Java直接内存读写的例子

在[b]Hotspot JVM[/b]上，我们能够直接对内存进行读写操作。该类的[b]allocateMemory[/b]方法用于申请分配内存，[b]putAddress[/b]和[b]getAddress[/b]方法用于对直接内存进行读写。

本文将通过sun.misc.Unsafe给出一个直接读写内存的例子。

[color=red][b]注意：[/b][/color][color=blue][b]这只是一个例子，只是用来验证通过sun.misc.Unsafe来实现直接读写内存的可能性。[/b][/color][color=red][b]但是，这样做并没有安全保证，而且稍微有点疏忽将可能导致JVM崩溃。[/b][/color]

Unsafe类的三个方法：allocateMemory，putAddress和getAddress如下：

/**
          * Fetches a native pointer from a given memory address.  If the address is
          * zero, or does not point into a block obtained from {@link
          * #allocateMemory}, the results are undefined.
          *
          * 
 If the native pointer is less than  bits wide, it is extended as
          * an unsigned number to a Java long.  The pointer may be indexed by any
          * given byte offset, simply by adding that offset (as a simple integer) to
          * the long representing the pointer.  The number of bytes actually read
          * from the target address maybe determined by consulting {@link
          * #addressSize}.
          *
          * @see #allocateMemory
          */
         public native long getAddress(long address);

         /**
          * Stores a native pointer into a given memory address.  If the address is
          * zero, or does not point into a block obtained from {@link
          * #allocateMemory}, the results are undefined.
          *
          * 
 The number of bytes actually written at the target address maybe
          * determined by consulting {@link #addressSize}.
          *
          * @see #getAddress(long)
          */
         public native void putAddress(long address, long x);

         /// wrappers for malloc, realloc, free:

         /**
          * Allocates a new block of native memory, of the given size in bytes.  The
          * contents of the memory are uninitialized; they will generally be
          * garbage.  The resulting native pointer will never be zero, and will be
          * aligned for all value types.  Dispose of this memory by calling {@link
          * #freeMemory}, or resize it with {@link #reallocateMemory}.
          *
          * @throws IllegalArgumentException if the size is negative or too large
          *         for the native size_t type
          *
          * @throws OutOfMemoryError if the allocation is refused by the system
          *
          * @see #getByte(long)
          * @see #putByte(long, byte)
          */
         public native long allocateMemory(long bytes);

1. long allocateMemory(long bytes)
申请分配内存
2. long getAddress(long address) 和void putAddress(long address, long x)
对直接内存进行读写。


[color=blue][b]因为Unsafe这个类的访问是受限的，只有rt.jar中的类才能使用Unsafe的功能，它的构造方法是私有的，所以，我们不能通过new来创建实例。但是，可以通过反射的方法来获取Unsafe实例。[/b][/color]

下面就是一个直接访问内存的一个例子：

import java.lang.reflect.Field;

import sun.misc.Unsafe;

public class DirectMemoryAccess {

	public static void main(String[] args) {

		/*
		 * Unsafe的构造函数是私有的，不能通过new来获得实例。
		 * 
		 *  通过反射来获取
		 */
		Unsafe unsafe = null;
		Field field = null;
		try {
			field = sun.misc.Unsafe.class.getDeclaredField("theUnsafe");
			/*
			 * private static final Unsafe theUnsafe = new Unsafe();
			 * 
			 * 因为field的修饰符为 private static final,
			 * 需要将setAccessible设置成true，否则会报java.lang.IllegalAccessException
			 */
			field.setAccessible(true);
			unsafe = (Unsafe) field.get(null);
		} catch (SecurityException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (NoSuchFieldException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalArgumentException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

		long oneHundred = 100;
		byte size = 1;

		/*
		 * 调用allocateMemory分配内存
		 */
		long memoryAddress = unsafe.allocateMemory(size);

		/*
		 * 将100写入到内存中
		 */
		unsafe.putAddress(memoryAddress, oneHundred);

		/*
		 * 内存中读取数据 
		 */
		long readValue = unsafe.getAddress(memoryAddress);

		System.out.println("Val : " + readValue);
	}
}

[b]输出结果：[/b]
[b]Val : 100[/b]

如果，想要查阅Unsafe的源代码，请参考下面的链接.
[url]http://www.docjar.com/html/api/sun/misc/Unsafe.java.html[/url]