sun.misc.Unsafe 操作数组(原创)

sun.misc.Unsafe 操作数组的例子, 参照 jdk 的 java.util.concurrent.ConcurrentHashMap的源代码

package org.fantasy.unsafe;

public interface Array {

	/**
	 * 根据下标获取指定的元素
	 * @param index
	 * @return
	 */
	public T getObject(int index);
	/**
	 * 根据下标修改指定的元素
	 * @param index
	 * @param element
	 * @return
	 */
	public boolean setObject(int index, T element);
	/**
	 * 添加元素
	 * @param element
	 * @return
	 */
	public boolean addObject(T element);
	/**
	 * 删除元素
	 * @param element
	 * @return
	 */
	public T remove(T element);
	/**
	 * 根据下标删除元素
	 * @param index
	 * @return
	 */
	public boolean remove(int index);

}

 

package org.fantasy.unsafe;

import java.lang.reflect.Field;
import java.util.Arrays;

import sun.misc.Unsafe;

public class ArrayUnsafe implements Array {
	
	private static final int DEFAULT_INITIAL_CAPACITY = 10;
	private transient volatile Object[] elements;
	private volatile int size = 0;

	private static final Unsafe UNSAFE;
	private static final long ARRAY_OFFSET;
	private static final int ARRAY_SHIFT;
	private static final int INDEX_SCALE;
	private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
	private static final long SIZE_OFFSET;

	static {
		try {
			Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
			unsafeField.setAccessible(true);
			UNSAFE = (Unsafe)unsafeField.get(null);
			ARRAY_OFFSET = UNSAFE.arrayBaseOffset(Object[].class);
			INDEX_SCALE = UNSAFE.arrayIndexScale(Object[].class);
			ARRAY_SHIFT = 31 - Integer.numberOfLeadingZeros(INDEX_SCALE);
			SIZE_OFFSET = UNSAFE.objectFieldOffset(ArrayUnsafe.class.getDeclaredField("size"));
		} catch (Exception e) {
			throw new Error(e);
		}
	}

	public ArrayUnsafe() {
		this(DEFAULT_INITIAL_CAPACITY);
	}
	
	public ArrayUnsafe(int initialCapacity) {
		this.elements = new Object[initialCapacity];
	}

	public T getObject(int index) {
		return (T)UNSAFE.getObjectVolatile(elements, getIndexScale(index));
	}

	public boolean setObject(int index, T element) {
		rangeCheck(index);
		T oldElement = getObject(index);
		UNSAFE.putOrderedObject(elements, getIndexScale(index), element);
		if(oldElement != null && element == null) {
			int numMoved = size - index - 1;
			if(numMoved > 0) {
				System.arraycopy(elements, index + 1, elements, index, numMoved);
			}
			UNSAFE.compareAndSwapInt(this, SIZE_OFFSET, size, size - 1);
			UNSAFE.putOrderedObject(elements, getIndexScale(size), null);
		}
		return true;
	}
	
	private long getIndexScale(long index) {
		return (index << ARRAY_SHIFT) + ARRAY_OFFSET;
	}
	
	private void ensureCapacityInternal(int minCapacity) {
		if(minCapacity - elements.length > 0)
			grow(minCapacity);
	}
	
	private void grow(int minCapacity) {
		int oldCapacity = elements.length;
		int newCapacity = oldCapacity + (oldCapacity >> 1);
		if(newCapacity - minCapacity < 0)
			newCapacity = minCapacity;
		if(newCapacity - MAX_ARRAY_SIZE > 0)
			newCapacity = hugeCapacity(minCapacity);
		elements = Arrays.copyOf(elements, newCapacity);
	}
	
	private static int hugeCapacity(int minCapacity) {
		if(minCapacity < 0)
			throw new OutOfMemoryError();
		return minCapacity > MAX_ARRAY_SIZE ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
	}

	public boolean addObject(T element) {
		ensureCapacityInternal(size + 1);
		UNSAFE.putOrderedObject(elements, getIndexScale(size), element);
		UNSAFE.compareAndSwapInt(this, SIZE_OFFSET, size, size + 1);
		return true;
	}

	private void rangeCheck(int index) {
		if(index >= size || index < 0) {
			throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
		}
	}

	public T remove(T element) {
		for(int i = 0; i < elements.length; i++) {
			int index = i;
			T e = getObject(index);
			if(e != null) {
				if(e.equals(element)) {
					setObject(index, null);
					return e;
				}
			}
		}
		return null;
	}

	public boolean remove(int index) {
		rangeCheck(index);
		T removedElement = getObject(index);
		if(removedElement != null)
			return setObject(index, null);
		return false;
	}

	public static void main(String[] args) {
		Array strArray = new ArrayUnsafe();
		for(int i = 0; i < 10; i++) {
			strArray.addObject(String.valueOf(i));
		}
		strArray.addObject("hello");
		strArray.addObject("world");
		String s0 = strArray.getObject(0);// get "0"
		String s2 = strArray.getObject(2);// get "2"
		strArray.setObject(1, null);
		String s6 = strArray.remove("6"); // remove "6"
		String sNull = strArray.remove("foo");// null
		boolean result = strArray.remove(9);
	}
}