Java直接內存讀寫的例子 - honey01－IT工程師數位筆記本

文章出處

在Hotspot JVM上，我們能夠直接對內存進行讀寫操作。該類的allocateMemory方法用于申請分配內存，putAddress和getAddress方法用于對直接內存進行讀寫。

本文將通過sun.misc.Unsafe給出一個直接讀寫內存的例子。

注意：這只是一個例子，只是用來驗證通過sun.misc.Unsafe來實現直接讀寫內存的可能性。但是，這樣做并沒有安全保證，而且稍微有點疏忽將可能導致JVM崩潰。

Unsafe類的三個方法：allocateMemory，putAddress和getAddress如下：

Java代碼

/**
* 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);

1. long allocateMemory(long bytes)
申請分配內存
2. long getAddress(long address) 和void putAddress(long address, long x)
對直接內存進行讀寫。

因為Unsafe這個類的訪問是受限的，只有rt.jar中的類才能使用Unsafe的功能，它的構造方法是私有的，所以，我們不能通過new來創建實例。但是，可以通過反射的方法來獲取Unsafe實例。

下面就是一個直接訪問內存的一個例子：

Java代碼

import java.lang.reflect.Field;
import sun.misc.Unsafe;
public class DirectMemoryAccess {
public static void main(String[] args) {
/*
* Unsafe的構造函數是私有的，不能通過new來獲得實例。
*
* 通過反射來獲取
*/
Unsafe unsafe = null;
Field field = null;
try {
field = sun.misc.Unsafe.class.getDeclaredField("theUnsafe");
/*
* private static final Unsafe theUnsafe = new Unsafe();
*
* 因為field的修飾符為 private static final,
* 需要將setAccessible設置成true，否則會報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;
/*
* 調用allocateMemory分配內存
*/
long memoryAddress = unsafe.allocateMemory(size);
/*
* 將100寫入到內存中
*/
unsafe.putAddress(memoryAddress, oneHundred);
/*
* 內存中讀取數據
*/
long readValue = unsafe.getAddress(memoryAddress);
System.out.println("Val : " + readValue);
}
}

輸出結果：
Val : 100

如果，想要查閱Unsafe的源代碼，請參考下面的鏈接.
http://www.docjar.com/html/api/sun/misc/Unsafe.java.html

原文轉自：http://blog.csdn.net/joe_007/article/details/38964407