sun.misc.Unsafe

Instantiating sun.misc.Unsafe via reflection

public static Unsafe getUnsafe() {
    try {
        Field unsafe = Unsafe.class.getDeclaredField("theUnsafe");
        unsafe.setAccessible(true);
        return (Unsafe) unsafe.get(null);
    } catch (IllegalAccessException e) {
        // Handle
    } catch (IllegalArgumentException e) {
        // Handle
    } catch (NoSuchFieldException e) {
        // Handle
    } catch (SecurityException e) {
        // Handle
    }
}

sun.misc.Unsafe has a Private constructor, and the static getUnsafe() method is guarded with a check of the classloader to ensure that the code was loaded with the primary classloader. Therefore, one method of loading the instance is to use reflection to get the static field.

Instantiating sun.misc.Unsafe via bootclasspath

public class UnsafeLoader {
    public static Unsafe loadUnsafe() {
        return Unsafe.getUnsafe();
    }
}

While this example will compile, it is likely to fail at runtime unless the Unsafe class was loaded with the primary classloader. To ensure that happens the JVM should be loaded with the appropriate arguments, like:

java -Xbootclasspath:$JAVA_HOME/jre/lib/rt.jar:./UnsafeLoader.jar foo.bar.MyApp

The foo.bar.MyApp class can then use UnsafeLoader.loadUnsafe().

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	get*Volatile, put*Volatile, 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`

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.