}

class UnsafeClass {
	private long a;

	public UnsafeClass() {
		this.a = 1;
	}

	public long a() {
		return this.a;
	}

	public boolean isEqual() {
		return 42 == a;
	}
}