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。

例子

VM "intrinsification." ie CAS (Compare-And-Swap) used in Lock-Free Hash Tables eg:sun.misc.Unsafe.compareAndSwapInt it can make real JNI calls into native code that contains special instructions for CAS read more about CAS here http://en.wikipedia.org/wiki/Compare-and-swap The sun.misc.Unsafe functionality of the host VM can be used to allocate uninitialized objects and then interpret the constructor invocation as any other method call. One can track the data from the native address.It is possible to retrieve an object’s memory address using the java.lang.Unsafe class, and operate on its fields directly via unsafe get/put methods! Compile time optimizations for JVM. HIgh performance VM using "magic", requiring low-level operations. eg: http://en.wikipedia.org/wiki/Jikes_RVM Allocating memory, sun.misc.Unsafe.allocateMemory eg:- DirectByteBuffer constructor internally calls it when ByteBuffer.allocateDirect is invoked Tracing the call stack and replaying with values instantiated by sun.misc.Unsafe, useful for instrumentation sun.misc.Unsafe.arrayBaseOffset and arrayIndexScale can be used to develop arraylets,a technique for efficiently breaking up large arrays into smaller objects to limit the real-time cost of scan, update or move operations on large objects http://robaustin.wikidot.com/how-to-write-to-direct-memory-locations-in-java

更多的参考资料在这里- http://bytescrolls.blogspot.com/2011/04/interesting-uses-of-sunmiscunsafe.html

有趣的是，我甚至从未听说过这个类(这可能是一件好事，真的)。

我想到的一件事是使用Unsafe#setMemory将包含敏感信息的缓冲区归零(密码、密钥等)。您甚至可以对“不可变”对象的字段执行此操作(然后，我再次假设普通的反射也可以在这里执行此操作)。但我不是安全专家，所以对此持保留态度。

对象的可用性似乎低于Java代码通常允许的级别。如果您正在编写一个高级应用程序，那么JVM将内存处理和其他操作从代码级别抽象出来，因此更容易编程。通过使用不安全库，您可以有效地完成通常由您完成的低级操作。

正如woliveirajr所述，“random()”使用Unsafe来播种，就像许多其他操作将使用Unsafe中包含的allocateMemory()函数一样。

作为程序员，你可能永远不需要这个库，但严格控制底层元素确实很方便(这就是为什么在主要产品中仍然有汇编代码和(在较小程度上)C代码的原因)

基于对使用eclipse进行引用跟踪的Java 1.6.12库的简要分析，似乎Unsafe的每个有用功能都以有用的方式公开了。

CAS操作通过Atomic*类公开。 内存操作函数通过DirectByteBuffer公开 同步指令(park,unpark)通过AbstractQueuedSynchronizer公开，而AbstractQueuedSynchronizer又由Lock实现使用。

它使用的一个例子是random方法，该方法调用不安全的方法来更改种子。

这个网站也有它的一些用途。