Java为什么会引入及如何使用Unsafe

import sun.misc.Unsafe;
import sun.nio.ch.DirectBuffer;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.reflect.Field;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
public class PingPongMapMain {
public static void main(String... args) throws IOException {
boolean odd;
switch (args.length < 1 ? "usage" : args[0].toLowerCase()) {
case "odd":
odd = true;
break;
case "even":
odd = false;
break;
default:
System.err.println("Usage: java PingPongMain [odd|even]");
return;
}
int runs = 10000000;
long start = 0;
System.out.println("Waiting for the other odd/even");
File counters = new File(System.getProperty("java.io.tmpdir"), "counters.deleteme");
counters.deleteOnExit();
try (FileChannel fc = new RandomAccessFile(counters, "rw").getChannel()) {
MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, 1024);
long address = ((DirectBuffer) mbb).address();
for (int i = -1; i < runs; i++) {
for (; ; ) {
long value = UNSAFE.getLongVolatile(null, address);
boolean isOdd = (value & 1) != 0;
if (isOdd != odd)
// wait for the other side.
continue;
// make the change atomic, just in case there is more than one odd/even process
if (UNSAFE.compareAndSwapLong(null, address, value, value + 1))
break;
}
if (i == 0) {
System.out.println("Started");
start = System.nanoTime();
}
}
}
System.out.printf("... Finished, average ping/pong took %,d ns%n",
(System.nanoTime() - start) / runs);
}
static final Unsafe UNSAFE;
static {
try {
Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafe.setAccessible(true);
UNSAFE = (Unsafe) theUnsafe.get(null);
} catch (Exception e) {
throw new AssertionError(e);
}
}
}