Java直接内存读写的例子
在Hotspot JVM上，我们能够直接对内存进行读写操作。该类的allocateMemory方法用于申请分配内存，putAddress和getAddress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。
注意：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。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。
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. * * <p> 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. * * <p> 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); 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 blocke.printStackTrace();} catch (NoSuchFieldException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalArgumentException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalAccessException e) {// TODO Auto-generated catch blocke.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);}}