Java Languagesun.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

  1. 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.

  2. 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

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:

UseAPI
Off heap / direct memory allocation, reallocation and deallocationallocateMemory(bytes), reallocateMemory(address, bytes) and freeMemory(address)
Memory fencesloadFence(), storeFence(), fullFence()
Parking current threadpark(isAbsolute, time), unpark(thread)
Direct field and or memory accessget* and put* family of methods
Throwing unchecked exceptionsthrowException(e)
CAS and Atomic OperationscompareAndSwap* family of methods
Setting out memorysetMemory
Volatile or concurrent operationsget*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`