转自：http://blog.csdn.net/dfdsggdgg/article/details/51543545
sun.misc.Unsafe包下载
http://download.csdn.net/detail/dfdsggdgg/9535347
实例化私有类
正常情况下没法实例化一个私有构造函数的类，但是Unsafe可以做到。
import java.lang.reflect.Field;
import sun.misc.Unsafe;
public class UnsafePlayer {
public static void main(String[] args) throws Exception {
//通过反射实例化Unsafe
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
Unsafe unsafe = (Unsafe) f.get(null);
//实例化Player
Player player = (Player) unsafe.allocateInstance(Player.class);
player.setName("li lei");
System.out.println(player.getName());
}
}
class Player{
private String name;
private Player(){}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
} cas原子级操作&&通过内存偏移地址修改变量值
import java.lang.reflect.Field;
import sun.misc.Unsafe;
public class UnsafePlayer {
public static void main(String[] args) throws Exception {
//通过反射实例化Unsafe
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
Unsafe unsafe = (Unsafe) f.get(null);
//实例化Player
Player player = (Player) unsafe.allocateInstance(Player.class);
player.setAge(18);
player.setName("li lei");
for(Field field:Player.class.getDeclaredFields()){
System.out.println(field.getName()+":对应的内存偏移地址"+unsafe.objectFieldOffset(field));
}
System.out.println("-------------------");
int ageOffset= 8;
//修改内存偏移地址为8的值（age）,返回true,说明通过内存偏移地址修改age的值成功
System.out.println(unsafe.compareAndSwapInt(player, ageOffset, 18, 20));
System.out.println("age修改后的值："+player.getAge());
System.out.println("-------------------");
//修改内存偏移地址为8的值，但是修改后不保证立马能被其他的线程看到。
unsafe.putOrderedInt(player, 8, 33);
System.out.println("age修改后的值："+player.getAge());
System.out.println("-------------------");
//修改内存偏移地址为12的值，volatile修饰，修改能立马对其他线程可见
unsafe.putObjectVolatile(player, 12, "han mei");
System.out.println("name修改后的值："+unsafe.getObjectVolatile(player, 12));
}
}
class Player{
private int age;
private String name;
private Player(){}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
} 创建超级数组
java中数组的最大长度为Integer.MAX_VALUE，正常情况下如果想创建一个大于Integer.MAX_VALUE的数组是做不到的，但是Unsafe可以，通过对内存进行直接分配实现。
import java.lang.reflect.Field;
import sun.misc.Unsafe;
public class SuperArray {
public static void main(String[] arg) throws Exception{
//通过反射实例化Unsafe
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
Unsafe unsafe = (Unsafe) f.get(null);
//只要内存够大，可以把这个调大，大于Integer.MAX_VALUE
long size = (long)Integer.MAX_VALUE/2 ;
long addr = unsafe.allocateMemory(size);
System.out.println("unsafe address :"+addr);
for (int i = 0; i < size; i++) {
unsafe.putByte(addr+i, (byte)6);
if(unsafe.getByte(addr+i) !=6){
System.out.println("failed at offset");
}
}
}
}把size调大，size = (long)Integer.MAX_VALUE*2,错误信息如下。
unsafe address :15817328
#
# A fatal error has been detected by the Java Runtime Environment:
#
# EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x61cc0350, pid=31240, tid=31324
#
# JRE version: Java(TM) SE Runtime Environment (8.0_45-b14) (build 1.8.0_45-b14)
# Java VM: Java HotSpot(TM) Client VM (25.45-b02 mixed mode windows-x86 )
# Problematic frame:
# V[thread 30484 also had an error]
[jvm.dll+0x40350]
#
# Failed to write core dump. Minidumps are not enabled by default on client versions of Windows
#
# An error report file with more information is saved as:
# C:\welab\workspace\Person\hs_err_pid31240.log 线程挂起与恢复
将一个线程进行挂起是通过park方法实现的，调用 park后，线程将一直阻塞直到超时或者中断等条件出现。unpark可以终止一个挂起的线程，使其恢复正常。整个并发框架中对线程的挂起操作被封装在 LockSupport类中，LockSupport类中有各种版本pack方法，但最终都调用了Unsafe.park()方法。
public class LockSupport {
/**
* 恢复阻塞线程
*/
public static void unpark(Thread thread) {
if (thread != null)
unsafe.unpark(thread);
}
/**
* 一直阻塞当前线程，调用Unsafe.park()方法
*/
public static void park(Object blocker) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(false, 0L);
setBlocker(t, null);
}
/**
* 阻塞当前线程nanos纳秒
*/
public static void parkNanos(Object blocker, long nanos) {
if (nanos > 0) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(false, nanos);
setBlocker(t, null);
}
}
public static void parkUntil(Object blocker, long deadline) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(true, deadline);
setBlocker(t, null);
}
/**
* 一直阻塞当前线程
*/
public static void park() {
unsafe.park(false, 0L);
}
/**
* 阻塞当前线程nanos纳秒
*/
public static void parkNanos(long nanos) {
if (nanos > 0)
unsafe.park(false, nanos);
}
public static void parkUntil(long deadline) {
unsafe.park(true, deadline);
}
} 最后看下阻塞和恢复的例子
import java.util.concurrent.locks.LockSupport;
public class Lock {
public static void main(String[] args) throws InterruptedException {
ThreadPark threadPark = new ThreadPark();
threadPark.start();
ThreadUnPark threadUnPark = new ThreadUnPark(threadPark);
threadUnPark.start();
//等待threadUnPark执行成功
threadUnPark.join();
System.out.println("运行成功....");
}
static class ThreadPark extends Thread{
public void run(){
System.out.println(Thread.currentThread() +"我将被阻塞在这了60s....");
//阻塞60s，单位纳秒 1s = 1000000000
LockSupport.parkNanos(1000000000l*60);
System.out.println(Thread.currentThread() +"我被恢复正常了....");
}
}
static class ThreadUnPark extends Thread{
public Thread thread = null;
public ThreadUnPark(Thread thread){
this.thread = thread;
}
public void run(){
System.out.println("提前恢复阻塞线程ThreadPark");
//恢复阻塞线程
LockSupport.unpark(thread);
}
}
}