How is null represented in memory in java

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What exactly is null in Java memory

Does anyone know how a null value is represented in the memory for an object or anything. In dot net it is represented by null indicator Boolean value and value of the corresponding type (Refer here). So does any one know how it is done in java ??

The question you referred to is somewhat misleading - the accepted answer is all about `Nullable`, not null *references*. — Jon Skeet, Jan 10 '13 at 12:21
check the accepted answer of this question.will help you to understand wt is null in java.http://stackoverflow.com/questions/2707322/what-is-null-in-java — Abin Manathoor Devasia, Jan 10 '13 at 12:24

score 9 · Accepted Answer · answered Jan 10 '13 at 12:22

9

From the JVM specification, section 2.4:

The Java virtual machine specification does not mandate a concrete value encoding null.

So it could be anything, but it's very likely to be a reference value which is all zeroes. References themselves can be simple pointers (i.e. just a virtual address in the same way as a native pointer, of the appropriate size for the architecture) but again don't have to be. Some JVMs have very smart ways of handling references - for example, Hotspot is able to use 32-bit values for references in many situations, even when running in an x64 process.

answered Jan 10 '13 at 12:22

Jon Skeet

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"The Java virtual machine specification does not mandate a concrete value encoding null." <-- hmm, in this case, how is it {de,}serializable? – fge Jan 10 '13 at 12:32
2

@fge: That's the *in-memory* representation, which is completely separate from serialization. – Jon Skeet Jan 10 '13 at 12:48

Evgeniy Dorofeev · Answer 2 · 2013-01-10T12:58:49.123

In theory it is a JVM secret, in practice it is 0 (0x00000000) on 32-bin VM. If you use Oracle JVM you can use sun.misc.Unsafe, which can read memory content at any address, to verify.

public class Test {
    int i1 = 100;
    String s = null;
    int i2 = 200;

    public static void main(String[] args) throws Exception {
        Test test = new Test();
        System.out.println(getInt(test, "i1"));
        System.out.println(getInt(test, "s"));
        System.out.println(getInt(test, "i2"));
    }

    static int getInt(Test test, String name) throws Exception {
        Unsafe u = getUnsafe();
        long offset = u.objectFieldOffset(Test.class.getDeclaredField(name));
        return u.getInt(test, offset);
    }

    static Unsafe getUnsafe() throws Exception {
        Field f = Unsafe.class.getDeclaredField("theUnsafe");
        f.setAccessible(true);
        return (Unsafe) f.get(null);
    }
}

output

100
0
200

this only proves that `Unsafe` (eventually) returns zero when trying to read a null reference as int. And also consider that piece of the documentation of getInt "...However, the results are undefined if that variable is not in fact of the type returned by this method..." — user85421, Jan 10 '13 at 13:25

Peter Lawrey · Answer 3 · 2018-02-24T08:07:34.863

The exact representation is not specified but I would assume it is just 0x0 like it is on other C like languages. It is a special value which cannot occur otherwise, not an additional field or extra piece of information.

Note: when you have a String it is a reference not an Object. This 4-byte value can refer to an object or in the case of null is a value which is not a valid object.

BTW: Most 64-bit JVMs use 32-bit references unless the heap is 32 GB or more. This is possible in an OpenJDK JVM as all objects are 8 byte (or 16 byte) aligned. As the lower 3 or 4 bits are always 0 they don't need to be stored expanding the address range by 8x or 16x the typical 4 GB range.

@HannesHertach I have added an explanation as to why 32 GB is addressable with a 32 bit reference. — Peter Lawrey, Feb 24 '18 at 08:09

score 1 · Answer 4 · answered Jan 10 '13 at 12:24

No memory should be allocated for the null itself, because null is not actually a valid object instance. It is simply a placeholder that indicates that the object reference is not currently referring to an object.

When you do something like:

String str = null;

The only thing allocated is a reference to a string (this is analogous to a string-pointer in languages which contain pointer-types). This reference is a 32 or 64 bit variable (depending upon your hardware architecture and Java version) which resides on the stack (probably, depending upon the exact context in which your declaration is placed).

And what is the reference in you conception? Just abstract number? — Andremoniy, Jan 10 '13 at 12:26

How is null represented in memory in java

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