整理了幾個曾經從網上記錄sum.msic.Unsafe類的演示樣例。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 {
		// 通過反射獲取rt.jar下的Unsafe類
		Field field = Unsafe.class.getDeclaredField("theUnsafe");
		field.setAccessible(true);
		unsafe = (Unsafe) field.get(null);
	}

	/**
	 * allocateInstance初始化對象不調用構造方法
	 * 
	 * @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
	}

	/**
	 * 改動private變量的值
	 * 
	 * @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();
		}
	}

	/**
	 * 獲取對象的大小
	 * @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
	}

	/**
	 * 淺拷貝：該方法能夠用來拷貝不論什麽類型的對象。動態計算它的大小。
	 * 
	 * @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];
	}

	/**
	 * 密碼隱藏：通常我們會將用完的密碼設置為null，可是在設置為null到GC垃圾回收該對象，是有一定的時間。在這段時間內，該對象還存在於內存中。
	 * 就在這段時間內。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("[email protected]