not safe.park()和not safe.unpark()用于构建自定义并发控制结构和协作调度机制。

I was recently working on reimplementing the JVM and found that a surprising number of classes are implemented in terms of Unsafe. The class is mostly designed for the Java library implementers and contains features that are fundamentally unsafe but necessary for building fast primitives. For example, there are methods for getting and writing raw field offsets, using hardware-level synchronization, allocating and freeing memory, etc. It is not intended to be used by normal Java programmers; it's undocumented, implementation-specific, and inherently unsafe (hence the name!). Moreover, I think that the SecurityManager will disallow access to it in almost all cases.

简而言之，它的存在主要是为了允许库实现者访问底层机器，而不必在某些类(如AtomicInteger native)中声明每个方法。在常规Java编程中不需要使用或担心它，因为重点是使其余的库足够快，从而不需要这种访问。

我自己没有使用过它，但我认为如果你有一个变量，只是偶尔被多个线程读取(所以你真的不想让它volatile)，你可以在主线程中写入它时使用putObjectVolatile，在从其他线程中进行罕见的读取时使用readObjectVolatile。

堆外集合对于分配大量内存并在使用后立即释放内存而不受GC干扰可能很有用。我基于sun.misc.Unsafe编写了一个用于处理堆外数组/列表的库。

使用它来有效地访问和分配大量的内存，例如在您自己的体素引擎中!(例如《我的世界》风格的游戏。)

In my experience, the JVM is often unable to eliminate bounds-checking in place you truly need it. For example, if you're iterating over a large array, but the actual memory access is tucked underneath a non-virtual* method call in the loop, the JVM may still perform a bounds check with each array access, rather than once just before the loop. Thus, for potentially large performance gains, you can eliminate JVM bounds-checking inside the loop via a method which employs sun.misc.Unsafe to access the memory directly, making sure to do any bounds-checking yourself at the correct places. (You are gonna bounds check at some level, right?) *by non-virtual, I mean the JVM shouldn't have to dynamically resolve whatever your particular method is, because you've correctly guaranteed that class/method/instance are some combination of static/final/what-have-you.

对于我自己开发的体素引擎来说，这在块生成和序列化期间(在我同时读取/写入整个数组的低位置)带来了显著的性能提升。结果可能会有所不同，但如果缺乏界限消除是您的问题，那么这将解决它。

There are some potentially major problems with this: specifically, when you provide the ability to access memory without bounds-checking to clients of your interface, they will probably abuse it. (Don't forget that hackers can also be clients of your interface... especially in the case of a voxel engine written in Java.) Thus, you should either design your interface in a way such that memory access cannot be abused, or you should be extremely careful to validate user-data before it can ever, ever mingle with your dangerous interface. Considering the catastrophic things a hacker can do with unchecked memory access, it's probably best to take both approaches.

类不安全 用于执行低级、不安全操作的方法的集合。虽然类和所有方法都是公共的，但该类的使用是有限的，因为只有可信代码才能获得它的实例。

它的一种用法是在java.util.concurrent.atomic类中:

AtomicIntegerArray AtomicLongArray