Why does sun.misc.Unsafe exist, and how can it be used in the real world?

Question

I came across the sun.misc.Unsafe package the other day and was amazed at what it could do.

Of course, the class is undocumented, but I was wondering if there was ever a good reason to use it. What scenarios might arise where you would need to use it? How might it be used in a real-world scenario?

Furthermore, if you do need it, does that not indicate that something is probably wrong with your design?

Why does Java even include this class?

Answer 1

examples

1. 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

2. 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.

3. 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!

4. Compile time optimizations for JVM. HIgh performance VM using "magic", requiring low-level operations. eg: http://en.wikipedia.org/wiki/Jikes_RVM

5. Allocating memory, sun.misc.Unsafe.allocateMemory eg:- DirectByteBuffer constructor internally calls it when ByteBuffer.allocateDirect is invoked

6. Tracing the call stack and replaying with values instantiated by sun.misc.Unsafe, useful for instrumentation

7. 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

8. http://robaustin.wikidot.com/how-to-write-to-direct-memory-locations-in-java

more on references here - http://bytescrolls.blogspot.com/2011/04/interesting-uses-of-sunmiscunsafe.html

Answer 2

Just from running a search in some code search engine I get the following examples:

• Java Object Notation - use it for more efficient array processing, quoting the javadoc

Simple class to obtain access to the {@link Unsafe} object. {@link Unsafe} * is required to allow efficient CAS operations on arrays. Note that the versions in {@link java.util.concurrent.atomic}, such as {@link java.util.concurrent.atomic.AtomicLongArray}, require extra memory ordering guarantees which are generally not needed in these algorithms and are also expensive on most processors.

• SoyLatte - java 6 for osx javadoc excerpt

/** Base class for sun.misc.Unsafe-based FieldAccessors for static fields. The observation is that there are only nine types of fields from the standpoint of reflection code: the eight primitive types and Object. Using class Unsafe instead of generated bytecodes saves memory and loading time for the dynamically-generated FieldAccessors. */

• SpikeSource

/* FinalFields that are sent across the wire .. how to unmarshall and recreate the object on the receiving side? We don't want to invoke the constructor since it would establish values for final fields. We have to recreate the final field exactly like it was on the sender side. The sun.misc.Unsafe does this for us. */

There are many other examples, just follow the above link...