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。
下面将通过sun.misc.Unsafe演示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:
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
3.putByte、getByte方法,这两个方法deprecated了
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。
实例一:
package my.memory;
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);
}
}
输出结果:
Val : 100
实例二:
package my.memory;
import java.lang.reflect.Field;
import java.util.Arrays;
import sun.misc.Unsafe;
public class DirectMemoryTest {
private static int byteArrayBaseOffset;
public static void main(String[] args) throws SecurityException, NoSuchFieldException, IllegalArgumentException,
IllegalAccessException {
Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafe.setAccessible(true);
Unsafe UNSAFE = (Unsafe) theUnsafe.get(null);
System.out.println(UNSAFE);
byte[] data = new byte[10];
System.out.println(Arrays.toString(data));
byteArrayBaseOffset = UNSAFE.arrayBaseOffset(byte[].class);
System.out.println("byteArrayBaseOffset:" + byteArrayBaseOffset);
UNSAFE.putByte(data, byteArrayBaseOffset, (byte) 1);
UNSAFE.putByte(data, byteArrayBaseOffset + 5, (byte) 5);
System.out.println(Arrays.toString(data));
}
}
运行结果:
sun.misc.Unsafe@de6ced [0, 0, 0, 0, 0, 0, 0, 0, 0, 0] byteArrayBaseOffset:12 [1, 0, 0, 0, 0, 5, 0, 0, 0, 0]
实例三:
package my.memory;
import java.lang.reflect.Field;
import sun.misc.Unsafe;
public class SuperArray {
private final static int BYTE = 1;
private long size;
private long address;
public SuperArray(long size) {
this.size = size;
//得到分配内存的起始地址
address = getUnsafe().allocateMemory(size * BYTE);
}
public void set(long i, byte value) {
getUnsafe().putByte(address + i * BYTE, value);
}
public int get(long idx) {
return getUnsafe().getByte(address + idx * BYTE);
}
public long size() {
return size;
}
private static Unsafe getUnsafe() {
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();
}
return unsafe;
}
public static void main(String[] args) {
int sum = 0;
long SUPER_SIZE = (long)Integer.MAX_VALUE * 2;
SuperArray array = new SuperArray(SUPER_SIZE);
System.out.println("Array size:" + array.size()); // 4294967294
for (int i = 1; i <= 100; i++) {
array.set((long)i, (byte)3);
sum += array.get((long)i);
}
System.out.println("Sum of 100 elements:" + sum); // 300
}
}
运行结果:
Array size:4294967294 Sum of 100 elements:300
参考文章:
http://mouselearnjava.iteye.com/blog/1922390
http://tech.ddvip.com/2014-06/1403069715211221_2.html
PS:Unsafe的源代码,请参考下面的链接
http://www.docjar.com/html/api/sun/misc/Unsafe.java.html