从这里稍作修改：
import sun.misc.Unsafe;
import java.lang.reflect.*;
public class K {
private static Unsafe unsafe;
private static int fieldOffset;
private static K instance = new K();
static {
try {
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
unsafe = (Unsafe) f.get(null);
fieldOffset = unsafe.fieldOffset(K.class.getDeclaredField("obj"));
} catch (Exception ex) {
throw new RuntimeException(ex);
}
}
private Object obj;
public synchronized static long toAddress(int o) {
instance.obj = o;
return unsafe.getLong(instance, fieldOffset);
}
public synchronized static Object toObject(long address) {
unsafe.putLong(instance, fieldOffset, address);
return instance.obj;
}
public static void main(String[] args) {
int x = 20;
System.err.println("x was = " + x);
long addr = toAddress(x);
System.err.println("&x = " + addr);
x = 70;
System.out.println("x is now " + x);
long newAddr = toAddress(x);
System.err.println("new (&x) = " + newAddr);
System.out.println("Previous value of x is " + toObject(addr)); // !!!
}
}
我特别担心标有的行!!!。如果原始x垃圾被收集起来，那么其地址可能毫无价值。
我的问题是，JVM segfault还是会因（toObject(BOGUS_VALUE)总是NullPointerException被捕获）而失败？
我敢肯定你们大多数人都知道，这个Unsafe文件充其量是最多的。
我添加了一些可能在尝试检索丢失的对象之前可能导致GC的代码。
for (int i = 0; i < 1000000; i ++) {
new Object().toString();
}
System.out.println("Previous value of x is " + toObject(addr).getClass()); // !!!
并得到了
# SIGSEGV (0xb) at pc=0x000000010e627c46, pid=49698, tid=6403
您所拥有的地址可能不再指向可以被视为对象的内容。是的Unsafe。
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我来说两句