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。

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。

通过在一些代码搜索引擎中运行搜索，我得到了以下示例:

Java对象符号——使用它来进行更有效的数组处理，引用javadoc

类来获取对{@link Unsafe}对象的访问。{@link安全} 为了允许对数组进行有效的CAS操作，*是必需的。注意 {@link java.util.concurrent中的版本。原子}，例如{@link java.util.concurrent.atomic。AtomicLongArray}，需要额外的内存排序 这些保证在这些算法中通常是不需要的 在大多数处理器上都很昂贵。

SoyLatte - java 6 for osx javadoc excerpt

/** sun.misc的基类基于静态的不安全的FieldAccessors 字段。据观察，只有9种类型的 从反射代码的角度看字段:八个原语 类型和对象。使用不安全类而不是生成类 字节码节省内存和加载时间 动态生成的FieldAccessors。* /

SpikeSource

／* 通过线路发送的FinalFields ..如何解编和重新创建对象上 接收方?我们不想调用构造函数，因为它将为 最后一个字段。我们必须重新创建与发送端完全相同的最终字段。 太阳，杂项，不安全为我们做了这些。 * /

还有很多其他的例子，只要按上面的链接…

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

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。

为了有效地复制内存(至少对于短块，复制速度比System.arraycopy()快);由Java LZF和Snappy编解码器使用。它们使用'getLong'和'putLong'，这比逐字节复制要快;在复制16/32/64字节块时尤其有效。