sun.misc.Unsafe

Remarks#

The Unsafe class allows a program to do things that are not allowed by the Java compiler. Normal programs should avoid using Unsafe.

WARNINGS

If you make a mistake using the Unsafe APIs, your applications are liable to cause the JVM to crash and/or exhibit symptoms that are hard to diagnose.
The Unsafe API is subject to change without notice. If you use it in your code, you may need to rewrite the code when changing Java versions.

Instantiating sun.misc.Unsafe via reflection

Instantiating sun.misc.Unsafe via bootclasspath

Getting Instance of Unsafe

Unsafe is stored as a private field that cannot be accessed directly. The constructor is private and the only method to access public static Unsafe getUnsafe() has privileged access. By use of reflection, there is a work-around to make private fields accessible:

public static final Unsafe UNSAFE;

static {
    Unsafe unsafe = null;

    try {
        final PrivilegedExceptionAction<Unsafe> action = () -> {
            final Field f = Unsafe.class.getDeclaredField("theUnsafe");
            f.setAccessible(true);

            return (Unsafe) f.get(null);
        };

        unsafe = AccessController.doPrivileged(action);
    } catch (final Throwable t) {
        throw new RuntimeException("Exception accessing Unsafe", t);
    }

    UNSAFE = unsafe;
}

Uses of Unsafe

Some uses of unsafe is s follows:

Use	API
Off heap / direct memory allocation, reallocation and deallocation	`allocateMemory(bytes)`, `reallocateMemory(address, bytes)` and `freeMemory(address)`
Memory fences	`loadFence()`, `storeFence()`, `fullFence()`
Parking current thread	`park(isAbsolute, time)`, `unpark(thread)`
Direct field and or memory access	`get` and `put` family of methods
Throwing unchecked exceptions	`throwException(e)`
CAS and Atomic Operations	`compareAndSwap*` family of methods
Setting out memory	`setMemory`
Volatile or concurrent operations	`getVolatile`, `putVolatile`, `putOrdered*`

The get and put family of methods are relative to a given object. If the object is null then it is treated as an absolute address.

// Putting a value to a field
protected static long fieldOffset = UNSAFE.objectFieldOffset(getClass().getField("theField"));
UNSAFE.putLong(this, fieldOffset , newValue);

// Puting an absolute value
 UNSAFE.putLong(null, address, newValue);
 UNSAFE.putLong(address, newValue);

Some methods are only defined for int and longs. You can use these methods on floats and doubles using floatToRawIntBits, intBitsToFloat, doubleToRawLongBits, longBitsToDouble`